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Ecosystem effects on harvested populations : lower trophic level dynamics in the northeast pacific and… Baumann, Michael 1998

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Ecosystem Effects on Harvested Populations: Lower Trophic Level Dynamics in the Northeast Pacific and Its Implications on Sockeye Salmon (Oncorhynchus nerka) Survival by Michael Baumann M.Sc, The University of Vienna, Austria A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF T H E REQUIREMENTS FOR T H E D E G R E E OF  Doctor of Philosophy in T H E F A C U L T Y OF G R A D U A T E STUDIES (Department of Earth and Ocean Sciences) We accept this thesis as conforming to the reauired standard  The University of British Columbia September 1998 © Michael Baumann, 1998  In  presenting  degree  this  at the  thesis  in partial  University of  freely available for reference copying  of  department  this or  fulfilment  British Columbia, and study.  of  I agree  I further agree  thesis for scholarly purposes by  his  or  her  the  may be  representatives.  It  is  requirements that the  for an advanced  Library shall make it  that permission for extensive granted  by the  understood  that  publication of this thesis for financial gain shall not be allowed without permission.  Department of  iVA  The University of British Columbia Vancouver, Canada  Date  DE-6 (2/88)  21 <S*aP  OCfcAM  So(=MC£Ss  head  of my  copying  or  my written  Abstract Almost all epipelagic fish species in the Northeast Pacific show an increase in population size between the late 1950s and the 1980s. The complexity of pelagic ecosystems makes speculations on the causes of these increases easy to justify, and thus various conjectures on the chain of events leading to increased fish survival have been put forward. In this thesis I try to explain the variability in cohort survival, abundance and distribution of sockeye salmon (Oncorhynchus nerka) - the fish species that has experienced the largest increase in abundance and biomass of all epipelagic fish species in the Northeast Pacific between the late 1950s and the 1980s - by ecosystem effects. I assumed that sockeye salmon total survival rate is largely determined in early marine life due to exposure to predators, which is set by the time at risk of predation, itself a function of sockeye prey, i.e. mesozooplankton, abundance. I then developed two simple food chain models with three and four trophic levels, respectively, which include lower trophic level dynamics but not fish itself. Both population models were calibrated and tested for two locations in the Northeast Pacific through mean field simulations driven by abiotic environmental forcings. Using a 4-hour time step from 1950 to 1990, both calibrated population models were then run as spatially-explicit simulations with a resolution of one degree latitude and longitude for the whole area of the Northeast Pacific, a total of 1240 open ocean fields. To assess the relative importance of biological processes versus physical advection both population models were simulated with and without surface currents. I have tried to design the best models within reason utilizing the best information on environmental forcings and biological processes available at the time. Simulation results do not suggest a clear linkage between prey density in the oceanic environment and sockeye salmon cohort survival. However, there are two fundamental lessons to be learned from this modeling  ii  exercise: First, categorization of ecosystem components into trophic levels with no regard of the many life history strategies is one of the worst aggregation errors in ecology, one that implicitly includes errors of hierarchical organization as well as of spatio-temporal stability. And second, the complexity of ecosystems will always make results from trophodynamic simulations interpretable, even if these results bear no relationship to the natural system.  iii  Table of Contents Abstract  ii  Table of Contents  iv  List of Tables  vi  List of Figures  vii  Acknowledgments  xvii  1. INTRODUCTION  1  1.1. In Which I Provide the Context  1  1.2. Interannual Variability: Facts and Speculations  5  1.3. Seasonal Variability: A Summary of the Current Paradigm and Some of Its Flaws  26  1.4. Approach, Assumptions and Anticipation  32  2. S O C K E Y E S A L M O N A N D T H E MARINE ENVIRONMENT  42  2.1. Ocean Feeding Ecology of Sockeye Salmon (Oncorhynchus nerka)  42  2.2. Ecosystems of the Northeast Pacific  49  2.2.1. The Central Subarctic Domain  51  2.2.2. The Coastal Downwelling and the Transitional Domain  76  2.3. Physical Oceanography of the Northeast Pacific  3.  POPULATION  MODELS,  M E A N FIELD SIMULATIONS  ENVIRONMENTAL  79  FORCINGS,  AND 85  iv  3.1. Essential State Variables  85  3.2. Environmental Forcings  92  3.2.1. Observed Variables  92  3.2.2. Derived Variables  93  3.3. Population Models 3.3.1. Point Equilibria and Stability Analysis 3.4. Mean Field Simulations  108 118 124  3.4.1. Simulation Results: 3-Trophic Levels Model  130  3.4.2. Simulation Results: 4-Trophic Levels Model  138  3.5. Sensitivity Analyses  144  3.5.1. Sensitivity Analyses: 3-Trophic Levels Model  144  3.5.2. Sensitivity Analyses: 4-Trophic Levels Model  150  4. SPATIALLY-EXPLICIT SIMULATIONS  161  4.1. Spatio-Temporal Resolution and Advection  161  4.2. Simulation Results and Analysis  177  4.2.1. Empirical Validation  177  4.2.2. Operational Validation  212  5. CONCLUSIONS  232  References  247  v  List of Tables  Table 1.1: Descriptive statistics of the return size for different stocks of the Fraser River system and different river systems of Bristol Bay. n is number of years in time series.  6  Table 2.1: Diet composition of maturing sockeye salmon in terms of trophic levels of prey items (% composition of stomach contents by weight).  47  Table 2.2: Life history characteristics of North American Pacific salmon species (Oncorhynchus spp.). Adapted from Pearcy (1992).  73  Table 3.1: Rate Equations for State Variables  110  Table 3.2: Important symbols used in models and simulations.  Ill  Table 3.3: 'Standard Run' initial conditions and parameter values for the mean field simulations of the 3- and 4-Trophic Levels Models.  131  vi  List of Figures  Fig.  1.1:  S u r v i v a l f o r 12 s o c k e y e s a l m o n s t o c k s o f the F r a s e r R i v e r s y s t e m ( B r i t i s h C o l u m b i a ,  C a n a d a ) f o r the b r o o d years 1 9 4 8 - 1 9 8 8 .  Fig.  1.2:  7  S u r v i v a l f o r 8 s o c k e y e s a l m o n r i v e r systems o f the B r i s t o l B a y area ( A l a s k a , U S A )  the b r o o d years 1 9 5 6 - 1 9 8 7 .  Fig.  1.3:  10  Histograms of correlation coefficients f r o m cross-correlations of survival indices for  different sockeye s a l m o n stocks.  Fig.  1.4:  12  C o h o r t s u r v i v a l f o r c o m b i n e d s t o c k s o f the F r a s e r R i v e r s y s t e m ( b r o o d years:  1948-  1988) a n d c o m b i n e d r i v e r systems o f the B r i s t o l B a y area ( b r o o d years: 1 9 5 6 - 1 9 8 7 ) .  Fig.  1.5:  Distribution and abundance  of combined  late j u v e n i l e  a n d adult stages o f  14  sockeye  s a l m o n for the periods 1 9 5 5 - 1 9 5 8 and 1980-1989.  Fig.  1.6:  Escapement index for c o m b i n e d stocks of Fraser R i v e r system (female spawners)  B r i s t o l B a y area (total escapement).  Fig.  1.7:  for  16  and 18  S u m m e r (15 J u n e - 31 J u l y ) z o o p l a n k t o n (size: > 3 5 0 p m ) b i o m a s s c o n c e n t r a t i o n s (in g  w e t w e i g h t / 1 0 0 0 m" ) f r o m c o m p o s i t e data. 3  19  vii  Fig.  1.8:  A n n u a l m e a n c h l o r o p h y l l - a c o n c e n t r a t i o n s at O c e a n W e a t h e r S t a t i o n P ( 5 0 ° N  145°W)  f r o m 1964 to 1991.  20  Fig.  1.9:  F l o w d i a g r a m o f p o s s i b l e energy transfers i n e c o s y s t e m s o f the N o r t h e a s t P a c i f i c .  37  Fig.  2.1:  E c o l o g i c a l upper zone d o m a i n s and p r e v a i l i n g currents i n the N E - P a c i f i c O c e a n .  50  Fig.  2.2:  C h l o r o p h y l l - a c o n c e n t r a t i o n at O c e a n W e a t h e r  Station P (50°N 145°W), as m o n t h l y  averages 1964-1991.  52  Fig.  2.3:  Seasonal  Fig.  2.4:  S p a t i a l distribution o f the onset o f i n c r e a s e d p r i m a r y p r o d u c t i v i t y i n the N E - P a c i f i c . 5 8  Fig.  2.5:  1  4  C p r i m a r y p r o d u c t i v i t y at S t a t i o n P f r o m c o m p o s i t e d a t a 1 9 8 4 - 9 0 .  Seasonal change  i n total biomass  o f net zooplankton  (mesh  e x c l u d e d ) f r o m c o m p o s i t e data at S t a t i o n P 1 9 7 1 - 1 9 7 4 .  Fig. 2.6:  A n n u a l life cycles o f  Neocalanus plumchrus a n d /V. cristatus  distribution (dark shading indicates higher abundance).  Fig.  2.7:  S e a s o n a l c h a n g e i n p r o t o z o a d e n s i t y at S t a t i o n P ( 1 9 6 6 - 1 9 6 8 ) .  s i z e 3 5 0 [im,  56  salps 64  w i t h respect to depth 66  70  viii  Development  a n d deterioration o f seasonal t h e r m o c l i n e s i n the o p e n N E - P a c i f i c .  Fig.  2.8:  Fig.  3.1: F l o w d i a g r a m s f o r t w o p o p u l a t i o n m o d e l s .  Fig.  3.2:  82  87  S e a s o n a l a n d i n t e r a n n u a l v a r i a b i l i t y i n s e a s u r f a c e ( m i x e d l a y e r ) t e m p e r a t u r e at S t a t i o n  P ( 5 0 ° N 1 4 5 ° W ) a n d at 5 0 ° N 130°W f o r 1981 t o 1 9 8 4 .  94  Fig.  3.3:  S e a s o n a l v a r i a b i l i t y i n the e x t i n c t i o n c o e f f i c i e n t k i n the N o r t h e a s t P a c i f i c .  95  Fig.  3.4:  S e a s o n a l a n d i n t e r a n n u a l v a r i a b i l i t y i n s e a s u r f a c e i n s o l a t i o n at S t a t i o n P ( 5 0 ° N 1 4 5 ° W )  a n d at 5 0 ° N 130°W f o r 1981 t o 1 9 8 4 .  Fig.  3.5:  Upper  panel:  Observed  monthly  97  mean  mixed  layer depth  plus/minus  o n e standard  d e v i a t i o n at S t a t i o n P ( 5 0 ° N 145°W) f o r the p e r i o d 1 9 4 7 - 1 9 6 3 .  Fig.  3.6:  S e a s o n a l a n d i n t e r a n n u a l v a r i a b i l i t y i n m i x e d l a y e r d e p t h at S t a t i o n P ( 5 0 ° N  a n d at 50°N 130°W f o r 1981 t o 1984.  Fig.  3.7:  99  145°W) 101  S e a s o n a l a n d i n t e r a n n u a l v a r i a b i l i t y i n E k m a n p u m p i n g at S t a t i o n P ( 5 0 ° N 145°W) a n d  at 5 0 ° N 1 3 0 ° W f o r 1 9 8 1 t o 1 9 8 4 .  104  ix  Fig.  3.8:  1969)  Seasonal variability i n the c a r b o n - t o - c h l o r o p h y l l - a ratio (data points f r o m M c A l l i s t e r  a n d t w o regression models  (carbon-to-chlorophyll-a  ratio as a f u n c t i o n o f s e a surface  i n s o l a t i o n ) f o r S t a t i o n P.  Fig.  3.9:  106  S e a s o n a l a n d interannual v a r i a b i l i t y i n t h e c a r b o n - t o - c h l o r o p h y l l - a ratio at S t a t i o n  (50°N 145°W) a n d at 50°N 130°W f o r 1981 t o 1984.  P  107  Fig.  3.10:  Possible functional responses o f a predator to prey density.  115  Fig.  3.11:  S t a b i l i t y a n a l y s i s o f the 3 - T r o p h i c - L e v e l s M o d e l .  120  Fig.  3.12:  S t a b i l i t y a n a l y s i s o f the 4 - T r o p h i c - L e v e l s M o d e l .  121  Fig.  3.13:  S i m u l a t e d c a r b o n c o n c e n t r a t i o n s f o r p h y t o p l a n k t o n (Po),  microzooplankton (H),and  m e s o z o o p l a n k t o n ( C O at S t a t i o n P (50°N 145°W) a n d at 5 0 ° N 1 3 0 ° W f o r 1981 t o 1 9 8 4 .  Fig.  3.14:  Simulated  microzooplankton  production  per  cubic  (H), and mesozooplankton  meter  and  day  ( d ) at S t a t i o n P  for  (50°N  phytoplankton  3.15:  S e a s o n a l p r i m a r y p r o d u c t i v i t y at S t a t i o n P ( 5 0 ° N 1 4 5 ° W ) a n d a t 5 0 ° N 1 3 0 ° W  1 9 8 1 - 8 4 s i m u l a t i o n results.  (P ), 0  145°W) a n d at 5 0 ° N  130°W f o r 1 9 8 1 t o 1 9 8 4  Fig.  132  135  from 136  x  Fig.  3.16:  Simulated  mass-specific  clearance  or filtration  rates  [liters  (mg C)"  d" ] f o r  1  1  m e s o z o o p l a n k t o n ( Q ) at S t a t i o n P ( 5 0 ° N 145°W) a n d at 5 0 ° N 1 3 0 ° W f o r 1981 t o 1 9 8 4 .  Fig.  3.17:  Simulated  mesozooplankton  carbon  concentrations  ( C O , and macrozooplankton  for phytoplankton  microzooplankton  (Po),  ( C ) at S t a t i o n P 2  139  (H),  ( 5 0 ° N 1 4 5 ° W ) a n d at 5 0 ° N  130°W f o r 1981 to 1984.  Fig.  3.18:  Simulated  microzooplankton  140  production  per  (H), mesozooplankton  cubic  meter  and  day  for  ( C O , and macrozooplankton  (C2)  phytoplankton  (P ),  at S t a t i o n P  (50°N  0  145°W) a n d at 5 0 ° N 1 3 0 ° W f o r 1981 t o 1 9 8 4 .  Fig.  3.19:  141  S i m u l a t e d m a s s - s p e c i f i c c l e a r a n c e rates o r f i l t r a t i o n rates [liters ( m g C ) " d" ] f o r 1  mesozooplankton  ( C O and macrozooplankton  ( C ) at S t a t i o n P 2  (50°N  1  145°W) a n d at 5 0 ° N  130°W f o r 1981 to 1984.  143  Fig.  3.20:  S e n s i t i v i t y t o i n i t i a l c o n d i t i o n s i n the 3 - T r o p h i c - L e v e l s M o d e l .  145  Fig.  3.21:  S e n s i t i v i t y t o photosynthetic e f f i c i e n c y a i n the 3 - T r o p h i c - L e v e l s M o d e l .  147  Fig.  3.22:  S e n s i t i v i t y t o s p e c i f i c r e s p i r a t i o n o r n o n - p r e d a t o r y death rates i n t h e 3 - T r o p h i c - L e v e l s  Model.  148  xi  Fig.  3.23:  S e n s i t i v i t y to predation parameters i n the 3 - T r o p h i c - L e v e l s M o d e l .  149  Fig.  3.24:  S e n s i t i v i t y to the f u n c t i o n a l r e s p o n s e i n the 3 - T r o p h i c - L e v e l s M o d e l .  151  Fig.  3.25:  S e n s i t i v i t y to i n i t i a l c o n d i t i o n s i n the 4 - T r o p h i c - L e v e l s M o d e l .  153  Fig.  3.26:  S e n s i t i v i t y to photosynthetic e f f i c i e n c y a i n the 4 - T r o p h i c - L e v e l s M o d e l .  154  Fig.  3.27:  S e n s i t i v i t y to s p e c i f i c r e s p i r a t i o n or n o n - p r e d a t o r y death rates i n the 4 - T r o p h i c - L e v e l s  Model.  156  Fig.  3.28:  S e n s i t i v i t y to predation parameters i n the 4 - T r o p h i c - L e v e l s M o d e l .  157  Fig.  3.29:  S e n s i t i v i t y to predation parameters i n the 4 - T r o p h i c - L e v e l s M o d e l .  158  Fig.  3.30:  S e n s i t i v i t y to the f u n c t i o n a l r e s p o n s e i n the 4 - T r o p h i c - L e v e l s M o d e l .  160  Fig.  4.1:  S i m u l a t e d b i o m a s s c o n c e n t r a t i o n [ m g C m" ] 3  A p r i l , July and October 1951.  Fig.  4.2:  for zero boundary conditions for January, 167  S i m u l a t e d b i o m a s s c o n c e n t r a t i o n [ m g C m" ] f o r z e r o - g r a d i e n t b o u n d a r y c o n d i t i o n s f o r  January, A p r i l , July and October 1951.  3  169  xii  Fig.  4.3:  Simulated  biomass  concentration  [mg C  m~ ] 3  for combined  zero  /  zero-gradient  b o u n d a r y c o n d i t i o n s (see text) f o r January, A p r i l , J u l y a n d O c t o b e r 1 9 5 1 .  Fig.  4.4:  Simulated biomass concentration [mg C  m" ] f o r O c t o b e r 3  1 9 6 0 (after  172  10 years  simulation).  Fig.  4.5:  Simulated  174  biomass  concentration  [mg C  m" ] 3  f r o m t w o runs w i t h r a n d o m  conditions.  Fig.  4.6:  4.7:  October  Simulated daily primary productivity  [mg C  m"  3  d" ] f o r J a n u a r y , A p r i l , J u l y a n d 1  179  S i m u l a t e d m i c r o z o o p l a n k t o n c o n c e n t r a t i o n [ m g C m" ] f o r January, A p r i l , J u l y a n d 3  o f 1 9 8 2 a n d 1983. S i m u l a t i o n : 4-trophic levels m o d e l s  with advection.  period: 1951-1990.  Fig.  4.8:  initial 175  October of 1982 and 1983.  Fig.  of  Simulated mesozooplankton  185  c o n c e n t r a t i o n [ m g C m" ] f o r J a n u a r y , A p r i l , J u l y a n d  O c t o b e r o f 1 9 8 2 a n d 1983. S i m u l a t i o n : 4-trophic levels m o d e l s period: 1951-1990.  Simulation  with advection.  Simulation 189  xiii  Fig.  4.9:  October  Simulated macrozooplankton  c o n c e n t r a t i o n [ m g C m~ ] f o r J a n u a r y , A p r i l , J u l y a n d 3  o f 1982 a n d 1983. S i m u l a t i o n : 4-trophic levels m o d e l s  with advection.  period: 1951-1990.  Fig.  4.10:  Simulation 193  S i m u l a t e d m i c r o z o o p l a n k t o n c o n c e n t r a t i o n [ m g C m" ] f o r J u l y o f 1 9 8 2 (upper panel) 3  and 1983 (lower panel). Simulation: 3-trophic levels models w i t h advection. S i m u l a t i o n period: 1951-1990.  198  S i m u l a t e d m i c r o z o o p l a n k t o n c o n c e n t r a t i o n [ m g C m" ] f o r J u l y o f 1 9 8 2 ( u p p e r p a n e l )  Fig.  4.11:  and  1983 (lower  3  panel).  S i m u l a t i o n : 4-trophic levels m o d e l s  without  advection.  period: 1951-1990.  199  S i m u l a t e d m i c r o z o o p l a n k t o n c o n c e n t r a t i o n [ m g C m" ] f o r J u l y o f 1 9 8 2 (upper p a n e l )  Fig.  4.12:  and  1983 (lower  3  panel).  Simulation:  3-trophic  levels models  without  period: 1951-1990.  Fig.  Simulation  4.13:  advection.  Simulation 200  S i m u l a t e d m e s o z o o p l a n k t o n c o n c e n t r a t i o n [ m g C m" ] f o r J u l y o f 1 9 6 2 ( u p p e r p a n e l ) 3  a n d 1963 (lower panel). S i m u l a t i o n : 4-trophic levels m o d e l s w i t h advection. S i m u l a t i o n period: 1951-1990.  Fig.  4.14:  202  S i m u l a t e d m e s o z o o p l a n k t o n c o n c e n t r a t i o n [ m g C m" ] f o r J u l y o f 1 9 8 2 (upper panel) 3  and 1983 (lower panel). Simulation: 3-trophic levels models w i t h advection. S i m u l a t i o n period: 1951-1990.  203  xiv  Fig.  4.15:  S i m u l a t e d m e s o z o o p l a n k t o n c o n c e n t r a t i o n [ m g C m" ] 3  for July and October of  (first t w o panels) and 1983 (second t w o panels). S i m u l a t i o n : 4 - t r o p h i c levels m o d e l s  1982  without  a d v e c t i o n . S i m u l a t i o n p e r i o d : 1951 - 1 9 9 0 .  Fig.  4.16:  204  S i m u l a t e d m a c r o z o o p l a n k t o n c o n c e n t r a t i o n [ m g C m" ] 3  for July and October of  (first t w o panels) a n d 1983 (second t w o panels). S i m u l a t i o n : 4 - t r o p h i c l e v e l s m o d e l s  1982  without  advection. Simulation period: 1951-1990.  Fig.  4.17:  206  M e a n s i m u l a t e d m e s o z o o p l a n k t o n c o n c e n t r a t i o n s [ m g C m" ] 3  for the m o n t h o f J u l y  1956 to 1959 (upper panel) a n d 1980 to 1989 ( l o w e r panel). S i m u l a t i o n : 4 - t r o p h i c l e v e l s m o d e l s with advection. Simulation period: 1951-1990.  Fig.  4.18:  210  M e a n s i m u l a t e d m e s o z o o p l a n k t o n c o n c e n t r a t i o n s [ m g C m" ] f o r the m o n t h o f A u g u s t 3  1956 to 1959 (upper panel) a n d 1980 to 1989 ( l o w e r panel). S i m u l a t i o n : 4 - t r o p h i c l e v e l s m o d e l s with advection. Simulation period: 1951-1990.  Fig.  4.19:  Monthly  simulated mesozooplankton concentrations [mg  211  C  m" ] 3  for July  1960  to  F e b r u a r y 1961 ( l o w s u r v i v a l year for Fraser R i v e r s o c k e y e salmon). N o t e the change i n scale f o r November  to F e b r u a r y m a p s . S i m u l a t i o n : 4 - t r o p h i c levels m o d e l s w i t h a d v e c t i o n . S i m u l a t i o n  period: 1951-1990.  214  xv  Fig.  4.20:  February November  Monthly 1984  simulated mesozooplankton  concentrations [mg  (high survival year for Fraser R i v e r  salmon).  C  Note  •3  m" ] f o r J u l y  the change  1983  in  to February maps. S i m u l a t i o n : 4-trophic levels models w i t h advection.  scale  4.21:  Monthly  for  Simulation  period: 1951-1990.  Fig.  to  218  simulated mesozooplankton  concentrations [mg  C  m" ] f o r J u l y  1971  to  F e b r u a r y 1972 ( l o w s u r v i v a l year f o r B r i s t o l B a y s o c k e y e s a l m o n ) . N o t e the c h a n g e i n scale f o r November  to February maps. S i m u l a t i o n : 4-trophic levels models w i t h advection.  Simulation  period: 1951-1990.  Fig.  4.22:  Monthly  223  simulated mesozooplankton  concentrations  [mg  C  m" ] 3  for July  1979  to  F e b r u a r y 1980 ( h i g h s u r v i v a l year for B r i s t o l B a y sockeye salmon). N o t e the change i n scale for November  to F e b r u a r y m a p s . S i m u l a t i o n : 4 - t r o p h i c l e v e l s m o d e l s w i t h a d v e c t i o n .  Simulation  period: 1951-1990.  Fig.  5.1:  Mean  month of July  simulated mesozooplankton  227  concentrations [mg  C  m" ]. 3  Upper panel: For  1 9 5 6 t o 1 9 5 9 as a r e s u l t o f t h e 4 - t r o p h i c l e v e l s s i m u l a t i o n w i t h o u t  the  advection.  L o w e r p a n e l : F o r t h e m o n t h o f J u l y 1 9 8 0 t o 1 9 8 9 as a r e s u l t o f t h e 4 - t r o p h i c l e v e l s s i m u l a t i o n with advection.  243  xvi  Acknowledgments  "All right. I think it's amazing that you've done as well as you have. You've got hardly any theory of social organization, astonishingly backward economic systems, no grasp of the machinery of historical prediction, and very little knowledge about yourselves. Considering how fast your world is changing, it's amazing you haven't blown yourselves to bits by now. That's why we don't want to write you off just yet. You humans have a certain talent for adaptability at least in the short term." I n C . S a g a n ( 1 9 8 5 ) Contact, p p . 3 6 0 - 3 6 1  I apologize  tom y wife Nani  and  m y children Phoebe  a n d M a i a that I h a v e  chosen a  p r o f e s s i o n w h i c h I t h o u g h t w o u l d b e a r s o m e c u l t u r a l as w e l l as s o c i o - e c o n o m i c r e l e v a n c e i n t h e future of h u m a n k i n d . I was wrong. I t h a n k A n d r e S t e i n h a u s e n w h o m I h a v e n o t s e e n f o r m o r e t h a n 18 y e a r s a n d w h o i n s p i r e d m y interest a n d f a s c i n a t i o n f o r " t h e order, the h a r m o n y , the u n i f o r m i t y a n d the u n i v e r s a l i t y o f the l a w s o f nature" (Chandrasekhar 1990). H e w a s w r o n g , too. I t h a n k P a u l L e B l o n d f o r a c c e p t i n g m e as a g r a d u a t e s t u d e n t a t a t i m e w h e n n o b o d y  else  w o u l d , a n d f o r p r o v i d i n g the a c a d e m i c f r e e d o m to e x p l o r e s c i e n t i f i c space. T h i s w o r k w a s f u n d e d i n part b y a grant to M . C . H e a l e y , P . H . L e B l o n d , a n d C . J . W a l t e r s ; a n d b y a grant to P . H . L e B l o n d . Input data u s e d i n the s i m u l a t i o n s (raw a n d spatially interpolated), s i m u l a t i o n s o u r c e c o d e , s i m u l a t i o n r e s u l t s , a s w e l l as t h e N E - P a c i f i c M a p V i e w e r c a n b e o b t a i n e d f r o m m e u p o n r e q u e s t .  Michael Baumann icos Research Labs. icos@salzburg.co.at  xvii  1. INTRODUCTION  "Cause-and-ejfect assertions ... are forever dubious because of the logical flaw o f p o s t h o c e r g o p r o p t e r h o c reasoning." G . Hardin (1985)  1.1. In Which I Provide the Context It i s t r i t e t o s a y t h a t a n o r g a n i s m i s u l t i m a t e l y d e p e n d e n t o n i t s e n v i r o n m e n t  a n d that t h e  c o m p o n e n t s o f this environment are biotic, i.e. food, competitors, predators, as w e l l asabiotic, s u c h as nutrients a n d c l i m a t i c factors. H o w e v e r , traditionally different b i o l o g i c a l organization levels a n d associated processes a n d patterns have b e e n studied i n different s u b - d i s c i p l i n e s o f ecology  (Odum  Ecology (Odum  1971). F o r e x a m p l e , energy  circuits  and biogeochemical cycles i n Systems  1983), food webs a n d spatio-temporal diversity i n C o m m u n i t y  Ecology, and  abundance a n d distribution o f individuals i n Population E c o l o g y . Thus, t o o little consideration has been g i v e n t o the effects o f populations, c o m m u n i t i e s , a n d ecosystems onto e a c h  other  ( Y o d z i s 1989). A d d i t i o n a l l y , there i s n o consensus o n the a n s w e r t o the m o s t f u n d a m e n t a l q u e s t i o n i n the environmental  sciences:  H o w to deal w i t h the complexity o f ecosystems (e.g. K r e b s 1 9 9 5 ;  O k s a n e n 1991; Peters 1977)? Different scientists have adopted different approaches: S o m e u s e statistical analyses (e.g. B e a m i s h & B o u i l l o n 1993; C y r & P a c e 1993; Francis & H a r e 1 9 9 4 ; Moen  & Oksanen  1991; X i e & H s i e h 1989), others suggest n o n - l i n e a r processes b e t w e e n a n  a b i o t i c a n d a b i o t i c v a r i a b l e ( e . g . A d k i s o n et al. 1 9 9 6 ; G a r g e t t 1 9 9 7 ; H i n c h et al. 1 9 9 5 ; H s i e h et al.  1 9 9 1 ; W e l c h et al.  1995), a n d a g a i n others study t h e d y n a m i c s (e.g. L a w t o n & P i m m  1978;  M a y 1 9 7 2 b ; M a y 1 9 7 6 b ; P i m m 1 9 8 2 ; P i m m & L a w t o n 1 9 7 7 ; P i m m et al. 1 9 9 1 ; S a u n d e r s 1 9 7 8 )  1  a n d e n e r g y transfers i n f a i r l y d e t a i l e d f o o d w e b m o d e l s (e.g. C h r i s t e n s e n & 1 9 9 3 ; K r e m e r & N i x o n 1 9 7 8 ; O d u m 1 9 8 3 ; P a u l y et al.  1996; W a l s h  Pauly 1995;  Frost  1981). B u t although  most  authors a c k n o w l e d g e that their analyses c a n n o t r e v e a l a l l aspects o f nature, o n l y f e w s t r a t e g i e s o n h o w t o d e a l w i t h i t s f u l l c o m p l e x i t y ( e . g . L e i r s et al. I n t h i s t h e s i s I t r y t o e x p l a i n v a r i a b i l i t y at t h e p o p u l a t i o n  demonstrate  1997). l e v e l b y the v a r i a b i l i t y i n  the  e c o s y s t e m s as a w h o l e , i.e. b i o t i c a n d a b i o t i c v a r i a b l e s , m o r e s p e c i f i c a l l y , the v a r i a b i l i t y i n c o h o r t survival, abundance and distribution of sockeye salmon  {Oncorhynchus nerka)  by biological and  p h y s i c a l p r o c e s s e s o c c u r r i n g i n t h e N o r t h e a s t P a c i f i c O c e a n . It i s a n a t t e m p t " t o p r y o p e n  the  b l a c k b o x o f r e c r u i t m e n t " (Steele 1996) i n order to understand l o n g - t e r m trends a n d to i d e n t i f y w h a t to l o o k for i n cases w h e r e s p e c i f i c predictions s h o u l d be made. M y study thus f o l l o w s the recommendation biology  that o n e needs " t o l o o k deeper  involved"  (Sugihara  1996)  ...  to m o r e f u n d a m e n t a l  i n order to r e s o l v e the l o n g - s t a n d i n g  studies o f the basic debate between  the  ' b i o t i c ' a n d the ' c l i m a t e ' s c h o o l i n e c o l o g y ( w i t h the first d e c l a r i n g internal b i o t i c m e c h a n i s m s a n d t h e l a t e r e x t e r n a l e n v i r o n m e n t a l f o r c i n g s as u l t i m a t e c a u s e s f o r p o p u l a t i o n l i m i t a t i o n ( S t e e l e &  Henderson  1994;  Sugihara  1995)  B u r k e n r o a d debate ( H i l b o r n & Walters  -  a controversy  known  in  f i s h e r i e s as t h e  Thompson-  1992).  T h e c h o i c e o f s o c k e y e s a l m o n p o p u l a t i o n s has s e v e r a l reasons (apart f r o m the v e r y p r a c t i c a l one  that  I  was  partially  funded  by  a  strategic grant  which  focused  on  this  species  (see  Acknowledgements): (1) S o c k e y e s a l m o n i s a h a r v e s t e d a n a d r o m o u s  species whose management requires  stock-specific (British C o l u m b i a , Canada) or river-specific ( A l a s k a , U S A )  abundance  annual  estimates  f o r a d u l t s r e t u r n i n g t o t h e i r p a r e n t a l s t r e a m s o r l a k e s y s t e m s . Its e c o n o m i c i m p o r t a n c e  makes  sockeye s a l m o n a w e l l studied species w i t h a wealth of available information.  2  (2) S o c k e y e s a l m o n i s s e m e l p a r o u s w i t h a c o n s t a n t l i f e c y c l e a n d c l e a r l y d e f i n e d l i f e h i s t o r y stages i n different habitats ( B u r g n e r 1991), w h i c h m a k e it easier t o study than s p e c i e s w i t h m i x e d l i f e - h i s t o r y stages i n a single habitat. (3) O f a l l P a c i f i c s a l m o n s p e c i e s s o c k e y e s a l m o n s h o w s t h e l o w e s t p r o p o r t i o n o f v a r i a b i l i t y i n t o t a l m o r t a l i t y that i s a c c o u n t e d f o r b y the f r e s h w a t e r stage ( 4 3 % ; B r a d f o r d (4) B e t w e e n t h e p e r i o d s 1 9 5 5 - 1 9 5 8  and 1980-1989  1995).  sockeye salmon experienced a 3.0-fold  increase i n abundance, the largest o f all epipelagic f i s h species i n the Northeast P a c i f i c & Ware  (Brodeur  1995).  A n d (5), available composite distribution data f o r sockeye salmon, i.e. s u m m e r months  data  o f s e v e r a l y e a r s , s h o w that t h e m a i n i n c r e a s e i n a b u n d a n c e b e t w e e n t h e p e r i o d s 1 9 5 5 - 1 9 5 8 a n d 1980-1989  occurred south o f the A l a s k a Peninsula w i t h a decline i n abundance o f f the B r i t i s h  Columbia  coast  represents  a s p a t i a l s c a l e that  environmental  (Brodeur  Ware  1995).  makes  conditions derived from  1 9 8 9 ; W o o d r u f f et al. The  &  Northeast  T h e physical  distance  between  it possible to distinguish regional available oceanographic  these  locations  abiotic and biotic  data (Ingraham &  Miyahara  1987) and a spatially-explicit e c o s y s t e m m o d e l .  Pacific  also  provides  some  advantages  to study  ecosystem  effects  on  populations: (1) L a r g e s p a t i a l l y - e x p l i c i t o b s e r v a t i o n a l as w e l l as m o d e l d a t a - s e t s a r e a v a i l a b l e f o r m a n y p h y s i c a l ( I n g r a h a m & M i y a h a r a 1 9 8 9 ; W o o d r u f f et al.  1987) a n d biological variables  1988; B r o d e u r & W a r e 1992; Brodeur & W a r e 1995; F a l k o w s k i & W i l s o n P a r s o n s et al.  1 9 6 6 ; P e a r c y et al.  1988; Sugimoto  & Tadokoro  (Brodeur  1992; Parsons  1972;  1997; X i e & H s i e h 1995), i n  a d d i t i o n t o t h e l o n g - t e r m p o i n t m e a s u r e m e n t s at O c e a n W e a t h e r S t a t i o n P at 5 0 ° N 1 4 5 ° W ( e . g . M i l l e r 1993b; M i l l e r  etal.  1991b; Parsons 1972; Parsons & L a l l i 1988; W o n g  etal.  1995).  3  (2)  D u r i n g their marine  phase N o r t h A m e r i c a n salmon reside almost entirely w i t h i n  Northeast P a c i f i c . T h u s , the Northeast P a c i f i c represents a large e n o u g h spatial scale  the  (Steele  1 9 9 1 ) t o i n v e s t i g a t e t h e i m p o r t a n t e c o l o g i c a l m e c h a n i s m s f o r a n a d r o m o u s a s w e l l as m a r i n e f i s h population  regulation, s t o c k o r r i v e r s p e c i f i c v a r i a t i o n i n s u r v i v a l rates, a n d  spatio-temporal  distribution. And  (3),  t w o international m u l t i m i l l i o n dollar projects ( P I C E S  1 9 9 3 ) , G L O B E C ( d e Y o u n g et relationship  between  al.  climate and  (Hargreaves  &  Sugimoto  1994)) have been l a u n c h e d i n recent years to investigate the biological production.  Progress  in  these  projects  can  be  i n c o r p o r a t e d i n m y e c o s y s t e m m o d e l s and, representing the i n f o r m a t i o n f l u x c o u n t e r f l o w , results f r o m m y w o r k p o t e n t i a l l y c o u l d be u s e d to m o d i f y the o b s e r v a t i o n a l p r o g r a m s o f these projects w i t h respect to b i o l o g i c a l organization l e v e l , area and/or  t i m e o f i n t e r e s t . A f t e r a l l , as  Oleg  G r i t s e n k o r e m a r k e d , "the north P a c i f i c O c e a n m a y be the best laboratory i n the w o r l d to study how  c a r r y i n g c a p a c i t y [a p o p u l a t i o n v a r i a b l e ]  relates to f l u c t u a t i o n s i n c l i m a t e [the  ...  ultimate  f o r c i n g function i n an ecosystem]." ( M a c C a l l 1996, annotations i n brackets by yours truly)  4  1.2. Interannual Variability: Facts and Speculations Sockeye Salmon Interannual variability i n sockeye salmon survival for 12 Fraser R i v e r stocks and 8 B r i s t o l B a y r i v e r s y s t e m s ( T a b l e 1.1) c a n b e s e e n i n t h e s u r v i v a l t i m e s e r i e s p l o t t e d i n F i g . 1.1 a n d F i g . 1.2, r e s p e c t i v e l y . ( P l e a s e n o t e t h a t a l t h o u g h t h e b a s i c b i o l o g i c a l u n i t o f m a n a g e m e n t a n d data availability i s different  f o r the F r a s e r R i v e r (stock) a n d B r i s t o l B a y  reasons o f clarity I w i l l refer t o sockeye populations  (river  thus  system), f o r  as ' s t o c k s ' i n the f o l l o w i n g d i s c u s s i o n .  N e v e r t h e l e s s , I a c k n o w l e d g e that the u n i t s t o c k , i.e. a c o l l e c t i o n o f i n d i v i d u a l s that i s d o m i n a t e d b y birth a n d death rather than m i g r a t i o n processes, i s not easy t o identify i n practice (Walters 1986)). Fraser R i v e r stocks s h o w no clear temporal " r e g i m e s " (Steele 1996) o funusually l o w o r h i g h s u r v i v a l e x c e p t that a l l c o h o r t s o f the b r o o d years 1957 to 1962 s e e m to h a v e h a d a p e r i o d o f l o w s u r v i v a l . O n the other h a n d , stocks f r o m B r i s t o l B a y r i v e r systems c o n s i s t e n t l y s h o w a n interval o f l o w survival between 1967 and 1972 f o l l o w e d b y five years o f h i g h s u r v i v a l w h e n , i n 1977, m o s t stocks returned very suddenly to a l o w s u r v i v a l phase w i t h a trend t o w a r d s  improved  s u r v i v a l thereafter. T h e m o r e consistent t e m p o r a l v a r i a t i o n i n the s u r v i v a l i n d e x a m o n g s t o c k s i n the B r i s t o l B a y r i v e r systems is also reflected i n the c r o s s - c o r r e l a t i o n s b e t w e e n different s t o c k s w i t h i n the s a m e r i v e r s y s t e m w h e r e o n l y 3 6 % o f t h e F r a s e r R i v e r ( F i g . 1.3a) b u t 7 1 % o f t h e B r i s t o l B a y s t o c k s (Fig.  1.3b)  are s i g n i f i c a n t l y p o s i t i v e l y c o r r e l a t e d . A l t h o u g h  stock survival index  correlations  a m o n g r i v e r s y s t e m s ( F i g . 1.3c) s h o w a r a n g e f r o m s i g n i f i c a n t l y n e g a t i v e t o s i g n i f i c a n t l y p o s i t i v e w i t h the m a j o r i t y b e i n g u n c o r r e l a t e d , i t s h o u l d b e n o t e d that the t w o largest s t o c k s f r o m e a c h r i v e r s y s t e m , i.e. the A d a m s s t o c k o f the F r a s e r R i v e r a n d the K v i c h a k s t o c k s o f the B r i s t o l B a y  5  Table  1.1: D e s c r i p t i v e  statistics o f the return size f o r different s t o c k s o f the F r a s e r R i v e r s y s t e m  a n d different r i v e r systems o f B r i s t o l B a y . n is n u m b e r o f years i n the t i m e series.  n  Median  Mean  S. D .  (in 1000s)  (in 1000s)  (in 1000s)  81.6  2051.2  3322.2  41  1046.8  1283.3  1029.2  41  Fraser River System (Stocks) Adams C h i l k o River and North E n d L a k e Horsefly River  6.7  823.1  2169.4  41  Stellako River  356.3  463.9  352.3  41  57.8  423.4  721.0  41  Birkenhead River  261.3  377.2  323.2  41  E a r l y Stuart  173.6  317.7  383.3  41  Weaver Creek  152.4  238.6  307.3  41  L a t e Stuart  Seymour River  68.8  152.7  190.8  41  Cultus Lake  54.8  75.8  87.2  41  U p p e r Pitt R i v e r  61.1  72.0  51.5  41  Bowron River  28.1  50.0  50.6  41  11801.4  14970.6  35  4882.3  5055.6  36  Bristol Bay River Systems Kvichak  5160  Egegik  2857.5  Naknek  2501  3322.3  2518.1  35  Wood  1909  2313.6  1276.8  35  Ugashik  1110.5  2249.9  2423.6  36  Igushik  536  858.5  829.5  35  Branch  358  475.1  417.7  35  Togiak  377  474.3  304.2  35  6  Adams  Chilko River and North End Lake 4  Horsefly  5  t  0 II c  T  I M I I I I I I I I I I I I I I I II  o  t  c  w  i  M  o  c  c  o  o  o  c  i  D  -  c  c  o  o  r  I I I I I I I 1 I I I I 1 I I I i I I o  ^  t  j  ^  t  c  D  o  O  c  ^  o  c  o  c  o  Stellako 4  0 II c ^ O  Fig.  1.1:  I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I o o g c o O ' t f c o c M t o o ^ ' O • t O l O C O C O C O N - h . c O O O C > 0 ) 0 > 0 5 0 ) 0 ) 0 5 0 ) 0 5 0 ) 0  Brood Year  I  O O >  """"""""  Survival f o r 12 sockeye salmon stocks o f the Fraser R i v e r system (British C o l u m b i a ,  C a n a d a ) f o r the b r o o d years 1948-1988 w i t h : ( S u r v i v a l Index).. = In ( 1  Total Recruitment o fB r o o d Y e a r Class i — — ).. Female Escapement i n Y e a r l :  1  i a n d j denote brood year a n d stock, respectively. F o rgeographical location a n d coordinates o f l a k e s o r r i v e r s see G r o o t a n d M a r g o l i s ( 1 9 9 1 ) : G e o g r a p h i c a l I n d e x ,  p.523.  7  Late Stuart 6  c T G  o f >  c M e o o f m w < o e o < 0 ) 0 ) 0 3 0 >  c o 0  o e g t t " - f » > 0 ) 0 )  o o 0  o o >  o 0  ^  o >  c  c 0  o  o )  Early Stuart 4  Weaver Creek 5  -1 I [ C T O  ^  Fig. 1.1: Continued  I I I I I I I I I I I [ I I I I I I I I I I I I I I I I I I I I I I I I II I O C V J C O O ^ t c O C M t O O ^ C O f r w i o < o t o < o r - h » o o c o c o > 0 ) 0 ) 0 ) 0 ) 0 > 0 > 0 ) 0 ) 0 ) 0 )  "~  Sroo3 Year  I  Cultus Lake 4t  Upper Pitt  6roo3 Year  Fig. 1.1: C o n t i n u e d  Kvichak  Egegik  Naknek  Wood  Brood Year  Fig. 1.2: Survival for 8 sockeye salmon river systems of the Bristol Bay area (Alaska, USA) for the brood years 1956-1987 with: Total Recruitment of Brood Year Class i (Survival Index),. = In ( —— ).. Escapement in Year i i and j denote brood year and river system, respectively. For geographical location and coordinates of lakes or rivers see Groot and Margolis (1991): Geographical Index, p.523. :  1  1  10  Ugashlk  Igushik  t U 0  O ) )  t  O 0  O  ^ <  )  0  C O )  O  (  O  0  )  C h  M I )  0  l 0  O C )  ^  0 0  O  ( )  '  X 0  *  > )  Branch  -2 I  I I I I I I I I I I I I II  c m 0  o < )  o 0  o  < 0  ^  c  o )  o o  < 0  f  )  I I I I I I I I I I I I I I I I I  c  »  0  j  r  )  t 0  -  )  o -  0  c  o  o )  ^  c 0  -  o )  Toglak  -1 I  II  I I I I I I I I I I I I I I I I I I I I I I I I I I I I I  c  o  0  )  o 0  )  q 0  )  0  c  g  e  )  0  )  M 0  Brood Year  Fig. 1.2:  Continued  i )  o 0  o )  0  t )  (a) Within the Fraser River System, n=66 25 20 + >• 35 o Sio u. 5  JZL  0  -0.1325  -+-  •+-  0  -+-  0.1325  0.265  0.3975  0.53  0.6625  (b) Within the Bristol Bay System, n=28 10  T  8  1 + 6  3  §" 4 2 0  0.14  „,  0.28  0.42  0.7  0.56  (c) Among Fraser River and Bristol Bay Stocks, n=96  35 — 30 25 + >> £20  Il5 -  u.  10 5 0  Fig. 1.3: H i s t o g r a m s  -0.54  -0.36  ^-0.18, ^  a  „ . Q.18  x  Correlation Coefficient r  0.36  0.54  o f correlation coefficients f r o m cross-correlations o f survival indices for  d i f f e r e n t s o c k e y e s a l m o n s t o c k s , a ) S t o c k s w i t h i n t h e F r a s e r R i v e r s y s t e m (ro.o5(i),38 = 0 . 2 6 4 ) ; b r o o d years 1956-1987; b r o o d years  1948-1988;  b ) S t o c k s w i t h i n t h e B r i s t o l B a y a r e a (r .05(i),3o = 0 . 2 9 6 ) , b r o o d o  years  ( c ) A m o n g s t o c k s o f t h e F r a s e r R i v e r s y s t e m a n d B r i s t o l B a y a r e a (ro.os(2),3o ?= 0 . 3 4 9 ) , 1956-1987.  F o r d e f i n i t i o n s o f s u r v i v a l i n d i c e s s e e F i g . 1.1  a n d F i g . 1.2. V a l u e s i n  b r a c k e t s are the c r i t i c a l v a l u e s f o r the c o r r e l a t i o n c o e f f i c i e n t r at the 0 . 0 5 s i g n i f i c a n c e l e v e l . N o t e t h a t x - a x i s l a b e l s r e p r e s e n t c l a s s m a x i m a . S h a d e d b a r s are s i g n i f i c a n t .  12  s y s t e m , c o n t r i b u t i n g 3 2 . 5 % a n d 4 4 . 7 % t o the m e a n total return o f the r e s p e c t i v e r i v e r  system  ( T a b l e 1.1), h a v e a s i g n i f i c a n t p o s i t i v e c o r r e l a t i o n c o e f f i c i e n t o f 0 . 4 2 . T h e c o h o r t s u r v i v a l t i m e series for c o m b i n e d Fraser R i v e r and c o m b i n e d B r i s t o l B a y stocks ( F i g . 1.4) s h o w t h a t t h e l a t e r h a v e g e n e r a l l y a g r e a t e r v a r i a b i l i t y . C o m b i n e d F r a s e r R i v e r s t o c k s h a d a p h a s e o f p o o r s u r v i v a l f r o m 1956 to 1963 a n d that i n t e r a n n u a l v a r i a b i l i t y s e e m s to h a v e increased f r o m 1975 to 1988 c o m p a r e d to pre-1975. B r i s t o l B a y stocks h a d a l o w s u r v i v a l phase f r o m 1966 to 1971 and a f i v e year h i g h phase f r o m 1972 to 1977. M o s t i m p o r t a n t l y , there is n o i n d i c a t i o n o f e x c e p t i o n a l h i g h s u r v i v a l i n s o c k e y e after a h y p o t h e t i c a l c h a n g e i n c a r r y i n g c a p a c i t y 1 9 7 6 / 7 7 ( B r o d e u r & W a r e 1 9 9 5 ; E b b e s m e y e r et al. 1 9 9 1 ; I s h i d a et al. 1 9 9 3 ; K e r r 1 9 9 2 ; M a c C a l l 1 9 9 6 ; V e n r i c k et al. 1 9 8 7 ) as h a s b e e n c o n j e c t u r e d f o r m a n y f i s h p o p u l a t i o n s i n t h e  Northeast  P a c i f i c ( B e a m i s h 1993; B e a m i s h & B o u i l l o n 1993; B r o d e u r & W a r e 1995) a n d w h i c h has been attributed to a shift i n the o c e a n - a t m o s p h e r e  system i n general and i n the strength, extent  and  l o c a t i o n o f the A l e u t i a n L o w P r e s s u r e S y s t e m i n p a r t i c u l a r ( B e a m i s h 1995). N o t e that the l o w e r s u r v i v a l r a t e o f B r i s t o l B a y s o c k e y e i n F i g . 1.4 i s a c o n s e q u e n c e o f d i f f e r e n t e s c a p e m e n t i n d i c e s u s e d i n the c a l c u l a t i o n o f c o h o r t s u r v i v a l rate, i.e. f e m a l e s p a w n e r s f o r F r a s e r R i v e r a n d total escapement for B r i s t o l B a y stocks.  Inconsistencies I n s p i t e o f t h e a c c e p t a n c e b y m a n y s c i e n t i s t s o f (1) a c l i m a t e s h i f t e v e n t i n t h e  Northeast  P a c i f i c i n the m i d - 1 9 7 0 s a n d (2) a l i n k b e t w e e n c l i m a t e a n d f i s h r e c r u i t m e n t v a r i a b i l i t y , there are s e v e r a l s e r i o u s p r o b l e m s ( B a u m a n n 1998) a s s o c i a t e d w i t h s o m e o f the data a n d c o n c l u s i o n s that h a v e b e e n d r a w n f r o m t h e m , as w e l l as c o n c e p t u a l f l a w s i n t h e i n t e r p r e t a t i o n h o w  ecosystems  w o r k , w h i c h I w i l l d i s c u s s i n the f o l l o w i n g paragraphs.  13  Fig.  1.4:  C o h o r t s u r v i v a l f o r c o m b i n e d s t o c k s o f the F r a s e r R i v e r s y s t e m ( b r o o d y e a r s :  1948-  1988) a n d c o m b i n e d r i v e r systems o f the B r i s t o l B a y area ( b r o o d years: 1 9 5 6 - 1 9 8 7 ) w i t h : ^ ( S u r v i v a l I n d e x ) , = In —  (Total Recruitment o f B r o o d Y e a r C l a s s i) . ;  ^ 2_! ( E s c a p e m e n t i n Y e a r i ) .  .  ;  i a n d j denote b r o o d year a n d stock, respectively. E s c a p e m e n t is total f e m a l e s p a w n e r s f o r the F r a s e r R i v e r s y s t e m a n d total escapement f o r the B r i s t o l B a y area stocks.  14  B e a m i s h & B o u i l l o n ( 1 9 9 3 ) state that " t h e r e w a s n o s i g n i f i c a n t c o r r e l a t i o n o b s e r v e d w h e n w e u s e d l i n e a r r e g r e s s i o n analysis to c o m p a r e the annual A l e u t i a n L o w a n n u a l N o r t h P a c i f i c O c e a n S a l m o n P r o d u c t i o n ..." using  doubtful  ambiguous  proxies  (i.e.  Pressure Index, and  the  S u r p r i s i n g about this result is that i n spite o f  "index-of-measurement  units, location and/or time of measurement)  error"  (Baumann  &  LeBlond  1996):  for s a l m o n p r o d u c t i o n (catch) and the  A l e u t i a n L o w ( s u m o f w i n t e r a n d s p r i n g m e a n s o f the area o f the N o r t h P a c i f i c O c e a n c o v e r e d b y the A l e u t i a n L o w pressure system w i t h less than 100.5 hPa) plus various s m o o t h i n g techniques, B e a m i s h and B o u i l l o n couldn't c o m e up w i t h a significant correlation. These authors also failed to report h o w m a n y d i f f e r e n t l y treated datasets they h a d s c a n n e d i n s e a r c h f o r a c o r r e l a t i o n , o n e o f the q u a l i t y c o n t r o l c r i t e r i a suggested b y W a l t e r s & C o l l i e (1988). T o m a k e m y s e l f clear: T h e r e m a y w e l l be a l i n k b e t w e e n c l i m a t e a n d f i s h p r o d u c t i o n i n the N o r t h e a s t P a c i f i c , but b e c a u s e o f the m a n y n o n - l i n e a r f u n c t i o n a l r e l a t i o n s h i p s i n the c a u s a l c h a i n o f e v e n t s it i s u n l i k e l y to b e detected b y linear regression analysis - a p r o b l e m w h i c h has already been addressed b y  others  (e.g. F r a n c i s & H a r e 1994). Brodeur abundance  &  Ware  (1995) report  b e t w e e n the periods  that s o c k e y e s a l m o n  1955-1958  and  experienced a 3.0-fold  1980-1989 (Fig.  1.5),  the largest  increase  in  population  g r o w t h o f a l l e p i p e l a g i c n e k t o n i n t h e N o r t h e a s t P a c i f i c . It c o u l d b e a r g u e d t h a t t h e i n c r e a s e i n a b u n d a n c e after the h y p o t h e t i c a l 1 9 7 6 / 7 7 c l i m a t e event s h o u l d be the largest f o r s o c k e y e s a l m o n o f a l l s a l m o n s p e c i e s b e c a u s e i t e n t e r s t h e o c e a n at a l a r g e r s i z e t h a t m a k e s i t p o s s i b l e t o e x p l o i t spatially  and  temporally  independent  production  patches  the  best.  Or  increased  freshwater  s u r v i v a l rates due to a temperature rise i n lakes i n A l a s k a c o u l d h a v e affected the increase i n total survival in some  sockeye salmon  stocks through higher  body  growth  rates a n d  thus  larger  15  Fig. 1.5: Distribution and abundance of combined late juvenile and adult stages of sockeye salmon for the periods 1955-1958 and 1980-1989. Abundance is estimated as the number offish caught per kilometer of surface gill net per 12 hours. Data were collectedfromMay to August in both periods. Adapted from Brodeur and Ware (1995).  16  b o d y s i z e w h e n e n t e r i n g the m a r i n e e n v i r o n m e n t m a k i n g it easier f o r this s p e c i e s to e x p l o i t p r e y p a t c h e s b e t t e r a n d e s c a p e p r e d a t o r s . H o w e v e r , a l o o k at e s c a p e m e n t ( F i g . 1.6) a n d c o h o r t s u r v i v a l d a t a ( F i g . 1.4) f o r c o m b i n e d F r a s e r R i v e r a n d c o m b i n e d B r i s t o l B a y s t o c k s r e v e a l s , f i r s t , t h a t total s o c k e y e a b u n d a n c e i n the N o r t h e a s t P a c i f i c is l a r g e l y d e t e r m i n e d b y A l a s k a n stocks, s e c o n d , that i n c r e a s e d a b u n d a n c e i n the 1980s m a y w e l l b e the result o f a n e x t r e m e l y  and large  e s c a p e m e n t i n 1 9 8 0 , a l t h o u g h c o h o r t s u r v i v a l w a s rather l o w f o r that b r o o d y e a r . I m u s t a d m i t that m y o b j e c t i o n s d o not r e s o l v e the i n c r e a s e d a b u n d a n c e o f o t h e r e p i p e l a g i c nekton (Brodeur  &  Ware  1995), o f w h i c h m a n y  are e x c l u s i v e l y m a r i n e , n o r the d o u b l i n g  in  s u m m e r z o o p l a n k t o n (size: > 3 5 0 ujn) standing s t o c k i n the N o r t h e a s t P a c i f i c b e t w e e n the p e r i o d s 1 9 5 6 - 1 9 6 2 a n d 1 9 8 0 - 1 9 8 9 ( F i g . 1.7; B r o d e u r & W a r e 1 9 9 2 ) . H o w e v e r , t h e r e h a v e a l s o b e e n n o e x p l a n a t i o n s w h y h i g h e r trophic l e v e l s d o not c o n s u m e the i n c r e a s e d z o o p l a n k t o n s t a n d i n g s t o c k n o r w h y it does not h a v e any, d e p e n d i n g o n the structure o f the f o o d c h a i n , p o s i t i v e o r n e g a t i v e consequences on phytoplankton concentrations (Fig.  Physics  1.8).  Fish  C h a n g e s at v a r i o u s s p a t i a l a n d l o n g e r - t h a n - s e a s o n a l t e m p o r a l s c a l e s h a v e b e e n r e p o r t e d f o r several s y s t e m s , or its c o m p o n e n t s , o f the N o r t h P a c i f i c . T h e s e reports c o m p r i s e i n t e r a n n u a l a n d interdecadal shifts i n radiation f l u x (Sugimoto & T a d o k o r o 1997), sea surface temperature  and  w i n d speed (Royer 1989; S u g i m o t o & T a d o k o r o 1997; W a r e 1995); i n strength, location, spatial extent, a n d d u r a t i o n o f the A l e u t i a n L o w Pressure S y s t e m w i t h the a s s o c i a t e d m o d i f i c a t i o n i n strength and direction o f ocean currents (Trenberth 1990), m i x e d layer depth a n d m i x i n g events (Polovina  et  al.  1994),  and  spatio-temporal changes in u p w e l l i n g  (Xie  &  Hsieh  1995);  in  17  Bristol Bay 40000  0 1  CO cn  I  I I I I I I I I I I I CM in cn T~  Fig.  1.6:  to m en  o CD O)  I I I I I I I I I I I M  T—  s  CO CD 05  CM O) .—  I I I I I I I I CD  cn  .—  o CO cn  I I II oo cn  I I 1  CO CO cn  Escapement index for c o m b i n e d stocks of Fraser R i v e r system (female spawners)  and  B r i s t o l B a y area (total escapement). N o t e the different scales o n the v e r t i c a l axes a n d the large escapement for B r i s t o l B a y sockeye i n 1980.  18  Fig.  1.7:  S u m m e r ( 1 5 J u n e - 3 1 J u l y ) z o o p l a n k t o n ( s i z e : > 3 5 0 p:m) b i o m a s s c o n c e n t r a t i o n s ( i n g  w e t w e i g h t / 1 0 0 0 m " ) f r o m c o m p o s i t e d a t a f o r (A) 3  1956-1959, (B)  1960-1962, and (C)  1980-  1989 (without 1986). A d a p t e d f r o m B r o d e u r and W a r e (1992).  19  Fig.  1.8:  A n n u a l m e a n c h l o r o p h y l l - a c o n c e n t r a t i o n s at O c e a n W e a t h e r S t a t i o n P ( 5 0 ° N  145°W)  f r o m 1 9 6 4 t o 1991. N o t e that v a r i a b i l i t y a r o u n d the m e a n i s n o t reported, thus n o c o n c l u s i o n s about seasonal a n d o r spatial variability (patchiness) i n the samples c a n b e inferred.  Adapted  f r o m W o n g et a l . (1995).  20  chlorphyll-a concentration (Sugimoto Falkowski PHOEBE  &  Wilson  1992;  in phytoplankton  1995; Francis &  Tadokoro  Falkowski & standing  ( B r o d e u r & W a r e 1 9 9 2 ; L o n g h u r s t et al. fish production (Beamish  &  Wilson  V e n r i c k et  1993;  Welch  stock ( M c A l l i s t e r  1972);  in  1987;  al.  1993;  Wong  zooplankton  but et  see  also  1995);  al.  concentration  1972; M c A l l i s t e r 1972; S u g i m o t o & T a d o k o r o 1997); i n  1993; B e a m i s h  1995; B e a m i s h &  Sibley 1991; H o l l o w e d &  i n d i c a t o r s ( E b b e s m e y e r et al.  1997;  Wooster  1 9 9 1 ; P o l o v i n a et al.  1992);  Bouillon  1993; Brodeur  and in many  &  Ware  other  environmental  1994). L a r g e - s c a l e , l o n g - t e r m  configurations  i n a b i o t i c a n d b i o t i c variables have b e e n t e r m e d " r e g i m e s " , w i t h the r e l a t i v e l y r a p i d ( w i t h respect to the m e a n persistence o f a regime) p o t e n t i a l l y reversible transformations b e i n g c a l l e d ' c l i m a t e e v e n t s ' ( P o l o v i n a et al.  1994), 'climate or regime shifts' (Francis & H a r e 1994; K e r r 1992; Steele  1996), or 'changes i n carrying capacity' (Brodeur 1 9 9 6 ; V e n r i c k et al.  &  Ware  1 9 9 5 ; I s h i d a et  al.  1993;  MacCall  1987), d e p e n d i n g o n the s y s t e m s t u d i e d . W a r e ( 1 9 9 5 ) has i d e n t i f i e d a b i o t i c  e n v i r o n m e n t a l o s c i l l a t i o n s at v a r i o u s t i m e s c a l e s w i t h p e r i o d s o f 2 - 3 y e a r s , 5 - 7 y e a r s , 2 0 - 2 5 y e a r s a n d 5 0 - 7 5 years. H e f o u n d that r e g i m e shifts ( l i k e that i n 1 9 7 6 / 7 7 ) o c c u r w h e n the t w o l o w e r f r e q u e n c y o s c i l l a t i o n s are i n p h a s e , a g e n e r a l c o n c l u s i o n that m a y b e q u e s t i o n e d c o n s i d e r i n g that o n l y 100 years of data were available. R e p o r t s o n r e g i m e shifts u s u a l l y i n c l u d e speculations o n either their u l t i m a t e causes o r the mechanistic c h a i n of events leading f r o m one phenomenon phenomenon.  to another  supposedly  dependent  S i m p l e (as o p p o s e d t o c o m p l e x , i . e . i n c l u d i n g f a c t o r i n t e r a c t i o n , f e e d b a c k l o o p s ,  time lags, thresholds, limits and breakpoints (Baumann complicated (many components),  & L e B l o n d 1996)), although  sometimes  l i n e a r e x p l a n a t i o n s i n v o k e e x t e r n a l m e c h a n i s m s that d r i v e  c h a n g e . E x a m p l e s are c h a n g e s i n the total or spectral output o f the s u n ( K e l l y & W i g l e y L a c i s & C a r l s o n 1992; M a d d o x  1995; Schlesinger & Ramankutty  a  1992;  1992), or proxies thereof such  21  as t h e s u n s p o t c y c l e ( F r i i s - C h r i s t e n s e n & L a s s e n 1 9 9 1 ; K e r r 1 9 8 7 ; K e r r 1 9 9 1 ) o r t h e a t m o s p h e r i c semi-diurnal tide (Cooper pers. c o m m . ) .  More  1993), or v a r i a t i o n i n the earth's angular m o m e n t u m  (Beamish  c o m p l e x externally forced causes for regime shifts i n v o l v e  1996  atmospheric  t e l e c o n n e c t i o n s , s u c h as E l N i n o - S o u t h e r n O s c i l l a t i o n e v e n t s ( K e r r 1 9 9 2 ; M a n n & L a z i e r 1 9 9 1 ; Parsons & L a l l i 1988; Trenberth 1990; W a r e 1995; W o o s t e r & Fluharty 1985), c l i m a t i c cycles ( L a t i f & Barnett 1994; W a r e 1995) a n d f o o d c h a i n considerations ( W a r e & T h o m s o n Further,  1991).  s p e c u l a t i o n s o n the c a u s a l c h a i n o f events are easy to i m a g i n e g i v e n the  many  c o m p o n e n t s a n d processes i n an e c o s y s t e m (Walters & C o l l i e 1988). E x a m p l e s f o r the p h y s i c a l a n d b i o l o g i c a l i n t e r a c t i o n s that h a v e b e e n i n v e s t i g a t e d i n order to u n d e r s t a n d the r e l a t i o n s h i p b e t w e e n c l i m a t e a n d f i s h p r o d u c t i o n are: a t m o s p h e r i c p r e s s u r e a n d f i s h p r o d u c t i o n ( B e a m i s h Bouillon  1 9 9 3 ) , w i n d a n d w a t e r c o l u m n s t a b i l i t y ( P o l o v i n a et  al.  (Blackett 1993), w i n d and chlorophyll concentration (Sugimoto primary production (Ware & Thomson Ware  1992),  wind  and  p r o d u c t i o n ( P a r s o n s et chlorophyll  fish survival al.  concentration  phytoplankton production and  w i n d and  Tadokoro  nutrients  1997), w i n d  1991), w i n d and z o o p l a n k t o n standing stock (Brodeur (Blackett  1993),  water  column  stability and  and &  primary  1966), water c o l u m n stability and f i s h p r o d u c t i o n (Gargett  1997),  and  1997),  zooplankton  standing  standing stock and fish production fish  &  1994),  &  stock  (Ware  &  (Sugimoto Thomson  &  Tadokoro  1991),  phytoplankton  production (Iverson 1990), and f i s h abundance and z o o p l a n k t o n  standing  s t o c k ( S u g i m o t o & T a d o k o r o 1997). W h a t these studies h a v e i n c o m m o n is the often e x p l i c i t but s o m e t i m e s o n l y i m p l i c i t r e f e r e n c e t o p r i m a r y p r o d u c t i o n as t h e u l t i m a t e c a u s e f o r c h a n g e s i n f i s h s u r v i v a l or g r o w t h . F o r e x a m p l e , increased water c o l u m n stability i n the northern P a c i f i c w i l l provide phytoplankton w i t h more light (Gargett  Northeast  1997), resulting i n higher  primary  p r o d u c t i o n w h i c h t h e n w i l l b e t r a n s f e r r e d u p the f o o d c h a i n to f i s h . H o w e v e r , the statement that  22  " f i s h production is ultimately dependent o n primary production" (Francis & S i b l e y 1991) is o n l y true i f t a k e n to its e x t r e m e : If there is no p r i m a r y p r o d u c t i o n i n the o c e a n there w i l l be n o f i s h production. A s s e r t i o n s about merely qualitative effects o f increased p r i m a r y p r o d u c t i o n o n fish s u r v i v a l a n d g r o w t h s h o u l d g e n e r a l l y b e distrusted. ( N o t e that p r i m a r y successfully  used  to  summarize  fish production  in  mass-balance  production has  models  (Baumann  been 1995;  C h r i s t e n s e n & P a u l y 1 9 9 5 ; P a u l y & C h r i s t e n s e n 1 9 9 5 a ; P a u l y & C h r i s t e n s e n 1 9 9 5 b ; P a u l y et  al.  1996), but the f o l l o w i n g t w o c r i t i c i s m s still apply.) Although  many  i n t e r a c t i o n s are  supported  by  regression and/or  more  process  oriented  analysis, m o s t authors d o not take into a c c o u n t t w o important factors r e g a r d i n g f i s h . F i r s t , the b i o l o g y o f o r g a n i s m s o n e v o l u t i o n a r y a n d e c o l o g i c a l t i m e scales: O r g a n i s m s are a d a p t e d to t h e i r environment and fish survival and production is not e x c l u s i v e l y determined by b o t t o m - u p effects F o r e x a m p l e , p h y t o p l a n k t o n species i n l o w light h i g h nutrients e n v i r o n m e n t s are u s u a l l y larger than species i n h i g h light l o w nutrients environments  ( c f . P a r s o n s et  al.  1984, F i g . 121).  Size  differences i n p r i m a r y p r o d u c e r s i n turn result i n different c o m m u n i t i e s ( c o m p a r e the e x t r e m e s o f the short f o o d c h a i n o f the P e r u C u r r e n t u p w e l l i n g s y s t e m ( W a r e 1992) w i t h the l o n g f o o d c h a i n o f the S u b a r c t i c P a c i f i c ( P a r s o n s & L a l l i 1 9 8 8 ) ; see a l s o r e f e r e n c e s f o r e c o s y s t e m c h a n g e s g i v e n i n S h a r p 1995) w h i c h determine the transfer o f p r i m a r y p r o d u c t i o n u p the f o o d c h a i n , i.e. the parameters i n the s i m p l e energy transfer equation:  P where  P  is production of trophic level  L  (Eq.1.1)  = E -P L  L  l  prim  and primary producers  (prim),  and  E  represents the m e a n  transfer e f f i c i e n c y (for a c r i t i c i s m o f the t r o p h o d y n a m i c c o n c e p t see C o u s i n s 1 9 8 7 ; P e t e r s 1 9 7 7 , as w e l l as C h a p t e r 5 : C o n c l u s i o n s ) . T r a n s f e r e f f i c i e n c y b e t w e e n o r g a n i s m s  of two  adjacent  t r o p h i c l e v e l s i s the p r o d u c t o f g r o w t h e f f i c i e n c y o f the h i g h e r t r o p h i c l e v e l , d e t e r m i n e d b y the  23  energy  allocation w i t h i n a predator organism (metabolism, b o d y growth, reproduction),  and  p r e d a t i o n e f f i c i e n c y , i.e. the p r o p o r t i o n o f p r e y p r o d u c t i o n t a k e n b y the predator. B o t h o f the later e f f i c i e n c i e s are c o n s e q u e n c e s o f l i f e h i s t o r y strategies that h a v e d e v e l o p e d i n e v o l u t i o n a r y t i m e a n d the t r a d e - o f f an o r g a n i s m has to m a k e b e t w e e n p r e d a t i o n r i s k a n d the b e n e f i t s o f activities i n e c o l o g i c a l t i m e , e.g. f o r a g i n g  (Lima  &  Dill  other  1990). A l s o , the m o r e e v o l v e d  an  o r g a n i s m i s , the less s u s c e p t i b l e it i s to the d i r e c t effects o f the p h y s i c a l e n v i r o n m e n t , w h i c h i s why  many  aquatic predators  are  migratory  and  can  thus  exploit  spatially and  temporally  independent production patches (Sharp 1995; Steele 1980). A n d s e c o n d , the spatial a n d t e m p o r a l scales o f m e c h a n i s m s a n d s u p p o r t i n g data: In analyses there is a r i s k o f c o m m i t t i n g an " i n d e x - o f - m e a s u r e m e n t  e r r o r " , i.e. a m b i g u o u s  many units,  l o c a t i o n a n d / o r t i m e o f m e a s u r e m e n t ( B a u m a n n & L e B l o n d 1996). A s s u m i n g that there e x i s t s a c r i t i c a l phase f o r c o h o r t s u r v i v a l i n P a c i f i c s a l m o n ( W a l t e r s & Juanes 1993) a n d that this phase is the e a r l y m a r i n e l i f e h i s t o r y stage, the space a n d t i m e scales o f p o s s i b l e m e c h a n i s m s , the c h a i n o f events, a n d s u p p o r t i n g data are c r u c i a l f o r a n u n d e r s t a n d i n g o f the e c o l o g i c a l p r o c e s s e s ( L e v i n 1 9 9 2 ) . B y a n a l o g y , w h y c a r e a b o u t a l o w a n n u a l m e a n t e m p e r a t u r e i n E u r o p e as l o n g a s t h e w e a t h e r i s f i n e w h e n y o u are i n R o m e o n v a c a t i o n . In s u m m a r y ,  one u l t i m a t e cause has been i d e n t i f i e d for i n c r e a s e d f i s h p r o d u c t i o n i n  the  N o r t h e a s t P a c i f i c b e t w e e n the late 1950s a n d 1980s, i.e. the s t r e n g t h e n i n g o f the A l e u t i a n L o w pressure s y s t e m prevalent d u r i n g the w i n t e r m o n t h s  (e.g. B e a m i s h &  B o u i l l o n 1993;  Gargett  1997). A l t h o u g h " c o u p l i n g o f [higher trophic level] s t o c k s to the p r o d u c t i o n base is an a l m o s t u n t o u c h e d research area" ( M i l l e r  1993a) m e c h a n i s m s connecting m e t e o r o l o g i c a l events  with  p r o d u c t i o n p r o c e s s e s , i . e . s u r v i v a l a n d g r o w t h , at h i g h e r t r o p h i c l e v e l s ( e . g . f i s h ) h a v e  been  f r e q u e n t l y s u g g e s t e d . I c o n t e n d that w i t h o u t t a k i n g i n t o a c c o u n t the b i o l o g y o f o r g a n i s m s  at  24  ecological  spatial  scales, and  evolutionary  and  ecological time  scales, and  without  the  d e v e l o p m e n t o f r i g o r o u s c o u p l e d p r o c e s s m o d e l s f o r h i g h e r t r o p h i c l e v e l s , i.e. e n v i r o n m e n t a l l y d r i v e n s p a t i a l l y - e x p l i c i t t r o p h o d y n a m i c s i m u l a t i o n s , s u c h s u g g e s t i o n s are m e r e o p i n i o n s .  25  1.3. Seasonal Variability: A Summary of the Current Paradigm and Some of Its Flaws Sockeye Salmon This  section  is a  summary  o f the current  interpretation  o f the principal  ecological  m e c h a n i s m s i n the Northeast P a c i f i c o n the seasonal time scale. Literature i s r e v i e w e d  with  respect to relevancy to sockeye salmon marine survival a n d distribution i n the Northeast Pacific O c e a n . A r g u m e n t s are presented qualitatively a n d some perceived inconsistencies are discussed. M o r e detailed a n d quantitative reviews o n sockeye salmon marine feeding ecology, ecosystems and p h y s i c a l oceanography o f the Northeast P a c i f i c c a n b e f o u n d i n Chapter 2: S o c k e y e S a l m o n and the M a r i n e  Environment.  U s u a l l y after o n e o r t w o years o f residence i n a l a k e ( B u r g n e r 1 9 9 1 ; P e a r c y 1 9 9 2 ) N o r t h A m e r i c a n j u v e n i l e s o c k e y e s a l m o n enter the Northeast P a c i f i c O c e a n f r o m early t o late s u m m e r (Fraser R i v e r stocks through Johnstone Strait i n June - J u l y ; B r i s t o l B a y stocks through passages i n the A l e u t i a n i s l a n d c h a i n i n J u l y - A u g u s t ; B u r g n e r 1991), w h e r e they stay near the coast ( D . Welch  1 9 9 8 pers. c o m m . ) .  ( F r e n c h et al. juvenile  A t this l i f e history stage a n d w i t h a b o d y l e n g t h o f about  10 c m  1976) sockeye s a l m o n feed m a i n l y o n m e s o z o o p l a n k t o n (size: 0 . 2 - 2 0 m m ) . A t sea,  sockeye s a l m o n basically drift w i t h the A l a s k a Current  around the A l a s k a n  Gyre.  S o m e t i m e a f t e r t h e f i r s t w i n t e r at s e a a n d w i t h a b o d y l e n g t h a r o u n d 2 0 c m ( F r e n c h et al. 1 9 7 6 ) sockeye  salmon  change  their  prey  size  preference  and switch  to  a  diet  of  mainly  m a c r o z o o p l a n k t o n ( 2 - 2 0 c m ) . F o r a g i v e n s i z e c l a s s , g e o g r a p h i c ( L e B r a s s e u r 1 9 6 6 ; P e a r c y et 1988) a n d t e m p o r a l differences (Favorite 1970; H e a l e y 1 9 9 1 ; M a n z e r reflect changes i n prey  al.  1968) i n diet c o m p o s i t i o n  availability rather than f o o d preferences. A s a consequence, lists o f  s t o m a c h contents items represent the availability a n d hence the relative abundance o f prey i n the environment  (Healey  1991).  In summary,  juvenile,  immature,  and maturing  ocean  sockeye  26  salmon  are  opportunistic  (within  L e B r a s s e u r 1 9 6 6 ; P e a r c y et al.  a  size  class)  polyphagous  planktivores  (Healey  1991;  1988), w i t h maturing sockeye additionally feeding o n squid  f i s h ( F a v o r i t e 1 9 7 0 ; L e B r a s s e u r 1 9 6 6 ; M a n z e r 1 9 6 8 ; see a l s o r e v i e w b y B r o d e u r  and  1990).  Prey, Competitors and Predators Food  availability for North  American  sockeye salmon  a d v e c t i o n processes i n the C e n t r a l S u b a r c t i c D o m a i n  is dominated  (Brodeur  &  by  production  Hollowed  and  1993; W a r e  &  M c F a r l a n e 1989; W i c k e t t 1967), the largest o f the four d o m a i n s o f the N o r t h e a s t P a c i f i c ( W a r e & M c F a r l a n e 1989). Here, w h i l e little i n f o r m a t i o n is available o n m a c r o z o o p l a n k t o n (2-20 partly b e c a u s e o f the p r o b l e m s  associated w i t h sampling of highly motile groups  cm),  within  z o o p l a n k t o n c o m m u n i t y , m e s o z o o p l a n k t o n ( 0 . 2 - 2 0 m m ) has b e e n studied e x t e n s i v e l y ( M a c k a s  the &  Frost 1993; Parsons & L a l l i 1988). M e s o z o o p l a n k t o n is d o m i n a t e d b y c o p e p o d species w i t h an a n n u a l l i f e c y c l e d u r i n g w h i c h the o r g a n i s m s c o m p l e t e ontogenetic v e r t i c a l m i g r a t i o n s et al.  1 9 9 3 ; M i l l e r et al.  (Mackas  1 9 8 4 ) w i t h e a r l y l i f e h i s t o r y s t a g e s a r r i v i n g at t h e s u r f a c e i n s p r i n g , a n d  c o p e p o d s r e a c h i n g their greatest b i o m a s s density i n M a y to J u n e (Parsons & L a l l i 1988), before  t h e y start t o m i g r a t e t o d e p t h ( M i l l e r et  microzooplankton  (20-200  pm)  and  are  al.  believed  1984). to  Mesozooplankton  control  m i c r o p h y t o p l a n k t o n ( 2 0 - 2 0 0 pirn) s t a n d i n g s t o c k s ( B o o t h et al. L a n d r y et al. Nauplii  1 9 9 3 a ; L a n d r y et al. of  large  nanophytoplankton  copepods  (2-20  p:m)  1 9 9 3 b ; M a c k a s et al. may, and  at  thus  least  in  microzooplankton 1993; D a g g  1 9 9 3 ; M i l l e r et al. principle,  mainly  be  assist m i c r o z o o p l a n k t o n  as  shortly feed  on  well  as  1993; G i f f o r d  1993;  1991b).  capable  of  feeding  upon  control  nanophytoplankton  s t a n d i n g s t o c k i n s p r i n g , w h i l e the later l i f e h i s t o r y stages o f c o p e p o d s a l s o f e e d u p o n e a r l i e r l i f e history  stages o f o r g a n i s m s  o f the s a m e  adult  size class. M i c r o z o o p l a n k t o n  standing  stock  27  c o n s i s t s o f s m a l l h e t e r o t r o p h i c f l a g e l l a t e s a n d c i l i a t e s ( B o o t h et al. its largest d e n s i t y i n w i n t e r  (LeBrasseur &  Kennedy  1972)  1993; Frost 1987) a n d attains  because of  m e s o z o o p l a n k t o n p r e d a t o r s ( D a g g 1 9 9 3 ; G i f f o r d 1 9 9 3 ; M a c k a s et al.  the  absence of  its  1993). In winters w i t h l o w  p r i m a r y p r o d u c t i v i t y , i.e. l a r g e r d e p t h o f m i x i n g , m i c r o z o o p l a n k t o n m a y b e a b l e to m a i n t a i n h i g h densities b y s h i f t i n g to a diet o f particulate organic matter ( P O M )  + associated bacteria ( M o r e l  et  al.  1 9 9 1 ) . M i c r o z o o p l a n k t o n are b e l i e v e d to c o n t r o l n a n o p h y t o p l a n k t o n s t a n d i n g s t o c k ( B o o t h  et  al.  1 9 9 3 ; D a g g 1 9 9 3 ; M i l l e r et al. Phytoplankton  standing  1 9 9 1 b ; S t r o m et al.  stock  in  the  1 9 9 3 ; W e l s c h m e y e r et al.  Northeast  Pacific  t h r o u g h o u t t h e y e a r ( P a r s o n s & L a l l i 1 9 8 8 ; W o n g et al. p r i m a r y p r o d u c t i v i t y f r o m w i n t e r t o s u m m e r ( W o n g et al.  Ocean  is  1993).  supposedly  constant  1995) w i t h an a p p r o x i m a t e d o u b l i n g i n 1995), a result o f increased insolation  a n d the stratification o f the water c o l u m n . P r i m a r y p r o d u c t i o n per unit b i o m a s s does not s e e m to b e b o t t o m - u p l i m i t e d t o a n y e x t e n t ( W e l s c h m e y e r et  al.  1993), w h i c h is reflected i n the never  d e p l e t e d nitrate p o o l ( P a r s l o w 1981), thus g r a z i n g a n d s i n k i n g d e t e r m i n e the s t a n d i n g stock. T h e d o m i n a n t s i z e class o f p r i m a r y producers i n the N o r t h e a s t P a c i f i c i s n a n o p h y t o p l a n k t o n (size 2 2 0 p : m ; B o o t h et al.  1993; Parsons 1972) w h i c h outcompetes larger p h y t o p l a n k t o n species, due to  i r o n l i m i t a t i o n o f t h e l a t t e r ( M a r t i n & F i t z w a t e r 1 9 8 8 ; M a r t i n et  al.  s t a n d i n g s t o c k i s g r a z e r - c o n t r o l l e d b y m i c r o z o o p l a n k t o n ( B o o t h et al. al.  1 9 9 1 b ; S t r o m et al.  1 9 9 3 ; W e l s c h m e y e r et al.  f o o d s o u r c e ( M i l l e r et al.  1994).  Nanophytoplankton  1993; D a g g 1993; M i l l e r  et  1993), w h i c h has h i g h e r g r o w t h rates than their  1991b). B e c a u s e their feeding apparatus is too coarse, later life history  s t a g e s o f m e s o - a n d m a c r o z o o p l a n k t o n a r e n o t a b l e t o c o n s u m e n a n o p h y t o p l a n k t o n ( M i l l e r et  al.  1991a). H o w e v e r , e p i s o d i c a t m o s p h e r i c d e p o s i t i o n o f i r o n , i.e. i r o n i n p u t e v e n t s , c a n result i n a d o m i n a n c e o f m i c r o p h y t o p l a n k t o n s p e c i e s o v e r n a n o p h y t o p l a n k t o n ( D o n a g h a y et al. &  Tindale  1991; Duce  1991). M i c r o p h y t o p l a n k t o n w i l l the be either nitrate l i m i t e d o r g r a z e r - l i m i t e d  by  28  m e s o z o o p l a n k t o n . M i c r o z o o p l a n k t o n i s t h e s a m e s i z e c l a s s as m i c r o p h y t o p l a n k t o n a n d i s t h u s t o o s m a l l t o c o n t r o l t h e l a r g e r p r i m a r y p r o d u c e r s ( M i l l e r et al. Only  1991b).  little quantitative information i s available o ncompetitors and predators o f sockeye  s a l m o n . M a i n c o m p e t i t o r s i n the N o r t h e a s t P a c i f i c are o t h e r s a l m o n s p e c i e s ( G r o o t & M a r g o l i s 1991), e s p e c i a l l y p i n k s a l m o n ( B u r g n e r 1991). O t h e r c o m p e t i t o r s i n the m a r i n e e n v i r o n m e n t  are  the saury, a s m a l l p e l a g i c f i s h , a n d the larger p o m f r e t , b o t h o f w h i c h are s u m m e r v i s i t o r s to the N o r t h e a s t P a c i f i c ( B r o d e u r 1 9 8 8 ; P e a r c y 1 9 9 3 ) a n d h a v e a b o u t t h e s a m e d i e t as s o c k e y e s a l m o n (Pauly  et al.  1996).  Important  predators o n immature  and maturing  s o c k e y e s a l m o n are  the  d a g g e r t o o t h ( P a u l y et al. 1 9 9 6 ) , t h e s a l m o n s h a r k , a n d t h e b l u e s h a r k , a s u m m e r v i s i t o r ( B r o d e u r 1988). L i t t l e is k n o w n about predators o n j u v e n i l e sockeye s a l m o n , except for stocks o n the w e s t coast o f V a n c o u v e r Island w h i c h i n w a r m years suffer h i g h mortality f r o m n o r t h w a r d  expanding  mackerel stocks from California.  Inconsistencies T h e r e are s o m e i n c o n s i s t e n c i e s i n the current t h e o r e t i c a l f r a m e w o r k o f the e c o s y s t e m s o f the Northeast  Pacific,  a n d the q u e s t i o n o f w h a t c o n t r o l s what,  where  and  when  a n s w e r e d satisfactorily yet, a l t h o u g h syntheses h a v e b e e n attempted (Frost F r o s t 1 9 9 3 ; M i l l e r 1 9 9 3 a ; M i l l e r et al.  1 9 9 1 a ; M i l l e r et al.  1991b).  vertial  fora moment  following  migrations  o f mesozooplankton  and  has  not  1987; Frost  Ignoring  been 1991;  the ontogenetic  a simple  food  chain  a r g u m e n t ( H a i r s t o n et al. 1 9 6 0 ) , i f m e s o z o o p l a n k t o n b i o m a s s i n c r e a s e s i n s u m m e r t h i s m e a n s that c o p e p o d d e n s i t y is b o t t o m - u p c o n t r o l l e d rather than t o p - d o w n . M i c r o z o o p l a n k t o n b i o m a s s , w h i c h f o r m s the m a i n f o o d source o f m e s o z o o p l a n k t o n i n the C e n t r a l S u b a r c t i c D o m a i n , i s thus t o p - d o w n c o n t r o l l e d a n d s h o u l d be r e d u c e d to a l e v e l w h e r e it is u n a b l e to c o n t r o l p h y t o p l a n k t o n  29  density, hence p h y t o p l a n k t o n b i o m a s s s h o u l d increase u n t i l it is b o t t o m - u p l i m i t e d . A n increase i n m e s o z o o p l a n k t o n standing s t o c k has b e e n o b s e r v e d i n s u m m e r (Parsons & L a l l i 1988) a n d so has the d e c l i n e i n m i c r o z o o p l a n k t o n density ( L e B r a s s e u r & K e n n e d y  1972). H o w e v e r ,  p r o d u c t i o n per unit b i o m a s s is not b o t t o m - u p l i m i t e d i n s u m m e r ( W e l s c h m e y e r  et al.  primary  1993)  p h y t o p l a n k t o n s t a n d i n g s t o c k h a s b e e n r e p o r t e d c o n s t a n t t h r o u g h o u t t h e y e a r ( W o n g et al.  and  1995).  It h a s a l s o b e e n s p e c u l a t e d t h a t t h e l i f e - h i s t o r y - i n d u c e d S e p t e m b e r m i n i m u m i n c o p e p o d d e n s i t y m i g h t c a u s e the o b s e r v e d slight increase i n p h y t o p l a n k t o n Parsons &  Lalli  s t a n d i n g s t o c k ( M i l l e r et  al.  1988). Y e t , the l i f e - h i s t o r y i n d u c e d S e p t e m b e r m i n i m u m i n c o p e p o d  1984; density  s h o u l d decrease rather than increase the p h y t o p l a n k t o n standing s t o c k i n O c t o b e r . One  e x p l a n a t i o n f o r the i n c o n s i s t e n c y i n the c o n c e p t u a l f r a m e w o r k  phytoplankton  standing  stock  is  based  on  chlorophyll-a  and  not  is that the  phytoplankton  constant carbon  c o n c e n t r a t i o n s . In a s e e m i n g l y forgotten p u b l i c a t i o n , M c A l l i s t e r ( 1 9 6 9 ) reported c h a n g e s i n the c a r b o n - t o - c h l o r o p h y l l - a ratio that, together w i t h n e w data o n c h l o r o p h y l l - a c o n c e n t r a t i o n s ( W o n g et  al.  1995), indicate a possible fivefold increase in phytoplankton  standing stock in  summer,  w h i c h is supported by carbon-based summer estimates of phytoplankton concentrations ( B o o t h al.  et  1993). A n o t h e r e x p l a n a t i o n m a y be f o u n d i n the l a c k o f r e s o l u t i o n o f the data w i t h respect to  s p a t i o - t e m p o r a l variability i n population c o n t r o l processes. B e c a u s e o f the coarse s p a t i o - t e m p o r a l scale o f the data u s e d i n m y study, I w i l l not be able to address this s e c o n d e x p l a n a t i o n , but f o r a p r o m i s i n g a p p r o a c h see Steele & H e n d e r s o n (1992a). A n o t h e r i n t e r e s t i n g a s p e c t r e l a t e d t o t r o p h i c c a s c a d i n g ( C a r p e n t e r et al.  1985), i.e. the direct  a n d i n d i r e c t effects o f i n t e r a c t i o n s w i t h i n a f o o d c h a i n ( P i m m 1992), i s that a l t h o u g h  Brodeur  and W a r e report a d o u b l i n g i n s u m m e r m e s o z o o p l a n k t o n b i o m a s s (Brodeur & W a r e 1992) and a similar increase i n many fish populations  (Brodeur  &  Ware  1995)  i n the Northeast  Pacific  30  b e t w e e n t h e l a t e 1 9 5 0 s a n d t h e 1 9 8 0 s , W o n g et a l . ( W o n g et al.  1995) c o u l d not detect any l o n g -  t e r m s i g n a l i n c h l o r p h y l l - a at O c e a n W e a t h e r S t a t i o n P f r o m 1 9 6 4 - 1 9 9 1 . D a t a c a n p o s s i b l y b e r e c o n c i l e d w i t h c u r r e n t u n d e r s t a n d i n g b y l o o k i n g at t h e s p a t i o - t e m p o r a l s c a l e s o f o b s e r v a t i o n s ( f o r e x a m p l e c o m p a r e B r o d e u r & W a r e ( 1 9 9 2 ) w i t h L o n g h u r s t et al.  (1972)).  31  1.4. Approach, Assumptions and Anticipation My on  attempt i n this thesis i s t o g o b e y o n d the speculations o f the effects o f p h y s i c a l f o r c i n g s  fish survival  a n d distribution  (e.g.  Adkison  et al.  1996;  Beamish  1993;  Beamish  1995;  B e a m i s h & B o u i l l o n 1 9 9 3 ; B e a m i s h et al. 1 9 9 4 ; B l a c k e t t 1 9 9 3 ; B r o d e u r & W a r e 1 9 9 5 ; W e l c h  et  al. 1 9 9 5 ; X i e & H s i e h 1 9 8 9 ) e v e n i f t h e s e a r e s o m e t i m e s e c o l o g i c a l l y m o r e i n v o l v e d ( F r a n c i s & S i b l e y 1 9 9 1 ) . I w i l l try to put s o m e o f the p r e s u m e d c a u s e s o f v a r i a b i l i t y i n f i s h s u r v i v a l t o a test using  ecological coupled  (Woodruff  et  al.  1987)  process  models  a n d a surface  that  aredriven  current  model  b y abiotic environmental  (Ingraham  & Miyahara  datasets  1989). T h e  c o m p l e x i t i e s i n these s i m u l a t i o n s arise w h e n c o n s i d e r i n g the i n t e r a c t i o n b e t w e e n s p e c i e s b e f o r e the b a c k g r o u n d o f a f l u c t u a t i n g e n v i r o n m e n t a n d a d v e c t i o n . L i s t e d b e l o w are 4 g e n e r a l a s s u m p t i o n s that I m a k e , e a c h f o l l o w e d b y e x p l i c i t e x p l a n a t i o n s o n its v a l i d i t y , f r o m w h i c h the c o m p l e x w o r k i n g h y p o t h e s i s o f this thesis has b e e n s y n t h e s i z e d :  Conjecture:  E c o s y s t e m processes i n the N o r t h e a s t P a c i f i c l a r g e l y d e t e r m i n e  the v a r i a b i l i t y i n  sockeye s a l m o n cohort survival. These ecosystem processes consist o f biotic processes s u c h as f o r a g i n g , c o m p e t i t i o n a n d predation, a n d the a s s o c i a t e d b e h a v i o r a l responses at e c o l o g i c a l a n d e v o l u t i o n a r y t i m e s c a l e s , as w e l l as a b i o t i c e n v i r o n m e n t a l f o r c i n g s s u c h as w a t e r c o l u m n s t a b i l i t y and currents.  32  Assumption #1: marine,  S u r v i v a l r a t e s at d i f f e r e n t l i f e h i s t o r y s t a g e s o f s o c k e y e s a l m o n ( e . g . l a k e , e a r l y  sub-adult)  exhibit  interannual  variability,  with  t h e largest  interannual  variability  o c c u r r i n g i n early m a r i n e life, w h i c h thus w i l l determine relative y e a r class s u r v i v o r s h i p . T h e n u m b e r o f i n d i v i d u a l s f r o m a c o h o r t s u r v i v i n g t o l i f e h i s t o r y s t a g e t, i . e . t h e r e c r u i t s t o t, is g i v e n b y the s i m p l e equation:  ( E q . 1.2)  N, = s s s ...s,N Q  l  2  G  w h e r e TV a n d s r e p r e s e n t p o p u l a t i o n s i z e a n d s u r v i v a l r a t e , r e s p e c t i v e l y , a n d s u b s c r i p t s i n d i c a t e l i f e h i s t o r y stages ( o r age classes). T h e p r o d u c t o f a l l s u r v i v a l rates i s c a l l e d s u r v i v o r s h i p  and  represents the p r o b a b i l i t y that an i n d i v i d u a l s u r v i v e s t o v a r i o u s stages. T h e s u r v i v a l rate f o r a n y p a r t i c u l a r l i f e h i s t o r y stage i s u s u a l l y a c o n s e q u e n c e o f b o t h d e n s i t y i n d e p e n d e n t  and  density  dependent effects ( B e v e r t o n & H o l t 1957; P e t e r m a n 1978). T h e l i f e h i s t o r y stage that s h o w s the greatest v a r i a b i l i t y i n s u r v i v a l rate f o r different c o h o r t s w i l l d o m i n a t e v a r i a b i l i t y i n s u r v i v o r s h i p , w h i c h m u l t i p l i e d w i t h the respective i n i t i a l c o h o r t s i z e w i l l d e t e r m i n e y e a r - c l a s s strength. N o t e h o w e v e r that the l i f e h i s t o r y stage w i t h the largest v a r i a b i l i t y i n s u r v i v a l m a y w e l l v a r y  from  c o h o r t t o c o h o r t f o r a g i v e n p o p u l a t i o n , as w e l l as f r o m p o p u l a t i o n t o p o p u l a t i o n f o r a g i v e n y e a r class. While survival  the contributions o f sockeye salmon  o f freshwater and marine are still unresolved  conditions encountered i n individual environments  phase t o t h e variation i n total  (Bradford a t [the]  1995), "the  annual  p o p u l a t i o n s . " ( B u r g n e r 1991)  variations i n  e a r l y sea l i f e stage are  b e l i e v e d t o b e l a r g e l y r e s p o n s i b l e f o r the v a r i a t i o n seen i n o v e r a l l m a r i n e  cohort  generally  survival o f cohort  o r e v e n i n total c o h o r t s u r v i v a l (see a l s o F r a n c i s & H a r e  1994;  H e a l e y 1 9 9 1 ; P e a r c y 1 9 9 2 ; W a l t e r s & J u a n e s 1 9 9 3 a n d r e f e r e n c e s t h e r e i n , a n d W a l t e r s et  al.  1 9 7 8 f o r a d i s c u s s i o n ) . N o t e a l s o , that a l t h o u g h o n l y 4 3 % o f the v a r i a b i l i t y i n t o t a l m o r t a l i t y i s  33  a c c o u n t e d f o r b y the freshwater stage o f s o c k e y e s a l m o n , the p r o p o r t i o n o f total m o r t a l i t y f r o m freshwater is higher (58%)  t h a n that f r o m the m a r i n e e n v i r o n m e n t d u e the r e s i d e n c e o f s o c k e y e  s a l m o n i n a l a k e d u r i n g its first year(s) ( B r a d f o r d 1995). H o w e v e r , components  o f recruitment, Bradford  f o r the c a l c u l a t i o n o f  ( 1 9 9 5 ) a s s u m e s that i n s t a n t a n e o u s  m o r t a l i t y rates  i n d e p e n d e n t o f d e n s i t y a n d o f habitat, i.e. f r e s h w a t e r a n d m a r i n e e n v i r o n m e n t ,  the vary  which might  not  be valid.  Assumption #2: cohort  E x p o s u r e to predators i n the early m a r i n e l i f e o f s o c k e y e s a l m o n  s u r v i v a l rate, i.e. n o  f i s h starves to death. T h e  amount of prey  per  determines  sockeye  salmon  d e t e r m i n e s t h e t i m e at r i s k a n d t h u s t h e e x p o s u r e t o p r e d a t o r s . B e c a u s e m o r t a l i t y r i s k i n early m a r i n e l i f e o f s o c k e y e s a l m o n , i.e. the t i m e b e t w e e n  smolt  o c e a n e n t r y i n s u m m e r a n d t h e e n d o f t h e f i r s t w i n t e r at s e a , i s v e r y h i g h e v e r y w h e r e , i t h a s b e e n a r g u e d t h a t t h e b e s t s t r a t e g y f o r j u v e n i l e s o c k e y e s a l m o n i s t o g r o w as q u i c k l y as p o s s i b l e t o outgrow  its predators  probably homogeneous  (M.  Healey  1995  pers. c o m m . ) .  Nevertheless, while mortality  risk is  o v e r a l a r g e r s c a l e , s a y 1 0 0 m t o 1 0 k m , m o r t a l i t y r i s k at t h e s m a l l s c a l e  o f f i s h s c h o o l s m i g h t w e l l b e v a r y i n g i n s u c h a w a y that f i s h near the c e n t e r o f a s c h o o l h a v e a s p a t i o - t e m p o r a l r e f u g e f r o m p r e d a t i o n a n d a c t i v e f o r a g i n g at t h e s c h o o l b o u n d a r i e s e x p o s e s t h e i n d i v i d u a l to predation r i s k ( W a l t e r s & Juanes 1993; M . H e a l e y 1995 pers. c o m m . ) . T h i s v i e w is a l s o s u p p o r t e d b y results f r o m H e a l e y ( 1 9 9 1 ) w h o f o u n d that e s t i m a t e d d a i l y rations o f j u v e n i l e pink, c h u m and sockeye s a l m o n i n Hecate Strait (British C o l u m b i a , Canada) were s m a l l e n o u g h to l i m i t g r o w t h  rates, this i n  spite o f  the  fact that,  "unless  z o o p l a n k t o n p r o d u c t i o n i s a v a i l a b l e t o t h e s a l m o n " ( W a l t e r s et  only al.  a small fraction of  total  1978), ocean limitation  of  s a l m o n is u n l i k e l y . S o it is i m p o r t a n t to note that " d u r i n g a n y g i v e n day, an a n i m a l m a y f a i l to  34  obtain a meal and go hungry,  but i n the l o n g t e r m , the d a y ' s s h o r t c o m i n g s m a y h a v e m i n i m a l  i n f l u e n c e o n l i f e t i m e f i t n e s s . F e w f a i l u r e s , h o w e v e r , a r e as u n f o r g i v i n g as t h e f a i l u r e t o a v o i d a predator: b e i n g k i l l e d greatly decreases future fitness." ( L i m a & D i l l 1990). In s u m m a r y , s u r v i v a l is d e t e r m i n e d b y the a m o u n t o f t i m e an i n d i v i d u a l e x p o s e s i t s e l f to r i s k o f p r e d a t i o n , a c o m p l e x i n t e r a c t i o n o f (1) a v a i l a b i l i t y o f p r e y ( b o t t o m - u p a n d m i d d l e - o u t e f f e c t s ) , (2)  requirements  and  allocation of energy  w i t h i n the f o r a g i n g  organism,  (3)  abundance  of  p r e d a t o r s ( a n d p a r a s i t e s , t o p - d o w n e f f e c t s ) . It s h o u l d b e e m p h a s i z e d t h a t t h e e s t i m a t i o n o f  an  o r g a n i s m ' s p r e d a t i o n r i s k is not a t r i v i a l p r o b l e m , neither f o r the o r g a n i s m ( A b r a m s 1994) n o r f o r the scientist w h o  has to deal w i t h several different  spatial and temporal  scales.  Behavioral  response to predation r i s k c a n l e a d to s o m e interesting p o p u l a t i o n effects, e x p e c t e d (Carpenter al.  1 9 8 7 ; W e r n e r et  al.  1983)  and unexpected (Walters &  Juanes 1993). B e c a u s e little or  et no  information is available on predator abundance and distribution, their behavior, and behavioral r e s p o n s e s o f j u v e n i l e s a l m o n as a r e s u l t o f p r e d a t i o n r i s k , a n d a l t h o u g h d e e m e d i n s u f f i c i e n t b y s o m e ( W a l t e r s & Juanes 1993), I w i l l f o c u s o n the a v a i l a b i l i t y o f s o c k e y e s a l m o n p r e y i n  my  s t u d y . I h o p e t h a t b y t h i s a p p r o a c h I at l e a s t w i l l b e a b l e t o i d e n t i f y e x t r e m e l y p o o r a n d g o o d s u r v i v a l years for s o c k e y e cohorts and shed s o m e light o n e c o s y s t e m f u n c t i o n i n the Northeast Pacific.  Assumption #3:  Prey  for  juvenile  sockeye  salmon  is  represented  by  the  size  class  m e s o z o o p l a n k t o n ( 0 . 2 - 2 0 m m ) . F o r the p e l a g i c e c o s y s t e m s o f the N o r t h e a s t P a c i f i c s i z e classes o f p l a n k t o n organisms d o represent trophic levels. T r a d i t i o n a l l y the a p p r o a c h to large e c o s y s t e m s i n m o d e l i n g a n d f i e l d studies has b e e n  to  s t u d y o n l y a f e w p a r t s at a t i m e b y e i t h e r t a k i n g a s u b s e t o f h i g h t a x o n o m i e r e s o l u t i o n f r o m a  35  community,  or  studying  a  whole  community  and  lumping  species  into  higher  systematic  c a t e g o r i e s ( Y o d z i s 1989), e.g. invertebrates, vertebrates. O t h e r a p p r o a c h e s h a v e u s e d g u i l d s , i.e. f u n c t i o n a l d i v i s i o n s s u c h as t r o p h i c l e v e l s ( F i e l d et al. aggregations  (Ulanowicz  B o u d r e a u et al.  &  Piatt  1985),  e.g.  1 9 8 9 ; H a i r s t o n et al.  body-size  classes  1960), or a t a x o n o m i c  (Boudreau  &  Dickie  1992;  1 9 9 1 ; T h i e b a u x & D i c k i e 1993). F o r this study, I w i l l a s s u m e that f o r p l a n k t o n  o r g a n i s m s different size classes actually represent different trophic levels, and f l o w s o f  energy  a r e o n l y u s e d as t h e y r e l a t e t o p r i m a r y p r o d u c t i o n , w h i l e f o r h e r b i v o r e s a n d h i g h e r t r o p h i c l e v e l s I h a v e i n c l u d e d b e h a v i o r a l responses to their environment.  A  c o n c e p t u a l m o d e l o f the t r o p h i c  p a t h w a y s i n t h e e c o s y s t e m s o f t h e N o r t h e a s t P a c i f i c i s s h o w n i n F i g . 1.9. T h e classification of plankton organisms into different size classes representing trophic levels s e e m s t o b e a v a l i d c o n c e p t f o r p e l a g i c e c o s y s t e m s ( O k s a n e n 1 9 9 1 ; S h e l d o n et al. et  al.  1987):  First, it represents  well  'Pimm's  principle'  (Pimm  1982)  1977;  which  Walters  says that  a  c o m b i n a t i o n o f p r e d a t i o n a n d c o m p e t i t i o n b y the s a m e species s h o u l d l e a d to the e x t i n c t i o n o f the v i c t i m a n d thus to a structural s i m p l i f i c a t i o n o f the f o o d w e b ( O k s a n e n 1988). A n d s e c o n d , it takes i n t o a c c o u n t l i f e h i s t o r y b y a d d r e s s i n g c h a n g e s i n the p r e y c o m p o s i t i o n as v i e w e d f r o m the p e r s p e c t i v e o f the d e v e l o p i n g o r g a n i s m s ( C a s w e l l 1989), a n d the fact that w h a t an  individual  a n i m a l e a t s d e p e n d s o n t h e c a p a b i l i t i e s at i t s p a r t i c u l a r l i f e h i s t o r y s t a g e a s w e l l as t h o s e o f i t s p r e y o r g a n i s m s ( R i c e 1995). C o m p a r e this t r o p h o d y n a m i c v i e w p o i n t w h i c h states that e c o l o g i c a l e f f i c i e n c i e s are the p r o d u c t o f t r o p h i c structure, not v i c e v e r s a ( H a i r s t o n - J r . & H a i r s t o n - S r . supplemented  with evolutionary  approach ( O d u m capable of  1971):  feeding  upon  theory  (Gould  &  F o r example, Parsons & its c o m p e t i t o r s  should  Lewontin  1979),  to the s y s t e m s - e c o l o g i c a l  L a l l i (1988) h a v e argued that a n include them  1993,  i n its diet w h e n  individual  the  transfer  e f f i c i e n c y t h r o u g h the n e w l o n g e r f o o d c h a i n b e c o m e s larger than t h r o u g h the short f o o d c h a i n .  36  Fig.  1.9:  F l o w d i a g r a m o f p o s s i b l e energy transfers i n ecosystems o f the N o r t h e a s t  Environmental  forcings (circles; Physical Subsystem)  determine  Pacific.  w h i c h o f the p o s s i b l e  food  c h a i n s ( b o x e s ; B i o l o g i c a l S u b s y s t e m ) i s r e a l i z e d . N o t e that f o r p l a n k t o n o r g a n i s m s I a s s u m e that size class represents trophic level.  A l s o note the respective s p a t i a l a n d t e m p o r a l  scales  of  p r o c e s s e s at t h e d i f f e r e n t g u i l d l e v e l s . H a t c h e d b o x e s a n d c i r c l e s r e p r e s e n t s t a t e v a r i a b l e s a n d e n v i r o n m e n t a l f o r c i n g s i n c l u d e d i n the s i m u l a t i o n s . N o t s h o w n to s i m p l i f y the d i a g r a m :  import  and export through sinking, advection, immigration and emigration; autocatalytic processes and heat s i n k s ; sources f o r a n d regeneration o f nutrients, and d i s s o l v e d ( D O M ) organic matter (POM);'"mesophytoplankton",  and particulate  e.g. c h a i n - f o r m i n g m i c r o p h y t o p l a n k t o n , w h i c h c a n  b e c o n s u m e d b y fish d i r e c t l y .  37  (Transfer  e f f i c i e n c y i s d e f i n e d here as b i o m a s s p r o d u c t i o n  p r o d u c t i o n o f its prey  Assumption  #4:  o f the predator  per unit  biomass  organism.)  Spatio-temporal  scope and detail used i n m y spatially-explicit single-layer  simulations are sufficient to capture the effects o f ecosystem processes i n the m i x e d upper layer o f the Northeast P a c i f i c o n s o c k e y e s a l m o n survival. E c o s y s t e m processes i n the m i x e d upper layer o f the Northeast P a c i f i c are simulated o n a g e o r e f e r e n c e d 1° x 1° ( l o n g i t u d e , l a t i t u d e ) g r i d o f u n e q u a l - s i z e d a r e a s e n c o m p a s s i n g t h e P a c i f i c O c e a n b e t w e e n 1 8 0 t o 125°W a n d 3 5 t o 62°N. B e c a u s e the N o r t h e a s t P a c i f i c i s n o t a d i s t i n c t b a s i n o f the P a c i f i c O c e a n , b u t i s rather d e l i m i t e d b y the variable extent o f o c e a n currents, it h a s been defined here b y the approximate range i n ocean distribution o f N o r t h A m e r i c a n salmon  species (Groot & M a r g o l i s  1991;Welch  et  al.  1995). T h e spatial resolution o f the  simulations is a c o m p r o m i s e between the spatial resolution o f the input data (abiotic ( W o o d r u f f et al.  and resolution,  probably Bering  forcings  1987) a n d the surface current m o d e l (Ingraham & M i y a h a r a 1989)), the relevant  scales o f b i o l o g i c a l processes, a n d computation scope  Pacific  especially when  time. There  considering  determined i n or near the coastal domains  S e a is only  that  are t w o s h o r t c o m i n g s  sockeye  salmon  cohort  i n spatial survival is  ( s e e A s s u m p t i o n s #1 a n d # 2 ) : F i r s t , t h e  partially covered, a n d second, input  data lack a h i g h  resolution coastal  circulation model. T h e c h o i c e o f the 'right' spatial scales i n e c o l o g i c a l studies i s n o t a t r i v i a l p r o b l e m 1992)  (Levin  a n d because the spatial a n d temporal variability o f different ecosystem properties i s a  f u n c t i o n o f s p a t i a l scale as w e l l as the e c o s y s t e m property itself, it m a y b e a r g u e d that there i s " n o right w a y to d o i t " ( C . W a l t e r s 1994 pers. c o m m . ) . S o the spatial resolution a n d scope o f m y  38  s i m u l a t i o n s s h o u l d b e v i e w e d as o n e a t t e m p t t o a n s w e r t h e q u e s t i o n : W h a t a r e t h e  relevant  spatial and t e m p o r a l scales o f e c o l o g i c a l processes important for sockeye s a l m o n s u r v i v a l ? In m y s i m u l a t i o n s I h a v e t r i e d t o i n c o r p o r a t e p r o c e s s e s at s m a l l e r s p a t i a l s c a l e s t h a n g r i d s i z e  by  t r e a t i n g t h e m s p a t i a l l y - i m p l i c i t l y (e.g. T y p e U I f u n c t i o n a l r e s p o n s e o f p r e d a t o r c o n s u m p t i o n to p r e y d e n s i t y i m p l i c i t l y r e p r e s e n t s t h e e f f e c t o f a p a r t i a l r e f u g e f o r t h e p r e y ( B e g o n et al.  1990);  see a l s o S e c t i o n 3.3: P o p u l a t i o n M o d e l s ) . S i m i l a r a r g u m e n t s a p p l y f o r t e m p o r a l s c a l e s a n d n e s t e d m o d e l design was used w h e n appropriate. I d o n o t p r e t e n d that m y m o d e l s a n d s i m u l a t i o n s w i l l c a p t u r e m o s t o f the b i o l o g i c a l a n d p h y s i c a l i n t r i c a c i e s that m a y  o c c u r i n t h e e c o s y s t e m s o f t h e N o r t h e a s t P a c i f i c at t h e  many  different spatial a n d t e m p o r a l scales, neither d o I have the k n o w l e d g e to d o so n o r d o I t h i n k that this i s t h i s the p u r p o s e o f m o d e l i n g (e.g. C a s w e l l 1 9 8 8 ; S t a r f i e l d & B l e l o c h 1 9 9 1 ; W a l t e r s  1986,  but see C a s t i 1997). E v e r y o n e s t u d y i n g c o m p l e x , adaptive s y s t e m s is f a c e d w i t h c r i t i c a l c h o i c e s i n t h e d e v e l o p m e n t o f m o d e l s , a n d t h e q u e s t i o n s as w e l l as t h e d e s i r e d a c c u r a c y o f t h e a n s w e r s w i l l d e t e r m i n e the r e s o l u t i o n , i.e. s c o p e a n d d e t a i l ( S t a r f i e l d & B l e l o c h 1991), o f the m o d e l s a n d s i m u l a t i o n s . F o r e x a m p l e , i n c a s e s w h e r e a c c u r a t e p r e d i c t i o n w a s d e e m e d n e c e s s a r y (as a r e s u l t o f a lot o f m o n e y b e i n g i n v o l v e d ) s o m e have tried to simulate nature b y a c c o u n t i n g for  every  d e t a i l that m i g h t o c c u r i n a g i v e n s y s t e m ( C a s t i 1997). In other cases m o d e l e r s h a v e p o n d e r e d the r e s o l u t i o n o f the i n p u t data (e.g. K i r k i l i o n i s 1995), a n d h a v e e v e n s u g g e s t e d to d i r e c t l y l i n k e c o s y s t e m m o d e l s to satellite remote  sensing to m a k e better p r e d i c t i o n s (predictions  whose  purpose w a s not clear to me). In  most  ecosystem  studies  emphasis  has  been  on  understanding,  not  prediction,  c o n c e p t u a l f l o w d i a g r a m s ( l i k e t h a t i n F i g . 1.9) h a v e f r e q u e n t l y b e e n d e v e l o p e d a n d e v e n m u c h greater d e t a i l (e.g. B r o d e u r 1 9 8 8 ; B r o d e u r & P e a r c y 1 9 9 2 ; K r e m e r & N i x o n 1 9 7 8 ;  and in  Odum  39  1 9 8 3 ; P i a t t et  al.  1981). U s u a l l y the c o m p l e x i t y o f these m o d e l s a n d the i n t r i n s i c p r o b l e m s  n o n l i n e a r e q u a t i o n s (e.g. C o h e n  1976b;  Stone  to d e s c r i p t i v e f u n c t i o n a l a n a l y s i s ( B r i a n d  1983;  Y o d z i s 1989) o r l i n e a r n e t w o r k a n a l y s i s o f m a s s - b a l a n c e m o d e l s (e.g. C h r i s t e n s e n & P a u l y  1995;  1993)  1 9 9 5 ; C r u t c h f i e l d et  h a v e restricted the scientific procedure  L a e v a s t u & L a r k i n s 1 9 8 1 ; O d u m 1 9 7 1 ; P a u l y et  al.  al.  1986; G l e i c k 1987; M a y  of  1 9 9 6 ; W u l f f et  al.  1989; for an interesting  a p p r o a c h see K l e p p e r 1 9 9 5 ) . O n the other h a n d , s o m e w h o l e e c o s y s t e m s i m u l a t i o n s h a v e d e v e l o p e d w i t h v a r y i n g a c c e p t a n c e i n t h e s c i e n t i f i c c o m m u n i t y ( P i a t t et al.  been  1981). A s an example  c o n s i d e r t h e ' G e n e r a l E c o s y s t e m M o d e l o f t h e B r i s t o l C h a n n e l a n d S e v e r n E s t u a r y ' (as d e s c r i b e d in  Piatt  et  al.  1981)  with  225  parameters  and  more  than  2.25-10  1 0 7  possible  parameter  combinations in a 'brute-force' sensitivity analysis where each parameter can only assume one  of  three values. A s a c o n s e q u e n c e , the g u i d i n g p r i n c i p l e i n the d e v e l o p m e n t  o f m y s i m u l a t i o n s has been  s i m p l i f y the natural s y s t e m w h i l e still trying to capture the essence o f n o n - l i n e a r processes. M o d e l i n g  to  real-world  o f c o m p l e x ecosystems is a l o n g and tedious process o n the n a r r o w  path  between mathematical games and intractable pattern i m i t a t i o n ( B a u m a n n 1998). E x c l u s i o n a n d aggregation  of interacting components  classes o f components acknowledging  (Starfield  &  s h o u l d o c c u r at t h e d e t a i l e d l e v e l s a n d n o t at  Bleloch  1991,  but  see R i c e  1995), w h i l e  -  higher  as u s u a l  that the real w o r l d is m o r e c o m p l i c a t e d . I h a v e t r i e d to i n c o r p o r a t e  the  -  most  i m p o r t a n t e c o s y s t e m processes a n d pathways w i t h respect to f o o d a v a i l a b i l i t y to j u v e n i l e s o c k e y e s a l m o n b y s i m p l i f y i n g (1) the s p a t i a l - t e m p o r a l v a r i a t i o n i n m e s o z o o p l a n k t o n  p r o d u c t i o n i n the  N o r t h e a s t P a c i f i c w i t h i n a f o o d c h a i n c o n t e x t , (2) the t r a n s p o r t o f b i o l o g i c a l p r o d u c t i o n  within  the N o r t h e a s t P a c i f i c , a n d (3) b e h a v i o r o f z o o p l a n k t o n a n d the e f f e c t s o n a v a i l a b i l i t y to j u v e n i l e s a l m o n m i g r a t i n g a l o n g the A l a s k a n G y r e . B e c a u s e o f the c o m p l i c a t i o n s stated i n  Assumption #2  40  (Walters & Juanes 1993, M . H e a l e y 1995 pers. c o m m . ) I have not e x p l i c i t l y i n c l u d e d s a l m o n i n my  model. In s u m m a r y , here is the attempt: P r o g r a m the s i m p l e s t p l a u s i b l e s p a t i a l l y - e x p l i c i t s i n g l e - l a y e r  e c o s y s t e m s i m u l a t i o n t o e x p l a i n , at l e a s t i n p a r t , t h e i n t e r a n n u a l v a r i a b i l i t y i n s o c k e y e s u r v i v a l b y the s p a t i o - t e m p o r a l v a r i a b i l i t y i n prey a v a i l a b i l i t y . A incorporate animal life history and individual daily behavior,  salmon  d y n a m i c m o d e l l i k e this must seasonal biological  production  p r o c e s s e s i n t h e o c e a n , f o o d c h a i n d y n a m i c s , a d v e c t i o n o f b i o l o g i c a l p r o d u c t i o n , a s w e l l as o t h e r p h y s i c a l f o r c i n g s . It t h u s n o t o n l y  has to integrate various  organization  levels  (ecosystems,  c o m m u n i t i e s , g u i l d s , p o p u l a t i o n s , i n d i v i d u a l s ) , but also the different spatial a n d t e m p o r a l scales o n w h i c h r e g u l a t o r y p r o c e s s e s o c c u r . T h i s i s n o t a n e a s y t a s k ( L e v i n 1 9 9 2 ; L e v i n et al.  1997)  the attempt is thus v u l n e r a b l e to c r i t i q u e f r o m e a c h o f the different s u b - d i s c i p l i n e s that I tried to integrate.  C r i t i c i s m is anticipated. H o w e v e r ,  the g o a l  of  my  work  is an  and have  increased  understanding o f what does and what does not control s a l m o n s u r v i v a l , rather than p r e d i c t i o n , a n d I h o p e the reader w i l l agree that this e x e r c i s e leads to a d e e p e r u n d e r s t a n d i n g .  41  2. SOCKEYE SALMON AND THE MARINE ENVIRONMENT  "If no use is made of the labors of the past, the world must remain always in the infancy of knowledge." Cicero (106 - 43 B C )  "Just as we suffer from excess in all things, so we suffer from excess in literature." Seneca (4B C - A D 65)  In the f o l l o w i n g three sections I w i l l r e v i e w data together w i t h current " u n d e r s t a n d i n g " o f the ocean feeding ecology o f sockeye salmon, a n d the biological a n d physical characteristics o f the ecosystems o f the N E - P a c i f i c . T h i s fairly detailed i n f o r m a t i o n w a s s u m m a r i z e d w i t h respect t o i t s r e l e v a n c y f o r e c o s y s t e m e f f e c t s o n s o c k e y e s a l m o n m a r i n e l i f e i n C h a p t e r 1.  2.1.  Ocean Feeding Ecology of Sockeye Salmon (Oncorhynchus nerka) U n d e r s t a n d i n g the trophic p o s i t i o n o f sockeye s a l m o n requires k n o w l e d g e o f the f o o d items  e a t e n at d i f f e r e n t l i f e h i s t o r y stages. I d e f i n e these stages s o m e w h a t a r b i t r a r i l y as j u v e n i l e ( 1 . 0 1.1 f i s h ; f o r a g e c l a s s i f i c a t i o n s y s t e m s i n s a l m o n s e e G r o o t & M a r g o l i s 1 9 9 1 ) a n d , d u e t o t h e similarity i n their diet (Brodeur  1990), i m m a t u r e a n d m a t u r i n g f i s h c o m b i n e d (1.1 s a l m o n a n d  older). A l t h o u g h b o d y length and weight i n sockeye salmon change continuously (Burgner 1991; F r e n c h et al.  1976), the clustering o f the ratio o f predator t o prey  size f o r various  pelagic  o r g a n i s m s a r o u n d a constant (ratio o f e q u i v a l e n t s p h e r i c a l d i a m e t e r o f p r e d a t o r t o that o f p r e y ~ 15 ( S h e l d o n et al.  1 9 7 7 ) ) a n d t h e b i o m a s s ( B o u d r e a u & D i c k i e 1 9 9 2 ; B o u d r e a u et al. 1 9 9 1 ;  T h i e b a u x & D i c k i e 1 9 9 3 ) a s w e l l a s p a r t i c l e s i z e s p e c t r a ( P a r s o n s & L e B r a s s e u r 1 9 7 0 ; P a r s o n s et al. 1 9 8 4 ; S h e l d o n & P a r s o n s 1 9 6 7 ) i n t h e a q u a t i c e n v i r o n m e n t s u g g e s t t h a t t h e s i z e o f s o c k e y e  42  s a l m o n p r e y o r g a n i s m s c h a n g e s a b r u p t l y s o m e t i m e a f t e r t h e f i r s t w i n t e r at s e a , w h e n l e n g t h g r o w t h r a t e s t a r t s t o f a l l o f f ( F r e n c h et al. However, biomass  size)  development  the establishment  1976). distribution  vs.  levels  and  the  a n d e v o l u t i o n o f its constituent species, a l l u n d e r the f o r c i n g c o n d i t i o n s o f  the  is a consequence  physical environment.  of  sockeye  a certain biomass  of  the  spectrum  interaction between  (body  various  mass trophic  B i o m a s s s p e c t r a are thus e m e r g e n t p r o p e r t i e s w h o s e e x p l i c i t t h e o r e t i c a l  treatment has o n l y recently been initiated (Thiebaux & D i c k i e 1993).  Juvenile sockeye salmon (age class: 1.0) A f t e r entering the o c e a n , j u v e n i l e o r g a n i s m s i n the c o a s t a l e n v i r o n m e n t are s t o m a c h contents v o l u m e  sockeye salmon prey upon a very  broad  (Pearcy 1992). These i n c l u d e (Brodeur  1990;  spectrum  of  percentages  proportions):  euphausiids (Class: Malacostraca)  larval and juvenile fishes  (11%)  amphipods (54%; Malacostraca)  squid larvae  copepods  decapod larvae (15%; Malacostraca,)  pteropods (Class: Gastropoda) D i e t c o m p o s i t i o n is quite variable i n different geographic locations, e.g. larvaceans (Class: Appendicularia)  insects  (11%)  chaetognaths  cladocerans (Class:  Phyllopoda)  h a v e b e e n i d e n t i f i e d i n v a r i o u s l o c a t i o n s i n the Strait o f G e o r g i a , B r i t i s h C o l u m b i a  (Healey  1978). H e a l e y (1991) reports large interannual variability i n diet organisms, e.g. c o p e p o d s  with  percentage v o l u m e contributions of 3 % and 3 6 % for 1986 and 1987, respectively, even though the frequency o f occurrence o f copepods i n stomachs o f j u v e n i l e s o c k e y e w a s l o w e r i n 1987.  43  G e n e r a l l y s o c k e y e j u v e n i l e s s h o w e d a p o s i t i v e s e l e c t i o n f o r n e u s t o n (i.e. o r g a n i s m s  living  o n l y w i t h i n c e n t i m e t e r s b e l o w to the surface l a y e r (Ott 1988)) c o m p a r e d to o t h e r p e l a g i c g u i l d s ( B r o d e u r 1990). A l t h o u g h the density o f neuston o r g a n i s m s m a y be v e r y h i g h , their total b i o m a s s o v e r t h e w h o l e w a t e r c o l u m n i s l o w b e c a u s e o f t h e t h i n n e s s o f t h e l a y e r t h e y i n h a b i t ( P a r s o n s et al. 1 9 8 4 ) . H o w e v e r , as v i s u a l p r e d a t o r s ( B u r g n e r 1 9 9 1 ) s o c k e y e s a l m o n s u c c e s s f u l l y e x p l o i t t h i s c o m m u n i t y i n the sunlit surface layer. Overall, spatial and temporal differences  i n the  variability i n diet c o m p o s i t i o n c a n s i m p l y b e attributed t o  availability o f species w i t h i n  preferences (Brodeur  the  preferred  size  class  rather  than  food  1 9 9 0 ; H e a l e y 1 9 9 1 ) . U s i n g t h e s i z e c l a s s i f i c a t i o n s c h e m e i n P a r s o n s et al.  ( 1 9 8 4 ) the a b o v e p r e y o r g a n i s m s c a n be c l a s s i f i e d as m e s o z o o p l a n k t o n (0.2 - 2 0  mm).  Immature and maturing sockeye salmon (age class: 1.1 and older) F a v o r i t e (1970) reports e x p l i c i t stomach contents v o l u m e proportions (percentages g i v e n i n brackets b e l o w ) for i m m a t u r e and maturing sockeye s a l m o n f r o m samples taken i n the A l a s k a n S t r e a m area a n d B r i s t o l B a y f o r M a y to A u g u s t  1960:  euphausiids (12%)  copepods  (7%),  amphipods (43%),  pteropods  (2%),  fish (18%),  crustacean larvae  squid (16%),  pelagic polychaetes  H o w e v e r , s a m p l e s taken i n w i n t e r 1964 (January to F e b r u a r y ) across the A l a k a n G y r e s h o w overwhelming dominance o f fish (71%)  and squid (27%)  an  i n the diet o f s o c k e y e r a n g i n g i n size  f r o m 26.5 to 5 9 c m ( M a n z e r 1968).  44  E v e n m o r e uncertainty i s added b y L e B r a s s e u r ' s (1966) report o n differences i n stomach contents f o r different oceanic regimes f r o m samples taken between M a y a n d June 1958. sockeye caught i n the Subarctic Pacific f e dmainly upon immature  a n d maturing  sockeye,  respectively),  While  squid (75 a n d8 9 % b y weight for  i n the coastal region  they  depended / o n  e u p h a u s i i d s (48 a n d 6 0 % ) , s q u i d (31 a n d 9 % ) , a n d f i s h ( 1 2 a n d 1 6 % ) . W i t h i n the A l a s k a n S t r e a m area s o c k e y e preyed o n euphausiids ( 5 0 a n d 2 1 % ) a n d a m p h i p o d s ( 5 0 % f o r immature)  or fish  (60% for maturing). Maturing sockeye migrating through the transition zone c o n s u m e d  mainly  euphausiids (71%)and amphipods (27%). T h e s e c o n f l i c t i n g r e p o r t s w e r e s o m e w h a t r e c o n c i l e d b y a m a j o r s t u d y b y P e a r c y et al. w h o collected data o n s a l m o n stomach contents (sockeye, p i n k coho  (O. kisutch),  155°W  (1984,  a n d steelhead  1985),  a n d 55°N  (O. mykiss)) (1982,  (O. gorbuscha),  chum  (O. keta),  o n s i x J u l y cruises a l o n g 145°W (1980,  1983).  They  c o n c l u d e d that s a l m o n  (1988)  species  1981), forage  opportunistically w i t h large overlap between species (except chum). Prey c h o i c e was not r a n d o m but selective f o r a certain size class. In  summary,  spatial a n d temporal  variability i n the diet c o m p o s i t i o n o f i m m a t u r e a n d  maturing sockeye s a l m o n c a n b e attributed t odifferences i n the availability o f species w i t h i n the preferred size class (Brodeur  1 9 9 0 ; L e B r a s s e u r 1 9 6 6 ; P e a r c y et al.  1988). (Because o f l a c k o f  independent support a n d its somewhat counterintuitive conclusions, I disregard here a study b y B e a c h a m ( 1 9 8 6 as c i t e d i n B r o d e u r ( 1 9 9 0 ) w h i c h suggests that w i t h i n c r e a s i n g s o c k e y e s i z e , t h e m e a n s i z e o f invertebrate p r e y decreases w h i l e that o f f i s h p r e y increases.) P r e y o r g a n i s m s f o r i m m a t u r e a n d m a t u r i n g sockeye represent the larger size fraction o f m e s o z o o p l a n k t o n (e.g. the copepod  Neocalanus cristatus  w i t h a m a x i m u m adult length o f 10 m m (Parsons & L a l l i  m a c r o z o o p l a n k t o n , a n d " m a c r o " n e k t o n ( b y d e f i n i t i o n : 2 - 2 0 c m ( P a r s o n s et al.  1988)),  1984)).  45  D i e t c o m p o s i t i o n for sockeye s a l m o n has also been reported i n terms o f trophic levels o f prey items (Table 2.1; L e B r a s s e u r 1972; Sanger 1972). A n interesting change i n the trophic p o s i t i o n occurs when Domain.  maturing  s o c k e y e migrate f r o m the C e n t r a l S u b a r c t i c D o m a i n i n t o the  U s i n g data g i v e n i n L e B r a s s e u r (1972), I c a l c u l a t e d the f r a c t i o n a l t r o p h i c l e v e l s  s o c k e y e s a l m o n i n the t w o regions primary  Coastal  producers  =  as 4 . 6  and 4.1, respectively (where the trophic l e v e l  of of  1). T h e l a t t e r v a l u e i s c o n s i s t e n t w i t h a f r a c t i o n a l t r o p h i c l e v e l o f  3.9  calculated f r o m data g i v e n i n Sanger (1972), a s s u m i n g these data have been c o l l e c t e d i n  the  c o a s t a l z o n e . T h i s is a n effect o f the shorter f o o d c h a i n o f the C o a s t a l D o m a i n . H o w e v e r , the analysis o f f i s h s t o m a c h contents is v e r y tedious a n d w h i l e p r e y species (or higher taxa) c a n be identified b y persistent efforts o f taxonomists, their size c a n hardly ever be reconstructed f r o m the partially digested organisms  found in  the  stomachs,  let alone  their  (fractional) t r o p h i c levels. A l s o s a l m o n tend to regurgitate f o o d w h e n caught i n g i l l nets (Favorite 1970) w h i c h p o s e s a d d i t i o n a l p r o b l e m s to the interpretation o f c e r t a i n results (e.g. F a v o r i t e L e B r a s s e u r 1 9 6 6 ; P e a r c y et  al.  1970;  1988). F u r t h r m o r e , it w a s p r e v i o u s l y b e l i e v e d that c o p e p o d s  the N E - P a c i f i c were m o s t l y herbivorous  (LeBrasseur  1972), w h i l e m o r e recent studies  s h o w n t h a t t h e y p r e y m o s t l y o n m i c r o z o o p l a n k t o n ( s e e S e c t i o n 2 . 2 ; B o o t h et 1 9 9 3 ; G i f f o r d 1 9 9 3 ; L a n d r y et al.  1 9 9 3 a ; L a n d r y et al.  1 9 9 3 b ; M a c k a s et al.  al.  1993;  in  have Dagg  1 9 9 3 ; M i l l e r et  al.  1991b) w h i c h increases the trophic l e v e l o f s o c k e y e s a l m o n a c c o r d i n g l y .  46  Table 2.1: D i e t  composition o f maturing sockeye salmon i n terms o f trophic levels o f prey items  (% c o m p o s i t i o n o f s t o m a c h c o n t e n t s b y w e i g h t ) .  Life History Stage  Herbivores  (Location)  Primary  Secondary  Carnivores  Carnivores  Reference  Sanger(1972)  not available  15  80  5  maturing  3  30  67  LeBrasseur (1972)  6  82  12  LeBrasseur (1972)  (oceanic)  maturing (coastal)  47  D i e t c o m p o s i t i o n and foraging analysis for fish faces inherent (Brodeur  1990).  U s u a l l y fish caught  contents a n a l y s i s , w h e r e the v o l u m e  in  some  methodological  f i s h e r i e s are r a n d o m l y  sampled  or weight contribution of a specific food  problems  for  stomach  i t e m to total  s t o m a c h contents a n d the f r e q u e n c y o f o c c u r r e n c e o f a s p e c i f i c f o o d i t e m i n different s t o m a c h s are d e t e r m i n e d .  The  overall objective of  most  studies is to determine  the degree  of  food  p r e f e r e n c e . F o o d p r e f e r e n c e m e a n s that the p r o p o r t i o n o f a f o o d i t e m i n the d i e t i s greater t h a n t h e p r o p o r t i o n o f t h e s a m e i t e m a v a i l a b l e t o t h e f o r a g i n g a n i m a l ( B e g o n et  al.  1991). Unfortunately  food  m a n y studies rely o n  stomach analyses solely, or on  1990;  Healey  availability  studies c o n d u c t e d s o m e w h e r e or s o m e t i m e else. W i t h o u t proper reference to actual a v a i l a b i l i t y o f f o o d i t e m s , studies o n p r e f e r e n c e a n d o p t i m a l f o r a g i n g are not p o s s i b l e i n p r i n c i p l e . A l l that c a n b e i n f e r r e d i s w h e t h e r o r n o t f i s h o f d i f f e r e n t r e g i o n s o r c a u g h t at d i f f e r e n t t i m e s h a v e s i m i l a r stomach  contents. Still,  even  in  simultaneously and independently  studies where ( P e a r c y et  al.  the  availability of  1988)  one  food  cannot be  items  is  measured  sure that the  sample  r e p r e s e n t s w h a t i s a c t u a l l y a v a i l a b l e to the f o r a g i n g a n i m a l , i.e. the s e l e c t i v i t y o f the s a m p l i n g gear m a y affect what m a y seem "available".  48  2.2. Ecosystems of the Northeast Pacific H e r e , I define the Northeast P a c i f i c as the range i n ocean distribution o f N o r t h Pacific salmon species (Groot & Margolis region  into  four  Downwelling  (Oncorhynchus  spp.), i.e. a p p r o x i m a t e l y  1 9 9 1 ; W e l c h et al.  4 0 - 6 6 ° N a n d 175°E  domains  ( F i g . 2.1): T h e borders  (Alaska and North British Columbia),  Transitional (Central  andCoastal Upwelling Domains (Washington,  are s p a t i a l l y transient a n d d e t e r m i n e d  -125°W  1995). W a r e & M c F a r l a n e (1989) h a v e c l a s s i f i e d this  ecological upper-zone  Columbia, andWashington)  American  o f the Coastal  andSouth Oregon,  British  California)  b y the bifurcation o f the eastward Subarctic  Current  offshore o f the N o r t h A m e r i c a n continent into the n o r t h w a r d A l a s k a C u r r e n t a n d the s o u t h w a r d C a l i f o r n i a Current. T h e fourth d o m a i n , the Central Subarctic, represents the oceanic p r o v i n c e o f the N E - P a c i f i c a n d i s the m a i n feeding g r o u n d (Brodeur 1990; Burgner  f o r maturing North A m e r i c a n Pacific  salmon  1991).  T h e s e b i o g e o g r a p h i c a l provinces are characterized b y different p h y s i c a l properties a n d as a c o n s e q u e n c e p r o d u c t i v i t y patterns ( f l o w  o f energy a n d nutrients) a n d e c o l o g i c a l c o m m u n i t i e s  (species c o m p o s i t i o n , size classes; L e B r a s s e u r 1966; W a r e & M c F a r l a n e 1989). I assert ( i n the f o r m o f a c o m p l e x w o r k i n g h y p o t h e s i s ) that m a r i n e s u r v i v a l a n d p o s s i b l y o v e r a l l c o h o r t - s u r v i v a l of Pacific  salmon i s largely determined  b y t h e e n d o f their first w i n t e r at s e a . S u r v i v a l i s  c o n t r o l l e d b y p h y s i c a l - b i o l o g i c a l processes (e.g. f o o d production, predation risk) i n the C o a s t a l Downwelling,  Transitional  (advection, migration) 1993; W a r e  and Central  Subarctic  Domains,  as w e l l  as b y the transport  o f organisms a n d nutrients a m o n g those d o m a i n s (Brodeur  & McFarlane  1989;Wickett  1967).  Because only  f e wsalmon  & Hollowed  stocks enter t h e  C o a s t a l U p w e l l i n g D o m a i n during their migration, I have e x c l u d e d this area f r o m the f o l l o w i n g discussion.  49  60°N  50°N  40°N  170°W  Fig.  2.1:  150°W  130°W  E c o l o g i c a l upper zone domains and prevailing currents i n the Northeast P a c i f i c O c e a n .  1 - Coastal Downwelling Domain, 2 - Transitional Domain, 3 - Coastal U p w e l l i n g Domain, 4 C e n t r a l Subarctic P a c i f i c , 5 - B e r i n g Sea. D o t t e d lines represent variable boundaries  between  d o m a i n s . T h e S u b a r c t i c B o u n d a r y i s a frontal region w h i c h separates t h e S u b a r c t i c P a c i f i c t o the north f r o m the Subtropic P a c i f i c to the south (Thomson,  1981). A f t e r D o d i m e a d  et a l . ( 1 9 6 3 ) ,  L e B r a s s e u r (1966), Sanger (1972a), T h o m s o n (1981), and W a r e and M c F a r l a n e (1989).  50  2.2.1. The Central Subarctic  Domain  Phytoplankton P h y t o p l a n k t o n s t a n d i n g s t o c k s i n the C e n t r a l S u b a r c t i c D o m a i n as e s t i m a t e d b y c h l o r o p h y l l - a c o n c e n t r a t i o n s a t O c e a n W e a t h e r S t a t i o n P ( 5 0 ° N 1 4 5 ° W , h e n c e f o r w a r d c a l l e d S t a t i o n P) s h o w little s e a s o n a l v a r i a b i l i t y ( F i g . 2 . 2 ) . M e a n C h l - a c o n c e n t r a t i o n s at S t a t i o n P b e t w e e n  1958-1991  were around 0.4 m g C h l - a m " throughout the year (Parslow 1981; Parsons & L e B r a s s e u r 3  W o n g et al. 1 9 9 5 ) , a l t h o u g h J u n e a n d O c t o b e r v a l u e s a p p e a r s o m e w h a t  1968;  above the respective  adjacent months, w i t h the October c h l o r o p h y l l - a " m a x i m u m " possibly caused b y the life-history i n d u c e d S e p t e m b e r m i n i m u m i n c o p e p o d b i o m a s s ( M i l l e r et al. 1 9 8 4 ; P a r s o n s & L a l l i 1 9 8 8 ) . W o n g et al. ( 1 9 9 5 ) d o n o t r e p o r t t h e v a r i a b i l i t y a r o u n d t h e m o n t h l y m e a n s , t h u s  short-term  increase i n C h l - a concentrations cannot be completely excluded and n o conclusions can be drawn about  short-term  and/or  spatial variability (patchiness)  i n the samples  (see also Parsons  L e B r a s s e u r 1968 their F i g . 3(A)). H o w e v e r , cumulative composite data f o r Station P f r o m  &  1959-  1 9 7 0 r e v e a l that there are n o p h y t o p l a n k t o n b l o o m s (here d e f i n e d as c o n c e n t r a t i o n s > 2 m g C h l - a m"  a n d n o t as i n c r e a s e d p r i m a r y p r o d u c t i v i t y ) i n t h e C e n t r a l S u b a r c t i c D o m a i n a n d that 1 m g  C h l - a m " i s o n l y e x c e e d e d o c c a s i o n a l l y ( M i l l e r et al. 1 9 8 4 ; M i l l e r et al. 1 9 9 1 a ; M i l l e r et al. 3  1991b).  O n the other  hand,  a n independent  1964-1976  time  series ( P a r s l o w  1981) shows  intermittent events o fvery high C h l - a concentrations w i t h m a x i m a o f 3-5 m g C h l - a m " occurring 3  abruptly w i t h o u t a n y i n d i c a t i o n i n the data t a k e n o n l y days before the events.  51  Fig.  2.2:  C h l o r o p h y l l - a c o n c e n t r a t i o n at O c e a n W e a t h e r S t a t i o n P  (50°N 145°W), as m o n t h l y  averages 1 9 6 4 - 1 9 9 1 . N o t e that v a r i a b i l i t y a r o u n d the m e a n i s n o t r e p o r t e d , thus n o c o n c l u s i o n s about temporal and/or spatial variability (patchiness) i n the samples c a n b e inferred.  Adapted  f r o m W o n g et a l . ( 1 9 9 5 ) .  52  A l t h o u g h it i s s o m e w h a t u n u s u a l that t w o p u b l i s h e d data-sets o f the s a m e r e g i o n S t a t i o n P is n o m i n a l l y a point, due to a d v e c t i o n processes measurements taken there  (although represent  r e g i o n a l data) w i t h o v e r l a p p i n g t i m e periods present s e e m i n g l y different results, I t h i n k these m e a s u r e m e n t s m i g h t be r e c o n c i l e d b y the patchiness apparent i n p l a n k t o n c o m m u n i t i e s a n d the inherent delay o f zooplankton-control of phytoplankton standing stock. A  somewhat different picture emerges w h e n phytoplankton  standing stock is estimated  by  c a r b o n c o n c e n t r a t i o n s . B e c a u s e p h y t o p l a n k t o n c o n c e n t r a t i o n s are d i f f i c u l t to m e a s u r e i n u n i t s o f c a r b o n (P. H a r r i s o n 1997 pers. c o m m . ) c h l o r o p h y l l - a data m u s t be m u l t i p l i e d b y the c a r b o n c h l o r o p h y l l - a ratio. B e c a u s e the C / C h l - a  ratio varies w i t h light intensity  p h y t o p l a n k t o n s t a n d i n g s t o c k s at S t a t i o n P c o u l d v a r y f r o m 5 . 2 5 m g C m " 15) t o > 2 7 . 5 m g C m "  3  ( J u n e C / C h l - a = 5 0 ) , u s i n g W o n g et al.'s  3  (McAllister  /  1969)  (January C / C h l - a  =  (1995) C h l - a and M c A l l i s t e r ' s  ( 1 9 6 9 ) C / C h l - a d a t a , i.e. a f i v e f o l d i n c r e a s e (see a l s o the v a r i a b i l i t y o f the c a r b o n - t o - c h l o r o p h y l l a ratio e s t i m a t e s o b t a i n e d d u r i n g the s u m m e r S U P E R - c r u i s e s i n F r o s t ( 1 9 9 3 ) , h i s T a b l e 3). T h e m e t h o d s u s e d to estimate s e a s o n a l v a r i a b i l i t y i n C / C h l - a ratios are s o m e w h a t  arbitrary  ( M c A l l i s t e r 1969) a n d c o u l d be m i s l e a d i n g , thus q u e s t i o n i n g the n u m e r i c a l v a l i d i t y o f the ratios a s w e l l as t h e o v e r a l l c o n c l u s i o n o f t h e p o s s i b l e f i v e f o l d i n c r e a s e i n p h y t o p l a n k t o n  standing  stock. In general, scientific k n o w l e d g e about the c a r b o n - t o - c h l o r o p h y l l ratio is surrounded great u n c e r t a i n t y  (Banse  1977), the r e a s o n f o r w h i c h s e e m s  to b e  d y n a m i c p r o c e s s e s a r e o f t e n u s e d i n o n e c o r r e l a t i o n a n a l y s i s ( P i a t t et al.  that d a t a f r o m  by  different  1981). T. Parsons  (1997  pers. c o m m . ) has p o i n t e d out that the i n c r e a s e i n the C / C h l - a ratio c o u l d w e l l b e attributed to a n i n c r e a s e d s t a n d i n g s t o c k o f detritus, i.e. n o n - l i v i n g p a r t i c u l a t e o r g a n i c m a t t e r ( P O M ) bacteria (Parsons  et  al.  1984), d u r i n g the s u m m e r  months.  A l s o , because the  + associated phytoplankton  c e l l u l a r C / C h l - a ratio is to s o m e extent c o n t r o l l e d b y seawater nitrate concentrations w h i c h s h o w  53  a decrease but n o d e p l e t i o n i n the C e n t r a l S u b a r c t i c D o m a i n i n s u m m e r , p h y t o p l a n k t o n s t a n d i n g stocks  should  remain  more  or  less  constant throughout  the  year.  On  the  other  r e l a t i o n s h i p b e t w e e n nitrate concentrations a n d the C / C h l - a ratio i s b a s e d o n P a r s o n s et al.  1 9 8 4 t h e i r T a b l e 10) w h i c h r e p r e s e n t < 1 0 %  C e n t r a l S u b a r c t i c D o m a i n ( M i l l e r et al. in  units  of  carbon  has  also been  Ecosystem Research; M a y  the  diatoms  (see  o f the p r i m a r y p r o d u c e r b i o m a s s i n the  1991a). A l s o , h i g h variability i n p h y t o p l a n k t o n m e a s u r e d  observed  and August  hand,  1984,  during  the  SUPER-cruises (Subarctic  1988, June and September  1991b)) w i t h a m e a n standing stock of 20 m g C m"  3  1987  ( M i l l e r et  a n d a m a x i m u m o f 7 4 ( B o o t h et al.  Indirect evidence for an increased phytoplankton  Pacific  standing stock in summer  al.  1993).  comes  from  d i l u t i o n e x p e r i m e n t s c o n d u c t e d d u r i n g t w o S U P E R - c r u i s e s i n t h e G u l f o f A l a s k a ( L a n d r y et 1 9 9 3 b ; M i l l e r et  al.  1991b). W h i l e the s p e c i f i c rates f o r p h y t o p l a n k t o n c o m m u n i t y g r o w t h  al. and  m i c r o z o o p l a n k t o n g r a z i n g w e r e a p p r o x i m a t e l y t h e s a m e ( 0 . 3 5 d" ) i n J u n e 1 9 8 7 a n d M a y  1988,  phytoplankton  1988.  1  (0.49  d" ) 1  by  far  exceeded  microzooplankton  (0.26  d" ) 1  in  August  I n t e r e s t i n g l y , t h i s r e s u l t h a s b e e n i n t e r p r e t e d as m i c r o g r a z e r s c o n t r o l l i n g p h y t o p l a n k t o n " i n d y n a m i c a n d v a r i a b l e f a s h i o n " ( M i l l e r et al.  a  1991b), rather than a c o n s e q u e n c e o f m o r e c o m p l e x  f o o d w e b interactions w h e r e l i f e - h i s t o r y - i n d u c e d changes i n the a b u n d a n c e o f m e s o z o o p l a n k t o n s e a s o n a l l y i n t e n s i f y a n d a l l e v i a t e g r a z i n g p r e s s u r e u p o n s m a l l e r z o o p l a n k t o n , w h i c h i n t u r n are u n a b l e o r apt to c o n t r o l p h y t o p l a n k t o n s t a n d i n g stock. Whatever  the case, the p o s s i b i l i t y o f a 5 - f o l d increase i n s u m m e r p h y t o p l a n k t o n  standing  s t o c k w h e n m e a s u r e d i n c a r b o n has important i m p l i c a t i o n s f o r the r e a l i z e d f o o d c h a i n structure (see l a t e r c h a p t e r s ) . A f t e r a l l , c o n s u m e r m e t a b o l i s m d e p e n d s o n r e d u c e d c a r b o n c o m p o u n d s  and  not o n c h l o r o p h y l l - a .  54  C o n s i d e r a b l e s e a s o n a l v a r i a b i l i t y i n p r i m a r y p r o d u c t i v i t y at S t a t i o n P ( f r o m -20 to = 3 5 0 m g C m"  2  in December  d" i n J u l y ( M c A l l i s t e r 1969)) has recently b e e n disputed. U s i n g the C e n t r e f o r 1  O c e a n C l i m a t e C h e m i s t r y c o m p o s i t e d a t a s e t f o r 1 9 8 4 - 1 9 9 0 ( F i g . 2 . 3 ) W o n g et al. mean of 283 for winter (December-February)  a n d 4 6 6 m g C m~  2  d"  1  for summer  (1995) report a (June-August),  a l t h o u g h the w i n t e r v a l u e m u s t be v i e w e d w i t h c a u t i o n since it is b a s e d o n o n l y 2 s a m p l e s taken on  subsequent  days i n late February  primary productivity o f 661 m g C m" M i l l e r et al. ( B o o t h et al.  1989.  2  d"  1  Furthermore,  a  mean  during their s u m m e r cruises (1984, 1987 and  1988;  1991b), w i t h values up to 1 0 0 0 m g C m" 1 9 9 3 ; W e l s c h m e y e r et al.  2  SUPER-scientists obtained  d" or a p p r o x i m a t e l y one d o u b l i n g per day 1  1993).  R e c e n t e s t i m a t e s f o r an a n n u a l p r i m a r y p r o d u c t i o n i n the C e n t r a l S u b a r c t i c D o m a i n are g C m"  2  y"  1  ( W o n g et al.  1995) and 170 g C m"  2  y" ( W e l s c h m e y e r 1  et al.  1993), w i t h the s e c o n d  p r o b a b l y b i a s e d due to s a m p l i n g i n s u m m e r only. B o t h these estimates l i e w i t h i n o n e deviation o f an independently Welch  derived, though  1993), estimate ( F a l k o w s k i & W i l s o n  previously reported values of 45-72 g C m"  2  y"  140  strongly debated ( F a l k o w s k i &  standard  Wilson  1993;  1992), a n d are t w o - to t h r e e f o l d h i g h e r than the  1  ( M c A l l i s t e r 1 9 7 2 ; see a l s o S a n g e r 1 9 7 2 h i s F i g s .  1 to 4). T h e c a u s e s f o r these d i s c r e p a n c i e s are u n k n o w n  a n d m a y b e attributed to the c l e a n e r  s a m p l i n g t e c h n i q u e s w i t h w h i c h the m o r e recent data h a v e b e e n c o l l e c t e d . H o w e v e r , a l l o f the above measurements were taken f r o m Station P only and therefore m a y reflect spatio-temporal v a r i a b i l i t y i n o c e a n o g r a p h i c conditions rather than b a s i n - w i d e changes i n p r i m a r y  production.  T h e seasonal onset o f increased p h y t o p l a n k t o n p r o d u c t i v i t y c a n be attributed to the seasonal increase i n i n s o l a t i o n a n d the f o r m a t i o n o f the seasonal t h e r m o c l i n e , a n d a n i n c r e a s e d c r i t i c a l depth and a s h a l l o w e r depth o f m i x i n g (Parsons 1988). T h e spatial distribution o f this event the N E - P a c i f i c is s u c h that w a t e r c o l u m n s t a b i l i z a t i o n i n c o a s t a l areas o c c u r s i n a b o u t  in  March  55  1000  H?  Fig.  2.3:  900  Seasonal  1 4  C p r i m a r y p r o d u c t i v i t y at S t a t i o n P f r o m c o m p o s i t e d a t a 1 9 8 4 - 9 0 . A , B , a n d  C levels represent historical annual p r i m a r y p r o d u c t i o n estimates o f 6 0 g C m" 1972), 140 g C r n  2  y  1  ( W o n g et a l . , 1 9 9 5 ) , a n d 2 3 0 g C r n  2  y  1  2  y"  1  (McAllister,  ( W e l s c h m e y e r et a l . , 1 9 9 1 * ) ,  respectively. T h e third value is based o n S U P E R - d a t a f r o m s u m m e r cruises o n l y ( S 1 - S 6 )  and  t h u s i s l i k e l y t o b e b i a s e d . L a r e d a t a f r o m B o o t h et a l . 1 9 8 8 * . N o t e a g a i n t h a t v a r i a b i l i t y a r o u n d the s e a s o n a l m e a n s is not reported, thus n o c o n c l u s i o n s about t e m p o r a l a n d / o r s p a t i a l v a r i a b i l i t y (patchiness) i n the s a m p l e s c a n b e i n f e r r e d . R e f e r e n c e s w i t h a s t e r i s k are g i v e n i n W o n g et a l . ( 1 9 9 5 ) . A d a p t e d f r o m W o n g et a l . ( 1 9 9 5 ) .  56  (possibly  influenced b y haline  stratification d u e to meltwater  run-off  from  the American  c o n t i n e n t ) p r o g r e s s i n g o f f s h o r e s o that the c e n t e r o f the A l a s k a n G y r e i s r e a c h e d i n M a y  (Fig.  2 . 4 ; P a r s o n s et al.  day-  length during  1966; Parsons & L e B r a s s e u r 1968). H o w e v e r , m i x e d layer depth and  spring a n d summer  phytoplankton  doubling  cruises c o u l d only partly ( 2 5 % ) explain the variability i n  rates ( B o o t h  et  al.  1993).  Primary  production  per unit biomass o f  n a n o p h y t o p l a n k t o n , the d o m i n a n t s i z e class, does not s e e m to be b o t t o m - u p l i m i t e d t o a n y extent, t h u s g r a z i n g ( B a n s e 1 9 9 4 ) a n d s i n k i n g d e t e r m i n e t h e s t a n d i n g s t o c k ( W e l s c h m e y e r et al.  1993),  the p r o d u c t i o n base. T h e species c o m p o s i t i o n o f p h y t o p l a n k t o n i n the C e n t r a l S u b a r c t i c D o m a i n i s h i g h l y v a r i a b l e o n a l lt i m e scales w i t h a f e w species b e i n g present independent  o f the season ( f o r a detailed  l i s t i n g s e e P a r s o n s a n d L a l l i ( 1 9 8 8 ) ) . T h e d o m i n a n t s i z e c l a s s ( > 9 0 % o f b i o m a s s ( M i l l e r et 1991a)) o f primary ( B o o t h et al.  producers i n the Central Subarctic is nanophytoplankton  10  4  (size 2 - 2 0 | j m  1 9 9 3 ; P a r s o n s 1 9 7 2 ) ) , u s i n g t h e c l a s s i f i c a t i o n s c h e m e i n P a r s o n s et al. ( 1 9 8 4 , t h e i r  F i g . 3 ) , w h i c h o c c u r s at d e n s i t i e s o f u p t o 1 0 species  Emiliana huxleyi  6  c e l l s l " a n d are d o m i n a t e d b y the c o c c o l i t h o p h o r i d 1  (Parsons & L a l l i 1988). Concentrations o f very s m a l l diatoms g o u p t o  c e l l s l " a n d that o f the large ( m i c r o p h y t o p l a n k t o n : 2 0 - 2 0 0 (lm) d i a t o m s p e c i e s 1  criophilum  al.  Corethron  s h o w e d 6 0 0 0 c e l l s l " i n J u l y ( P a r s o n s & L a l l i 1 9 8 8 ). 1  Because nanophytoplankton species have a lower M i c h a e l i s - M e n t e n constant f o r a m m o n i u m u p t a k e ( A . M i l l i g a n 1997 pers. c o m m . ) as w e l l as f o r nitrate (Parsons & T a k a h a s h i 1973) outcompete  the larger  microphytoplankton  f o r nitrogen.  Microphytoplankton  nitrate, w h i c h i s never depleted i n the C e n t r a l Subarctic D o m a i n ,  could  they  convert  into a m m o n i u m using the  e n z y m e nitrate reductase, w h i c h requires the m i c r o n u t r i e n t i r o n (Fe) i n m i n u t e quantities ( M a r t i n 1 9 9 1 ; M a r t i n et al.  1 9 9 4 ; M a r t i n & F i t z w a t e r 1 9 8 8 ; M o r e l et al.  1991). Iron i s p r o v i d e d t o the  57  Fig.  2.4:  Broken  S p a t i a l d i s t r i b u t i o n o f the onset o f i n c r e a s e d p r i m a r y p r o d u c t i v i t y i n the line  marks  approximate  temporal  progression  of  the  formation  of  NE-Pacific.  the  seasonal  t h e r m o c l i n e ( d e r i v e d f r o m the least f a v o r a b l e c o n d i t i o n s i n o r d e r to s h o w the greatest d i f f e r e n c e w i t h i n the area o f the N E - P a c i f i c (Parsons a n d L a l l i , 1988). D o t t e d areas represent c o p e p o d w e t weights  for April.  The  hatched horseshoe-shaped  area represents the interpolated r e g i o n  of  m a x i m u m c o p e p o d w e t w e i g h t i n A p r i l . A d a p t e d f r o m P a r s o n s et a l . ( 1 9 6 6 ) .  58  euphotic zone ( D o n a g h a y et  o f the ocean m a i n l y b y 1991; Duce  al.  &  atmospheric deposition and only  Tindale  1991)  in s m a l l quantities  which makes microphytoplankton  production  bottom-up iron limited. N a n o p h y t o p l a n k t o n standing stock is c o n t r o l l e d t h r o u g h g r a z i n g b y m i c r o z o o p l a n k t o n 2 0 0 u r n ) , i . e . s m a l l h e t e r o t r o p h i c f l a g e l l a t e s a n d c i l i a t e s ( B o o t h et al. al.  1 9 9 3 b ; S t r o m et al.  d o u b l i n g s d"  1  (20-  1993; Frost 1987; Landry  et  1993). M i c r o z o o p l a n k t o n c a n m a i n t a i n g r o w t h rates o f u p to m o r e t h a n 5  ( M i l l e r et  al.  1 9 9 1 b ) w h i c h are h i g h e r t h a n the g r o w t h rates o f t h e i r f o o d s o u r c e  ( B a n s e 1994) due to t w o reasons: First, m i c r o z o o p l a n k t o n is capable o f c e l l d i v i s i o n 2 4 hours per d a y ( M i l l e r et al.  1 9 9 1 a , a n d references c i t e d therein), a n d s e c o n d , it i s s p a r e d the e n e r g e t i c costs  o f s y n t h e s i z i n g b a s i c b i o l o g i c a l m o l e c u l e s , s u c h as s u g a r s , p r o t e i n s a n d f a t s w h i c h i t f i n d s i n i t s food (Miller  etal.  1991b).  T h e l o w g r o w t h rates o f m i c r o p h y t o p l a n k t o n , c a u s e d b y the c o m b i n a t i o n o f i r o n l i m i t a t i o n and  resource  (ammonium)  mesozooplankton  use  ( 2 0 0 - 2 0 0 0 urn)  competition  with  nanophytoplankton,  has  the  can effectively control microphytoplankton  effect  standing  that stock,  w h i l e m i c r o z o o p l a n k t o n due to its s m a l l s i z e a n d a f e e d i n g apparatus restricted to i n g e s t i o n o f organisms  <10  p:m  microphytoplankton  in  diameter  (Miller  et  al.  1991b)  cannot  exert  standing stock. Nevertheless, sediment trap data f r o m  largest b i o m a s s o f d i a t o m frustules f r o m M a y to A u g u s t (Parsons & highest  any  mesozooplankton  density  in  the  euphotic  mesozooplankton control on microphytoplankton  zone.  In  order  Lalli to  control  on  Station P  the  reveal  1988), the t i m e be  consistent  standing stocks, these frustules must  of  with come  f r o m o r g a n i s m s that h a v e not b e e n c o m p l e t e l y i n g e s t e d o r d i g e s t e d , o r b e l o n g to the s m a l l e r s i z e class of nanophytoplankton.  59  It h a s b e e n s p e c u l a t e d t h a t t h e l i f e - h i s t o r y - i n d u c e d S e p t e m b e r m i n i m u m i n c o p e p o d might  c a u s e the slight increase i n p h y t o p l a n k t o n  s u b s e c t i o n ; M i l l e r et al.  s t a n d i n g s t o c k (see first p a r a g r a p h  density of  this  1984; Parsons & L a l l i 1988). Y e t , i f phytoplankton consists m a i n l y  of  n a n o p h y t o p l a n k t o n w h i c h i s b e l i e v e d t o b e c o n t r o l l e d b y m i c r o z o o p l a n k t o n ( M i l l e r et al.  1991a),  w h i c h i n turn is controlled b y mesozooplankton  Gifford  1993; Parsons &  Lalli  a r g u m e n t ( H a i r s t o n et al.  1988; W a r e &  (mainly  McFarlane  large copepods; D a g g 1993;  1989), and f o l l o w i n g a s i m p l e f o o d  chain  1960), the l i f e - h i s t o r y i n d u c e d S e p t e m b e r m i n i m u m i n c o p e p o d d e n s i t y  s h o u l d rather decrease than increase the p h y t o p l a n k t o n standing s t o c k i n O c t o b e r .  Nutrients P h y t o p l a n k t o n - n u t r i e n t interactions i n the C e n t r a l S u b a r c t i c D o m a i n c a n b e c h a r a c t e r i z e d b y 3 n u t r i e n t s : t h e m a c r o n u t r i e n t s n i t r a t e (NO3) (Fe).  In  general  nitrate  and  iron  and a m m o n i u m ( N H / )  concentrations  are  governed  a n d the m i c r o n u t r i e n t by  external  iron  processes  (i.e.  u p w e l l i n g , a n d a t m o s p h e r i c d e p o s i t i o n i n the case o f i r o n ) w h i l e r e c y c l i n g p r o c e s s e s r e g u l a t e a m m o n i u m ( M i l l e r 1 9 9 3 a ; M i l l e r et al.  1991a).  Y e a r - a r o u n d d a t a 1 9 6 6 - 1 9 7 6 f r o m S t a t i o n P s h o w a m e a n n i t r a t e - m a x i m u m o f a b o u t 15 | i M i n t h e s u r f a c e l a y e r i n e a r l y M a r c h at t h e e n d o f t h e w i n t e r m i x i n g s e a s o n ( P a r s l o w  1981).  B e t w e e n M a r c h a n d S e p t e m b e r the seasonal t h e r m o c l i n e reduces the already m e a g e r (due to a p e r m a n e n t h a l o c l i n e at a r o u n d 1 0 0 m ) s u p p l y o f n i t r a t e t o t h e m i x e d l a y e r b y c r e a t i n g a f u r t h e r b a r r i e r , a b o v e t h e h a l o c l i n e , t o u p w a r d a d v e c t i v e - d i f f u s i v e f l u x e s . It i s e s t i m a t e d t h a t > 7 0 % the total transport o f s u b - h a l o c l i n e concentrations o f 3 0 to 4 5  into the euphotic z o n e is due to  a d v e c t i v e f l u x , i . e . u p w e l l i n g ( a d v e c t i v e f l u x : 1.6 m m o l n i t r a t e m nitrate m"  2  d"  1  (Miller  et al.  1991b)).  During  the  same  of  time  2  d" ; diffusive flux: 0.6 1  some  mmol  nitrate is taken u p  by  60  p h y t o p l a n k t o n w h i c h thus results i n a steady d e c l i n e o f nitrate concentration a n d a m e a n minimum  o f «7  uM  b y S e p t e m b e r ( M i l l e r et  1991b; P a r s l o w 1981; W h e e l e r  al.  t h e r e i s v a r i a b i l i t y at a l l t i m e s c a l e s at S t a t i o n P, b e l o w 5 pJVl a n d n o s i n g l e m e a s u r e m e n t b e l o w  1993).  NO3While  nitrate i s n e v e r c o m p l e t e l y u s e d u p , i.e. n o m e a n 1.5 pJVI h a s b e e n r e c o r d e d i n t h e p e r i o d  1966-  1 9 7 6 (see P a r s l o w 1981 h i s F i g s . 28 a n d 29). M i l l e r et al. the M a y  (1991b) and W h e e l e r (1993) report a m m o n i u m  c o n c e n t r a t i o n s at O S W  1988 S U P E R cruise w i t h values f r o m a l m o s t 0 to 3.9 p M . A m m o n i u m  a r e w i l d l y f l u c t u a t i n g ( s e e M i l l e r et  al.  from  P  concentrations  1 9 9 1 b t h e i r F i g . 12) b u t l i t t l e i s k n o w n a b o u t t h e e x a c t  nature o f the s e a s o n a l , interannual, a n d spatial v a r i a b i l i t y . H o w e v e r , it is s p e c u l a t e d that tight c o u p l i n g ( t r o p h o d y n a m i c phasing) b e t w e e n p r i m a r y producers a n d their c o n s u m e r s leads to r a p i d nutrient c y c l i n g i n v o l v i n g particulate nitrogen a n d the m i c r o b i a l l o o p , a n d p r o v i d i n g  ammonium  b a c k to phytoplankton  ammonium  (NH  4  +  ( M i l l e r et  al.  1991b; W h e e l e r  1993). P h y t o p l a n k t o n prefers  ) o v e r n i t r a t e (NO3") b e c a u s e i t i s a l r e a d y i n a n r e d u c e d s t a t e t h u s s a v i n g e n e r g y e x p e n s e s  for s o m e b i o c h e m i c a l redox-reactions w h i c h w o u l d be required for nitrate reduction. I r o n c o n c e n t r a t i o n s i n the C e n t r a l S u b a r c t i c D o m a i n  a r e v e r y l o w (<0.1  nM  (Morel  1 9 9 1 , P . H a r r i s o n 1 9 9 3 p e r s . c o m m . ) ) . It i s s u p p l i e d t o t h e e u p h o t i c z o n e o f t h e o c e a n  et  al.  through  r o c k w e a t h e r i n g a n d subsequent transport i n rivers (T. P e d e r s e n 1995 pers. c o m m . ) , input  from  the deep o c e a n t h r o u g h u p w e l l i n g , a n d d e p o s i t i o n f r o m the atmosphere after w i n d transport  from  l a n d ( D o n a g h a y et al.  1991; D u c e & T i n d a l e 1991). B e c a u s e i r o n is e f f e c t i v e l y r e m o v e d f r o m the  w a t e r c o l u m n d u r i n g e s t u a r i n e m i x i n g ( B o y l e et al.  1 9 7 7 ; F l e t c h e r et al.  1983) it is e s t i m a t e d that  a p p r o x i m a t e l y 7 5 % o f a l l F e - i n p u t to the e u p h o t i c z o n e o f the oceans c o m e s f r o m the a t m o s p h e r e ( D u c e & T i n d a l e 1 9 9 1 ) a n d o n l y i n s m a l l q u a n t i t i e s . S e e a l s o a r g u m e n t s i n B o y d et al.  (1998).  61  T h e a v a i l a b i l i t y o f i r o n l i m i t s the synthesis o f the e n z y m e nitrate reductase i n p h y t o p l a n k t o n , an e n z y m e n e e d e d t o reduce nitrate t o a m m o n i u m .  A k e y strategy f o r p h y t o p l a n k t o n t o ensure  g r o w t h i n l o w ( m i c r o - ) n u t r i e n t a r e a s i s s m a l l c e l l s i z e ( M o r e l et al. 1 9 9 1 ) , a n d t h u s i n t h e C e n t r a l Subarctic D o m a i n  nanophytoplankton  outcompete  the larger m i c r o p h y t o p l a n k t o n  n u t r i e n t s . N a n o p h y t o p l a n k t o n i s c o n t r o l l e d b y m i c r o z o o p l a n k t o n ( B o o t h et al. al.  1 9 9 3 b ; S t r o m et al.  to  nanophytoplankton.  for a l l the  1993; Landry  et  1993) a n d the resulting tight t r o p h o d y n a m i c p h a s i n g p r o v i d e s a m m o n i u m T h e untouched  high  nitrate  concentration  cannot  be utilized b y  m i c r o p h y t o p l a n k t o n after a l l , s i m p l y because i r o n is not a v a i l a b l e . In  summary,  microphytoplankton  the combination f o r nitrogen  of:  1)  nanophytoplankton  outcompeting  a n d iron, because o f lower macronutrient  constants as w e l l as l o w e r i r o n requirements  o f nanophytoplankton  the  larger  Michael-Menten  (P. Harrison  1998  pers.  c o m m . ) ; 2 ) a m m o n i u m a v a i l a b i l i t y s u p p r e s s i n g n i t r a t e u p t a k e i n a l l p h y t o p l a n k t o n ( M i l l e r et  al.  1 9 9 1 b ; W h e e l e r & K o k k i n a k i s 1 9 9 0 , A . M i l l i g a n 1 9 9 7 p e r s . c o m m . , b u t s e e a l s o P r i c e et  al.  1991); and, 3) p h y t o p l a n k t o n b e i n g b o t t o m - u p i r o n l i m i t e d i n the p r o d u c t i o n o f the  enzyme  nitrate-reductase, w h i c h i s essential f o r the u t i l i z a t i o n o f nitrate, m a k e s t h e C e n t r a l S u b a r c t i c D o m a i n o n e o f the three k n o w n h i g h - n u t r i e n t - l o w - c h l o r o p h y l l ( H N L C ) r e g i o n s i n t h e W o r l d O c e a n ( L o n g h u r s t 1996; M i l l e r 1993a; T h e others arethe eastern E q u a t o r i a l P a c i f i c and t h e Southern Ocean). H o w e v e r , a l o w c h l o r o p h y l l concentration does n o t necessarily m e a n a l o w p h y t o p l a n k t o n s t a n d i n g s t o c k as t h e C / C h l - a r a t i o v a r i e s s e a s o n a l l y ( s e e  Phytoplankton).  While  the a v a i l a b i l i t y o f certain micronutrients c o u l d set the r e a l i z e d size-class o r other g u i l d s o f primary  producers  (Armstrong  1994)  there  i s s t r o n g i n d i c a t i o n that  the standing  stock o f  n a n o p h y t o p l a n k t o n i n t h e N o r t h e a s t P a c i f i c (as m e a s u r e d i n C h l - a c o n c e n t r a t i o n ) m a y n o t a t a l l b e n u t r i e n t - , b u t r a t h e r l i g h t - a n d g r a z e r - l i m i t e d ( B a n s e 1 9 9 4 ; B o o t h et al.  1993).  62  Zooplankton Zooplankton  standing  stocks at S t a t i o n  P  show  strong  seasonal  variability  (Fig. 2.5).  C o p e p o d s d o m i n a t e z o o p l a n k t o n b i o m a s s i n t h e C e n t r a l S u b a r c t i c D o m a i n ( M a c k a s et al.  1993;  P a r s o n s & L a l l i 1 9 8 8 ; W a r e & M c F a r l a n e 1 9 8 9 ) , a n d d e m o n s t r a t e J a n u a r y a n n u a l l o w s at a r o u n d 0.44 m g C m" been  reported  3  and M a y - J u n e annual highs o f3 m gC r n (Mackas  & Frost  1993)).  Carbon  (but v a l u e s u p t o 2 0 m g C m "  3  values  were  calculated from  3  have  1971-1974  c o m p o s i t e m e a n w e t w e i g h t concentrations i n P a r s o n s & L a l l i (1988) a n d the c o n v e r s i o n factors: ( d r y w e i g h t ) / ( w e t w e i g h t ) = 0.1 ( P a r s o n s & L a l l i 1 9 8 8 ) ; ( C a r b o n w e i g h t ) / ( d r y w e i g h t ) = 0 . 4 ; ( P a r s o n s et al. 1 9 8 4 t h e i r T a b l e 1 1 ) . T h e v a r i a b i l i t y w i t h i n t h e m o n t h l y s a m p l e s i s c o n s i d e r a b l e w i t h ranges 0.12-0.84 i n January and 0.44-15.12  m g C m"  3  i n M a y . B o t h might b e attributed to  the s p a t i a l l y p a t c h y d i s t r i b u t i o n o f c o p e p o d s . M a y d a t a are s i m i l a r t o the v a l u e s p u b l i s h e d b y M c A l l i s t e r ( 1 9 6 9 ) , S a n g e r ( 1 9 7 2 ) , P e a r c y et al. ( 1 9 8 8 ) , a n d b y B r o d e u r & W a r e ( 1 9 9 2 ) , w i t h t h e later h a v i n g a n a l y z e d large spatial datasets f o r the N E - P a c i f i c f o r the p e r i o d s 1980-1989  f o r samples  taken  between  15 June  and 3 0 July  o f each  1956-1962 a n d  year.  Estimates f o r  z o o p l a n k t o n p r o d u c t i o n h a v e b e e n 1 1 - 1 3 g C m~ y" ( M c A l l i s t e r 1 9 6 9 ; M c A l l i s t e r 1972). 2  The dominant  1  g r o u p o f z o o p l a n k t o n i c b i o m a s s i n the e u p h o t i c z o n e o f the w h o l e  S u b a r c t i c D o m a i n are c o p e p o d s , i n the s i z e class m e s o z o o p l a n k t o n ( 0 . 2 - 2 0 m m ) .  Central  8 0 - 9 5 % o f the  t o t a l b i o m a s s ( M a c k a s et al. 1 9 9 3 ; P a r s o n s & L a l l i 1 9 8 8 ; W a r e & M c F a r l a n e 1 9 8 9 ) c o n s i s t o f t h e large copepod species Wen  1995  Neocalanus plumchrus  pers. c o m m . ) ,  N. cristatus  (5.5 m m ) ,  (10 mm;  N. flemingeri  M a c k a s & Frost  Parsons & L a l l i 1988; W a r e & M c F a r l a n e 1989), and  (Miller  1991a, M .  et al.  1993;  (Mackas & Frost  1993;  1993;  Eucalanus bungii  et al.  Mackas  M a c k a s et al. 1 9 9 3 ) w h i c h a l l u n d e r g o o n t o g e n e t i c v e r t i c a l m i g r a t i o n s .  63  150  Fig.  2.5:  r  Seasonal change  ,0WS P  i n total biomass  e x c l u d e d ) f r o m c o m p o s i t e d a t a at S t a t i o n P  of  net z o o p l a n k t o n  (mesh  size 350  (im,  salps  1 9 7 1 - 1 9 7 4 . C o n v e r s i o n u s e d i n text: ( m g C m" ) 3  =  0 . 0 4 ( m g w e t w e i g h t m" ). N o t e the r e l a t i v e l y h i g h z o o p l a n k t o n s t a n d i n g s t o c k i n w i n t e r . D e c l i n e 3  in copepods from M a y  to O c t o b e r m a y b e c a u s e d b y e m i g r a t i o n to depth or c o n s u m p t i o n  by  predatory mesozooplankton. A d a p t e d f r o m Parsons and L a l l i (1988).  64  A f t e r the f e m a l e s o f  Neocalanus  s p p . s p a w n y o l k y e g g s at a r o u n d 4 0 0 m s o m e t i m e b e t w e e n  S e p t e m b e r a n d J a n u a r y , t h e y d i e ( M i l l e r et al.  1984, M . W e n 1995 pers. c o m m . ) . E a r l y larval  stages m i g r a t e t o w a r d s the surface w h e r e they arrive b e t w e e n N o v e m b e r a n d M a r c h ( F i g . 2.6). I n surface waters l a r v a l d e v e l o p m e n t proceeds f r o m C o p e p o d i t e I t o V i n the first h a l f o f the year. L a t e r l a r v a l stages carry out their m i g r a t i o n to depth i n the m o n t h M a y to J u l y f o r and July to September for  N. cristatus,  N.  plumchrus  w i t h the adult f o r m s h a v i n g r e d u c e d m o u t h parts ( R .  G o l d b l a t t 1995 pers. c o m m . ) and therefore not b e i n g able t o feed but rather u s i n g a c c u m u l a t e d o i l r e s e r v e s ( M i l l e r et  al.  1 9 9 1 a ; M i l l e r et al.  1991b;  Parsons & L a l l i  1988). T h i s  annual,  Eucalanus  bungii,  s e m e l p a r o u s l i f e c y c l e i s contrasted b y the b i e n n i a l , iteroparous l i f e c y c l e o f another  copepod,  w h i c h reproduces  i n the m i x e d  layer  i n early  M a y a n d early  o v e r w i n t e r s i n d i a p a u s e ( C o p e p o d i t e s t a g e s I I I - V I ) at d e p t h o f 2 5 0 - 5 0 0 m ( M i l l e r et al. In  late  summer  a n d fall  the smaller  copepod  Calanus pacificus  July and 1984).  (3 m m ) dominates  m e s o z o o p l a n k t o n b i o m a s s and seems most abundant i n waters w i t h >13°C. H o w e v e r , sea surface temperature c o u l d induce  Neocalanus spp.  vertical migrations, to avoid high metabolic loss due  to h i g h t e m p e r a t u r e o r due t o any o f the 13 p o s s i b i l i t i e s that h a v e b e e n s u g g e s t e d (see M a n g e l a n d C l a r k ( 1 9 8 8 ) p . 1 4 9 - 1 5 1 ) , thus r e n d e r i n g the temperature effect o n  C. pacificus  indirect.  S m a l l e r c o p e p o d species, w h i c h m a y h a v e m o r e than one g e n e r a t i o n p e r year, are m o s t a b u n d a n t i n late fall a n d w i n t e r a n d generally have a higher density o f i n d i v i d u a l s than larger species w h i c h d o m i n a t e the b i o m a s s a n d w h i c h m a y at t i m e s f e e d u p o n t h o s e s m a l l e r s p e c i e s ( P a r s o n s & L a l l i 1988). It h a s b e e n s h o w n i n l a b e x p e r i m e n t s t h a t m e s o z o o p l a n k t o n i s o m n i v o r o u s a n d t h a t i t p r e f e r s microzoo-  and microphytoplankton  over  nanophytoplankton  (Dagg  1993;Gifford  1993).  65  Neocalanus plumchrus  Neocalanus cristatus  Fig.  2.6:  A n n u a l l i f e c y c l e s o f Neocalanus  distribution  (dark  shading  indicates  higher  plumchrus  and  abundance).  N. cristatus  Note  different  w i t h respect t o depth depth  scales. F o r  e x p l a n a t i o n s s e e t e x t . A d a p t e d f r o m M i l l e r et a l . ( 1 9 8 4 ) .  66  H o w e v e r , because m i c r o p h y t o p l a n k t o n represents o n l y a s m a l l p r o p o r t i o n o f the p h y t o p l a n k t o n s t a n d i n g s t o c k i n the C e n t r a l S u b a r c t i c D o m a i n ,  and because nanophytoplankton  h a d to  be  p r e s e n t e d i n t h e l a b e x p e r i m e n t s at m u c h h i g h e r c o n c e n t r a t i o n s t h a n f o u n d at S t a t i o n P i n o r d e r f o r m e s o z o o p l a n k t o n to thrive, m e s o z o o p l a n k t o n m u s t be p r i m a r i l y c a r n i v o r o u s i n the C e n t r a l Subarctic D o m a i n  (Dagg  1993;  Gifford  1993;  M i l l e r et  1991a;  al.  Parsons  A d d i t i o n a l l y i t w a s f o u n d t h a t t h e g r a z i n g c a p a c i t y o f c o p e p o d s at S t a t i o n P  &  Lalli  1988).  was never  large  e n o u g h t o m a t c h p h y t o p l a n k t o n g r o w t h r a t e s a n d t h a t t h e a m o u n t o f i n g e s t e d p h y t o p l a n k t o n (as m e a s u r e d b y the a m o u n t o f p h y t o p l a n k t o n p i g m e n t s i n c o p e p o d guts) w a s not large e n o u g h s u p p o r t m e s o z o o p l a n k t o n r e s p i r a t i o n r a t e s ( D a g g 1 9 9 3 ; G i f f o r d 1 9 9 3 ; M i l l e r et al. data  in  Miller  et  al.  (1991b)  calculation  of  the  ratio  of  chlorophyll  to  1991b). U s i n g  removal  trough  m i c r o z o o p l a n k t o n g r a z i n g to r e m o v a l through macrograzers s h o w s an increase f r o m 2.3 to 13.3 from M a y August  to A u g u s t (but note that data c o m e f r o m 4 c r u i s e s m a d e b e t w e e n J u n e 1987  1988).  T h i s increase c a n be attributed to several factors i n c l u d i n g  mesozooplankton  s w i t c h i n g preferred p r e y - s i z e f r o m s m a l l to larger prey d u r i n g their ontogenetic greater  availability of  larger  phytoplankton  species in  late  spring  and  seems to b e c o n t r o l l e d b y m i c r o z o o p l a n k t o n w h i c h i n turn is c o n s u m e d b y  et al.  1 9 9 3 ; D a g g 1 9 9 3 ; G i f f o r d 1 9 9 3 ; L a n d r y et al.  1 9 9 3 ; M i l l e r et al.  development,  early  ontogenetic m i g r a t i o n to depth o f large c o p e p o d s i n s u m m e r a n d fall. H o w e v e r ,  ( B o o t h et al.  and  1 9 9 3 a ; L a n d r y et al.  summer,  or  phytoplankton  mesozooplankton 1993b; M a c k a s  1991b)  T h i s v i e w h a s b e e n c a l l e d t h e " m i x i n g a n d m i c r o g r a z e r h y p o t h e s i s " ( M i l l e r et al.  1 9 9 1 b ) , i.e.  a s h a l l o w m i x e d layer i n w i n t e r supports steady p r i m a r y p r o d u c t i o n and thus the m i c r o g r a z e r c o m m u n i t y w h i c h , d u e to its h i g h g r o w t h rates o f u p to m o r e t h a n 5 d o u b l i n g s p e r d a y ( M i l l e r al.  et  1 9 9 1 b ) , c a n c o n t r o l n a n o p h y t o p l a n k t o n s t a n d i n g s t o c k b e f o r e e n v i r o n m e n t a l c o n d i t i o n s (i.e.  67  seasonal m i x e d layer, increased i l l u m i n a t i o n , h i g h nutrient levels) c o u l d cause a b l o o m et  al.  that  (Landry  1993b). T h e " m i x i n g a n d rnicrograzer h y p o t h e s i s " has r e p l a c e d the c l a s s i c a l e x p l a n a t i o n  Neocalanus s p p . l i f e h i s t o r y c a u s e s m e s o z o o p l a n k t o n t o a r r i v e at t h e s u r f a c e j u s t i n t i m e t o  c o n t r o l p h y t o p l a n k t o n standing s t o c k (for a d i s c u s s i o n see P a r s o n s & L a l l i 1988). T h e i n c r e a s e i n mesozooplankton ( W o n g et  biomass  immediately  after the increase i n p r i m a r y  productivity  in  spring  1995) is probably caused b y a c o m b i n a t i o n o f the arrival o f seasonally, v e r t i c a l l y  al.  m i g r a t i n g c o p e p o d s as w e l l as h i g h l y c o u p l e d g r a z i n g at t h e s e c o n d t r a n s f e r l e v e l . It h a s a l s o b e e n s u g g e s t e d t h a t m e s o z o o p l a n k t o n c o u l d f e e d o n s m a l l p h y t o p l a n k t o n c e l l s t h a t a r e a t t a c h e d t o p a r t i c l e s ( D a g g 1 9 9 3 a , P . B o y d 1 9 9 5 p e r s . c o m m . ) o r l a r g e P O M per et  al.  se  (Mackas  1 9 9 3 ) . B e c a u s e c o h o r t - s u r v i v a l o f m a n y s p e c i e s i s d e t e r m i n e d e a r l y i n l i f e ( B e g o n et  1990), the abundance  of  al.  Neocalanus s p p . i s p r o b a b l y c o n t r o l l e d b y p r o c e s s e s at g r e a t d e p t h  w h i c h m i g h t i n v o l v e P O M , but w h i c h are yet u n k n o w n . M i c r o z o o p l a n k t o n (i.e. h e t e r o t r o p h i c f l a g e l l a t e s a n d c i l i a t e s ) p l a y s a n i m p o r t a n t r o l e i n the transfer o f energy u p the f o o d c h a i n a n d apparently i n the c o n t r o l o f n a n o p h y t o p l a n k t o n s t o c k ( B o o t h et published  al.  1 9 9 3 ; L a n d r y et  d a t a ( S t r o m et  al.  1993)  s t a n d i n g s t o c k a r o u n d 6.5 m g C m" 10.3-10  6  al.  3  1 9 9 3 b ; M i l l e r et  I have  al.  1 9 9 1 b ; S t r o m et  estimated microzooplankton  (or 13-10 c e l l s m" ) 6  3  al.  standing  1993).  spring and  From  summer  f o r M a y / J u n e a n d 4.8 m g C m~  c e l l s m" ) f o r A u g u s t / S e p t e m b e r , w i t h r e s p e c t i v e c o e f f i c i e n t s o f v a r i a t i o n o f 6 5 % 3  3  (or  (78%)  a n d 2 5 % ( 2 2 % ) . H o w e v e r , m y e s t i m a t e s are l o w e r t h a n the m e a n n e a r - s u r f a c e c o n c e n t r a t i o n s o f 15 m g C m "  3  r e p o r t e d b y B o o t h et al.  ( 1 9 9 3 ) . N o t e t h a t P a u l y et al.  (1996) i n their mass-balance  m o d e l o f the A l a s k a G y r e use a m i c r o z o o p l a n k t o n d e n s i t y that i s t o o l o w b y a f a c t o r o f 3, a n e r r o r attributable to t a k i n g i n t o a c c o u n t the c i l i a t e c o m p o n e n t o f m i c r o z o o p l a n k t o n o n l y , w h i c h i s usually <40%  (Booth  et  al.  1993). Highest abundance  of microzooplankton  occurs  between  68  N o v e m b e r a n d M a r c h / A p r i l ( F i g . 2.7 ( L e B r a s s e u r &  K e n n e d y 1972), w h i c h is consistent w i t h  decreased predation through copepods i n fall and winter (Dagg 1993; G i f f o r d 1993; M a c k a s al.  1993).  A l s o , i n case nanophytoplankton  is not  r e a d i l y a v a i l a b l e (e.g.  in winters  et  and/or  locations w i t h (depth o f m i x i n g ) > (critical depth)) m i c r o z o o p l a n k t o n is able to m a i n t a i n a h i g h density b y s h i f t i n g to a diet o f P O M c a l l e d " M i c r o b i a l L o o p " ( A z a m et LeBrasseur &  al.  a n d associated b a c t e r i a (a m o d e o f e c o l o g i c a l i n t e r a c t i o n 1 9 8 3 ; M o r e l et  al.  1991)). N o t e that the d a t a g i v e n  K e n n e d y ( 1 9 7 2 ) a r e 3 - 4 o r d e r s o f m a g n i t u d e s m a l l e r t h a n t h o s e i n S t r o m et  in al.  (1993) w h i c h m a y b e attributed to the coarse m e s h s i z e o f 4 4 pim u s e d i n the f o r m e r study a n d the m o d e r n a n a l y z i n g e q u i p m e n t a n d thus higher r e s o l u t i o n for s m a l l s i z e classes i n the later. A s s u m i n g there is n o bias i n L e B r a s s e u r &  K e n n e d y ' s (1972) errors.Fig. 2.7 s h o w s seasonal  changes i n p r o t o z o a n u m b e r s rather than absolute concentrations. A l t h o u g h the n o t i o n o f a constant standing s t o c k o f p h y t o p l a n k t o n t h r o u g h o u t the year has a l r e a d y a c q u i r e d t h e s t a t u s o f i n d i s p u t a b l e t r u t h ( M i l l e r et  al.  1991b; P a r s l o w 1981; Parsons  &  L a l l i 1 9 8 8 ; P a r s o n s & L e B r a s s e u r 1 9 6 8 ; W o n g et al.  1995) and although this has been attributed  t o g r a z i n g l i m i t a t i o n b y m i c r o z o o p l a n k t o n ( B o o t h et  al.  S t r o m et al.  1 9 9 3 ; W e l s c h m e y e r et al.  1 9 9 3 ; D a g g 1 9 9 3 ; M i l l e r et  al.  1991b;  1993) doubts r e m a i n whether m i c r o z o o p l a n k t o n is capable  o f c o n t r o l l i n g t h e n a n o p h y t o p l a n k t o n s t a n d i n g s t o c k ( s e e s e c t i o n Phytoplankton a n d e x p e r i m e n t s b y L a n d r y et al.  1993b).  L i t t l e is k n o w n about other z o o p l a n k t o n groups a n d s i z e classes i n the C e n t r a l S u b a r c t i c Domain:  Non-crustacean  herbivorous  suspension-feeding  mesozooplankton  (e.g.  pteropods,  salps, l a r v a c e a n s ) w h i c h d e p l o y m u c o u s nets to capture f o o d p a r t i c l e s ( p l a n k t o n a n d  POM);  69  50000 -I  f  ro O o o o  oo O o o o  number m-3  40000 -  \  1  10000 -  0 -•  Jan  I• I^ I Mar  E I"  !  May  1  iVi  Jul  \l\ —  i Bj M i  Sep  1  -0  Nov  Fig. 2.7: S e a s o n a l c h a n g e i n p r o t o z o a d e n s i t y at S t a t i o n P ( 1 9 6 6 - 1 9 6 8 ) . N o t e t h e h i g h  standing  s t o c k o f m i c r o z o o p l a n k t o n i n w i n t e r . F o r a d i s c u s s i o n a b o u t t h e q u a l i t y o f t h e d a t a a s w e l l as m o r e recent estimates see text. D a t a f r o m L e B r a s s e u r a n d K e n n e d y (1972).  70  chaetognaths  which  feed  o n other  mesozooplankton  a n d especially  o n their  smaller  d e v e l o p m e n t a l stages; e u p h a u s i i d s w h i c h are u s u a l l y o m n i v o r o u s but d u e to l o w c o n c e n t r a t i o n s o f m i c r o p h y t o p l a n k t o n i n the C e n t r a l S u b a r c t i c D o m a i n  are m a i n l y c a r n i v o r o u s , f e e d i n g o n  m i c r o - a n d m e s o z o o p l a n k t o n ; a n d g e l a t i n o u s z o o p l a n k t o n w h i c h i n c e r t a i n z o n e s at c e r t a i n t i m e s d o m i n a t e z o o p l a n k t o n b i o m a s s ( P e a r c y et al. 1 9 8 8 ) . F o r a d e t a i l e d l i s t i n g o f z o o p l a n k t o n s p e c i e s i n t h e C e n t r a l S u b a r c t i c D o m a i n s e e P a r s o n s & L a l l i ( 1 9 8 8 ) o r P e a r c y et al.  (1988).  While  juvenile salmon mostly feed on mesozooplankton, immature and maturing Pacific salmon rely a l s o o n l i t t l e s t u d i e d s q u i d a n d m a c r o z o o p l a n k t o n as t h e i r f o o d s o u r c e ( B r o d e u r 1 9 9 0 ) ; s e e a l s o : Section  2.1.  Feeding  Ecology  o f Sockeye  Salmon).  Lack  o f information  o n squid a n d  m a c r o z o o p l a n k t o n is due t o the p r o b l e m s associated w i t h the s a m p l i n g o f h i g h l y m o t i l e a n d o f g e l a t i n o u s g r o u p s w i t h i n the z o o p l a n k t o n c o m m u n i t y .  Fish and Higher Trophic Levels Many  o f the estimates f o r f i s h a n d higher t r o p h i c l e v e l s i n the C e n t r a l S u b a r c t i c D o m a i n  c o m e f r o m P a u l y et al. ( 1 9 9 6 ) . U n f o r t u n a t e l y , I h a v e i d e n t i f i e d m a n y n u m e r i c a l e r r o r s i n t h i s w o r k s h o p report e s p e c i a l l y f o r v a r i a b l e s f o r w h i c h alternative s o u r c e s are r e a d i l y a v a i l a b l e . T h u s , m o s t v a l u e s r e f e r e n c e d u n d e r " P a u l y et al. 1 9 9 6 " a r e n o t b e y o n d d o u b t . T r i t e s & H e i s e ' s ( 1 9 9 6 ) s e c t i o n i n P a u l y et al. ( 1 9 9 6 ) i s a c a r e f u l r e v i e w o f m a r i n e m a m m a l s i n t h e N E - P a c i f i c w h i c h I thus reference separately. T h e t o t a l s t a n d i n g s t o c k o f f i s h i n the e p i p e l a g i c z o n e o f the C e n t r a l S u b a r c t i c D o m a i n has b e e n e s t i m a t e d to b e a r o u n d 3 g w e t w e i g h t m"  (plus around 4.5 g wet w e i g h t m  m i g r a t i n g m e s o p e l a g i c s ; P a u l y et al. 1 9 9 6 ) , o r 0 . 4 g C m  2  (plus 0.6 g C m  2  o f diurnally  mesopelagics), using  I v e r s o n ' s ( 1 9 9 0 ) f i s h c a r b o n to w e t w e i g h t ratio o f 0 . 1 3 . T o t a l a n n u a l f i s h p r o d u c t i o n has b e e n  71  e s t i m a t e d at = 3 . 9 g w e t w e i g h t m al.  2  y"  1  (plus 3.2 g wet w e i g h t m"  2  y " f r o m m e s o p e l a g i c s ; P a u l y et 1  1996), o r around 0.5 g C m " y" (plus 0 . 4 mesopelagics). T h e s e recent estimates o f f i s h 2  production (Parsons  i n the Gulf  1  o f A l a s k a open  ocean ecosystem are >10 times previous  1986). T h i s d i s c r e p a n c y c a n p a r t i a l l y b e attributed t o the r e v i s e d p r i m a r y  estimates  productivity  e s t i m a t e s ( W o n g et al. 1 9 9 5 ) a n d t r a n s f e r u p t h e f o o d c h a i n , a n d n e w i n f o r m a t i o n o n t h e g r o u p o f small pelagics w h i c h make up =80%  o f t h e t o t a l f i s h p r o d u c t i o n ( P a u l y et al.  W i t h a standing stock o f <0.05 g C m" al.  1996)  anadromous  2  and an annual production o f 0.06 g C m  N o r t h A m e r i c a n P a c i f i c s a l m o n represent =10%  standing stock as w e l l as total  fish  1996). y"  2  1  ( P a u l y et  o f both thetotal fish  production, e x c l u d i n g mesopelagics i n b o t h cases.  While  s a l m o n s p e c i e s are rather i n s i g n i f i c a n t i n f i s h b i o m a s s a n d p r o d u c t i o n f o r the C e n t r a l S u b a r c t i c D o m a i n their trophic niche certainly is important.  {Oncorhynchus gorbuscha),  chum  (O. keta),  T h ecommercially important  species  pink  and sockeye salmon undertake extensive migrations  i n this r e g i o n ( G r o o t & M a r g o l i s 1991) and d u r i n g their early m a r i n e l i f e history stages b i o l o g i c a l p r o d u c t i o n processes i n the C e n t r a l S u b a r c t i c D o m a i n m a y d e t e r m i n e y e a r - c l a s s strength a n d thus c a t c h i n the fishery w h e n adults return t o their s p a w n i n g g r o u n d s a f e w years later ( B r o d e u r &  Hollowed  1993; Burgner  1991; the c o m p l e x w o r k i n g hypothesis o f m y thesis). C o h o a n d  c h i n o o k s a l m o n a l s o m i g r a t e i n t o the o c e a n but i n h a b i t the C o a s t a l a n d T r a n s i t i o n a l rather than t h e C e n t r a l Subarctic D o m a i n ,  domains,  during their oceanic phase. T a b l e 2 . 2 summarizes  Pacific salmon life history characteristics. T h e d o m i n a n t s m a l l p e l a g i c f i s h o f the N E - P a c i f i c is saury  (Cololabis saira),  w h i c h visits the  N E - P a c i f i c i n the s u m m e r ( B r o d e u r 1988; P e a r c y 1993). B e c a u s e o f its s m a l l s i z e ( L  T C  = 35 c m  72  Table 2.2: L i f e h i s t o r y c h a r a c t e r i s t i c s o f N o r t h spp.). A d a p t e d f r o m P e a r c y ( 1 9 9 2 ) .  Species  Pink  A m e r i c a n Pacific salmon species  (Oncorhynchus  Freshwater  Month of  Size at  Estuarine  Ocean  Residence Time  Ocean Entry  Ocean Entry  Residence Time  Residence Time  days-weeks  May-Jun  30-40 mm  <1 week  1.6 years  days-weeks  Mar-Jun  30-40 mm  1-2 weeks  2-4 years  0-2 years  May-Jun  60-100 mm  <1 week  1-5 years  0-4 years  May-Jun  60-120 mm  <1 week  0.5-1.5 years  0-2 years  May-Oct  40-110 mm  <1 week-months  0.5-6 years  (O. gorbuscha)  Chum (O. keta)  Sockeye (0. nerka)  Coho* (0. kisutch)  Chinook* (0. tshawytscha)  * Species does not inhabit Central Subarctic Domain during oceanic phase.  73  total length) a n d the use o f g i l l nets i n surveys, a b u n d a n c e estimates are d i f f i c u l t to o b t a i n f o r this species. Saury m o s t l y feeds o n copepods ( 5 0 % a m p h i p o d s , a n d s m a l l e r f i s h ( P a u l y et al.  o f stomach contents b y weight),  euphausiids,  1996).  A n e c o l o g i c a l l y important f i s h species i s the p o m f r e t  (Brama japonica)  w h i c h again is a  s u m m e r v i s i t o r ( B r o d e u r 1988; P e a r c y 1993) a n d has the highest a b u n d a n c e (catch rates o f u p t o >400 fish (km"  gillnet)  1  (12 h)" (Brodeur 1  & Ware  1995)) o f a l l vertebrate  species i n the  epipelagic z o n e o f the o p e n ocean. P o m f r e t have an asymptotic total length ( L o ) o f 61 c m and a n a s y m p t o t i c w e i g h t ( W o o ) o f 3 8 6 0 g a n d a r e l o n g l i v e d ( 9 y e a r s ( P a u l y et al. consists mainly o f cephalopods and fish (>50%  1996)). P o m f r e t  prey  b y weight), and euphausiids, amphipods a n d  d e c a p o d s ( 1 1 - 4 9 % b y w e i g h t ( P a u l y etal. 1 9 9 6 ) ) . A n i m p o r t a n t f i s h predator m i g h t b e the daggertooth total length)  (Anotopterus pharao),  b a t h y p e l a g i c f i s h that preys o n adult, i m m a t u r e  a large (85 c m  andpossibly juvenile  salmon.  Daggertooth slash marks have been found o n 1 2 % o f adult sockeye s a l m o n returning to B r i t i s h C o l u m b i a ( W e l c h et a l . 1 9 9 1 as c i t e d i n P a u l y et al. ( 1 9 9 6 ) . L i t t l e i s k n o w n a b o u t a b u n d a n c e , l i f e h i s t o r y , d i e t a n d p o p u l a t i o n d y n a m i c s o f t h e d a g g e r t o o t h ( P a u l y et al.  1996).  L a r g e r f i s h predators i n the C e n t r a l S u b a r c t i c D o m a i n are the s a l m o n s h a r k w h i c h c a n be f o u n d a l l year r o u n d , a n d the b l u e shark warmer waters (Brodeur  1 9 8 8 ; P a u l y et al.  G u l f o f A l a s k a areunknown; metric tons k m " salmon (coho  2  (Prionace glauca),  (Lamna  a summer visitor from  1996; P e a r c y 1993). T h e densities o f sharks i n the  however, bycatch data provide l o w e r b o u n d  estimates o f 0.05  f o r both sharks c o m b i n e d . S a l m o n sharks prey u p o n immature  (O. kisutch),  ditropis),  a n d mature  sockeye, pink, and chum), other pelagic, and m e s o p e l a g i c fish. B l u e  sharks feed m a i n l y o n squid, mesopelagics, saury and pomfret (Brodeur  1988).  74  S e a b i r d p o p u l a t i o n s f o r the total C e n t r a l S u b a r c t i c D o m a i n (here S a n g e r ' s d e f i n i t i o n , w i t h a n area o f 3.79 1 0 k m 6  birds k m  2  2  ( S a n g e r 1972a)) h a v e b e e n e s t i m a t e d at 0 . 5 7 b i r d s k m "  2  i n winter and 4.40  i n s u m m e r , o r 0.33 a n d 2.41 k g l i v e w e i g h t k m " , r e s p e c t i v e l y (Sanger 1972a). A 2  2 recent estimate f o r the s u m m e r seabird p o p u l a t i o n i s 9.49 b i r d s k m " ( P a u l y et al.  more  2 and 6 k g live weight km"  1996). A v a i l a b l e data d o not a l l o w the e s t i m a t i o n o f a p r o d u c t i o n / b i o m a s s ratio but  i n d i c a t e a v e r y h i g h f o o d c o n s u m p t i o n / b i o m a s s r a t i o o f 1 0 1 ( P a u l y et al.  1996), w h i c h c a n b e  attributed to the h i g h m e t a b o l i c requirements o f these s m a l l endotherms. D i e t c o m p o s i t i o n s h o w s that s m a l l p e l a g i c f i s h ( 4 8 % ) a n d c e p h a l o p o d s c o n s u m p t i o n f o r a l l b i r d s c o m b i n e d ( P a u l y et al.  (45%) make  u p the bulk  o f the total  1996). M a r i n e birds prey u p o n j u v e n i l e  food  salmon  w h i l e s a l m o n m i g r a t e t h r o u g h the C o a s t a l D o w n w e l l i n g D o m a i n d u r i n g their s e a w a r d m i g r a t i o n , h o w e v e r , at w h a t stage i n t h e m a r i n e e n v i r o n m e n t j u v e n i l e s a l m o n a t t a i n a l a r g e e n o u g h size and thus escape speed to reduce avian predation remains Thirteen species o f marine m a m m a l Subarctic  Domain  (Trites  &  Heise  body  unknown.  s p e c i e s a r e at least t e m p o r a r y r e s i d e n t s o f the C e n t r a l 1996).  Because  marine  mammal  standing  stock and  p r o d u c t i o n , as w e l l as diet c o m p o s i t i o n a n d i n g e s t i o n quantity data are v e r y sparse o n l y t w o s p e c i e s m a y d i r e c t l y i m p a c t s a l m o n a n d are thus s u m m a r i z e d here. A m o r e e x p l i c i t d i s c u s s i o n o n marine m a m m a l s can b e found i n Trites & Heise (1996). The Northern Fur Seal September  with population  (Callorhinus  sizes estimated  between A p r i l and September c o n d i t i o n s i n that r e g i o n )  ursinus)  i s present i n the A l a s k a n G y r e f r o m A p r i l t o  at 1 3 0 0 0 0 i n d i v i d u a l s  during  their  migrations  a n d a m e r e 5 0 0 0 ( i f a n y at a l l c o n s i d e r i n g t h e h a r s h  i n t h e rest o f t h e year.  T h e production  p i n n i p e d s i s e s t i m a t e d at 6 % p e r y e a r ( w i t h a m a x i m u m  winter-  / biomass  ratio o f these  o f 0.12 y" ). S u m m e r  diet f o r these  1  75  pinnipeds i s dominated b y squid (78%)  a n d o n l y a f e w s a l m o n are c o n s u m e d ( 1 1 %  (Trites  &  H e i s e 1 9 9 6 ) ; s t o m a c h c o n t e n t s p e r c e n t a g e i s n o t d e f i n e d as w e i g h t o r v o l u m e ) . Little is k n o w n  orca),  about a third and n e w l y discovered subspecies o f k i l l e r whales  (Orcinus  the o c e a n i c k i l l e r w h a l e , w h i c h apart f r o m h a v i n g b e e n o b s e r v e d m i g r a t i n g t o w a r d s  o p e n o c e a n has s o m e m o r p h o l o g i c a l traits related t o its d o r s a l f i n (J. F o r d 1 9 9 4 pers.  the  comm.).  F r o m their intense h u n t i n g c o m m u n i c a t i o n , s i m i l a r to resident k i l l e r w h a l e s a n d u n l i k e the rather q u i e t l y h u n t i n g m a m m a l - e a t i n g transients, i t has b e e n i n f e r r e d that o c e a n i c s m u s t b e f i s h e a t i n g . F u t u r e r e s e a r c h w i l l h o p e f u l l y s h e d s o m e light o n the e c o l o g y o f this subspecies. B e c a u s e the t o p i c o f interest i n this study i s the interannual v a r i a b i l i t y i n s o c k e y e  salmon  ( m a r i n e ) s u r v i v a l a n d because h i g h e r t r o p h i c l e v e l s i n the C e n t r a l S u b a r c t i c D o m a i n f e e d m o s t l y o n later l i f e h i s t o r y stages o f s a l m o n , h i g h e r t r o p h i c l e v e l s i n the C e n t r a l S u b a r c t i c  Domain  p r o b a b l y h a v e a m i n o r i m p a c t o n s a l m o n c o h o r t - s u r v i v a l , w h i c h I c o n j e c t u r e t o b e set e a r l y i n marine life.  2.2.2. The Coastal Downwelling and the Transitional Domain T h e C o a s t a l D o w n w e l l i n g D o m a i n reaches f r o m C a p e S c o t t at the n o r t h tip o f V a n c o u v e r Island t o the A n d r e a n o f  Islands i n the A l e u t i a n c h a i n ( F i g . 2.1).  Its w i d t h f r o m the c o a s t l i n e  f o l l o w s the continental shelf and ranges f r o m a f e w k i l o m e t e r s (off the Q u e e n C h a r l o t t e Islands, B r i t i s h C o l u m b i a ) t o m o r e than 2 0 0 k m northeast o f K o d i a k Island ( A l a s k a ) . T h i s d o m a i n c a n be characterized as a non-tropical shelf ecosystem with a n annual primary production o f 200-300  m g C m " ^ ( P a u l y & C h r i s t e n s e n 1 9 9 5 b ; W a r e & M c F a r l a n e 1 9 8 9 ) a n d m i c r o p h y t o p l a n k t o n at t h e base  o f a three-  to four-level  food  chain (Ware  p r o d u c t i o n i s i n the order o f 1 0 - 5 0 g C m "  2  y"  1  & McFarlane  1989).  Annual  zooplankton  ( W a r e & M c F a r l a n e 1989). B e c a u s e o f onshore  76  a d v e c t i o n f r o m the C e n t r a l S u b a r c t i c D o m a i n ( B r o d e u r & H o l l o w e d 1 9 9 3 ; W a r e & 1989;  Wickett  1967)  the  zooplankton  community  i n the  Coastal Downwelling  McFarlane Domain  is  d o m i n a t e d b y t h e s a m e s p e c i e s as t h e o c e a n i c e n v i r o n m e n t , e x c e p t f o r t h e s u m m e r w h e n s m a l l e r neritic copepods b e c o m e more abundant ( W a r e & M c F a r l a n e 1989, and references cited therein). D o m i n a n t f i s h s p e c i e s i n t h i s r e g i o n are W a l l e y e p o l l o c k , P a c i f i c c o d , S a b l e f i s h , a n d P a c i f i c halibut  (Ware  &  McFarlane  1989).  Their  bentho-pelagic  life  suggests  that  trophodynamic  p h a s i n g ( P a r s o n s 1 9 8 8 ; P a r s o n s & K e s s l e r 1 9 8 7 ; P a r s o n s & L a l l i 1 9 8 8 ; P a r s o n s et  al.  1984)  in  t h e e u p h o t i c z o n e m a y n o t b e as t i g h t h e r e as i n t h e C e n t r a l S u b a r c t i c D o m a i n , i . e . o r g a n i c m a t t e r i s e x p o r t e d f r o m t h e s u r f a c e a n d d r i v e s a b e n t h i c f o o d c h a i n at d e p t h . P a c i f i c h e r r i n g a n d  of  course j u v e n i l e s a l m o n o n their m i g r a t i o n into the C e n t r a l S u b a r c t i c D o m a i n f o r m the p e l a g i c f i s h g r o u p ( W a r e & M c F a r l a n e 1989). Unfortunately, little information is available o n this ecosystem ( W a r e & M c F a r l a n e  1989).  N e v e r t h e l e s s , the i m p o r t a n c e o f r e g i o n a l z o o p l a n k t o n p r o d u c t i o n a n d a d v e c t i o n f r o m the C e n t r a l Subarctic D o m a i n  i n t o t h i s r e g i o n as w e l l as o f r e s i d e n t p r e d a t o r p o p u l a t i o n s f o r s u r v i v a l  j u v e n i l e s a l m o n a n d h e n c e its y e a r - c l a s s strength s h o u l d b e e m p h a s i z e d ( B u r g n e r 1991; Parsons  etal.  1984; Pearcy 1992; Peterman 1978; Walters  etal.  of  1991; Healey  1978).  S e a b i r d p o p u l a t i o n s for the total C o a s t a l D o m a i n (using S a n g e r ' s d e f i n i t i o n , w i t h an area o f 1.36 1 0  6  km  2  ( S a n g e r 1 9 7 2 a ) ) h a v e b e e n e s t i m a t e d at 1.6 b i r d s k m "  2  i n w i n t e r a n d 7.8 birds k m "  2  i n s u m m e r , o r 0.63 a n d 4.79 k g l i v e w e i g h t k m " , r e s p e c t i v e l y (Sanger 1972a). D i e t data f r o m the 2  C e n t r a l S u b a r c t i c D o m a i n i n d i c a t e that m a r i n e b i r d s m o s t l y f e e d u p o n s m a l l p e l a g i c f i s h ( P a u l y et al.  1996) a n d s h o w that the S o o t y shearwater p o p u l a t i o n m a y c o n s u m e = 1 5 0 k g o f s m a l l f i s h  km"  o v e r the s u m m e r h a l f year, w h i c h is the e q u i v a l e n t o f 10 0 0 0 j u v e n i l e f i s h k m " , a s s u m i n g a  2  2  77  rather h i g h b o d y w e i g h t o f 15 g p e r j u v e n i l e f i s h , w h i c h i s e q u i v a l e n t t o t h e h i g h e s t m e a n b o d y w e i g h t a s o c k e y e smolt m a y attain ( B u r g n e r 1991). Resident killer whales  (Orcinus orca)  l i v e c l o s e t o the coast a n d p o p u l a t i o n s i z e i s e s t i m a t e d  at 2 4 0 i n d i v i d u a l s ( T r i t e s & H e i s e 1 9 9 6 ) . T h e p r o d u c t i o n / b i o m a s s r a t i o o f t o o t h e d w h a l e s i s e s t i m a t e d t o b e -3%  per year (with a m a x i m u m  o f 0.04 y" ). S t o m a c h contents data are not 1  available f o r the G u l f o f A l a s k a but inferring f r o m different sources Trites & H e i s e  (1996)  e s t i m a t e that 8 0 % o f t h e s u m m e r a n d 6 0 % o f t h e w i n t e r diet c o n s i s t s o f s a l m o n . B a c k o f t h e e n v e l o p e c a l c u l a t i o n s s h o w that r e s i d e n t k i l l e r w h a l e s c o n s u m e =5 m i l l i o n s a l m o n p e r year, h a r d l y a n u m b e r that c o u l d d o m i n a t e s a l m o n c o h o r t - s u r v i v a l c o n s i d e r i n g that these c o u l d b e produced b y only 2000 female spawners. T h e Transitional D o m a i n i s a somewhat arbitrary construction o f a z o n e characterized b y high seasonal a n dinterannual variability i n oceanographic conditions, caused b y the bifurcation of  the eastward  Subarctic Current  into  the northward  A l a s k a Current  and the southward  C a l i f o r n i a C u r r e n t . N u t r i e n t a n d p h y t o p l a n k t o n s a m p l e s t a k e n a l o n g L i n e P ( f r o m the s o u t h tip o f V a n c o u v e r Island o u t t o S t a t i o n P (50°N 145°W) s h o w the f o l l o w i n g s e q u e n c e ( P . H a r r i s o n 1 9 9 5 pers. c o m m . ) : I r o n l i m i t a t i o n o f m i c r o p h y t o p l a n k t o n w e s t o f 140°W. N o nitrate l i m i t a t i o n i n a n approximately  50  k m  wide  band  from  the  coast  with  diatoms  as  the  dominating  m i c r o p h y t o p l a n k t o n g r o u p . I n - b e t w e e n , a z o n e w h e r e nitrate i s l i m i t i n g a n d d i a t o m s a s w e l l as d i n o f l a g e l l a t e s c a n b e f o u n d . T h e s e results d e m o n s t r a t e that the T r a n s i t i o n a l D o m a i n has i t s o w n ecological characteristics, whose influences o n salmon (marine) survival is u n k n o w n .  78  2.3. Physical Oceanography of the Northeast Pacific T h e N o r t h e a s t P a c i f i c i s n o t a distinct b a s i n o f the P a c i f i c O c e a n b u t i s rather d e f i n e d b y t h e variable extent o f ocean currents, especially the Subarctic Current. H o w e v e r , f o r the purpose o f t h i s s t u d y the N o r t h e a s t P a c i f i c i s d e f i n e d as the o c e a n a r e a b e t w e e n 4 0 a n d 6 6 ° N , a n d 175°E a n d 125°W, i.e. t h e a p p r o x i m a t e  range  i n ocean distribution o f North  s p e c i e s ( G r o o t & M a r g o l i s 1 9 9 1 ; W e l c h et al.  American Pacific  salmon  1995). F o r c l a s s i f i c a t i o n o f the N o r t h e a s t P a c i f i c  i n t o f o u r u p p e r z o n e d o m a i n s see S e c t i o n 2.2. a n d F i g . 2 . 1 . T w o aspects o f the p h y s i c a l o c e a n o g r a p h y o f the N o r t h e a s t P a c i f i c p l a y a n i m p o r t a n t r o l e f o r s o c k e y e s a l m o n : First, the seasonal change i n the vertical and h o r i z o n t a l temperature a n d salinity structure w h i c h i s c r u c i a l to water c o l u m n stratification, and thus p r i m a r y p r o d u c t i o n (Parsons & L a l l i 1 9 8 8 ) , a n d o c e a n d i s t r i b u t i o n o f s a l m o n ( B r e t t et al. 1 9 6 9 ; W e l c h et al. 1 9 9 5 ) . A n d s e c o n d , the m a j o r c i r c u l a t i o n patterns w h i c h transport b i o l o g i c a l p r o d u c t i o n ( B r o d e u r & H o l l o w e d  1993;  W a r e & M c F a r l a n e 1 9 8 9 ; W i c k e t t 1 9 6 7 ) a n d i n f l u e n c e m i g r a t i o n r o u t e s o f n e k t o n ( S c a n d o l et  al.  1996). A d d i t i o n a l l y t w o restrictions apply: First, due to the lack o f a coastal circulation m o d e l , i n connection w i t h salmon survival especially needed for the Coastal D o w n w e l l i n g  Domain and  B e r i n g S e a , I h a v e n o t i n c l u d e d the c o a s t a l p h y s i c a l o c e a n o g r a p h y o f the N o r t h e a s t P a c i f i c i n this discussion;  l a c k o f b i o l o g i c a l data justifies a similar argument ( W a r e  S e c o n d , because sockeye s a l m o n are v i s u a l predators  (Burgner  & McFarlane  1991) I have  1989).  only  discussed  processes w i t h i n the m i x e d upper layer, i.e. the euphotic zone, although i t h a s been  suggested  t h a t s a l m o n o c c a s i o n a l l y f o r a g e i n w a t e r s b e l o w 1 5 0 m ( P e a r c y et al.  1988).  79  Temperature and salinity distribution In w i n t e r the o p e n Northeast P a c i f i c is characterized b y an i s o t h e r m a l , i s o h a l i n e upper layer w i t h temperatures a r o u n d 5 ° C a n d l o w salinities a r o u n d 32.7 parts per t h o u s a n d w h i c h e x t e n d s d o w n t o a d e p t h o f 1 0 0 - 2 0 0 m ( D o d i m e a d et al. 1 9 6 3 ; T h o m s o n 1 9 8 1 ) . B e l o w t h e m i x e d u p p e r l a y e r lies a n a r r o w ( f e w meters) but steep t h e r m o c l i n e w i t h a total temperature decrease o f 1°C w h i c h tops the c o l d - w a t e r sphere, the vast z o n e o f s l o w but c o n t i n u o u s temperature decrease w i t h depth. T h e thermocline is o n top of a thicker (around 50 m) permanent halocline w i t h a total salinity increase o f 1 part per thousand. Just l i k e the c h a n g e i n temperature, t h o u g h w i t h o p p o s i t e s i g n , b e l o w the h a l o c l i n e salinity increases s l o w l y but c o n t i n u o u s l y to a d e p t h o f u p to 4 0 0 0 m (Thomson A  1981).  different picture emerges i n s u m m e r w h e n a shallow (10 - 2 0 m ) isothermal layer w i t h  t e m p e r a t u r e s o f 1 2 - 15°C  overlies a t h i c k e r (around 5 0 m ) very steep t h e r m o c l i n e w i t h a total  t e m p e r a t u r e d i f f e r e n c e u p t o 10°C  b e l o w w h i c h temperature again decreases s l o w l y and steadily,  i.e. t e m p e r a t u r e rate o f i n c r e a s e is faster t h a n m i x i n g . W i t h i n the m i x e d l a y e r s a l i n i t y i n c r e a s e s s t e p w i s e f r o m a b o u t 3 2 . 5 t o 3 3 . 0 p a r t s p e r t h o u s a n d at t h e t o p o f t h e h a l o c l i n e at 1 0 0 - 2 0 0 m , a n d f u r t h e r to 33.7 parts p e r t h o u s a n d i n the a p p r o x i m a t e l y 5 0 m w i d e h a l o c l i n e . T h u s there i s little seasonal v a r i a b i l i t y i n the h a l o c l i n e , h e n c e the t e r m " p e r m a n e n t " h a l o c l i n e . Notwithstanding  that  there  i s spatial a n dtemporal  variation  i n the annual  cycle o f  s t r a t i f i c a t i o n ( P a r s o n s et al. 1 9 6 6 ; P a r s o n s & L e B r a s s e u r 1 9 6 8 ) , a n a v e r a g e l o c a t i o n w i t h i n t h e o p e n N o r t h e a s t P a c i f i c i n a n a v e r a g e y e a r c o u l d b e c h a r a c t e r i z e d as f o l l o w s : A t t h e e n d o f t h e w i n t e r m i x i n g season a r o u n d M a r c h i n c r e a s e d solar r a d i a t i o n heats u p the surface layer a n d w i n d a n d w a v e a c t i o n transport heat to depth. R e d u c e d m i x i n g due to a w e a k e n i n g i n w i n d s d u r i n g the s u m m e r m o n t h results i n a s h a l l o w i s o t h e r m a l s t r a t u m to a d e p t h o f 10 - 2 0 m that o v e r l i e s a  80  n u m b e r o f layers o f rapid temperature decrease, the remains o f the seasonal thermoclines ( F i g . 2.8). A f t e r A u g u s t c o o l e r a i r temperatures l e a d t o a net heat transfer f r o m t h e o c e a n i n t o t h e atmosphere a n dthe c o o l e r a n dthus heavier surface water parcels g i v e rise t o c o n v e c t i v e m i x i n g o f the upper layer, w h i c h a l l o w s c o l d e r water to penetrate deeper than w i n d m i x i n g alone w o u l d . However, the simultaneous action o f convective, w i n d and wave m i x i n g during fall a n d winter r e s u l t s i n a n i s o t h e r m a l s u r f a c e l a y e r b y J a n u a r y that e x t e n d s t o t h e t o p o f the h a l o c l i n e , i . e . t h e l a y e r o f s a l t - c o n t r o l l e d stability o f the water c o l u m n ( T h o m s o n 1981). A l t h o u g h t h e p e r m a n e n t h a l o c l i n e m a y n o t s e e m t o b e a s p e c t a c u l a r feature I w a n t t o e m p h a s i z e that i t i s the result o f n o n t r i v i a l d y n a m i c h y d r o l o g i c a l processes s u c h as freshwater input (rainfall, c o n t i n e n t a l r u n - o f f ) , e v a p o r a t i o n , a n d w i n d , w a v e a n d c o n v e c t i v e m i x i n g , a n d that its p r e s e n c e has l a r g e i m p l i c a t i o n s for the b i o l o g y o f the C e n t r a l Subarctic D o m a i n (see S e c t i o n 2.2. E c o s y s t e m s o f the Northeast P a c i f i c ; Parsons & L a l l i 1988). S p a t i a l patterns i n the Northeast P a c i f i c i n w i n t e r are a l m o s t z o n a l f o r temperature  from  a p p r o x i m a t e l y 12°C a t 4 0 ° N t o 4 ° C at 5 5 ° N w i t h i s o t h e r m a l s b e n d i n g n o r t h w a r d n e a r t h e c o a s t . S a l i n i t y i s spatially m o r e structured w i t h m a x i m a o c c u r r i n g w i t h i n the A l a s k a n S t r e a m ( F i g . 2.1) i n winter a n d the center o f the A l a s k a n G y r e i n summer.  Coast-near l o w salinities are d u e to  c o n t i n e n t a l f r e s h w a t e r r u n - o f f . S u m m e r temperature d i s t r i b u t i o n i s l a t i t u d i n a l u p t o 4 5 ° N (15°C) and  then  describes concentric circles  temperatures  and high  centered  around  10°C  i n the A l a s k a n  salinities i n the Coastal U p w e l l i n g D o m a i n  Stream. L o w  ( F i g . 2.1) are caused b y  u p w e l l i n g o f c o l d deep water.  81  Fig.  2.8:  D e v e l o p m e n t ( A ) a n d deterioration (B) o f seasonal t h e r m o c l i n e s i n the o p e n N o r t h e a s t  Pacific. This qualitative m o d e l  has b e e n  derived by T h o m s o n  (1981) f r o m  single day  data  c o l l e c t e d i n A u g u s t 1 9 7 7 a n d F e b r u a r y 1 9 7 8 at S t a t i o n P, a n d f r o m a r g u m e n t s i n D o d i m e a d e t a l . (1963). A d a p t e d f r o m T h o m s o n (1981).  82  Currents M a i n c u r r e n t patterns i n the N o r t h e a s t P a c i f i c are s h o w n i n F i g . 2 . 1 . T h e S u b a r c t i c C u r r e n t o r i g i n a t e s i n t h e K u r o s h i o - O y a s h i o s y s t e m o f f J a p a n a n d t r a v e l s e a s t w a r d at o n l y 5 - 1 0  c m s" . 1  A p p r o a c h i n g the N o r t h A m e r i c a n continent and i n a z o n e w h i c h , due to its large v a r i a b i l i t y i n currents o n a l l space and t i m e scales, is c a l l e d T r a n s i t i o n a l D o m a i n (Fig. 2.1),  the S u b a r c t i c  C u r r e n t then bifurcates into the southeastward C a l i f o r n i a C u r r e n t ( m e a n s p e e d =20  c m s" ) 1  and  the n o r t h w e s t w a r d A l a s k a Current. T h e A l a s k a Current continues a l o n g the coasts o f B r i t i s h C o l u m b i a a n d A l a s k a at m e a n s p e e d s o f 2 5 - 3 5 c m s " i n s u m m e r a n d w i n t e r , r e s p e c t i v e l y , b u t 1  southeast w i n d s i n w i n t e r m a y accelerate it u p to 7 5 c m s" . S o u t h o f the A l e u t i a n i s l a n d c h a i n the 1  A l a s k a C u r r e n t b e c o m e s a n a r r o w a n d f a s t (> 1 m s " ) w e s t w a r d b o u n d a r y c u r r e n t , t h e n c a l l e d t h e 1  A l a s k a n S t r e a m , w h i c h feeds water masses into the B e r i n g S e a a n d also s o u t h w a r d . S u b a r c t i c C u r r e n t , A l a s k a C u r r e n t a n d A l a s k a n S t r e a m f o r m the c y c l o n i c A l a s k a n G y r e , w h i c h represents t h e m a i n p a r t o f t h e C e n t r a l S u b a r c t i c D o m a i n ( F i g . 2 . 1 ) , w i t h E k m a n p u m p i n g at i t s c e n t e r . W i t h the e x c e p t i o n o f a f e w c o a s t a l c u r r e n t s , w h i c h are d e n s i t y - d r i v e n , m a j o r c u r r e n t s i n the Northeast  Pacific  are w i n d - d r i v e n .  Wind  direction and  speed  are m a i n l y  controlled by  the  i n t e n s i t y , i.e. s p a t i a l extent a n d strength, l o c a t i o n a n d d u r a t i o n o f t w o p r e s s u r e s y s t e m s ,  the  A l e u t i a n L o w a n d the N o r t h P a c i f i c H i g h . T h e A l e u t i a n L o w develops a r o u n d the A l e u t i a n Island f r o m A u g u s t to J a n u a r y , w h e n it reaches its p e a k , a n d t h e n shifts to the w e s t a n d s i m u l t a n e o u s l y gets w e a k e r u n t i l it i s f i n a l l y u n d e t e c t a b l e i n J u l y . T h e n the c y c l e starts a g a i n . W i n t e r w i n d s i n the G u l f o f A l a s k a a n d o n parts o f the N o r t h A m e r i c a n W - c o a s t Washington,  (Alaska, British  Columbia,  O r e g o n ) are f r o m s o u t h w e s t to southeast. In c o n t r a s t , the N o r t h P a c i f i c H i g h  is  l o c a t e d at a b o u t 3 5 ° N a n d r e m a i n s p r e s e n t y e a r r o u n d , a l t h o u g h w i t h v a r i a b l e s p a t i a l e x t e n t . It r e a c h e s a m a x i m u m , i n t e r m s o f s t r e n g t h as w e l l as h o r i z o n t a l d i m e n s i o n s , i n J u l y a n d  August  83  ( F a v o r i t e et  al.  1976). M a i n w i n d d i r e c t i o n i n the N o r t h e a s t P a c i f i c u n d e r the i n f l u e n c e o f the  N o r t h P a c i f i c H i g h is southeastward. A l o o k at s m a l l e r s c a l e s r e v e a l s m e s o s c a l e a n d s m a l l e r e d d i e s ( T h o m s o n et al. their characteristic shorter time  scales and  e c o l o g i c a l effects. H o w e v e r ,  1990) all w i t h  because  of  the  1°  l o n g i t u d e x 1° l a t i t u d e r e s o l u t i o n o f t h e s p a t i a l l y - e x p l i c i t s i m u l a t i o n s ( C h a p t e r 4 ) , s m a l l s c a l e o c e a n features have been ignored i n this study. I have also o m i t t e d teleconnections w i t h respect to E N S O - e v e n t s  (El Nino  - Southern  Oscillation) w h i c h can have large physical  (Dodimead  1985; H a m i l t o n & E m e r y 1985; H u y e r & S m i t h 1985; K e r r 1992; Tabata 1985; Trenberth  1990),  b i o g e o g r a p h i c a l and b i o l o g i c a l effects (Brodeur & P e a r c y 1992; F u l t o n & L e B r a s s e u r 1985). g o o d general d i s c u s s i o n o n the sequence o f events o f E N S O  c a n be f o u n d i n (ann  &  A  Lazier  ( 1 9 9 1 ) , a n d o n its s p e c i f i c effects i n the N o r t h e a s t P a c i f i c i n W o o s t e r & F l u h a r t y (1985).  84  3.  POPULATION  MODELS,  ENVIRONMENTAL  FORCINGS,  AND  MEAN FIELD SIMULATIONS  "There is no unique way to find out the characteristic properties of a system. The most important source remains intuition." R.E. U l a n o w i c z and G . Radach (1981)  In this C h a p t e r I w i l l introduce t w o population m o d e l s each o f w h i c h w i l l then b e used f o r a mean  field ecosystem simulation using abiotic environmental  f o r c i n g s at t w o sites ( S t a t i o n  P  (50°N 145°W) a n d a n e a r - c o a s t l o c a t i o n at 5 0 ° N 130°W) f r o m 1 9 8 1 t o 1984. T h e o b j e c t i v e o f t h e m e a n f i e l d s i m u l a t i o n s i s t o e x p l o r e a n d t o " t u n e " ( P i a t t et al.  1981) population m o d e l s f o r  the s p a t i a l l y - e x p l i c i t s i m u l a t i o n s t o f o l l o w i n C h a p t e r 4 i n w h i c h b o t h p o p u l a t i o n m o d e l s w i l l b e c o u p l e d t o s p a t i a l p h y s i c a l e n v i r o n m e n t a l d a t a s e t s ( W o o d r u f f et al. model  (Ingraham & Miyahara  simulations  to model  structure,  relationships o f component Northeast  Pacific have  1989). I n order  been  initial  to explore  conditions,  interactions, components  1987) a n d a surface  current  the sensitivity o f the ecosystem-  biological  parameters,  a n d functional  a n d processes o f the ecosystems o f the  radically s i m p l i f i e d t o the bare  essentials deemed  necessary to  explain variability i nsockeye salmon cohort survival.  3.1. Essential State Variables While  i n predictive studies o n e usually justifies the i n c l u s i o n o f system c o m p o n e n t s  into a  d y n a m i c m o d e l (e.g. r e s o u r c e m a n a g e m e n t ( H o l l i n g 1 9 7 8 ; W a l t e r s 1 9 8 6 ) ) , I f i n d that i n h i n d c a s t s t u d i e s o f s y s t e m s f o r w h i c h a b u n d a n t i n f o r m a t i o n i s a l r e a d y a v a i l a b l e i t i s m o r e u s e f u l t o start out w i t h a s y n t h e s i s o f the current m e c h a n i s t i c u n d e r s t a n d i n g i n t h e f o r m o f a c o m p l i c a t e d f l o w  85  d i a g r a m , a n d t h e n to j u s t i f y the e x c l u s i o n o f v a r i o u s state v a r i a b l e s a n d s u b p r o c e s s e s c o n s i d e r e d n o t n e c e s s a r y f o r the c o m p l e x p r o c e s s u n d e r i n v e s t i g a t i o n (see a l s o C h a p t e r  1 in Starfield  &  B l e l o c h 1991). A l t h o u g h the r e m a i n i n g ' m i n i m u m m o d e l ' still represents a subjective c h o i c e o f a l l available i n f o r m a t i o n , this c h o i c e seems less arbitrary because e c o s y s t e m c o m p o n e n t s  and  p r o c e s s e s h a v e b e e n r a t i o n a l l y e x c l u d e d rather than s i m p l y left out. T h e B i o l o g i c a l S u b s y s t e m i n F i g . 1.9 ( C h a p t e r different energy  1) s h o w s a c o n c e p t u a l f l o w d i a g r a m o f t h e  p a t h w a y s i n the e c o s y s t e m s o f the N o r t h e a s t P a c i f i c . T h e c o m p l e x  working  h y p o t h e s i s o f this thesis, i.e. the v a r i a b i l i t y i n s o c k e y e s a l m o n s u r v i v a l c a n b e e x p l a i n e d b y the variability  in  mesozooplankton  processes i n the open  availability for  Northeast Pacific  juvenile  ( s e e S e c t i o n 1.4:  sockeye, a  function  Conjecture),  The  dominant  nanophytoplankton >90%  size  ecosystem  a l l o w s the  e x c l u s i o n s , w h i c h r e p r e s e n t a set o f a s s u m p t i o n s f o r t h e p o p u l a t i o n m o d e l s ( c o m p a r e F i g s . 3.1 a n d  of  following  described  below  1.9): class  of  primary  producers  ( s i z e 2 - 2 0 p m ( B o o t h et al.  o f t h e b i o m a s s ( M i l l e r et al.  in  the  Central  Subarctic  Domain  is  1993; Parsons 1972)) w h i c h typically represents  1991a). T h i s d o m i n a n c e is the result o f the c o m p l e x nutrient  d y n a m i c s i n t h e C e n t r a l S u b a r c t i c as w e l l as t h e c a p a b i l i t y o f c o p e p o d s t o i m m e d i a t e l y  graze  d o w n any small-scale short-term increase in microphytoplankton. Thus microphytoplankton  has  b e e n c o m p l e t e l y e x c l u d e d f r o m the models.  T h e o b j e c t i v e o f t h i s t h e s i s i s t o e x p l a i n t h e v a r i a b i l i t y i n s o c k e y e s a l m o n s u r v i v a l , as d e r i v e d f r o m s t o c k - r e c r u i t m e n t data, b y the v a r i a b i l i t y i n m e s o z o o p l a n k t o n a v a i l a b i l i t y to the j u v e n i l e s . H o w e v e r , b e c a u s e o f the c o m p l i c a t i o n s i n m o d e l i n g p r o c e s s e s at h i g h e r t r o p h i c l e v e l s i n  an  e c o s y s t e m c o n t e x t ( s e e S e c t i o n 1.4) I h a v e n o t e x p l i c i t l y i n c l u d e d f i s h s p e c i e s i n m y m o d e l s .  As  a consequence, neither p i s c i v o r e s nor fisheries c a n be m o d e l e d i n any m e a n i n g f u l w a y and thus  86  Model 1  Model 2  Macrozooplankton  j l f l | l | Carnivores 2 (C2) Type III  ||f|||f j CI  Mesozooplankton  Type III  Carnivores 1 (CI) Type III  H Donor Control  Microzooplankton  Herbivores (H)  Donor Control P  Nanophytoplankton  Primary Producers (P)  Fig. 3.1: Flow diagrams for two population models. Model 1 includes three, Model 2 four trophic levels. Biomass transfers between trophic levels are labeled according to the functional form of predation rate to prey density (for explanations see Section 3.3).  87  h a v e b e e n left out. Nutrient (Donaghay  dynamics et  al.  in  1991)  the and  Northeast  Pacific  are  the  result  of  eco-physiological processes ( M o r e l  complex et  al.  biogeochemical  1991;  Wheeler  &  K o k k i n a k i s 1990) a n d thus h a v e r e c e i v e d a lot o f attention (e.g. M i l l e r 1 9 9 3 b ; P a r s o n s 1 9 8 8 ; f o r d e t a i l s see S e c t i o n 2.2.). H o w e v e r , w h i l e the a v a i l a b i l i t y o f c e r t a i n m i c r o n u t r i e n t s c o u l d set the realized  size-class  of primary  o b s e r v a t i o n s ( B o o t h et  al.  producers  1 9 9 3 ; L a n d r y et  (Armstrong al.  1994)  there  is strong  indication  from  1 9 9 3 b ) as w e l l as f r o m m o d e l i n g s t u d i e s ( F r o s t  1 9 9 1 ; F r o s t 1993) that p h y t o p l a n k t o n s t a n d i n g s t o c k i n the N E - P a c i f i c m a y not b e n u t r i e n t - , b u t is rather light- and g r a z e r - l i m i t e d (Banse does not s e e m to be b o t t o m - u p ( W e l s c h m e y e r et al.  1994). In fact, p r i m a r y p r o d u c t i o n per unit  biomass  l i m i t e d to any extent d u r i n g the s p r i n g a n d s u m m e r  months  1 9 9 3 ) , t h e p e r i o d o f t i m e i n w h i c h at l e a s t n u t r i e n t l i m i t a t i o n i s m o s t l i k e l y  to occur. C o n s e q u e n t l y , standing stock and d y n a m i c s o f nutrients have b e e n e x c l u d e d f r o m the p o p u l a t i o n m o d e l s b e l o w . F u r t h e r m o r e , s i n c e n u t r i e n t r e c y c l i n g s e e m s t o o c c u r at a f a s t e r r a t e t h a n w o u l d b e l i m i t i n g f o r p r i m a r y p r o d u c t i o n , i.e. p h y t o p l a n k t o n s t a n d i n g s t o c k n e v e r r e a c h e s a l e v e l w h e r e nutrient uptake is greater than nutrient supply, c o m p o n e n t s o f the m i c r o b i a l r e c y c l i n g process  (dissolved  (DOM)  and  particulate organic matter  (POM),  and  bacteria)  have  been  e x c l u d e d f r o m t h e m o d e l s as w e l l . T h e e x c l u s i o n o f nutrients f r o m a plankton m o d e l might s e e m unusual a n d thus deserves further justification: In general, different aquatic ecosystems have been m o d e l e d b y variations o f nutrient-phytoplankton-zooplankton ( N - P - Z ) models (Steele & H e n d e r s o n  1992) to g a i n insight  i n t o b i o g e o c h e m i c a l p r o c e s s e s (e.g. K i s h i & K a w a m i y a 1 9 9 5 ) , p o p u l a t i o n d y n a m i c s (e.g. W a l t e r s et  al.  1 9 8 7 ) , o r b o t h o f t h e s e a s p e c t s ( e . g . F r o s t 1 9 9 3 ) , as w e l l as c o m m u n i t y  Armstrong  1994).  H i s t o r i c a l l y the first p l a n k t o n m o d e l s  were developed  structure (e.g.  i n the early  1940s  88  (Banse  1994)  f o r r e g i o n s w h e r e o b s e r v a t i o n a l data w e r e r e a d i l y a v a i l a b l e , i.e. the c o n t i n e n t a l  shelves a n d the N o r t h A t l a n t i c . Incidentally these regions e x h i b i t regular seasonal p h y t o p l a n k t o n b l o o m s a s s o c i a t e d w i t h a d e c r e a s e i n n u t r i e n t c o n c e n t r a t i o n s ( B a n s e 1 9 9 4 ; P a r s o n s et al.  1984),  a n d c o n s e q u e n t l y , m o d e l s o f t h e s e o c e a n r e g i o n s h a v e i n c l u d e d n u t r i e n t s as a s t a t e v a r i a b l e . K n o w l e d g e o b t a i n e d f r o m these coastal e c o s y s t e m s , i n c o m b i n a t i o n w i t h L i e b i g ' s L a w o f the M i n i m u m ( O d u m 1 9 7 1 ) a n d t h e R e d f i e l d r a t i o ( P a r s o n s et al.  1984), w a s then e x t r a p o l a t e d to the  o p e n o c e a n situation a n d nutrient l i m i t a t i o n w a s also p r e s u p p o s e d there. Today,  there is a m p l e  e v i d e n c e that o p e n  ocean phytoplankton  community  structure is  c o n d i t i o n e d b y trace element or micronutrient a v a i l a b i l i t y and its standing stock is grazer- rather than nutrient-limited (Armstrong  1994; B a n s e 1994). Consequently, recent realistic e c o s y s t e m  m o d e l s f o r the N o r t h e a s t P a c i f i c d o not c o n t a i n d e p e n d e n c e o n nutrient c o n c e n t r a t i o n i n  the  p h y t o p l a n k t o n rate e q u a t i o n (Frost 1993), w h i l e o l d e r (Frost 1987) or m o r e c o n v e n t i o n a l ones d o (Kishi  &  Kawamiya  1995;  Matear  1995).  For  most  open  ocean  systems  phytoplankton  c o n c e n t r a t i o n s (as m e a s u r e d i n C h l - a ) c h a n g e s e a s o n a l l y o n l y b y a f a c t o r o f t w o ( B a n s e  1994);  the e x c e p t i o n is the greater than one order o f m a g n i t u d e seasonal c h a n g e i n C h l - a f o r the N o r t h Atlantic  (Parsons  Furthermore,  low  macronutrient  &  Lalli  1988),  the  "oddball"  seasonal variability in  concentrations  in  the  open  respective  (Banse  1994)  ocean systems regions  among  seems  (Banse  to  1994),  temperate be  independent  indicating  l i m i t a t i o n w i t h r a p i d nutrient c y c l i n g that p r o v i d e s a m m o n i u m b a c k to p h y t o p l a n k t o n 1 9 9 3 ; M i l l e r et  al.  1991b; Wheeler  &  oceans. of  grazing (Frost  K o k k i n a k i s 1990; see also S e c t i o n 2.2.; N o t e that the  arguments o n the c a r b o n - t o - c h l o r o p h y l l - a ratio presented i n S u b s e c t i o n 2.2.1. a n d i n F r o s t ( 1 9 8 7 , 1993) a n d M c A l l i s t e r (1969) apply to open o c e a n systems i n general.)  89  It i s i m p o r t a n t t o d i s t i n g u i s h b e t w e e n t h e e f f e c t s o f n u t r i e n t s o n p h y t o p l a n k t o n structure,  e.g.  nanophytoplankton  species  outcompeting  the  larger  community  microphytoplankton  for  a m m o n i u m , a n d their effects o n p h y t o p l a n k t o n s p e c i f i c g r o w t h rates. A m o r e p r e c i s e usage o f the term 'nutrient limitation' i n scientific publications w o u l d be helpful. Further, b i o m a s s production at a n y t r o p h i c l e v e l c a n b e s a i d t o b e b o t t o m - u p o r t o p - d o w n c o n t r o l l e d . (I i g n o r e h e r e w h a t h a s b e e n c a l l e d " m i d d l e - o u t c o n t r o l " b e c a u s e c o m p e t i t i o n effects are b y d e f i n i t i o n n o t a d d r e s s e d b y the aggregation  of species into trophic levels.) B o t t o m - u p  physical forcings  or  lower  trophic  levels have  on  the  c o n t r o l describes the effects  specific growth  rate o f  a  that  particular  p o p u l a t i o n , w h i l e t o p - d o w n c o n t r o l refers to the p r e d a t i o n effects o f h i g h e r t r o p h i c l e v e l s o n the s t a n d i n g s t o c k o f that p o p u l a t i o n . ( P h y t o p l a n k t o n s t a n d i n g s t o c k c o u l d i n p r i n c i p l e b e c o n t r o l l e d b y p a t h o g e n s . H o w e v e r , b e c a u s e v i r u s e s are s p e c i e s s p e c i f i c a n d the p h y t o p l a n k t o n c o m m u n i t y i s v e r y d i v e r s e , v i r u s e s p r o b a b l y d o not r e m o v e m o r e than 3 % o f the d a i l y p r i m a r y p r o d u c t i o n Suttle  1998  pers.  comm.).)  Biomass  production  of  a population  or  any  other  (C.  biological  aggregation is given by  dN — at w h e r e Nrepresents  biomass, and  , . = r(A,B,..)N  (Eq.3.1)  r(A,B,...) i s t h e s p e c i f i c g r o w t h r a t e t h a t i s r e g u l a t e d b y f a c t o r s  A,B,.... T h e r e i s n o i n h e r e n t r e a s o n w h y p r o d u c t i o n c o u l d n o t b e r e g u l a t e d b y b o t h t e r m s o n t h e r i g h t h a n d s i d e o f E q . 3.1 at t h e s a m e t i m e o r i n a d y n a m i c a l l y a l t e r n a t i n g f a s h i o n , i . e . a b i o t i c f a c t o r s o r l o w e r t r o p h i c l e v e l s a f f e c t i n g r, p r e d a t i o n l i m i t i n g t h e p o p u l a t i o n The  N.  i n c l u s i o n o f a p a r t i c u l a r state v a r i a b l e i n a m o d e l i s n o t o n l y b o u n d b y the  natural  p r o c e s s e s b e i n g m o d e l e d (and the o b j e c t i v e s o f the m o d e l ) b u t a l s o b y t h e i r s p a t i a l a n d t e m p o r a l scales. A s a consequence, non-repeatable  and/or non-stationary phenomena  (Walters  1986)  at  90  v a r i o u s spatial a n d t e m p o r a l scales either h a v e to b e addressed e x p l i c i t e l y or e x c l u d e d altogether. C o n s e q u e n t l y I a s s u m e that s p a t i o - t e m p o r a l structural c h a n g e s i n the n a t u r a l e c o s y s t e m s o f the Northeast  Pacific  w i t h i n the  simulation period  1 9 5 0 - 1 9 9 0 are n e g l i g i b l e  u n r e a s o n a b l e a s s u m p t i o n ( S t e e l e & H e n d e r s o n 1 9 8 4 ; b u t s e e P a u l y et al.  (a not  completely  1998)).  91  3.2. Environmental Forcings Physical simulations  environmental f o r Station P  (evaporation),  variables (Section  that  have  been  explicitly included  3 . 4 ) are: s o l a r r a d i a t i o n ,  winds, and m i x e d layer depth (Fig.  i n the mean  sea surface temperature,  1.9 i n C h a p t e r  field clouds  1). I n a d d i t i o n t o these, the  s p a t i a l l y - e x p l i c i t simulations o f C h a p t e r 4 contain advection fields (currents).  Only  sea surface  temperature, w i n d s , a n d c l o u d i n e s s are o b s e r v e d v a r i a b l e s , a l l other a b i o t i c f o r c i n g s u s e d i n m y s i m u l a t i o n s are d e r i v e d f r o m t h e m , w i t h the e x c e p t i o n o f sea surface currents w h i c h c o m e  from  simulation results b y Ingraham & M i y a h a r a (1989). In a l l m y simulations, m o n t h l y  data where  simplification o f 3 0 days f o r each month, interpolation between  assigned t o the  15  t h  o f each month  thus o n l y 3 6 0 days per year)  (with  with linear  the  temporal  months.  3.2.1 Observed Variables M o n t h l y arithmetic means for sea surface temperature, scalar w i n d speed and total c l o u d i n e s s data  from  (COADS).  1950-1990 COADS  were  taken  i s a statistical  from  the Comprehensive  summary  o f global  Ocean-Atmosphere  marine  observations  Data Set  with  a spatial  r e s o l u t i o n o f 2° l o n g i t u d e x 2° l a t i t u d e ( e v e n - n u m b e r e d ) f o r e a c h m o n t h o f e a c h y e a r f r o m  1854  u p to the present (for details o n data c o l l e c t i o n , a r c h i v i n g , statistics, a n d q u a l i t y c o n t r o l see S l u t z et al.  1 9 8 5 a n d W o o d r u f f et al.  simulations (Chapter  1987).  B e c a u s e the spatial r e s o l u t i o n o f m y s p a t i a l l y - e x p l i c i t  4 ) i s 1° l o n g i t u d e x 1° l a t i t u d e a n d b e c a u s e d a t a w e r e n o t a v a i l a b l e f o r  every 2° x 2° b o x , I have spatially interpolated data f r o m C O A D S (Press  etal.  using bilinear  interpolation  1992).  92  Fig.  3.2 shows  surface temperature)  the seasonal a n d interannual  variability i n m i x e d  layer  temperature (sea  at S t a t i o n P ( 5 0 ° N 145°W) a n d f o r a n e a r - c o a s t l o c a t i o n at 5 0 ° N 1 3 0 ° W f o r  four successive years (1981-1984).  3.2.2. Derived Variables  Incident Solar Radiation (Insolation) T h e t o t a l d a i l y i n s o l a t i o n o n a h o r i z o n t a l s u r f a c e a t d e p t h z, h, i s g i v e n b y :  I =V  (Eq.  b  z  3.2a)  w h e r e Io i s t h e d a i l y s e a s u r f a c e i n s o l a t i o n a n d z i s g i v e n i n n e g a t i v e v a l u e s , i . e . a s a d e p t h coordinate, k represents the extinction coefficient, w h i c h is a f u n c t i o n o f the concentrations o f p a r t i c u l a t e a n d d i s s o l v e d m a t t e r , a n d w a t e r i t s e l f ( P a r s o n s et al.  1984). B e c a u s e the effects o f  c h l o r o p h y l l - a concentration onto the e x t i n c t i o n coefficient are very s m a l l (seeE q . (2) i n Frost 1987) a n d because detritus, w h i c h c a n have a substantial effect o n thee x t i n c t i o n coefficient, w a s not i n c l u d e d into t h e m o d e l s , the e x t i n c t i o n c o e f f i c i e n t f o r e a c h m o n t h w a s c a l c u l a t e d as the m e a n v a l u e (Fig. 3.3) o f the observed m o n t h l y m i n i m u m a n d m a x i m u m 1 9 6 4 , a s s u m m a r i z e d i n P a r s o n s et al. Further,  f o r the period  1960 to  (1966).  as suggested b y Frost (1993) 7 0 % o f the total solar radiation w a s considered as  p h o t o s y n t h e t i c a l l y a c t i v e r a d i a t i o n IPAR, , Z  u p f r o m t h e 5 0 % u s e d e a r l i e r ( F r o s t 1 9 8 7 ; P a r s o n s et  al.  1984):  I PAR,  = 0.7/,  (Eq.  3.2b)  93  Station P (50°N 146°W)  94  Fig. 3.3: Seasonal variability in the extinction coefficient k in the Northeast Pacific. The extinction coefficient for each month was calculated as the mean value of the observed monthly minimum and maximum for the period 1960 to 1964. DatafromParsons et al. (1966).  95  D a i l y s e a s u r f a c e i n s o l a t i o n (Io i n E q . 3 . 2 a ) w a s c a l c u l a t e d as a f u n c t i o n o f d a i l y i n s o l a t i o n at t h e t o p o f t h e c l o u d s (l ) a n d c l o u d i n e s s ( C ) b y : s  /  0  = /,(l-0.08875C)  ( E q . 3.2c)  T h i s i s a v a r i a t i o n o f t h e S v e r d r u p E q u a t i o n ( S v e r d r u p et al. 1 9 4 7 ) w i t h c l o u d i n e s s C i n u n i t s o f e i g h t s ( o r 1 2 . 5 % ) o f s k y c o v e r e d b y c l o u d s , as g i v e n b y C O A D S . N o t e h o w e v e r t h a t c l o u d i n e s s is a very crude estimator for light reflection and absorption due t o c l o u d s ( K r e m e r & N i x o n 1978). D a i l y i n s o l a t i o n at t h e t o p o f t h e t r o p o s p h e r e (I ) w a s d e r i v e d f r o m f i r s t p r i n c i p l e s ( f o r s  the  c o m p l e t e d e r i v a t i o n see P e i x o t o & O o r t ( 1 9 9 2 ) t h e i r C h a p t e r 6 ) w i t h a n a s s u m e d a t m o s p h e r i c t r a n s m i s s i v i t y a b o v e the t r o p o s p h e r e o f Ts=0.75 ( O t t 1 9 8 8 ; S c h n e i d e r 1 9 8 9 ) :  /  c  \  I =t S-2[r]s'm(psino + cos(pcosds'mri)s  43200  s  w h e r e S is the solar constant ( 1 3 6 0 W m "  2  ( E q . 3.2d)  ( P e i x o t o & O o r t 1 9 9 2 ) ) , 77 r e p r e s e n t s t h e h o u r a n g l e  f r o m t h e l o c a l m e r i d i a n at s u n r i s e a n d s u n s e t ( a f u n c t i o n o f t h e t i m e o f t h e y e a r ) , cp i s l a t i t u d e , and S is declination. E q . 3.2d integrates instant irradiance over one day. Further,  r\ = a r c c o s ( - t a n cp t a n S)  (Eq.  3.2e)  8 = arcsin(sinvsind)  (Eq.  3.2f)  w h e r e 2r\ i s d a y l e n g t h i n r a d i a n s , v r e p r e s e n t s t h e o b l i q u i t y o f t h e e c l i p t i c ( i . e . 23.45°) a n d d i s n u m b e r o f days after v e r n a l e q u i n o x . Fig.  3.4  s h o w s the seasonal and interannual variability i n d a i l y sea surface i n s o l a t i o n as  c a l c u l a t e d f r o m E q s . 3 . 2 c - f at S t a t i o n P a n d at a n e a r - c o a s t l o c a t i o n f o r 1 9 8 1 - 1 9 8 4 .  Station P  m o d e l results r e s e m b l e c l o s e l y the data s h o w n i n F r o s t (1993 his F i g . l ) .  96  Fig. 3.4: Seasonal and interannual variability in sea surface insolation at Station P (50°N 145°W) and at 50°N 130°W for 1981 to 1984. Sea surface insolation was calculated from first principles and COADS cloudiness data.  97  Mixed Layer Depth L a c k i n g a l o n g - t e r m a n d s p a t i a l l y - e x p l i c i t d a t a s e t o f t h e m i x e d l a y e r d e p t h (ZML, n e g a t i v e v a l u e s as m i x e d l a y e r d e p t h w a s regarded as a d e p t h c o o r d i n a t e ) i n t h e N o r t h e a s t P a c i f i c I t o o k t h e m o n t h l y s t a t i s t i c a l s u m m a r i e s o f m i x e d l a y e r d e p t h at S t a t i o n P ( 5 0 ° N 1 4 5 ° W ) f o r t h e y e a r s 1947-1963  published  i n Parsons  &  LeBrasseur  (1968).  From  the C o m p r e h e n s i v e  Ocean-  A t m o s p h e r e D a t a Set ( C O A D S ; see S u b s e c t i o n 3.2.1.) I c a l c u l a t e d the m o n t h l y m e a n sea surface temperature  (T)  a n d scalar w i n d speed (w) f o r the same period. N e x t I performed  a linear  r e g r e s s i o n a n a l y s i s f o r m i x e d l a y e r d e p t h as a f u n c t i o n o f sea surface temperature ( E q . 3.3a) a n d scalar w i n d speed (Eq. 3.3b), respectively (coefficient o f determination i n brackets):  z  = 1 1 . 9 9 7 ; - 1 7 8 . 5 9 (r  z  = - 1 0 . 4 6 w + 2 3 . 5 3 (r  ML  ML  Furthermore,  2  2  = 0.83)  ( E q . 3.3a)  = 0.26)  (Eq. 3.3b)  I fitted a t w o variable (sea surface temperature  a n d scalar w i n d speed)  linear-  n o r m a l m o d e l ( B r o w n & R o t h e r y 1993) t o the data u s i n g the least squares m e t h o d :  z  ML  = 11.107/-3.33w-139.80  (r  2  =0.85)  F o r c o m p a r i s o n , I t o o k a n e m p i r i c a l m o d e l o r i g i n a l l y o b t a i n e d b y T a b a t a et al.  ( E q . 3.3c) (1965) for the  s u m m e r i s o t h e r m a l surface l a y e r a n d extrapolated it o v e r the w h o l e year: Z  ML  = - 2 . 0 6 w + 2.3 ( r  2  = 0.26)  (Eq. 3.3d)  F i g . 3.5 s h o w s the data f r o m Parsons & L e B r a s s e u r (1968) a n d results obtained f r o m the different models. A l t h o u g h the seasurface temperature m o d e l ( E q . 3.3a) explains 8 3 % o f the variability i n the data the t w o variable linear-normal m o d e l ( E q . 3.3c) w a s used f o r the m e a n field  simulations  f o r Station P  (Section  3.4) as w e l l  as the s p a t i a l l y - e x p l i c i t s i m u l a t i o n s  98  Fig,  3.5:  Upper panel: Observed  d e v i a t i o n at S t a t i o n F ( 5 ( T N  monthly  m e a n m i x e d layer depth plus/minus  one  standard  I 4 5 W > f o r t h e p e r i o d 1 9 4 7 - 1 9 6 3 . D a t a f r o m P a r s o n s et a l . ( 1 9 6 8 ) . U  L o w e r panels V a r i o u s m o d e l s for m i x e d layer depth u s i n g sea surface temperature  (7),  scalar  w i n d s p e e d (w), o r b o t h . T h e T a b a t a M o d e l ( T a b a t a et a l . 1 9 6 5 ) g i v e s t h e r e l a t i o n s h i p b e t w e e n summer isothermal surface layer and w i n d speed and performs poorly w h e n extrapolated beyond its v a l i d statistical u n i v e r s e o f i n f e r e n c e i n s u m m e r ( f o r d e t a i l s see text).  99  ( C h a p t e r 4) f o r the f o l l o w i n g reasons: F i r s t , o n l y p r e - a n a l y z e d , i.e. m o n t h l y statistical s u m m a r i e s , m i x e d layer depth data for o n l y one station were available for the regression analyses w h i c h thus g i v e s a v e r y n a r r o w p i c t u r e o f the w h o l e N o r t h e a s t P a c i f i c . C o n s e q u e n t l y , the m o d e l that fits the data e v e n o n l y m a r g i n a l l y better seems j u s t i f i e d . S e c o n d , testing f o r the d i f f e r e n c e s  between  m o n t h l y v a r i a n c e f o r the years 1947 to 1963 i n the m i x e d layer d e p t h data a n d m i x e d l a y e r d e p t h f r o m m o d e l c a l c u l a t i o n s r e v e a l e d that the t w o v a r i a b l e l i n e a r - n o r m a l m o d e l h a d o n l y f o u r  (Apr,  S e p - N o v ) s t a t i s t i c a l l y s i g n i f i c a n t d i f f e r e n c e s at t h e 5 % l e v e l , a n d t w o ( O c t , N o v ) at t h e 1 % l e v e l (2-tailed  variance  ratio  test ( Z a r  1996)),  while  all other  models  had  a  larger  number  of  s t a t i s t i c a l l y s i g n i f i c a n t d i f f e r e n c e s (note that here a s t a t i s t i c a l l y s i g n i f i c a n t d i f f e r e n c e m e a n s that the m o n t h l y v a r i a n c e s i n the data a n d the m o d e l d o not the c o m e f r o m the s a m e p o p u l a t i o n ) .  And  t h i r d , i n g e n e r a l the m e c h a n i s m s o f stratification i n v o l v e b o t h temperature a n d w i n d m i x i n g (see S e c t i o n 2.3.). N o t e that i n c a s e o f c a l m w i n d c o n d i t i o n s a n y sea surface t e m p e r a t u r e > 1 2 . 6 0 °C i n E q . 3.3c w i l l result i n a p o s i t i v e m i x e d l a y e r d e p t h c o o r d i n a t e . B e c a u s e temperatures greater t h a n that c a n o c c u r at l e a s t l o c a l l y a n y w h e r e i n t h e N o r t h e a s t P a c i f i c i n t h e s u m m e r a n d d o i n g e n e r a l o c c u r south  of  40°N in  winter  (Thomson  1981),  an  upper  maximum  for  the  mixed  layer  depth  c o o r d i n a t e o f - 1 5 m has b e e n a s s u m e d i n the s i m u l a t i o n s . F i g . 3 . 6 s h o w s t h e s e a s o n a l a n d i n t e r a n n u a l v a r i a b i l i t y i n m i x e d l a y e r d e p t h at S t a t i o n P at a n e a r - c o a s t l o c a t i o n f o r 1 9 8 1 - 1 9 8 4 .  and  S t a t i o n P m o d e l results a g a i n r e s e m b l e c l o s e l y the data  s h o w n i n Frost (1993 his F i g . l ) .  100  Station P (50°N 1 4 5 ^ )  Fig. 3.6: Seasonal and interannual variability in mixed layer depth at Station F (50°N 145°W) and at 50°N 130°W for 1981 to 1984. Mixed layer depth coordinate (negative values) was calculated as a function of sea surface temperature and scalar wind speed (bothfromCOADS) using a two variable linear-normal (Eq. 3.3c). Note that mixed layer depths are plotted before truncation (for details see text), resulting in a positive mixed layer depth at 50°N 130°W in 1983. Because the variability in mixed layer depth is largely explained (r = 0.83) by mixed layer temperature their seasonal and interannual patterns are similar (compare Fig. 3.2). 2  101  Advection, Divergence and Sinking Surface current data f o r the Northeast Pacific f r o m 1 9 5 0 t o 1 9 9 0 ,used i n the spatiallyexplicit simulations described i n Chapter 4, c o m e f r o m the Ocean Surface Current (OSCURS).  OSCURS  isa hydrodynamic  s i m u l a t i o n that c o m b i n e s C O A D S  Simulation  vector w i n d data  w i t h m e a n geostrophic currents a n d returns d a i l y values f o r currents i n the m i x e d u p p e r layer (Ingraham & M i y a h a r a 1989). T o save computer space currents w e r e averaged f o r e a c h m o n t h a n d t h e n l i n e a r l y i n t e r p o l a t e d i n t i m e f o r d a i l y v a l u e s ( S c a n d o l et al.  1996). O S C U R S  hasa  s p a t i a l r e s o l u t i o n o f 1° l o n g i t u d e x 1° l a t i t u d e a n d t h u s d o e s n o t s i m u l a t e s m a l l s c a l e o c e a n i c (e.g. m e s o s c a l e eddies) o r c o a s t a l p r o c e s s e s (e.g. tides, estuarine c i r c u l a t i o n ) . F r o m t h e m o n t h l y m e a n s o f O S C U R S c u r r e n t v e c t o r s (w, v ) I c a l c u l a t e d t h e d i v e r g e n c e  (D)  f o r e a c h 1° l o n g i t u d e x 1° l a t i t u d e f o r t h e p e r i o d 1 9 8 1 t o 1 9 8 4 , w i t h x, y a n d z a s a x e s i n a r i g h t handed coordinate system: Aw D = -  AM A v  — =—  Az Mean  divergence  (D=0.72  10~  3  Ax  Ay  d" , standard deviation:s = 2.28-10~ 1  (Eq.3.4)  +—  7  d" , sample 1  size:  n = 5 9 5 2 0 ) f o r t h e w h o l e area o f the N o r t h e a s t P a c i f i c (120-180°W, 35-62°N) f o r 1 9 8 1 - 1 9 8 4 shows excess export o f water, w h i c h is replaced b y E k m a n p u m p i n g f r o m depth. B e c a u s e i n m y s i m u l a t i o n s I a s s u m e that there i s n o l i f e b e l o w t h e surface m i x e d l a y e r (see S e c t i o n 3.4.) a n d b e c a u s e n u t r i e n t d y n a m i c s h a v e b e e n e x c l u d e d f r o m the s i m u l a t i o n s (see S e c t i o n 3.1.), u p w e l l i n g processes a n d the associated decrease i n concentrations o f b i o l o g i c a l products i s essentially accounted f o r b y surface export d u e t o advection. D o w n w e l l i n g , o n the other hand, c a n i n principle remove  organisms  divergence was very l o w (D  n e g  f r o m the m i x e d layer but because the mean = - 7 . 4 2 • 10~  3  value o f negative  d" , s a m p l e s i z e : n = 2 7 8 4 4 , i.e. o n a v e r a g e less t h a n 1  102  1 % o f the m i x e d l a y e r c o n c e n t r a t i o n gets e x p o r t e d e v e r y day), a n d b e c a u s e o c e a n i c s p e c i e s o f p h y t o p l a n k t o n and z o o p l a n k t o n have been selected for an array o f m o r p h o l o g i c a l , p h y s i o l o g i c a l , o r b e h a v i o r a l adaptations to prevent t h e m f r o m s i n k i n g out o f the m i x e d surface layer, v e r t i c a l e x p o r t o f l i v i n g p l a n k t o n f r o m the m i x e d l a y e r to d e p t h has b e e n a s s u m e d n e g l i g i b l e i n  my  s i m u l a t i o n s c o m p a r e d to changes due to b i o l o g i c a l p r o d u c t i o n a n d p h y s i c a l a d v e c t i o n processes. F i g . 3 . 7 s h o w s t h e s e a s o n a l a n d i n t e r a n n u a l v a r i a b i l i t y i n E k m a n p u m p i n g at S t a t i o n P a n d at a n e a r - c o a s t l o c a t i o n f o r 1 9 8 1 - 1 9 8 4 as c a l c u l a t e d f r o m t h e p r o d u c t o f d i v e r g e n c e (£>) a n d m i x e d layer depth  (ZML)-  Mean  Ekman  p u m p i n g for the s u m m e r  months  (Apr-Sep)  is 0.04  m  d"  1  (standard deviation: s = 0.13 m d" , sample size: n = 24, 1 9 8 1 - 1 9 8 4 ) w h i c h is w i t h i n the range o f 1  0 . 0 1 - 0 . 1 m d " r e p o r t e d b y M i l l e r etal. ( 1 9 9 1 b ) . 1  Carbon-to-Chlorophyll-a Ratio D e s p i t e the c o n s t a n t c h l o r o p h y l l - a c o n c e n t r a t i o n i n the N o r t h e a s t P a c i f i c (e.g. P a r s o n s L a l l i 1 9 8 8 ; W o n g et al.  1995) there is p r e s u m a b l y seasonal v a r i a t i o n i n p h y t o p l a n k t o n  stock w h e n m e a s u r e d i n c a r b o n c o n c e n t r a t i o n ( M c A l l i s t e r 1969, P. H a r r i s o n 1997 pers. T h i s is due to the v a r i a b i l i t y i n the i n t r a c e l l u l a r c a r b o n - t o - c h l o r o p h y l l - a ratio i n  &  standing comm.).  phytoplankton,  w h i c h i n c r e a s e s w i t h i n c r e a s i n g i n s o l a t i o n , i.e. u n d e r a h i g h l i g h t r e g i m e f e w e r a n d / o r  smaller  c h l o r o p l a s t s a r e a b l e t o m a i n t a i n t h e s a m e p h o t o s y n t h e t i c r a t e as m o r e a n d / o r l a r g e r c h l o r o p l a s t s u n d e r l o w l i g h t c o n d i t i o n s . B e c a u s e c h l o r o p l a s t s are c o m p l e x c y t o m o r p h o l o g i c a l s t r u c t u r e s a n d c h l o r o p h y l l - a i s a c o m p l e x m a c r o m o l e c u l e ( D e n f f e r et al.  1983), p o t e n t i a l l y h i g h e r g r o w t h rates  o f p h y t o p l a n k t o n c e l l s w i t h m o r e c h l o r o p h y l l - a are p r o b a b l y o f f s e t b y the h i g h m e t a b o l i c c o s t s o f its s y n t h e s i s a n d m a i n t e n a n c e .  103  -1 -I  1  Jan  1  1  Mar  1  1  May  1  1  Jul  1  1  Sep  1  Nov  Fig. 3.7: Seasonal and interannual variability in Ekman pumping at Station P (50°N 145°W) and at 50°N 130°W for 1981 to 1984. Positive Ekman pumping indicates upwelling, negative Ekman pumping downwelling. Variability in Ekman pumping reflects the variability in local advection processes. Compare the large interannual variability in Ekman transport to those in other environmental variables (Figs. 3.2,3.4, and 3.6).  104  T o a c c o u n t f o r the l a t i t u d i n a l v a r i a b i l i t y i n the c a r b o n - t o - c h l o r o p h y l l - a ratio c a u s e d b y  the  v a r i a b i l i t y i n i n s o l a t i o n , I p e r f o r m e d a linear regression analysis u s i n g the c a r b o n - t o - c h l o r o p h y l l a r a t i o (x) " d a t a " f o r S t a t i o n P i n M c A l l i s t e r ( 1 9 6 9 ) a n d m o n t h l y s e a s u r f a c e i n s o l a t i o n (Io, units M J m"  2  d" )  at S t a t i o n P f o r  1  1 9 8 1 - 1 9 8 4 as c a l c u l a t e d f r o m t h e m o d e l  described  in  above  (coefficient o f determination in brackets): X = 3 . 4 4 / + 5.68 ( r  (Eq.  3.5a)  C o n s i d e r i n g a time-lag between changes in sea-surface insolation and changes in m i x e d  layer  0  2  = 0.68)  d e p t h ( M c A l l i s t e r 1 9 6 9 ; c o m p a r e F i g s . 3 . 4 a n d 3 . 6 ) as w e l l as a p h y s i o l o g i c a l a d a p t a t i o n p e r i o d , I a l s o t r i e d a d e l a y e d r e g r e s s i o n ( F i g . 3.8):  X  moMh  = 4-07/ , 0  _ + 1 . 0 0 (r = 0 . 9 5 )  ( E q . 3.5b)  2  m o n ( / l  1  B e c a u s e E q . 3 . 5 b y i e l d s a n a n n u a l m i n i m u m c a r b o n - t o - c h l o r o p h y l l - a r a t i o o f % = 2 at 6 2 in January (with  I  0tDec  o f % = 1 3 3 at 4 3 N ° chlorophyll-a  ratio  Unfortunately  though,  = 0.26 M J m"  2  in July (with  was  l i m i t e d to  d" under f u l l overcast c o n d i t i o n s ) a n d an a n n u a l 1  I  0Jm  = 32.33  the range  M J m'  2  9 < ;£ < 9 0  d"  1  (P.  under clear sky) the Harrison  1993  b a s e d u p o n o n l y t w o o b s e r v a t i o n s (the w i n t e r m i n i m u m a n d the s u m m e r m a x i m u m ) from,  presumably  shipboard,  phytoplankton  cultures  with  interpolations  maximum carbon-to-  pers.  M c A l l i s t e r ' s (1969) estimates o f the c a r b o n - t o - c h l o r o p h y l l - a  N°  comm.). ratio  are  obtained  in-between,  which  m a k e s E q . 3.5b a w e a k f u n c t i o n a l statement. F i g . 3 . 9 s h o w s t h e s e a s o n a l a n d i n t e r a n n u a l v a r i a b i l i t y i n t h e c a r b o n - t o - c h l o r o p h y l l - a r a t i o at S t a t i o n P a n d at a n e a r - c o a s t l o c a t i o n f o r 1 9 8 1 - 1 9 8 4 a s c a l c u l a t e d f r o m E q . 3 . 5 b .  105  0  -I  1  Jan  Fig.  3.8:  1969)  1  —  Mar  H  1  May  1  1  1  Jul  1  Sep  Seasonal variability i n the carbon-to-chlorophyll-a ratio (data points f r o m  and two regression models  insolation) for Station P  (carbon-to-chlorophyll-a  McAllister  r a t i o as a f u n c t i o n o f s e a s u r f a c e  (for details see text). A r r o w s indicate the o n l y t w o observations  made  by M c A l l i s t e r (1969).  106  Fig. 3.9: Seasonal and interannual variability in the carbon-to-chlorophyll-a ratio at Station P (50°N 145°W) and at 50°N 130°W for 1981 to 1984. Note that because the carbon-tochlorophyll ratio is calculated as a function of sea surface insolation their seasonal and interannual patterns are similar (see Fig.3.4).  107  3.3. Population Models Fig.  3.1 ( S e c t i o n 3.1.)  s h o w s the c o n c e p t u a l f l o w d i a g r a m s  f o r two different  population  m o d e l s that I h a v e d e v e l o p e d f o r the l o w e r t r o p h i c l e v e l s o f the N o r t h e a s t P a c i f i c . State v a r i a b l e s e x p l i c i t l y i n c l u d e d i n the p o p u l a t i o n m o d e l s c a n b e s e e n i n a n e c o s y s t e m c o n t e x t i n F i g . 1.9 (hatched b o x e s there). T o test f o r effects o f l i n e a r c o m m u n i t y structure o n c o m m u n i t y d y n a m i c s , i.e. the s e n s i t i v i t y to m o d e l structure, I have d e v e l o p e d t w o different p o p u l a t i o n m o d e l s : M o d e l  1 contains three  state v a r i a b l e s , i.e. p r i m a r y p r o d u c e r s , h e r b i v o r e s , a n d p r i m a r y c a r n i v o r e s , a n d M o d e l 2 c o n t a i n s f o u r state v a r i a b l e s , i.e. a s e c o n d a r y c a r n i v o r e t r o p h i c l e v e l i s a d d e d . B e c a u s e u p p e r ( p r e d a t o r y ) closure has been f o u n d t o have m a r k e d effects o n m o d e l b e h a v i o r (Steele & H e n d e r s o n  1992)  u s i n g m o d e l s w i t h o d d and e v e n n u m b e r s o f trophic levels s h o u l d m a x i m i z e the contrast i n m o d e l behavior ( P i m m 1992). F o r e a c h state v a r i a b l e the f o l l o w i n g e q u a t i o n w i t h its c o m p o n e n t s a p p l i e s : d(Biomass) —  = Gain -  Loss + Import -  Gain  = Ingestion  Loss  = Egestion + Respiration + Deaths  (Eq.  3.6)  + Recruitment  Import = Immigration Export  Export  = Emigration  x  y  z  z  +  Retirement  + (Passive Import) + (Passive E x p o r t )  x y z  N o t e that b e c a u s e E q . 3.6 i s the rate e q u a t i o n f o r b i o m a s s , b i r t h s are not c o n s i d e r e d i n the " g a i n " t e r m , i.e. there i s n o b i o m a s s c h a n g e a s s o c i a t e d w i t h the b i r t h p r o c e s s . O n the o t h e r h a n d the c h a n g e i n b i o m a s s due t o deaths has t o b e i n c l u d e d . R e c r u i t m e n t a n d R e t i r e m e n t designate t e r m s u s e d i n a g e - s t r u c t u r e d p o p u l a t i o n m o d e l s (e.g. R i c e 1995) a n d h a v e o n l y b e e n a d d e d f o r c o m p l e t e n e s s but h a v e not b e e n i n c l u d e d i n the p o p u l a t i o n m o d e l s b e l o w . T h e terms f o r i m p o r t  108  f r o m a n d export to x - a n d y - d i m e n s i o n s , but not f r o m a n d to depths, w i l l b e a p p l i e d i n C h a p t e r 4: S p a t i a l l y E x p l i c i t S i m u l a t i o n s . E q . 3.6 says o f c o u r s e n o t h i n g about the details o f the b i o t i c a n d abiotic interactions but represents a useful general f r a m e w o r k for d e v e l o p i n g m o d e l s because it identifies a l l the p o s s i b l e f l o w s . T h e r a t e e q u a t i o n s f o r t h e state v a r i a b l e s b e l o w f o l l o w t h e s a m e s t r u c t u r e , w i t h f i r s t a t e r m f o r gross e n e r g y c o n s u m p t i o n , f o l l o w e d b y a t e r m w h i c h denotes the l o s s d u e to r e s p i r a t i o n , a n d at l a s t a t e r m f o r p r e d a t i o n . T h e f o l l o w i n g r a t e e q u a t i o n s a p p l y ( T a b l e 3 . 1 ; s e e T a b l e 3 . 2  for  symbols):  109  Table 3.1:  R a t e E q u a t i o n s for State V a r i a b l e s  Primary Producers (P )  ( E q . 3.7)  P = %P  0  0  CU  dH  Herbivores (H)  a W HC  — = b P - m Hp  dt  Carnivores 1 ( d )  Carnivores 2 (C ) 2  dC,  — = 1  dt  Q  2  H  1  2  K  +H  HC  a  nr  H  2  K +H 2  2  —-=  r  C, - m G 1  2  12  c  2  c  >  1  a  *  2  r r  ,'  Ci  2  C\  ( E q . 3.8)  -C  + C  C -m C 2  C,  2  2 2  2  ( E q . 3.9)  ( E q . 3.10)  110  Table 3.2: I m p o r t a n t  symbols used in models and simulations.  State Variables and Parameter Subscripts Ci  smaller carnivorous zooplankton (mesozooplankton)  c a r b o n c o n c e n t r a t i o n [ m g C m" ]  Cz  larger carnivorous zooplankton (macrozooplankton)  H  h e r b i v o r o u s z o o p l a n k t o n ( m i c r o z o o p l a n k t o n ) c a r b o n c o n c e n t r a t i o n [ m g C m" ]  Po  p h y t o p l a n k t o n c a r b o n c o n c e n t r a t i o n [ m g C m" ]  Pcu  p h y t o p l a n k t o n c h l o r o p h y l l - a c o n c e n t r a t i o n [ m g C h l - a m" ]  c a r b o n c o n c e n t r a t i o n [ m g C m" ] 3  3  3  3  Physical Forcings Io  d a i l y s e a s u r f a c e i n s o l a t i o n o n a h o r i z o n t a l p l a n e at s e a s u r f a c e [ M J m "  T  m i x e d layer temperature [°C]  ZML  m i x e d layer depth [m]  H  d a y l e n g t h [h]  k  extinction coefficient  d  d" ]  2  1  [m" ] 1  Parameters a  photosynthetic efficiency [mg C (mg Chl-a)"  X  c a r b o n - t o - c h l o r o p h y l l - a ratio  axY  m a x i m u m s p e c i f i c p r e d a t i o n rate o f predator Y o n p r e y X [ m g C ( m g C ) " d" ]  bp  s p e c i f i c g r o w t h rate f o r p h y t o p l a n k t o n [ m g C ( m g C ) " d ' ]  KXY  h a l f - s a t u r a t i o n c o n s t a n t f o r p r e d a t i o n o f p r e d a t o r Y o n p r e y X [ m g C m" ]  1  d" ] 1  1  1  1  1  3  mx  s p e c i f i c r e s p i r a t i o n or n o n - p r e d a t o r y death rate o f p o p u l a t i o n X [ m g C ( m g C ) "  PP  p h o t o s y n t h e t i c rate [ m g C ( m g C h l - a ) " d" ] 1  1  d" ] 1  1  111  Equation 3.7 P h y t o p l a n k t o n density i s a s s u m e d to b e s i m p l y a f u n c t i o n o f the s e a s o n a l l y v a r y i n g c a r b o n to-chlorophyll-a  ratio  x  (  Subsection  s e e  3.2.2.)  multiplied  b y the constant  c o n c e n t r a t i o n PCM o f 0 . 4 m g C h l - a m " f o r t h e N o r t h e a s t P a c i f i c ( W o n g et al. 3  chlorophyll-a 1995).  Further,  b e c a u s e s e a s o n a l l y the p h y t o p l a n k t o n c a r b o n c o n c e n t r a t i o n increases as a m a x i m u m f r o m 3 . 6 t o 36 m g C m"  3  (see S e c t i o n 3.2.2:  Carbon-to-Chlorophyll-a  Ratio),  p h o t o s y n t h e t i c rate i s g e n e r a l l y h i g h (e.g. 6 0 m g C ( m g C h l - a ) " (Welschmeyer  et al.  1  a n d because the observed  d" f o r Station P i n summer 1  1993)), the necessary increase i n phytoplankton c a r b o n concentration f r o m  the c a r b o n - t o - c h l o r o p h y l l - a ratio i s n e g l i g i b l e w i t h respect t o total p r i m a r y p r o d u c t i o n . H e n c e , I a s s u m e that a l l p r i m a r y p r o d u c t i o n is i m m e d i a t e l y a s s i m i l a t e d b y h e r b i v o r e s . Thus, phytoplankton standing stock is completely controlled b y herbivorous zooplankton i n that a n y i n c r e a s e i n p h y t o p l a n k t o n  c a r b o n c o n c e n t r a t i o n a b o v e t h e g r a z i n g t h r e s h o l d X^CM  is  i m m e d i a t e l y g r a z e d b y m i c r o z o o p l a n k t o n . M i c r o z o o p l a n k t o n g r o w t h rates o f u p t o m o r e t h a n 5 doublings  d" (Miller 1  et  1991b) m a k e  al.  d o u b l i n g d " ( W e l s c h m e y e r et al. 1  T h e a s s u m p t i o n o f a X^CM grazing  threshold  microzooplankton phytoplankton  control o f phytoplankton,  with growth  rates o f 1  1993), plausible even for l o w m i c r o z o o p l a n k t o n densities.  grazing threshold f o r herbivores is more difficult to justify. T h e  c o u l d arise f r o m patterns detection o f phytoplankton  a n d p r o c e s s e s at t h e m i c r o s c a l e : F o r e x a m p l e , c o u l d b e temperature  c a r b o n c o n c e n t r a t i o n s , i . e . X^CM,  i n summer  dependent  i n that  associated w i t h higher  higher  molecular  diffusivity due to increased temperature smear gradients o f organic c o m p o u n d s and thus m a k e it m o r e difficult to locate a n individual phytoplankton cell. O r , phases o f high a n d l o w feeding activity  by  herbivorous  microzooplankton  in  response  to  microscale  predation  m e s o z o o p l a n k t o n , c o u l d - v i a cascading effects o f predation pressure - provide temporal  by  refuges  112  f o r p h y t o p l a n k t o n ( C . W a l t e r s 1 9 9 8 p e r s . c o m m . ; s e e a l s o W a l t e r s et al. 1993). H o w e v e r ,  these arguments  1997; Walters & Juanes  are m e r e s p e c u l a t i o n s b e c a u s e o b s e r v a t i o n s o n s p a t i a l a n d  t e m p o r a l m i c r o s c a l e s i n the ocean e n v i r o n m e n t  are l o g i s t i c a l l y e x t r e m e l y d i f f i c u l t o r  simply  i m p o s s i b l e (in the sense o f H e i s e n b e r g ' s uncertainty principle), and lab e x p e r i m e n t s m a y m i m i c natural conditions (J. M i t c h e l l 1997  pers. c o m m . ,  R.  Luchsinger  1998  pers.  not  comm.).  N e v e r t h e l e s s , the f i r m e m p i r i c a l e v i d e n c e for a y e a r - r o u n d constant c h l o r o p h y l l - a c o n c e n t r a t i o n in  the  Northeast  Pacific  (e.g.  Parsons  &  p h e n o m e n o l o g i c a l g r a z i n g t h r e s h o l d o f X^CM,  Lalli  1988;  Wong  et  al.  1995)  implies  a  w h e t h e r its a s p a t i o - t e m p o r a l p r e d a t i o n effect, or  arises f r o m s y s t e m d y n a m i c a l or other b i o l o g i c a l causes. F r o m a h e u r i s t i c p o i n t o f v i e w , it is i m p o r t a n t to note that i n e n v i r o n m e n t a l l y - d r i v e n , s c a l e s p a t i a l l y - e x p l i c i t , n o n - e q u i l i b r i u m s i m u l a t i o n s , p a r a m e t e r a n d state v a r i a b l e  large-  combinations  m a y o c c u r t h a t w i l l e r a d i c a t e t h e p r i m a r y e n e r g y s o u r c e , a n d w h i c h m a y n o t b e o b v i o u s at a l l n o r e a s i l y detectable (for the u n p r e d i c t a b l e d y n a m i c s o f m u c h s i m p l e r systems see M a y u s e o f a d o n o r - c o n t r o l l e d b i o m a s s f l o w at t h e t r a n s f e r f r o m p h y t o p l a n k t o n t o w i l l prevent the p r i m a r y  energy  1976b). The  microzooplankton  source f r o m b e c o m i n g l o c a l l y extinct and thus represents  a  m e t h o d o l o g i c a l save-fail design ( H o l l i n g 1976).  Equation 3.8 In the first t e r m o n the r i g h t h a n d s i d e o f E q . 3.8 I a s s u m e that t h r o u g h o u t the y e a r a l l p r i m a r y production is immediately assimilated by herbivores. I hereby f o l l o w s u m m e r Booth  etal. ( 1 9 9 3 ) ,  observations  by  w h o c o n c l u d e d that the g r a z i n g c a p a c i t y o f h e t e r o t r o p h i c f l a g e l l a t e s a v e r a g e d  1 0 0 % o f the total p r i m a r y p r o d u c t i o n . B e c a u s e m i c r o z o o p l a n k t o n has h i g h e r g r o w t h rates than their  food  source  (Miller  et  al.  1991b)  the  effects o f  microzooplankton  biomass  on  food  113  c o n s u m p t i o n c a n b e n e g l e c t e d , i.e. d o n o r - c o n t r o l l e d p r e y c o n s u m p t i o n . T h u s w h e n P a n d H are i n e q u i l b r i u m E q . 3.8 f o l l o w s f r o m the s i m p l e fast v a r i a b l e a n a l y s i s :  where  P  and  H  ^  = b P -a (P-P )H  (Eq. 3.11a)  ^  = a (P-P )H-m H  (Eq. 3.11b)  p  0  PH  PH  are phytoplankton  0  0  H  a n dzooplankton  standing  stocks,  r e s p e c t i v e l y , a n d Po  represents the p h y t o p l a n k t o n t h r e s h o l d that c a n n o t b e g r a z e d b y h e r b i v o r o u s  microzooplankton  (parameters see T a b l e 3.2). A t e q u i l i b r i u m :  fti  P =^-+P e  (Eq. 3.11c)  0  PH  A  H ^ - ^ m  ( E q . 3.1 I d )  H  R e t u r n e d i n t o r a t e e q u a t i o n 3.1 l b :  dH m -r = a (—+Po-Po) at a  bP — -m H m  M  p  PH  H  PH  0  ( E q . 3.1 l e )  H  dH — = b P -m H at P  n  H  q . e . d . ( E q . 3.1 I f )  T h e s e c o n d t e r m o n the right side o f E q . 3.8 a c c o u n t s f o r r e s p i r a t i o n a n d n o n - p r e d a t o r y l o s s e s w h e r e m # i s a t e m p e r a t u r e d e p e n d e n t v a r i a b l e (see S e c t i o n 3.4.). T h e t h i r d t e r m s t a n d s f o r l o s s e s due t o predation b y m e s o z o o p l a n k t o n w i t h a T y p e H I f u n c t i o n a l response f o r the f u n c t i o n a l r e l a t i o n s h i p b e t w e e n p r e y c o n s u m p t i o n p e r predator p e r t i m e , i.e. the s p e c i f i c p r e d a t i o n rate, a n d prey density (Fig. 3.10).  114  Fig. 3.10: Possible functional responses of a predator to prey density (see text). Type I: Prey consumption per predator per time rises linearly up to a maximum, where further increase in prey density has no effect on the specific predation rate. Type II: Functional response follows the equation: aP  with f(P) prey consumption per predator per time, i.e. specific predation rate, a maximum specific predation rate, P prey density, and K the half-saturation constant, i.e. the prey density at which the specific predation rate equals all. Type HI: Functional response follows the equation: aP  2  with variables as for Type II. Values used to generate particular graphs: a = 1, AT = 0.5.  115  O r i g i n a l l y , the T y p e m  functional response w a s derived f r o m b e h a v i o r a l responses o f the  p r e d a t o r to v a r i a t i o n i n p r e y d e n s i t y ( H o l l i n g 1965), i.e. an i n c r e a s e i n the rate o f e f f e c t i v e s e a r c h (C.  Walters  consequence  1994 of  pers. c o m m . increased  ) or a decrease i n handling  availability  of  prey  organisms.  time  ( B e g o n et  al.  Phenomenologically  1990) a  as  a  Type  m  f u n c t i o n a l r e s p o n s e p r o v i d e s a p a r t i a l r e f u g e f o r p r e y o r g a n i s m s , i.e. a l o w s p e c i f i c p r e d a t i o n rate at l o w  prey  density.  Further,  d i s t r i b u t e d i n s p a c e ( B e g o n et  partial refuges al.  1990)  also occur whenever  prey  density  is  patchily  a n d p l a n k t o n i s u s u a l l y p a t c h i l y d i s t r i b u t e d at s c a l e s  greater t h a n 1 m (e.g. L e v i n 1 9 9 2 ; Steele 1980). A s a result the m a t h e m a t i c a l f o r m o f a T y p e  in  f u n c t i o n a l r e s p o n s e i s s u f f i c i e n t to a c c o u n t f o r the p a t c h y d i s t r i b u t i o n o f p l a n k t o n ( S t e e l e 1 9 8 5 ) . On  the  other  hand,  considerations (Holling  the 1959)  Type  U  functional  and does not show  response a low  was  derived  from  time-budget  s p e c i f i c p r e d a t i o n r a t e at l o w  prey  d e n s i t y ( s e e F i g . 3 . 1 0 ) . I n g e n e r a l , a T y p e II f u n c t i o n a l r e s p o n s e h a s a d e s t a b i l i z i n g w h i l e a T y p e LTI f u n c t i o n a l r e s p o n s e h a s a s t a b i l i z i n g e f f e c t o n p o p u l a t i o n d y n a m i c s . H o w e v e r , t h e s e e f f e c t s " d e p e n d o n the extent to w h i c h c o n s u m p t i o n rate accelerates or decelerates o v e r the r a n g e d e n s i t i e s n o r m a l l y e x p e r i e n c e d b y t h e p r e y p o p u l a t i o n . " ( B e g o n et al.  of  1990)  Equation 3.9 T h e first t e r m o n the right h a n d side o f E q . 3.9 stands f o r the a s s u m e d c o m p l e t e a s s i m i l a t i o n o f m i c r o z o o p l a n k t o n H,  i.e. n o energetic l o s s e s d u e to e g e s t i o n . A g a i n the s e c o n d t e r m represents  respiration and non-predatory  l o s s e s , w h e r e mci  d e p e n d e n c e f o r t h e s p e c i f i c r e s p i r a t i o n r a t e mcj  is a function of temperature and size. T h e size is different f o r M o d e l s 1 a n d 2 (see S e c t i o n 3.4.).  T h e t h i r d t e r m i n e q u a t i o n 3.9 denotes losses due to predation b y m a c r o z o o p l a n k t o n . F o r  Model  116  1 (Fig. 3.1)macrozooplankton density C  2  is zero b y definition w h i c h makes  mesozooplankton  loss due topredation zero.  Equation 3.10 T h e first t e r m o n the right h a n d side o f E q . 3.10 stands for the a s s u m e d c o m p l e t e a s s i m i l a t i o n of  ingested mesozooplankton,  where  the specific  predation  rate  again  follows  a Type  functional response. T h e second term denotes respiration and non-predatory losses w i t h f u n c t i o n o f t e m p e r a t u r e a n d s i z e . F o r M o d e l 1 ( F i g . 3.1) m a c r o z o o p l a n k t o n d e n s i t y C  2  nici  m as a  iszero b y  d e f i n i t i o n w h i c h m a k e s E q . 3.10 e q u a l to zero.  N o t e t h a t f o r t h e s y s t e m o f d i f f e r e n t i a l e q u a t i o n s i n T a b l e 3.1 u p p e r m o d e l c l o s u r e i s d e n s i t y i n d e p e n d e n t . D e n s i t y - d e p e n d e n c e states that the s p e c i f i c b i r t h , g r o w t h , d e a t h , a n d m i g r a t i o n rates are r e l a t e d t o p o p u l a t i o n density, w h i c h i n m a n y cases a r e n o t ( K r e b s frequently modeled  applied density dependent population  densities  1995). A l t h o u g h the  closure i s a convenient mathematical f o r m to regulate  a n d thus  prevent  numerical  explosion  o f state  variables,  m e c h a n i s m s l i m i t i n g z o o p l a n k t o n p o p u l a t i o n s (e.g. f o o d shortage, p r e d a t i o n , a l l e l o p a t h y ) are not w e l l s t u d i e d (see a l s o a r g u m e n t s  in  Equation 3.7  above). T h e a s s u m p t i o n f o r the system o f  d i f f e r e n t i a l e q u a t i o n s i n T a b l e 3.1 i s t h e f o l l o w i n g : I n t h e n a t u r a l w o r l d t h e r e e x i s t p r e d a t o r s t h a t feed o n the highest m o d e l e d trophic level. These predators themselves c a n b e c o m e prey for e v e n h i g h e r predators ( R i c e 1995) a n d thus a v o i d e x p o s u r e t o p r e d a t i o n r i s k (e.g. L i m a & D i l l W a l t e r s & J u a n e s 1 9 9 3 , s e e a l s o S e c t i o n 1.4. a n d C a r p e n t e r et al.  1990;  1 9 8 5 ) . C o n s e q u e n t l y t h e y are  u n a b l e t o regulate p o p u l a t i o n d e n s i t y o f the highest m o d e l e d t r o p h i c l e v e l (see M o d e l s 1 a n d 2 i n F i g . 3.1) w h i c h is t h e n l i m i t e d b y d e n s i t y - i n d e p e n d e n t factors.  117  3.3.1. Point Equilibria and Stability Analysis P o i n t e q u i l i b r i a ( s u b s c r i p t e) f o r t h e s y s t e m o f d i f f e r e n t i a l e q u a t i o n s i n T a b l e 3 . 1 c a n b e f o u n d b y setting:  dH  dG  —  = —  at  at  dC,  1  = — ^ = 0  (Eq. 3.12a)  at  Case 1: H>0, C, = C = 0 2  bP P  H  e  0  (Eq. 3.12b)  = - ^  Case 2: H>0, Ci>0, C = 0 (Model 1) 2  K  2  HC\  H. = HC  r  l  C=  —  le  (Eq. 3.12c)  — m  a  (b P -m H ) P  0  H  (Eq. 3.12d)  e  Case 3: H>0, Cj>0, C >0 (Model 2) 2  m  K  2  r C  "c,c H  e  2  (Eq. 3.12e)  C\Cl  ~c m  2  2  i s the s o l u t i o n o f the c u b i c equation:  h P V  2  P 0 ^HC  u  H - — 3  {b P ~a C )H  +  2  P  0  HC  u  K H2  HC  r  J  1  m  = 0  (Eq. 3.12f)  u  m C2  V  2  A V  / / C ,  _i_ +  t  JJ  2  i  e  m  C\  (Eq.3.12g)  118  F o r simple systems o f simultaneous homogeneous e q u a t i o n s (e.g. L o t k a 1 9 2 5 ; V o l t e r r a 1926) usually be done analytically (Murray  l -order nonlinear ordinary s t  differential  local stability analysis of e q u i l i b r i u m values  1993; R e n s h a w  1991; Y o d z i s  1989). T h e  can  mathematical  m e t h o d s i n v o l v e f i n d i n g the e q u i l i b r i u m values o f the s y s t e m o f n o n l i n e a r d i f f e r e n t i a l equations, l i n e a r i z i n g the s y s t e m , a n d s o l v i n g the l i n e a r i z e d s y s t e m b y c a l c u l a t i n g the e i g e n v a l u e s o f the community  m a t r i x at e q u i l i b r i u m v a l u e s ( B r o w n  &  Rothery  1993). T h e  s i g n o f the  largest  e i g e n v a l u e t h e n d e t e r m i n e s the d y n a m i c s o f a s y s t e m i n e q u i l i b r i u m that i s l o c a l l y p e r t u r b e d ( P i m m 1982), i.e. i f o n e e i g e n v a l u e i s p o s i t i v e the e q u i l i b r i u m i s u n s t a b l e . Further, l o c a l stability does not i m p l y g l o b a l stability, w h e r e a g l o b a l l y stable s y s t e m is d e f i n e d as o n e t h a t r e t u r n s t o e q u i l i b r i u m v a l u e s f r o m a n y i n i t i a l c o n d i t i o n s n o t j u s t c l o s e t o e q u i l i b r i u m values ( P i m m 1982), nor does local instability i m p l y global instability ( M a y e.g. stable l i m i t c y c l e s m a y  occur. A n a l y t i c a l techniques to determine  whether  1972a),  a system  d i f f e r e n t i a l e q u a t i o n s i s g l o b a l l y stable are s c a r c e a n d i n v o l v e f i n d i n g the L y a p u n o v  of  function,  w h i c h is s o d i f f i c u l t to d e t e r m i n e a n d interpret f o r m u l t i s p e c i e s m o d e l s that this a p p r o a c h is o n l y of l i m i t e d use ( P i m m 1982; R e n s h a w (Rosenzweig  &  M a c Arthur  1963)  and  1991). T h u s , w h i l e there is a w h o l e array o f g r a p h i c a l analytical mathematical procedures  available for  a n a l y s i s o f s i m p l e m o d e l s i n v o l v i n g o n e predator a n d one p r e y (e.g. B r o w n & C a s w e l l 1989; M u r r a y  1993; P i m m  1982; R e n s h a w  Rothery  the  1993;  1991; Y o d z i s 1989) m a t h e m a t i c a l l y exact  m e t h o d s f o r c o m p l e x m o d e l s r e m a i n scarce a n d one has to rely o n s i m u l a t i o n s instead ( L e v i n al.  et  1997). F i g . 3.11 a n d 3.12 s h o w the n u m e r i c a l stability analyses f o r p o p u l a t i o n M o d e l s 1 a n d 2  ( F i g . 3 . 1 , T a b l e 3.1).  119  Stab. Anal.: H disturbed  -H •C1  200  Fig. 3.11: Stability analysis of the 3-Trophic-Levels Model. Simulation runs with equilibrium state variables up to Time = 1 0 when perturbation of the respective state variable occurs. Magnitude of perturbation: Upper panel: 10-fold increase in microzooplankton (H). Lower panel: 90% decrease of mesozooplankton. Simulations run with standard run parameter values (Table 3.3), and biotic and abiotic environmental conditions at Station P in June (see Fig. 3.2 and 3.14), with b P = 10 mg C m' d" and m 0.5. Note different scales on the ordinates. 3  P  1  =  H  120  Stab. Anal.: C1 disturbed 25  2.5  .1 20 I 15  2 -  m CM  1 0.5  o  °. x§ o  - '"T"—i 50  1 1 — ! 100  1 1 150  1.5  Ij  8 5J a  -H -C2 •C1  200  Time  Stab. Anal.: C2 disturbed  Fig. 3.12: Stability analysis of the 4-Trophic-Levels Model. Simulation runs with equilibrium state variables up to Time =10 when perturbation of the respective state variable occurs. Magnitude of perturbation: Upper panel: 10-fold increase in microzooplankton (H). Middle and lower panel: 90% decrease in mesozooplankton or macrozooplankton respectively. Simulations run with standard run parameter values (Table 3.3), and biotic and abiotic environmental conditions at Station P in June (see Fig. 3.2 and 3.14), with b P = 10 mg C nr d" and m = 0.5. Note different scales on the ordinates. 3  P  1  H  121  F o r s t a n d a r d r u n p a r a m e t e r v a l u e s ( T a b l e 3.3) the because the microzooplankton  rate e q u a t i o n  microzooplankton concentrations  H.  (Eq.  Case 1  e q u i l i b r i u m point is g l o b a l l y stable  3.8) h a s a negative  U s i n g mathematical software  Case 2  s h o w that f o r p o s i t i v e b i o m a s s concentrations,  slope f o r a l l positive  (Maple V),  analytical results  has one solution w i t h t w o negative real  e i g e n v a l u e s a n d is thus stable. For C  2  Case 3  analytical results c o n f i r m the linear stability o f the e q u i l i b r i u m points:  H, Cj,  and  h a v e one real a n d t w o c o m p l e x roots ( f r o m the s o l u t i o n o f the c u b i c e q u a t i o n E q . 3.12f), a n d  three negative real eigenvalues. I n fact, H  e  w i l l have one real root and t w o c o m p l e x roots for any  c o m b i n a t i o n o f p a r a m e t e r s as l o n g as t h e y s a t i s f y t h e c o n d i t i o n ( B . B e r g e r s e n 1 9 9 8 p e r s . c o m m . ) :  d = r +q >0 2  (Eq. 3.12h)  3  where,  2a -9ab 3  + 21c  r =  a  2  ( E q . 3.121)  -3b  q = —-rj-  (Eq.3.12j)  a n d a, b, a n d c a r e t h e c o e f f i c i e n t s i n t h e c u b i c e q u a t i o n E q . 3 . 1 2 f . I s h o u l d m e n t i o n , that the s t a b i l i t y c o n c e p t e m e r g e d f r o m a n e q u i l i b r i u m v i e w o f s y s t e m s ( S h u b i k 1996) a n d that w h i l e s o m e b i o l o g i c a l c o m p o n e n t s o f a n e c o s y s t e m m i g h t h a v e  evolved  r e g u l a t i o n m e c h a n i s m s that are s t a b i l i z i n g t o p o p u l a t i o n s a n d c o m m u n i t i e s , o t h e r parts h a v e a d o p t e d d i f f e r e n t strategies w h i c h are at the m e r c y o f e n v i r o n m e n t a l  could  f o r c i n g s and i n fact  r e q u i r e n o n - e q u i l i b r i u m c o n d i t i o n s t o s u b s i s t (see S t e e l e 1 9 7 4 ; S t e e l e 1 9 8 0 ; S t e e l e 1 9 9 1 ; S t e e l e & Henderson  1994).  122  A s c a n b e calculated f r o m standard r u n parameter values (Table 3.3), a n df r o m 1981 to 1 9 8 4 e n v i r o n m e n t a l f o r c i n g s at S t a t i o n P ( 5 0 ° N 1 4 5 ° W ) a n d a t 5 0 ° N 1 3 0 ° W ( s e e S e c t i o n 3 . 2 . a n d 3 . 4 ) , a n d p r e s u m a b l y at m a n y o t h e r l o c a t i o n s i n t h e N o r t h e a s t P a c i f i c , e q u i l i b r i u m v a l u e s f o r C i (Eq.  3.12d) i n the 3-Trophic  between  October  and  environmentally-driven  Levels M o d e l  April.  parameters,  (Fig. 3.1, Table  Consequently,  equilibrium  as d e m o n s t r a t e d  3.1) c a n assume negative model  by Walters  et  simulation al.  (1987)  sign  through  for a  lake  ecosystem, cannot b y applied for the Northeast Pacific. Negative equilibrium values f o r C i a n d C  2  d o n o t o c c u r f o r the 4 - T r o p h i c - L e v e l s M o d e l . H o w e v e r , f o r a d v e c t i v e s y s t e m s a n e q u i l i b r i u m  approach w i l l b e justified only i f the local equilibration processes are m o r e rapid than the c h a n g e s c a u s e d b y a d v e c t i o n . T h i s i s n o t the case i n s i m u l a t i o n s that i n c l u d e currents  (Chapter  4).  123  3.4. Mean Field Simulations T h e k e y a s s u m p t i o n s o f the m e a n f i e l d a p p r o a c h i s that o n e p o i n t i n s p a c e , i.e. the m e a n f i e l d , is representative o f patterns a n d processes o v e r a m u c h larger area. O f c o u r s e , this i s n o t t o s a y that a l l l o c a t i o n s i n the N o r t h e a s t P a c i f i c e x p e r i e n c e t h e s a m e t i m i n g o f e v e n t s ; rather i n o r d e r t o tune the p o p u l a t i o n m o d e l s I ignore the spatial v a r i a b i l i t y i n a b i o t i c e n v i r o n m e n t a l f o r c i n g s i n the N o r t h e a s t P a c i f i c . B e c a u s e o f a l o n g t e r m a n d s t i l l o n g o i n g s a m p l i n g p r o g r a m at S t a t i o n P ( 5 0 ° N 145°W) I have  c h o s e n this l o c a t i o n as the reference p o i n t  for m y mean  field  simulations.  A d d i t i o n a l l y , f o r the purpose o f spatial c o m p a r i s o n I ran m e a n f i e l d s i m u l a t i o n s f o r the l o c a t i o n at 5 0 ° N 1 3 0 ° W , i . e . a p o s i t i o n a t t h e s a m e l a t i t u d e a s S t a t i o n P b u t a p p r o x i m a t e l y  1000k m  closer t o the C a n a d i a n coast. T h e b i o l o g i c a l variables i n the mean field simulations are driven b y abiotic  environmental  d a t a (see S e c t i o n 3.2.) f o r b o t h stations f o r the p e r i o d 1 9 8 1 t o 1984, w i t h a t i m e s t e p o f o n e d a y . Input data are: daily s e a surface insolation, m i x e d layer depth, m i x e d layer temperature, the e x t i n c t i o n c o e f f i c i e n t , d a y l e n g t h , a n d the c a r b o n - t o - c h l o r o p h y l l - a ratio. I n general, o b s e r v e d as w e l l as d e r i v e d i n p u t data h a v e a t e m p o r a l r e s o l u t i o n o f o n e m o n t h (values a s s i g n e d t o m i d month) w i t h linear interpolation between month f o r the o n e d a y timestep. U n i t s o f biomass c o n c e n t r a t i o n s are [ m g C m~ ]. 3  Physical structure T h e m o d e l ecosystem f o r the Northeast Pacific consists o f a m i x e d upper layer a n d a deep layer. B i o l o g i c a l processes are assumed to o c c u r only i n the m i x e d upper layer w h o s e  depth  varies seasonally. T h e underlying deep water layer i s assumed to b e v o i d o f life. H o r i z o n t a l a n d v e r t i c a l a d v e c t i o n a n d d i f f u s i o n are i g n o r e d (see S u b s e c t i o n 3.2.2). F u r t h e r , I a s s u m e that t h e  124  m i x e d l a y e r i s h o m o g e n e o u s l y m i x e d a n d t h a t m i x i n g t a k e s p l a c e at v e r y s h o r t t i m e s c a l e s s o t h a t b i o l o g i c a l p r o d u c t i o n at a n y d e p t h i s i m m e d i a t e l y The  above  assumptions  evidently  represent  homogenized. gross  simplifications: In the natural  phytoplankton persists a n d reproduces b e l o w the thermocline (Frost zooplankton  species undertake  diel and/or  seasonal and/or  system  1987) and m a n y fish and  ontogenetic  migrations  to  depth  ( M a n g e l & C l a r k 1988); some spend their entire life i n deep water (see S e c t i o n 2.2). A l s o , the s e a s o n a l t h e r m o c l i n e (the b o u n d a r y b e t w e e n the m i x e d u p p e r l a y e r a n d the deep w a t e r layer) i s not  a  step  function  phytoplankton  b u t rather  a  smooth  transition  zone  ( F i g . 2.8). D i l u t i o n  a n d zooplankton concentrations due to deepening  effects  on  o f m i x e d layer are a s s u m e d  n e g l i g i b l e because changes due to b i o l o g i c a l processes are m u c h larger i n magnitude.  However,  losses o f total b i o m a s s i n the water c o l u m n due to a s h a l l o w i n g o f the m i x e d layer have  been  accounted for i n the simulations.  Phytoplankton T h e b i o l o g i c a l process primarily affected b y abiotic environmental forcings i n the Northeast P a c i f i c O c e a n i s p r i m a r y p r o d u c t i o n , i.e. p h y t o p l a n k t o n g r o w t h rate The  photosynthetic  rate  of phytoplankton  [mg C  (pp,  (mg  (bpPo i n E q . 3 . 8 , [ m g C d " ] ) .  Chl-a)"  1  1  d" ] 1  is a function  of  p h o t o s y n t h e t i c a l l y a c t i v e r a d i a t i o n at d e p t h z (IPAR, [ J n i " d " ] ) : 2  z  1  ^ al PP = Pp.™  tann  M  1  ( E q . 3.13a)  PAR,z  V rp,max J where  pp,max i s t h e m a x i m u m p h o t o s y n t h e t i c r a t e ( t h e v a l u e o f pp at l i g h t s a t u r a t i o n ) , a n d  photosynthetic efficiency a ([mg C (mg Chl-a)" photosynthesis  vs. irradiance curve  1  (Jassby &  d" (J m " ) ] ) represents the initial slope o f the 1  Piatt  2  _ 1  1976). E q . 3 . 1 3 a i s the equation  for a  125  h y p e r b o l i c tangent a n d w a s f o u n d t o b e the best f i t t o e x p e r i m e n t a l p h o t o s y n t h e s i s v s . i r r a d i a n c e data o f eight different equations tested b y Jassby & Piatt (1976), a n d thus w a s preferred i n m y s i m u l a t i o n s o v e r o t h e r f o r m u l a t i o n s ( e . g . M i c h a e l i s - M e n t e n e q u a t i o n ( P i a t t et al. 1 9 8 1 ) , function (Smith  1936)). N o t e  that E q . 3 . 1 3 a d o e s n o t a c c o u n t f o r p h o t o - i n h i b i t i o n  Smith at h i g h  i r r a d i a n c e l e v e l s . H o w e v e r , b e c a u s e p r i m a r y p r o d u c t i o n w a s s u m m e d o v e r d e p t h at 1 m i n t e r v a l s s t a r t i n g at z = - 1 m , a n d b e c a u s e n o p r i m a r y p r o d u c t i o n b e l o w t h e t h e r m o c l i n e w a s a s s u m e d , t h e effects o fphoto-inhibition o n primary production are l i k e l y to b e compensated f o r i n the m o d e l . I n g e n e r a l , e q u a t i o n s d e s c r i b i n g p h o t o - i n h i b i t i o n e f f e c t s ( e . g . P i a t t et al. 1 9 8 1 ; S t e e l e 1 9 6 2 ) a r e only  rarely used i n ecosystem models  ( e . g . K a w a m i y a et al.  1995) simply because  i n h i b i t i o n r e q u i r e s t h e e s t i m a t i o n o f at l e a s t o n e m o r e i n d e p e n d e n t p a r a m e t e r ( P i a t t et al. T h e m a x i m u m p h o t o s y n t h e t i c rate s p e c i f i c g r o w t h rate f o r p h y t o p l a n k t o n  (pp  >max  (b  ,  P>max  [mg C (mg Chl-a)" [mg C ( m g C)"  1  d" ]) i s r e l a t e d t o the  1  1  photo1981).  maximum  d" ] b y the c a r b o n - t o - c h l o r o p h y l l - a 1  r a t i o (x [ m g C ( m g C h l - a ) - ] ) : 1  ( E q . 3.13b)  Pp,nM*=Xb .u P  Further,  bp  <tmx  m  i s a f u n c t i o n o f t h e d a i l y d o u b l i n g r a t e (bp^oubi [ d o u b l i n g s d " ] o f p h y t o p l a n k t o n 1  ( E q . 3 . 1 3 c ) , w h i c h i t s e l f i s a f u n c t i o n o f t e m p e r a t u r e (7/; E q . 3 . 1 3 d ( E p p l e y  **  b.  M  P  1972)):  =(2*'-*--l)fj  ( E q . 3.13c)  0 - 8 5 1 -10°  (Eq. 3.13d)  =  doubl  0 2 7 5 7  A s E q . 3.13d w a scalculated f o r laboratory cultures under 24-hour illumination (Eppley  1972),  E q . 3 . 1 3 c t a k e s i n t o a c c o u n t p h o t o - r e s p i r a t i o n a n d i t s l a s t t e r m c o r r e c t s f o r d a y l e n g t h (Hd [ h ] ) . E q u a t i o n 3 . 1 3 d has a Q i v a l u e o f 1.88 ( v e r y c l o s e t o the d e f a u l t v a l u e Q i = 2 s u g g e s t e d b y v a n ' t 0  0  H o f f s t e m p e r a t u r e r u l e f o r e n z y m e r e a c t i o n s ( D e n f f e r et al. 1 9 8 3 ) ) .  126  Because  o f different  insolation  at different  depths  a n d the assumed  fast  mixing, the  phytoplankton concentration change due to primary production f o rthe m i x e d layer is then:  b P = - — ]p P p  0  P  ML  Z  (Eq. 3.13e)  dz  CM  z  ML  w h e r e p h y t o p l a n k t o n c o n c e n t r a t i o n i n u n i t s o f c a r b o n (Po [ m g C m " ] ) r e l a t e s t o c h l o r o p h y l l - a c o n c e n t r a t i o n (PCM [ m g C h l - a m " ] ) b y : 3  (Eq.3.13f)  P = XP 0  CM  T h e f u l l e q u a t i o n f o r t h e g r o w t h r a t e o f p r i m a r y p r o d u c e r s p e r v o l u m e (bpP i n E q . 3 . 8 ) i n t h e m i x e d upper layer is:  C bP P  0  7  "ML  J Z  .  f  (2°-  85II0  °  0  2  7  5  r  v  -l)^tanh ' 24  \ al v  X  / ' 0 . 8 5 1 10 9  PAR,z 00 2 7 5 r  \ dz  _ A  H  (Eq.3.13g)  d  >2 4 ;  [  Zooplankton F o r t h e z o o p l a n k t o n rate equations ( E q s . 3 . 8 - 3 . 1 0 ) parameters u n d e r e n v i r o n m e n t a l c o n t r o l are t h e s p e c i f i c r e s p i r a t i o n o r n o n - p r e d a t o r y  death rates m  x  (X s t a n d s f o r t h e p o p u l a t i o n ) .  H e r b i v o r e r e s p i r a t i o n r a t e mu [d" ] w a s d e f i n e d a s a f u n c t i o n o f t e m p e r a t u r e (T): 1  H  M  ( E q . 3.14a)  = H,o °' M  2  lT  w h e r e m ,o r e p r e s e n t s t h e s p e c i f i c r e s p i r a t i o n o r n o n - p r e d a t o r y H  death rate at 0°C. T o p r o d u c e  m i c r o z o o p l a n k t o n g r o w t h e f f i c i e n c i e s ( d e f i n e d as p r o d u c t i o n p e r u n i t i n g e s t e d f o o d o r energy) s i m i l a r t o t h o s e f o u n d i n n a t u r e ( P a r s o n s & L a l l i 1 9 8 8 ) m ,o w a s s e t a t 0 . 2 5 f o r t h e s t a n d a r d r u n , H  i . e . a g r o w t h e f f i c i e n c y o f m i c r o z o o p l a n k t o n o f 5 0 % a t 10°C. T h e e x p o n e n t i n E q . 3 . 1 4 a r e v e a l s the a s s u m p t i o n that Q i o = 2 .  127  T h e r e s p i r a t i o n rate f o r m e s o z o o p l a n k t o n  (trici)  and macrozooplankton  (mci)  w a s d e f i n e d as  a f u n c t i o n o f b o d y - m a s s i n r e l a t i o n t o h e r b i v o r e r e s p i r a t i o n r a t e m#. U s i n g t h e m e a n l e n g t h o f t h e zooplankton  s i z e c l a s s e s ( P a r s o n s et al.  10.1 m m , m a c r o z o o p l a n k t o n :  110 m m )  1984; m i c r o z o o p l a n k t o n :  110u m , mesozooplankton:  and assuming half-sphere shaped organisms, respiration  per unit b o d y mass was calculated f r o m Eckert & Randall (1983):  (Eq. 3.14b)  m =aX^  25  x  w h e r e a i s a s p e c i e s - s p e c i f i c c o n v e r s i o n factor, here a s s u m e d t obe the same for a l l z o o p l a n k t o n ,  m  c  and X is b o d y mass. E q . 3.14b yields the ratios: —  m  L  H  1 mc 1 = — a n d — - = —. H o w e v e r , b e c a u s e i n 30 m 6 C ]  d y n a m i c f o o d c h a i n m o d e l s the natural t r o p h i c l e v e l s b e y o n d the s c o p e o f the m o d e l are f a c t u a l l y a s s u m e d c o l l a p s e d into the highest m o d e l trophic l e v e l , t o a c c o u n t f o r i m p l i c i t a d d i t i o n a l losses I d o u b l e d the m a s s - s p e c i f i c losses o f the highest m o d e l t r o p h i c l e v e l i n e a c h m o d e l . T h u s f o r the  m  m o d e l w i t h three trophic levels —  c L  m  H  1 mc l = — , a n d w i t h f o u r t r o p h i c l e v e l s — - = —. 15 m 3 C [  It m a y b e a r g u e d t h a t z o o p l a n k t o n b i o l o g y a n d l i f e h i s t o r y h a v e b e e n s i m p l i f i e d t o o m u c h i n M o d e l s 1 a n d 2 (Eqs. 3.8-3.10). F o r e x a m p l e , 8 0 - 9 5 % o f the total b i o m a s s o f m e s o z o o p l a n k t o n c o n s i s t s o f s p e c i e s that p e r f o r m o n t o g e n e t i c v e r t i c a l m i g r a t i o n s a n d h a v e rather c o m p l i c a t e d l i f e h i s t o r i e s ( M a c k a s et al.  1993; Parsons & L a l l i 1988; W a r e & M c F a r l a n e 1989; see also S e c t i o n  2 . 2 . ) , y e t , i n t h e s i m u l a t i o n s m e s o z o o p l a n k t o n i s r e p r e s e n t e d as a h o m o g e n e o u s  group inhabiting  the m i x e d upper layer. T h e s e s i m p l i f i c a t i o n s w e r e necessary because there a r eessentially n o a n s w e r s ( R . G o l d b l a t t 1998 pers. c o m m . ) to the f o l l o w i n g t w o questions: (1)  What  Neocalanus  determines  the time  o f ascent  a n d descent  i n the ontogenetic  migration  of  species?  128  Neocalanus plumchrus  It h a s b e e n s h o w n t h a t t h e p e a k b i o m a s s i n  i n the Northeast P a c i f i c i n  the 1990s i s about 6 0 days earlier than i n the 1950s ( D . M a c k a s 1997 pers. c o m m . ) . Further, i t i s a s s u m e d that the e n v i r o n m e n t at d e p t h h a r d l y c h a n g e s o v e r d e c a d e s , a n d t h u s , that the b i o m a s s pattern i s rather a c o n s e q u e n c e o f the t i m e o f descent than ascent. F o r of descent i s given b y the developmental  1998  spp. the t i m e  stage o f the o r g a n i s m (stage 4 o r 5 c o p e p o d i t e s ,  d e p e n d i n g o n t h e s p e c i e s , m i g r a t e t o d e p t h ( M i l l e r et al. development  Neocalanus  1984)) w h i c h i t s e l f is d e t e r m i n e d b y the  rate, a c o m p l e x f u n c t i o n o f processes o c c u r r i n g i n surface waters ( R . G o l d b l a t t  pers. c o m m . ) .  Whether  andh o w food  supply  (R. Goldblatt  1998  pers. c o m m . ) ,  total  temperature e x p o s u r e ( D . M a c k a s 1997 pers. c o m m . ) , s t a g e - s p e c i f i c m o r t a l i t y rates ( D . M a c k a s 1997 pers. c o m m . ) ,  o r other factors (e.g. p r e d a t i o n pressure) d e t e r m i n e  development  rate i s  unknown. (2) W h a t d e t e r m i n e s the s u r v i v a l o f  Neocalanus  s p e c i e s at d e p t h ?  E v e n less i s k n o w n t o a n s w e r this question. Intuition (under the density dependent p a r a d i g m ) suggests that i t i s l i k e l y that the n u m b e r o f n a u p l i i that a s c e n d i n w i n t e r a n d s p r i n g i s c o r r e l a t e d w i t h t h e n u m b e r o f stage 5 c o p e p o d i t e s ( C ) that d e s c e n d e d t h e p r e v i o u s year, b u t that t h e 5  n u m b e r o f l a r v a e t h a t s u r v i v e t o C5 v a r i e s s i g n i f i c a n t l y f r o m y e a r t o y e a r . " O r m a y b e n o t . N o - o n e k n o w s . " ( R . G o l d b l a t t 1998 pers. c o m m . ) L a c k o f i n f o r m a t i o n a l s o a p p l i e s to the w h o l e g r o u p o f m a c r o z o o p l a n k t o n (see S e c t i o n 2.2.).  Although  vertical migration  c a n easily be implemented  i n simulations  b y brute-force,  especially i n the absence o f data, this approach i s l i k e l y t o m a k e m y s i m u l a t i o n s s h i m m y almost any w a y I w a n t , a rather p o o r m o d e l i n g practice. Further, because I try e x p l a i n the v a r i a b i l i t y i n sockeye salmon survival b y the variability i n mesozooplankton  a v a i l a b i l i t y t o the j u v e n i l e s ,  129  w h i c h i m p l i e s a critical p e r i o d during the winter m o n t h (see C h a p t e r 4 ) v e r t i c a l l y m i g r a t i n g m e s o z o o p l a n k t o n species are n o t available f o rj u v e n i l e s o c k e y e s a l m o n i n the Northeast P a c i f i c ( F i g . 2 . 6 ) a n d m a y thus not e v e n b e r e l e v a n t f o r the o b j e c t i v e o f this thesis (see C h a p t e r 1).  3.4.1. Simulation Results: 3-Trophic-Levels Model T h e 'standard r u n ' s i m u l a t i o n (Table 3.3) s h o w s a seasonally v a r y i n g p h y t o p l a n k t o n standing stock (in units o f carbon) corresponding to the seasonally varying carbon-to-chlorophyll-a ratio ( F i g . 3.13, F i g . 3.9). P h y t o p l a n k t o n c o n c e n t r a t i o n p e a k s i n J u n e t o J u l y , a n d i t i s a s s u m e d that throughout the year  1 0 0 % o f the daily primary  production is consumed b y the herbivorous  microzooplankton w h i l e the reproducing phytoplankton  standing stock i s n o tgrazed  c o n t r o l l e d f l o w ; S e c t i o n 3.3.). F o r b o t h l o c a t i o n s t h e s i m u l a t e d s u m m e r p h y t o p l a n k t o n  (donorstanding  stock i s close t o the observed phytoplankton carbon concentrations during the S U P E R - c r u i s e s ( S u b a r c t i c P a c i f i c E c o s y s t e m Research; M a y a n d A u g u s t 1984, 1988, June a n d September 1987 ( M i l l e r et al. 1 9 9 1 b ) ) w i t h a m e a n s t a n d i n g s t o c k o f 2 0 m g C m " a n d a m a x i m u m o f 7 4 ( B o o t h et 3  al.  1993). T h e annual herbivore peak i s a result o f p r i m a r y p r o d u c t i v i t y a n d g r a z i n g pressure  from  m e s o z o o p l a n k t o n a n d trails b e h i n d t h e p h y t o p l a n k t o n p e a k at S t a t i o n P b u t p r e c e d e s i t at 50°N 130°W. S u m m e r c o n c e n t r a t i o n s m i c r o z o o p l a n k t o n f o r b o t h l o c a t i o n s are c l o s e t o o b s e r v e d m e a n o f 1 5 m g C m " a t S t a t i o n P ( B o o t h et al. 1 9 9 3 ) . M i c r o z o o p l a n k t o n c o n c e n t r a t i o n s > 3 0 m g C m " 3  have  only  been  rarely  observed  (Booth  et  al.  1993)a n d the high  3  microzooplankton  concentrations i n 1981 (Fig. 3.13) are a n artifact o f the initial c o n d i t i o n s o f the s i m u l a t i o n s (see S u b s e c t i o n 3.5.1.).  130  Table 3.3:  ' S t a n d a r d R u n ' i n i t i a l c o n d i t i o n s a n d parameter v a l u e s f o r the m e a n f i e l d s i m u l a t i o n s  o f the 3 - a n d 4 - T r o p h i c L e v e l s M o d e l s .  Initial Values [mg C m" ] 3  3-Trophic Levels Model  4-Trophic Levels Model  f(Io)  f(Io)  Po* H  1  1  0.5  0.5  -  0.1  3-Trophic Levels Model  4-Trophic Levels Model  c  2  Parameter [Units] [ m g C ( m g C h l - a ) " d" ]  a  1  1  m .o [d" ] mci/niu mci/mci a i [ m g C ( m e C ) " d" ] H  1  HC  K ci acici H  K ic2 C  *  1  [ m g C m" ] [ m g C ( m g C ) " d" ] 3  1  1  [mgCm ]  Phytoplankton  3  standing  stock  96  96  0.25  0.25  1/15  1/30  -  1/3  0.2  0.2  25  25  -  0.4  -  5  is calculated  as 0 . 4 m g C h l - a  m"  3  times  the carbon-to-  c h l o r o p h y l l - a ratio, a f u n c t i o n o f sea surface i n s o l a t i o n i n the p r e v i o u s m o n t h (see  Subsection  3.2.2.).  131  Station P (50°N 145°W),1981-1984  Fig. 3.13:  Simulated carbon concentrations f o r phytoplankton (P), microzooplankton (H), a n d  m e s o z o o p l a n k t o n (CO a t S t a t i o n P ( 5 0 ° N 1 4 5 ° W ) a n d a t 5 0 ° N 1 3 0 ° W f o r 1 9 8 1 t o 1 9 8 4 .  Note  different scales o n the ordinates.  132  A t S t a t i o n P a s w e l l a s at 5 0 ° N 1 3 0 ° W s i m u l a t e d m e s o z o o p l a n k t o n r e a c h e s i t s p e a k b y l a t e September, considerably later than the observed m e a n peak o f 3 m g C m " ( m a x i m u m 2 0 m g C 3  m" ) 3  i n May-June  (Mackas  & Frost  1993).  Maximum  simulated  standing  stocks  exceed  m a x i m u m o b s e r v e d m e s o z o o p l a n k t o n c o n c e n t r a t i o n s b y about o n e o r d e r o f m a g n i t u d e at S t a t i o n P a n d e v e n m o r e f o r t h e n e a r - c o a s t l o c a t i o n at 5 0 ° N 1 3 0 ° W . T h i s d i s c r e p a n c y c a n b e c a u s e d b y the f o l l o w i n g : (1) B e c a u s e , i n t r o p h o d y n a m i c  models the natural trophic levels b e y o n d the scope o f the  m o d e l are aggregated into the highest m o d e l trophic l e v e l , a n d because total energy m u s t b e conserved, theb i o m a s s density o f the highest m o d e l trophic l e v e l i s actually e x p e c t e d t o e x c e e d observedrvalues.,Compare  simulatedmesozooplankton,de4isities.-of;the '3-Trophic -LevelsiModeL. i  i  (Fig. 3.13) a n d o f the 4 - T r o p h i c - L e v e l s M o d e l ( F i g . 3.17). (2) T h e l a c k i o f i n f o r m a t i o n . ' o n m e s o z o o p l a n k t o n s p e c i e s that p e r f o r m o n t o g e n e t i c v e r t i c a l :  m i g r a t i o n s has. f o r c e d : m e to^^exclude;.this, group,completely;(see.Section3.4., 1  Zooplankton).jItis*  c l e a r that these s p e c i e s c o n s u m e large quantities o f m i c r o z o o p l a n k t o n d u r i n g their stay i n o r near the m i x e d u p p e r l a y e r d u r i n g the first h a l f o f the year, z o o p l a n k t o n b i o m a s s that t h e n m i g r a t e s t o d e p t h . I n fact, s i m u l a t i o n results ( F i g . 3 . 1 3 ) suggest that t h e first s i x m o n t h s h o u l d b e t h e t i m e w h e n i n t e r s p e c i f i c r e s o u r c e - u s e c o m p e t i t i o n i s at i t s m i n i m u m , a n d a r e ' t h u s c o n s i s t e n t , w i t h t h e life-history strategy o f ontogenetically m i g r a t i n g m e s o z o o p l a n k t o n w h i c h c o u l d have adapted to exploit the m i c r o z o o p l a n k t o n surplus, w h i c h then is not available f o r the m i x e d upper  layer  community. (3) O b s e r v e d densities o f larger z o o p l a n k t o n are u s u a l l y r e p o r t e d i n g r a m s w e t w e i g h t m " . 3  B e c a u s e the units u s e d i n simulations are m g C m " a c o n v e r s i o n o f 1 g w e t w e i g h t = 0 . 0 4 g C 3  (see S u b s e c t i o n 2.2.1.) w a s u s e d f o r c o m p a r i s o n o f o b s e r v a t i o n s a n d s i m u l a t i o n r e s u l t s . 0 . 0 4 i s a  133  rather conservative estimate o f the w e t w e i g h t to c a r b o n c o n v e r s i o n factor, a n d f o r the N o r t h P a c i f i c a m e a n o f 0.10 a n d a m a x i m u m o f 0.24 have been reported f o r crustacean p l a n k t o n (see P a r s o n s et al.  ( 1 9 8 4 ) their T a b l e U ) . T h e s e higher estimates o f the c o n v e r s i o n f a c t o r r e d u c e t h e  d i s c r e p a n c y b e t w e e n s i m u l a t i o n results a n d observations b y a factor o f 2.5 a n d 6, respectively. S i m u l a t e d daily primary productivity (Figs. 3.14) is consistent w i t h observations  (compare  F i g s . 3 . 1 5 a n d F i g . 2 . 3 ) . N o t e that s i m u l a t e d w i n t e r p r i m a r y p r o d u c t i v i t y f o r S t a t i o n P i s l o w e r t h a n t h a t r e p o r t e d b y W o n g et al. ( 1 9 9 5 ) . H o w e v e r , t h e i r w i n t e r e s t i m a t e s a r e b a s e d o n o n l y t w o o b s e r v a t i o n s o n t w o c o n s e c u t i v e days i n late F e b r u a r y  1 9 8 9 (almost t w o m o n t h after t h e w i n t e r  solstice) a n d are thus l i k e l y t o b e b i a s e d t o w a r d s h i g h e r v a l u e s . P r i m a r y p r o d u c t i v i t y f o r the n e a r coast location is higher because o f higher sea summer  temperature  there ( F i g . 3.2) a n d thus  s h a l l o w e r m i x e d layer depth ( F i g . 3.6). 1981 t o 1984 s i m u l a t e d m e a n annual p r i m a r y  production  w a s 1 1 4 g C m " at S t a t i o n P ( a l l f o u r y e a r s l i e w i t h i n ± 5 % o f the m e a n ) , a l i t t l e l o w e r t h a n t h e 2  observed 1984 to 1991 mean o f 140 g C m  2  ( W o n g et al.  1995). Nevertheless, because the fall  and winter season hasonly been sampled 5 times during the seven year period, the reliability o f Wong  et  al.'s  (1995) estimate is uncertain.  1981 to 1 9 8 4 simulated m e a n  annual  primary  p r o d u c t i o n at 5 0 ° N 1 3 0 ° W w a s 1 5 1 g C m " . H e r e t o o , a l l f o u r y e a r s l i e w i t h i n ± 5 % o f the m e a n . 2  Simulated  microzooplankton  net production,  i.e. ingestion  minus  respiration  andnon-  p r e d a t o r y l o s s , f o r b o t h l o c a t i o n s i s p o s i t i v e t h r o u g h o u t t h e year ( F i g . 3.14). T h i s m e a n s that t h e decline i n m i c r o z o o p l a n k t o n b i o m a s s i n fall i s caused b y m e s o z o o p l a n k t o n predation rather than excessive  microzooplankton  respiration  due to high  sea surface  temperatures.  Simulated  m e s o z o o p l a n k t o n net production f o r both locations i s positive i n spring a n d s u m m e r (Fig. 3.14)  134  Station P (50°N 145°W), 1981-1984  Fig.  3.14:  Simulated  production  per cubic  microzooplankton (H), a n dmesozooplankton  (CO  meter  a n d d a y for phytoplankton  at Station P  (50°N  (P),  145°W) a n d at 50°N  130°W f o r 1981 t o 1984. N o t e d i f f e r e n t scales o n the ordinates.  135  Station P (50°N 145°W) 800 700 ;f „ 600  1-6  500  1 E 400 o  c o) 300 200  •C  100 | " Jan  Mar  May  Sep  Nov  Sep  Nov  Jul  50°N 130 W 8  800 700 > „ 600  •g  if  I  E 400 £ » 300 £ E •c 200 a. 100 Jan  F i g . 3.15:  Mar  May  —t Jul  '?  S e a s o n a l p r i m a r y p r o d u c t i v i t y a t S t a t i o n P (50°N 145°W) a n d a t 50°N 130°W  from  1981-84 s i m u l a t i o n results. F o r the purposes o f c o m p a r i s o n t h e same f o r m a t w a s u s e d as i n F i g . 2.3.  136  a n d b e c o m e s n e g a t i v e i n the f a l l as a c o n s e q u e n c e o f t h e l o w e r p r i m a r y  p r o d u c t i o n that i s  transferred through m i c r o z o o p l a n k t o n to mesozooplankton, a n d through the effects o f h i g h sea surface temperatures (Fig. 3.2) o n the m a s s - s p e c i f i c respiration o r non-predatory  d e a t h rates o f  b o t h z o o p l a n k t o n size classes (Eq. 3.14). 1983 a n d 1984 m i c r o z o o p l a n k t o n transfer efficiencies calculated f r o m m e a n f i e l d s i m u l a t i o n s for Station P are 2 7 a n d 2 6 % respectively, close to the 2 2 % estimated f o r the open  Northeast  P a c i f i c (Parsons & L a l l i 1988). I define transfer e f f i c i e n c y o f a trophic l e v e l here as the b i o m a s s production  at that t r o p h i c  level divided  (Baumann  1995; see also Chapter  b y the biomass  1). H o w e v e r ,  production  o f its prey  organisms  1981 and 1982 microzooplankton  transfer  e f f i c i e n c i e s o f 7 a n d 8 % , r e s p e c t i v e l y , a p p e a r t o o l o w f o r S t a t i o n P. F o r t h e n e a r c o a s t l o c a t i o n a t 50°N 130°W, the 1981 t o 1984 m e a n m i c r o z o o p l a n k t o n transfer e f f i c i e n c y o f 5 5 % appears t o o h i g h . A s e x p e c t e d f o r a m a t u r e e c o s y s t e m , net c o m m u n i t y p r o d u c t i o n , i.e. net p r i m a r y p r o d u c t i o n m i n u s total heterotrophic respiration ( O d u m 1971), is close to zero f o rboth locations. In order f o r m o d e l m e s o z o o p l a n k t o n to have s o m e resemblance to its r e a l - w o r l d counterpart i t i s i m p o r t a n t t h a t m e s o z o o p l a n k t o n m a s s - s p e c i f i c c l e a r a n c e o r f i l t r a t i o n r a t e Fa  be close to the  o b s e r v e d r a n g e o f 0 . 4 t o 3 . 6 l i t e r s ( m g C V d " ( F r o s t 1 9 8 7 ) . FQI c a n b e c a l c u l a t e d f r o m t h e d a i l y 1  g r a z i n g r a t e Gci  1  [ m g C m " d" ] as f o l l o w s ( f o r s y m b o l s T a b l e 3 . 2 ; s e e a l s o rate e q u a t i o n s i n 3  1  Table 3.1):  HC  A  HL  ( E q . 3.15a)  L  1000  (Eq. 3.15b)  137  ^ HC Fa  i s i n d e p e n d e n t o f Ci a n d h a s a m a x i m u m  r u n parameter c h o i c e (Table 3.3)  F i,max C  F  c  =  i s 4 liters ( m g C)"  L  1  - 1 0 0 0 a t H=K aH  F o r the standard  d ' . Fig. 3.16 shows the simulated 1  m e s o z o o p l a n k t o n f i l t r a t i o n rate at S t a t i o n P a n d at 5 0 ° N 130°W f o r 1 9 8 1 - 1 9 8 4 .  3.4.2. Simulation Results: 4-Trophic Levels Model Simulated phytoplankton  concentration a n d productivity f o r the 4-Trophic  Levels  standard r u n ( T a b l e 3.3) i s the same as i n the s i m u l a t i o n o f 3 - T r o p h i c L e v e l s M o d e l  Model  (Compare  F i g . 3 . 1 7 a n d 3.13, F i g . 3.18 a n d 3.14; F i g . 3.15). T h i s i s a consequence o f the a s s u m p t i o n o f a %FCM g r a z i n g t h r e s h o l d a n d t h e d o n o r - c o n t r o l l e d b i o m a s s t r a n s f e r b e t w e e n p h y t o p l a n k t o n a n d m i c r o z o o p l a n k t o n ( S e c t i o n 3.3.). S u m m e r concentrations o f s i m u l a t e d m i c r o z o o p l a n k t o n are a p p r o x i m a t e l y t w i c e as h i g h f o r Station P a n d almost four times as h i g h f o r the near-coast l o c a t i o n , as the o b s e r v e d m e a n o f 15 m g C m " a t S t a t i o n P ( B o o t h et al. 1 9 9 3 ) . C o m p a r e d t o t h e 3 - T r o p h i c L e v e l s M o d e l ( S u b s e c t i o n 3  3.4.1.) this h i g h e r standing stock i s a c o n s e q u e n c e o f the t o p - d o w n c o n t r o l o f m a c r o z o o p l a n k t o n w h i c h releases m i c r o z o o p l a n k t o n f r o m p r e d a t i o n pressure b y m e s o z o o p l a n k t o n (see F i g . 3.1). A t S t a t i o n P m e s o z o o p l a n k t o n reaches its annual peak i n late s u m m e r , a little earlier than i nthe 3 T r o p h i c L e v e l s M o d e l but still m u c h later than theobserved peak i n M a y - J u n e ( M a c k a s & Frost 1993). A t the near-coast l o c a t i o n the t i m i n g o f the m e s o z o o p l a n k t o n peak f r o m the s i m u l a t i o n s c o i n c i d e s w i t h t h a t o b s e r v e d a t S t a t i o n P. S i m u l a t e d m a x i m u m m e s o z o o p l a n k t o n c o n c e n t r a t i o n s for both locations are a little l o w e r (except 1981, w h i c h i s a n initial conditions effect) than the  138  Station P (50°N 145°W),1981-1984 Mass-Specific Clearai ates [liters (mg C)-1 d-  0  / \  °  Jan  \  / / '»---'  Jul  / \  Jan  Jul  \  /  / \  Jan  Jul  \  / Jan  Jul  50°N 130°W,1981-1984 a  c T 5  2-6 8Z o o  it E  4 /  3  i g 2  *•  <g M  •  *  <?  **.  1  ra <H S | n ° Jan  Jut  i  •  \  •  1  '  •'  /  \  *  •  "«  • Jan  Jul  Jan  Jul  Jan  Jul  F i g . 3.16: Simulated mass-specific clearance or filtration rates [liters (mg C)" d"] for mesozooplankton (CO at Station P ( 5 0 ° N 145°W) and at 5 0 ° N 130°W for 1981 to 1984. 1  1  139  Station P (50°N 145°W),1981-1984  50°N 130°W,1981 -1984  Fig.  3.17:  Simulated  carbon  concentrations f o r phytoplankton  (P), microzooplankton (H),  m e s o z o o p l a n k t o n ( C O , a n d m a c r o z o o p l a n k t o n ( C ) at S t a t i o n P ( 5 0 ° N 145°W) a n d at 5 0 ° N 2  130°W f o r 1981 t o 1984. N o t e d i f f e r e n t scales o n the ordinates.  140  Fig.  3.18:  Simulated  production  per  cubic  meter  and  day  for  phytoplankton  microzooplankton ( H ) , mesozooplankton ( C , ) , a n d macrozooplankton ( C ) at Station P 2  (P), (50°N  145°W) a n d at 50°N 130°W f o r 1981 t o 1984. N o t e different scales o n the o r d i n a t e s .  141  o b s e r v e d M a y - J u n e m e a n p e a k o f 3 m g C m " at S t a t i o n P ( M a c k a s & F r o s t 1 9 9 3 ) . S i m u l a t e d 3  m a c r o z o o p l a n k t o n c o n c e n t r a t i o n s b e h a v e at l e a s t q u a l i t a t i v e l y as e x p e c t e d . L a c k o f d a t a ( s e e S e c t i o n 2.3.), h o w e v e r , d o n o t p e r m i t a n y c o m p a r i s o n t o the natural w o r l d . Because i n the 4-trophic levels m o d e l microzooplankton is released f r o m grazing  pressure,  microzooplankton  net production  becomes  negative  mesozooplankton (microzooplankton  r e s p i r a t i o n e x c e e d s p r i m a r y p r o d u c t i v i t y ) i n late s u m m e r f o r b o t h l o c a t i o n s w h e n s e a surface temperatures are high. A s a consequence, m i c r o z o o p l a n k t o n transfer e f f i c i e n c y i s o n l y  around  0 . 5 % , m u c h l o w e r than the 2 2 % estimated f o r the o p e n N o r t h e a s t P a c i f i c (Parsons & L a l l i 1988), or the "tried-and-true"  1 0 % (Slobodkin  1961; b u t see B a u m a n n  1995;Pauly &  Christensen  1995a; P a u l y & Christensen 1995b; S l o b o d k i n 1980). B e c a u s e m e s o z o o p l a n k t o n i s t o p - d o w n c o n t r o l l e d b y m a c r o z o o p l a n k t o n m u c h o f its p r o d u c t i o n is c o n s u m e d rather than u s e d u p b y respiration a n d non-predatory losses. Thus, m e s o z o o p l a n k t o n has a transfer e f f i c i e n c y o f around 8 0 % . A l t h o u g h these values intuitively appear w r o n g little i s k n o w n about the exact bioenergetic r e l a t i o n s h i p s i n the o p e n o c e a n e c o s y s t e m . A g a i n as e x p e c t e d net c o m m u n i t y p r o d u c t i o n , i.e. net primary production m i n u s total heterotrophic respiration ( O d u m 1971), f o r the 4-trophic levels m o d e l is close to zero forboth locations. Fig.  3.19 shows  the simulated  m a c r o z o o p l a n k t o n at S t a t i o n P  m a s s - s p e c i f i c f i l t r a t i o n rates  for mesozooplankton and  a n d at 5 0 ° N 1 3 0 ° W f o r 1 9 8 1 - 1 9 8 4 .  M a x i m u m mass-specific  c l e a r a n c e o r f i l t r a t i o n rates, as c a l c u l a t e d f r o m E q . 3 . 1 5 a n d u s i n g parameters o f the standard r u n (Table  3.3), are 4 liters ( m g C ) "  1  d"  1  for mesozooplankton  a n d 4 0 liters ( m g C Y  1  d" for 1  m a c r o z o o p l a n k t o n . N o t e that w h i l e m e s o z o o p l a n k t o n m a s s - s p e c i f i c f i l t r a t i o n rates g o u p t o i t s maximum  value o f 4 liters ( m g C ) " d " , m a c r o z o o p l a n k t o n 1  1  never reaches its mass-specific  filtration potential.  142  Station P (50°N 145°W),1981-1984  Jan  Jul  Jan  Jul  Jan  Jul  Jan  Jul  50°N 130°W,1981-1984  Fig.  3.19:  S i m u l a t e d m a s s - s p e c i f i c c l e a r a n c e rates o r filtration rates [liters ( m g C )  mesozooplankton  ( d ) and macrozooplankton  ( C ) at Station P 2  1  d" ] f o r 1  (50°N 145°W) a n d at 50°N  130°W f o r 1981 t o 1984. N o t e d i f f e r e n t s c a l e s o n the o r d i n a t e s .  143  3.5. Sensitivity Analyses 3.5.1. Sensitivity Analyses: 3-Trophic-Levels Model S e n s i t i v i t y analyses f o r the 3 - t r o p h i c - l e v e l s m o d e l  were conducted w i t h respect t o initial  c o n d i t i o n s , t h e b i o l o g i c a l p a r a m e t e r s a ( E q . 3 . 1 3 a ) , mn,o ( E q . 3 . 1 4 a ) , mci  ( E q . 3 . 1 4 b ) , <XHCI a n d  KHCI ( E q s . 3 . 1 8 a n d 3 . 1 9 ) , a n d t h e f u n c t i o n a l r e s p o n s e ( E q . 3 . 8 , F i g . 3 . 1 0 ) . R e s u l t s a r e p l o t t e d as d e v i a t i o n s f r o m the standard r u n , i.e. m o d i f i e d r u n results m i n u s s t a n d a r d r u n r e s u l t s , i n p e r c e n t o f standard run. Standard run initial conditions and parameter values c a n be f o u n d i n T a b l e 3.3.  Sensitivity to Initial Conditions S e n s i t i v i t y to i n i t i a l c o n d i t i o n s w a s tested b y d o u b l i n g the i n i t i a l v a l u e s o f o n e state v a r i a b l e at a t i m e a n d r u n n i n g t h e s i m u l a t i o n f o r S t a t i o n P f r o m  1981  t o 1984.  Simulated  biomass  d e n s i t i e s w e r e t h e n c o m p a r e d t o t h o s e o f t h e s t a n d a r d r u n at S t a t i o n P f o r t h e s a m e p e r i o d o f t i m e ( F i g . 3 . 1 3 ) . R e s u l t s i n F i g . 3 . 2 0 s h o w t h a t a d o u b l i n g i n m i c r o z o o p l a n k t o n i n i t i a l d e n s i t y (H) h a s almost  n o effect  o n both  microzooplankton  and mesozooplankton  densities  (Ci),  while  a  d o u b l i n g i n m e s o z o o p l a n k t o n i n i t i a l d e n s i t y has l a r g e r effects that persist f o r m o r e t h a n 2 years. C o n s e q u e n t l y , f o r the sensitivity analyses t o f o l l o w , o n l y the years 1983 a n d 1 9 8 4 h a v e  been  considered. T o a v o i d initial condition effects, spatially-explicit simulations i n Chapter 4 were g i v e n a t w o year pre-run time without advection before simulation results were recorded.  Sensitivity to Parameters P a r a m e t e r s that w e r e tested i n the s e n s i t i v i t y analyses w h e r e i n c r e a s e d b y 1 0 % c o m p a r e d t o their standard run values (Table  3.3).  144  Sens. Anal.: Init. Cond. H(0)=2 [mg C m-3] 4  1  ce  0  0.8  n > T3  -4  0.6  -8  0.4 o  3  •s  o c  "5 5?  0.2  -12 -16  1 1 1 1 1 1 1 1 1 1 n 1 1 1 1 1 1 1 1 1 i'fi i T H i T l n 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Jan  Jul  Jan  Jul  Jan  Jul  Jan  0  c  3  1I  -Hdev •Cldev  e  o a?  Jul  Sens. Anal.: Init. Cond. C1(0)=1 [mg C m-3] 4  j 100  •s(S  0  80  -4  60  -8  40  > TJ  • e ? S « 1  "5 -12 e -16  20  \  • 1 1 1 1 1 1 1 1 1 1 1 1 1 1 M 111 i 1 1 1 1 1 1 1 1 1 1 n i m » n n i l 1111 Jan  Jul  Jan  Jul  Jan  Jul  Jan  0  •E 3 K "S > •2 1  C1d >f Star  c  3  ce  -Hdev • Cldev  w  e  Jul  Fig. 3.20: Sensitivity to initial conditions in the 3-Trophic-Levels Model: Deviation of microzooplankton (Hdev) and mesozooplankton (Cidev) concentrations from the respective concentrations of the standard run after a doubling of the initial density in microzooplankton (upper panel) and mesozooplankton (lower panel). Simulation period: 1981-1984. Note different scales on the ordinates.  145  A n increase i n the photosynthetic e f f i c i e n c y a leads to a h i g h e r d e n s i t y o f the top-predator, i.e. p r i m a r y p r o d u c t i o n i s transferred t h r o u g h m i c r o z o o p l a n k t o n to m e s o z o o p l a n k t o n ( F i g . 3.21). Increase o f m e s o z o o p l a n k t o n  i s largest i n early summer,  (Cj)  when  l i g h t l i m i t a t i o n i s at a  m i n i m u m ( F i g s . 3 . 4 a n d 3 . 6 ) , w h i c h l e a d s t o a s u p p r e s s i o n i n m i c r o z o o p l a n k t o n d e n s i t y (H) i n late s u m m e r . C h a n g e s i n the specific respiration o r non-predatory  d e a t h r a t e s niu,o a n d ma  have  large  effects o n p o p u l a t i o n densities. A 1 0 % increase i n the m i c r o z o o p l a n k t o n s p e c i f i c r e s p i r a t i o n rate at 0 ° C ra#, , w h i c h t r i g g e r s a p a r a m e t e r c h a n g e i n ma 0  eradicates  the mesozooplankton  standing  stock  (see E q s . 3 . 1 4 ) , a l m o s t  ( F i g . 3.22, upper  completely  panel).  Although  m i c r o z o o p l a n k t o n d e n s i t i e s i n w i n t e r a n d s p r i n g are i n g e n e r a l a l i t t l e l o w e r t h a n i n the s t a n d a r d r u n as a c o n s e q u e n c e o f the larger losses due t o respiration, i n s u m m e r pressure  from  mesozooplankton  decreased predation  results i n a n increased m i c r o z o o p l a n k t o n  m e s o z o o p l a n k t o n r e s p i r a t i o n r a t e ma  stock.  Increased  r e p r e s e n t s t h e c a s e w h e n mu r e m a i n s u n c h a n g e d f r o m t h e  standard r u n s i m u l a t i o n a n d thus effects have less a m p l i t u d e ( F i g . 3.22, l o w e r panel). Changes  in  predation  parameters,  i.e. the m a x i m u m  m e s o z o o p l a n k t o n o n m i c r o z o o p l a n k t o n (aua) s p e c i f i c p r e d a t i o n r a t e (K a), H  specific  predation  rate  of  a n d the h a l f - s a t u r a t i o n constant o f the predator  h a v e l a r g e e f f e c t s o n b i o m a s s d e n s i t i e s . I n c r e a s e d ana  allows  m e s o z o o p l a n k t o n to increase its standing stock substantially c o m p a r e d to the standard run, w h i l e m i c r o z o o p l a n k t o n remains largely unaffected and is suppressed o n l y i n late s u m m e r (Fig.  3.23,  u p p e r panel), quite s i m i l a r i n pattern to the increase i n p h o t o s y n t h e t i c e f f i c i e n c y ( F i g . 3.21). T h i s m e a n s that f o r m o s t o f the year m e s o z o o p l a n k t o n c o n s u m e s m i c r o z o o p l a n k t o n b i o m a s s  that  w o u l d o t h e r w i s e b e l o s t t o m i c r o z o o p l a n k t o n r e s p i r a t i o n . O n t h e o t h e r h a n d , a n i n c r e a s e i n KHCI m a k e s m i c r o z o o p l a n k t o n less available t o m e s o z o o p l a n k t o n an thus reduces  mesozooplankton  146  Sens. Anal.: Photosynthetic Efficiency^05.6 [mg C (mg Chl-a)-1 d-1 (MJ m-2)-1] e 3  11 > 3  *  50 40 30 20  •C1dev  0  c -10  Fig. 3.21:  -Hdev  Jan  Jul  Jan  Jul  S e n s i t i v i t y to photosynthetic e f f i c i e n c y a i n the 3 - T r o p h i c - L e v e l s M o d e l : D e v i a t i o n o f  microzooplankton  (Hdev) a n dmesozooplankton  ( C i d e v ) concentrations f r o m  the respective  concentrations o fthe standard r u n after a 1 0 % increase i n photosynthetic efficiency. S i m u l a t i o n p e r i o d : 1 9 8 3 - 1 9 8 4 (see  Sensitivity to Initial  Conditions).  147  Sens. Anal.: mH0=0.275 100 50  si l i « CO  a  -Hdev  0  •Cldev  -50 -100 Jan  Jul  -i—i-  Jan  Jul  Sens. Anal.: mC1=1.1/15*mH •=• 100 50 -Hdev  •x -a  U a 5  •Cldev -50 t '•=• -100  CO  I  Jan  Jul  Jan  i  I  I  I  I  Jul  Fig. 3.22: Sensitivity to specific respiration or non-predatory death rates in the 3-Trophic-Levels Model: Deviation of microzooplankton (Hdev) and mesozooplankton (Cidev) concentrations from the respective concentrations of the standard run after a 10% increase in: Upper panel: herbivore specific respiration rate at 0 ° C {m }. Lower panel: the ratio of mesozooplankton (mci) to microzooplankton (m ) specific respiration rate. Simulation period: 1983-1984 (see Sensitivity to Initial Conditions). Hi0  H  148  Sens. Anal.: aHC1=0.22 [mg C (mg C)-1 d-1] 150 3 K  =1  100  raI £  .2  -Hdev •Cidev  0  <a V)  o  Q  50  -50  = . -100  H—I—I—I—I—I—h-  Jan  Jul  Jan  Jul  Sens. Anal.: KHC1=27.5 [mg C m-3] 150 3  100  K  =1  .2 M ra <=  in 5  -Hdev  50  •Cidev  0  Q ,  SS -50 =• -100  H—I—I—I—h-  Jan  Fig. 3.23:  Jul  Sensitivity to predation  microzooplankton  H—I—I—h  H—h  Jan  parameters  Jul  i n the3-Trophic-Levels  (Hdev) a n dmesozooplankton  (C dev) t  concentrations  Model: from  Deviation o f the respective  concentrations o f the standard r u n after a 1 0 % increase in: U p p e r panel: t h e m a x i m u m p r e d a t i o n rate o f m e s o z o o p l a n k t o n o n m i c r o z o o p l a n k t o n constant o f the predator  specific predation  rate  (K i). HC  {a i). HC  specific  L o w e r panel: the half-saturation  Simulation  period:  1983-1984 (see  Sensitivity to Initial Conditions).  149  b i o m a s s too v e r y l o w l e v e l s ( F i g . 3.23, l o w e r panel), a pattern v e r y s i m i l a r to that o b t a i n e d f r o m increased non-predatory  death rates ( F i g . 3.22). H i g h n o n - p r e d a t o r y  losses i n microzooplankton  k e e p i t s d e n s i t y t h e s a m e as i n t h e s t a n d a r d r u n s i m u l a t i o n s , e x c e p t f o r l a t e t h e s u m m e r  months.  Sensitivity to Functional Response H e r e I tested f o r the effects o f a T y p e II f u n c t i o n a l r e s p o n s e o f the s p e c i f i c p r e d a t i o n rate t o p r e y d e n s i t y (see d i s c u s s i o n i n S e c t i o n 3.3). mesozooplankton  (Cj)  A Type  U functional response f o r predation o f  o n m i c r o z o o p l a n k t o n (H) m a k e s t h e w h o l e s y s t e m r a t h e r u n s t a b l e  (Fig.  3.24). M i c r o z o o p l a n k t o n i s generally suppressed and w i l d l y fluctuates c o m p a r e d t o the standard run. M e s o z o o p l a n k t o n s h o w s peaks 4 3 times larger than the already h i g h c o n c e n t r a t i o n s i n the standard run.  3.5.2. Sensitivity Analyses: 4-Trophic-Levels Model S e n s i t i v i t y analyses f o r the 4 - t r o p h i c - l e v e l s m o d e l  were conducted w i t h respect t o initial  c o n d i t i o n s , t h e b i o l o g i c a l p a r a m e t e r s a ( E q . 3 . 1 3 a ) , mu.o ( E q . 3 . 1 4 a ) , ma  ana  and  Ka H  ( E q s . 3.8 a n d 3.9),  combinations (Eqs.  aaa  and  Kac2  a n d ma  (Eq.  3.14b),  ( E q s . 3.9 a n d 3.10), a n d f u n c t i o n a l r e s p o n s e  3 . 8 a n d 3.9, F i g . 3.10). A g a i n , results are p l o t t e d as d e v i a t i o n s f r o m  s t a n d a r d r u n , i.e. m o d i f i e d r u n results m i n u s standard r u n results, i n p e r c e n t o f s t a n d a r d  the run.  Standard run initial conditions and parameter values c a n be f o u n d i n T a b l e 3.3.  150  Sens. Anal.: Type II Functional Response  Fig. 3.24: Sensitivity to the functional response in the 3-Trophic-Levels Model: Deviation of microzooplankton (Hdev) and mesozooplankton (Cidev) concentrations from the respective concentrations of the standard run with a Type U functional response of the mesozooplankton specific predation rate to microzooplankton density. Simulation period: 1983-1984 (see Sensitivity to Initial Conditions). Note different scales on the ordinates.  151  Sensitivity to Initial Conditions S e n s i t i v i t y to i n i t i a l c o n d i t i o n s w a s tested b y d o u b l i n g the i n i t i a l v a l u e s o f o n e state v a r i a b l e at a t i m e a n d r u n n i n g t h e s i m u l a t i o n f o r S t a t i o n P f r o m  1981  t o 1984.  Simulated  biomass  d e n s i t i e s w e r e t h e n c o m p a r e d t o t h o s e o f t h e s t a n d a r d r u n at S t a t i o n P f o r t h e s a m e p e r i o d o f t i m e ( F i g . 3.17). R e s u l t s i n F i g . 3.25 s h o w that the effects o f a d o u b l i n g o f the i n i t i a l d e n s i t y o f a n y state v a r i a b l e w i l l h a v e e f f e c t i v e l y v a n i s h e d after t w o years. A g a i n , f o r the s e n s i t i v i t y a n a l y s e s to f o l l o w , o n l y the years 1983 a n d 1984 have thus b e e n c o n s i d e r e d . T o a c c o u n t f o r i n i t i a l c o n d i t i o n effects i n the 4 - t r o p h i c - l e v e l s m o d e l , s p a t i a l l y - e x p l i c i t simulations i n C h a p t e r 4 w e r e g i v e n a t w o year p r e - r u n time w i t h o u t advection before s i m u l a t i o n results w e r e recorded.  Sensitivity to Parameters P a r a m e t e r s that w e r e tested i n the sensitivity, analyses w h e r e i n c r e a s e d b y 1 0 % c o m p a r e d to their standard run values (Table Compared stable  3.3).  t o the standard r u n s i m u l a t i o n , i n c r e a s e d p h o t o s y n t h e t i c e f f i c i e n c y a leads t o a  higher  mesozooplankton  density  of  d e n s i t y (Ci),  microzooplankton  (H),  a  trophodynamically  and a generally higher macrozooplankton  fluctuating  density (Fig.  N o t e that, s o m e w h a t contrary to the s i m p l e v e r s i o n o f the t r o p h i c c a s c a d e a r g u m e n t ( P i m m  3.26). 1992)  a n d a s a c o n s e q u e n c e o f t h e T y p e in f u n c t i o n a l r e s p o n s e ( S e c t i o n 3 . 3 . ) , a l l t h r e e t r o p h i c l e v e l s show  increased standing  photosynthetic  stocks  efficiency.  from  Increase  fall in  1983  to summer  macrozooplankton  1984 is  as a result much  o f increased  smaller  than  in  m e s o z o o p l a n k t o n i n t h e 3 - t r o p h i c - l e v e l s m o d e l , j u s t as e x p e c t e d f r o m t h e b i o e n e r g e t i c l o s s e s t h a t o c c u r d u r i n g trophic transfers.  152  Sens. Anal.: Init. Cond. H(0)=2 [mg C m-3] 3  c  3 3  3  ^ ce  •s 3 1  o ?  3  co  •Cldev -C2dev  Hi-  o  -Hdev  o °  1 1 1 1 1 1 1 1 1 i 1111 n 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 M 1 1 1 1 1 1 1 1  Jan  Jul  Jan  Jul  Jan  Jul  Jan Jul  Sens. Anal.: Init. Cond. C1(0)=1 [mg C m-3] 3  300  BC  fl  "E n > -o » c  200  I  5 co3  100  x  •=• 3 CC  3 | o "g » co  -Hdev •Cldev C2dev  V i I i i i iII I  Jan  Jul  11  II I I I II II I I I I i i  Jan  Jul  Jan  11  I I i i I i I I II I I  Jul  I 11 I -100  111  Jan Jul  Sens. Anal.: Init. Cond. C2(0)=0.2 [mg C m-3] •=• 3  300  •s  200  ra  100  E 2  0  co  •s  c 3  CC  -1 11111111111111111111111111111111111111111111111  Jan  Jul  Jan  Jul  Jan  Jul  a -a  so i-g  is  -Hdev •Cldev -C2dev  ^- o  -100  Jan Jul  Fig. 3.25: Sensitivity to initial conditions in the 4-Trophic-Levels Model: Deviation of microzooplankton (Hdev), mesozooplankton (Cidev), and macrozooplankton (Cidev) concentrations from the respective concentrations of the standard run after a doubling of the initial density in microzooplankton (upper panel), mesozooplankton (middle panel), and macrozooplankton (lower panel). Simulation period: 1981-1984. Note different scales on the ordinates.  153  Sens. Anal.: Photosynthetic Efficiency=105.6 [mg C (mg Chl-a)-1 d-1 (MJ m-2)-1]  e 20 3  OC  II  -Hdev  10  •Cidev  -C2dev c  -10  H—I—I—I—I—I—IJan  Jul  H—I—I—I—I—I—H Jan  Jul  Fig. 3.26: Sensitivity to photosynthetic efficiency a in the 4-Trophic-Levels Model: Deviation of microzooplankton (Hdev), mesozooplankton (Cidev), and macrozooplankton (C dev) concentrations from the respective concentrations of the standard run after a 10% increase in photosynthetic efficiency. Simulation period: 1983-1984 (see Sensitivity to Initial Conditions). 2  154  In general, changes i n the specific respiration o r non-predatory  d e a t h r a t e s mn.o,  mci, nic2  have smaller effects o n population densities i n the 4 - than i n the 3-trophic-levels m o d e l  (Figs.  3.27 a n d 3.22). A g a i n the largest effect c o m e s w i t h a 1 0 % increase i n the m i c r o z o o p l a n k t o n s p e c i f i c r e s p i r a t i o n r a t e a t 0 ° C m ,o, H  w h i c h t r i g g e r s a p a r a m e t e r c h a n g e i n ma  a n d mci  (see E q .  3.14). H e r e , m i c r o z o o p l a n k t o n densities are g e n e r a l l y s u p p r e s s e d b y less t h e n 1 0 % c o m p a r e d t o the standard r u n s i m u l a t i o n ( F i g . 3.27, u p p e r panel), b u t o t h e r w i s e f o l l o w t h e s a m e s e a s o n a l p a t t e r n as t h e s t a n d a r d r u n ( F i g . 3 . 1 7 ) . I n c r e a s e d m e s o z o o p l a n k t o n r e s p i r a t i o n r a t e mci t h e c a s e w h e n m # r e m a i n s u n c h a n g e d a n d mci run  simulation  (Fig. 3.27, middle  panel),  r e p r e s e n t s t h e c a s e w h e n b o t h m # a n d mci  represents  i s c h a n g e d as w e l l ( E q . 3.14) f r o m the standard  increased macrozooplankton  r e s p i r a t i o n rate  mcz  r e m a i n u n c h a n g e d f r o m the standard r u n s i m u l a t i o n  ( F i g . 3.27, l o w e r panel). A g a i n d e v i a t i o n s f r o m standard r u n s i m u l a t i o n s h a v e less a m p l i t u d e the less effects a particular parameter c h a n g e has o n other parameters. C h a n g e s i n p r e d a t i o n parameters, i.e. m a x i m u m s p e c i f i c p r e d a t i o n rates o f c a r n i v o r e s  acici)  a n d t h e h a l f - s a t u r a t i o n c o n s t a n t s o f p r e d a t o r s p e c i f i c p r e d a t i o n r a t e s (KHCI,  (anci,  KciC2), h a v e  v e r y different effects o n the b i o m a s s densities d e p e n d i n g o n the t r o p h i c l e v e l w h e r e the changes o c c u r . I n c r e a s e i n a^ci  w i l l result i n the same pattern than a n increase i n p r i m a r y p r o d u c t i v i t y  ( c o m p a r e F i g s . 3 . 2 8 a n d F i g . 3 . 2 6 ) , a n d t h e e f f e c t , a t l e a s t i n p a t t e r n , o f i n c r e a s e d KHCI i s s i m i l a r to i n c r e a s e d non-predatory  d e a t h rates ( c o m p a r e F i g . 3.28. a n d F i g . 3.27). O n t h e o t h e r h a n d ,  i n c r e a s e i n t h e p r e d a t i o n p a r a m e t e r v a l u e s f o r m a c r o z o o p l a n k t o n (aac2, the s a m e effects o n predator ( m a c r o z o o p l a n k t o n )  Kcici)  have interestingly  a n d prey (mesozooplankton),  while  having  almost n o effect o n herbivorous m i c r o z o o p l a n k t o n (Fig. 3.29), again contrary t o the argument o f s i m p l e trophic c a s c a d i n g ( P i m m 1992).  155  Sens. Anal.: mH0=0.275  Hdev Cidev C2dev  Jan  Jul  Jan  Jul  Sens. Anal.: mC1=1.1/30*mH — 60 c 3  a: 40  i l  20  ra £ 0 > 3 -20 Ql OT  -40  =• -60  H—I—I—h  Jan  Jul  Jan  Jul  Sens. Anal.: mC2=1.1/3*mC1  Fig. 3.27:  Sensitivity to specific respiration o rnon-predatory  Model:  Deviation  of  microzooplankton  (Hdev),  d e a t h rates i n the mesozooplankton  4-Trophic-Levels (C]dev),  and  m a c r o z o o p l a n k t o n ( C d e v ) c o n c e n t r a t i o n s from t h e r e s p e c t i v e c o n c e n t r a t i o n s o f t h e s t a n d a r d r u n 2  a f t e r a 1 0 % i n c r e a s e i n : U p p e r p a n e l : h e r b i v o r e s p e c i f i c r e s p i r a t i o n r a t e a t 0 ° C (m )H0  panel: the ratio o f m e s o z o o p l a n k t o n  (m i) C  to microzooplankton  L o w e r p a n e l : t h e r a t i o o f m a c r o z o o p l a n k t o n (md)  (m ) H  Middle  specific respiration  t o m e s o z o o p l a n k t o n (m ) Cl  rate.  specific respiration  r a t e . S i m u l a t i o n p e r i o d : 1 9 8 3 - 1 9 8 4 ( s e e Sensitivity to Initial Conditions).  156  Sens. Anal.: aHC1=0.22 [mg C (mg C)-1 d-1] 50 3 OC  25  -Hdev  i l  •Cidev  0  -C2dev -25 =• -50 -I—i—I—i—i—I—H Jan  Jul  H—I—I—I—I—I—I—h Jan  H—I—I—I—h Jul  Sens. Anal.: KHC1=27.5 [mg C m-3] 50 3 OC  si  25  -Hdev  11 0) Q  •Cidev -C2dev  (0 o -25 =• -50  H—I Jan  1—I  1—IJul  H—I—I—h Jan  Jul  Fig. 3.28: Sensitivity to predation parameters in the 4-Trophic-Levels Model: Deviation of microzooplankton (Hdev), mesozooplankton (Cjdev), and macrozooplankton (C dev) concentrations from the respective concentrations of the standard run after a 10% increase in: Upper panel: the maximum specific predation rate of mesozooplankton on microzooplankton (axel). Lower panel: the half-saturation constant of the predator specific predation rate (K ci)Simulation period: 1983-1984 (see Sensitivity to Initial Conditions): 2  H  157  Sens. Anal.: aC1C2=0.44 [mg C (mg C)-1 d-1] •=•  c  12  c1 ra c >2 a (0 3  -Hdev  •2-S  •Cidev  -C2dev  °5  =. -12  H—I—H Jan  -I—I—h Jul  H—I—I—I—I—H Jan  Jul  Fig. 3.29: Sensitivity to predation parameters in the 4-Trophic-Levels Model: Deviation of microzooplankton (Hdev), mesozooplankton (Cidev), and macrozooplankton (C dev) concentrationsfromthe respective concentrations of the standard run after a 10% increase in: Upper panel: the maximum specific predation rate of macrozooplankton on mesozooplankton {acici)- Lower panel: the half-saturation constant of the predator specific predation rate (Kcia)Simulation period: 1983-1984 (see Sensitivity to Initial Conditions). 2  158  Sensitivity to Functional Response I  tested for  the  effects o f  various  functional  explicitly modeled trophic levels (Eqs. 3.8-3.10). A between mesozooplankton  Ci  response  combinations  between  the  three  T y p e U f u n c t i o n a l r e s p o n s e at t h e t r a n s f e r  and macrozooplankton  C  2  (Fig.  3.1)  will  eradicate both,  first  m e s o z o o p l a n k t o n a n d c o n s e q u e n t l y its predator (Fig. 3.30, u p p e r a n d l o w e r panel). A T y p e U / T y p e U I c o m b i n a t i o n f o r t h e m i c r o z o o p l a n k t o n (H) to m a c r o z o o p l a n k t o n  to m e s o z o o p l a n k t o n , a n d  transfer, respectively, leaves m i c r o z o o p l a n k t o n  mesozooplankton  a little suppressed  and  w i l d l y f l u c t u a t i n g c o m p a r e d t o t h e s t a n d a r d r u n . M e s o z o o p l a n k t o n s h o w s p e a k s 13 t i m e s l a r g e r standard r u n , a s o m e w h a t d a m p e d v e r s i o n o f the pattern i n the 3 - t r o p h i c - l e v e l s m o d e l ( F i g . 3.24). M a c r o z o o p l a n k t o n s h o w s o n average a slight increase c o m p a r e d to the standard run.  In s u m m a r y , the s m a l l s e n s i t i v i t y to i n i t i a l c o n d i t i o n s o f the 3 - a n d the 4 - t r o p h i c - l e v e l s m o d e l ( F i g . 3 . 1 , T a b l e 3.1) c a n b e c o m p e n s a t e d b y a 2 - y e a r p r e - r u n t i m e i n l o n g t e r m s i m u l a t i o n s . B o t h m o d e l s a r e m o s t l y s e n s i t i v e t o p r e d a t i o n p a r a m e t e r s at t h e b i o m a s s o r e n e r g y t r a n s f e r microzooplankton  and  mesozooplankton,  whose  effects are  similar i n pattern  but  between larger  in  m a g n i t u d e to c h a n g e s i n p r i m a r y p r o d u c t i v i t y a n d s p e c i f i c r e s p i r a t i o n rates. R e g a r d i n g stability, Type m  f u n c t i o n a l r e s p o n s e s appear to be a v a l i d a s s u m p t i o n . N e v e r t h e l e s s , it s h o u l d b e s a i d that  density-independent  m i g r a t i o n , s u c h as c a u s e d b y a d v e c t i o n , c a n e x e r t a s t a b i l i z i n g e f f e c t  on  p o p u l a t i o n s d y n a m i c s ( M c C a l l u m 1992; S t o n e 1993, but see Steele 1974).  159  Sens. Anal.: Type III / Type II Funct. Resp. 2 BC  "E ra  I* ? a x  to  •s c  =•-2  i  i  i  i  I I" I n|in.-f"T""1" 1  i i  Jan  Jul  Jan  Sens. Anal.: Type II / Type III Funct Resp. 1500  •s ra  1100  > "O  £  c 5 S  700  •s  300  X  e  3  >*  s?  CO  i  I i  I  I  -Hdev Cldev  -C2dev  5 °  -100  I'I  Jan  Sens. Anal.: Type II / Type II Funct. Resp. 2  r 1500  •=•  1 1 ra  1100  >*  c  3  3J = 5  c 3  BC  0  700  ra ?  > 8  •S-1  300  S3  s =•-2  o  i  Jan  I i i I i i I'I Jul  i  I  Jan  i  i i i  I I I  -100  -Hdev •Cldev  -C2dev  °4 .S  Jul  Fig. 330: Sensitivity to the functional response in the 4-Trophic-Levels Model: Deviation of microzooplankton (Hdev), mesozooplankton (Cidev), and macrozooplankton (C dev) concentrations from the respective concentrations of the standard run. Upper panel: Type in functional response of mesozooplankton specific predation rate to microzooplankton density. Type II functional response of macrozooplankton specific predation rate to mesozooplankton density. Middle panel: Type II / Type III. Lower panel: Type U / Type U. Simulation period: 1983-1984 (see Sensitivity to Initial Conditions). Note different scales on the ordinates. 2  160  4. SPATIALLY-EXPLICIT SIMULATIONS  Ockham's Razor: A plurality of reasons should not be posited without necessity. W i l l i a m of O c k h a m (1285-1349)  ' W o aphorism is more frequently repeated ... than that we must ask Nature few questions, or ideally, one question at a time. [I am] convinced that this view is wholly mistaken. ... Indeed if we ask [Nature] a single question, she will often refuse to answer until some other topic has been discussed." R.A. Fisher (1890-1962)  4.1. Spatio-Temporal Resolution and Advection Spatio-Temporal Resolution Spatially-explicit  simulations  o f ecosystem processes  i n the mixed  upper  layer  o f the  N o r t h e a s t P a c i f i c w e r e r u n o n a g e o r e f e r e n c e d 1° l o n g i t u d e x 1° l a t i t u d e g r i d c o v e r i n g t h e o c e a n surface between  1 8 0 t o 125°W a n d 3 5 t o 62°N. ( F o r p r i m a r y  production processes only, the  vertical spatial resolution was one meter f r o m the surface d o w n to the base o f the m i x e d  layer.)  T h e s p a t i o - t e m p o r a l r e s o l u t i o n o f the s i m u l a t i o n s i s a c o m p r o m i s e b e t w e e n t h e r e s o l u t i o n o f the i n p u t d a t a (see 3.2. E n v i r o n m e n t a l F o r c i n g s ) , t h e a s s u m e d relevant scales o f b i o l o g i c a l p r o c e s s e s , a n d c o m p u t a t i o n time. T h e r e are t w o shortcomings i n spatial scope a n d r e s o l u t i o n , e s p e c i a l l y w h e n c o n s i d e r i n g that s o c k e y e s a l m o n c o h o r t s u r v i v a l i s p r o b a b l y  determined i n o r near the  c o a s t a l d o m a i n s ( s e e S e c t i o n 1.4.): F i r s t , t h e B e r i n g S e a i s o n l y p a r t i a l l y c o v e r e d , a n d s e c o n d , input data lack a high resolution coastal circulation model. Every  1° x 1° f i e l d i n t h e s p a t i a l l y - e x p l i c i t s i m u l a t i o n s w a s i d e n t i f i e d b y t h e l o n g i t u d e a n d  latitude o f its s o u t h w e s t corner. A f i e l d w a s c l a s s i f i e d as o p e n o c e a n habitat i f a n d o n l y i f e a c h o f the f i e l d ' s four corners w a s represented i n the O c e a n Surface Current  Simulation  (OSCURS;  S u b s e c t i o n 3.2.2) a n d thus h a d t w o current vectors (one i n x - a n d o n e i n y-direction) assigned to  161  i t s g e o g r a p h i c c o o r d i n a t e s . O n l y f i e l d s c l a s s i f i e d as o p e n o c e a n h a b i t a t w e r e c o n s i d e r e d i n t h e s p a t i a l l y - e x p l i c i t e c o s y s t e m s i m u l a t i o n s , i n s u m 1240 fields. In order to c o m p a r e l o c a l b i o l o g i c a l to spatial a d v e c t i o n effects simulations were run without and w i t h a d v e c t i o n for b o t h , the 3 - and 4 - t r o p h i c l e v e l s m o d e l ( C h a p t e r 3). S i m u l a t i o n s w i t h o u t a d v e c t i o n w e r e r u n f r o m 1 9 4 9 to  1990  w i t h a t i m e step o f one day. B e c a u s e i n i t i a l c o n d i t i o n effects persisted for a p p r o x i m a t e l y  two  y e a r s i n the m e a n f i e l d s i m u l a t i o n s (see 3.5. S e n s i t i v i t y A n a l y s e s ) m o d e l o u t p u t w a s c o n s i d e r e d r e l i a b l e f r o m 1951 o n w a r d s . S i m u l a t i o n s that i n c l u d e d a d v e c t i o n w e r e r u n f r o m 1951 t o  1990,  a n d i n i t i a l c o n d i t i o n s f o r b i o l o g i c a l state v a r i a b l e s w e r e o b t a i n e d f r o m the s p a t i a l l y - e x p l i c i t s i m u l a t i o n s w i t h o u t a d v e c t i o n . A s i n the m e a n f i e l d s i m u l a t i o n s the t i m e step f o r b i o l o g i c a l p r o c e s s e s w a s o n e day. H o w e v e r , tests o n the O S C U R S i n p u t d a t a s h o w e d that s o m e f i e l d s i n the s p a t i a l l y e x p l i c i t s i m u l a t i o n s w o u l d e x p o r t a l m o s t t w i c e the b i o m a s s c o n c e n t r a t i o n they c o n t a i n w h e n r u n o n a 1° x 1° g r i d w i t h a d a i l y t i m e s t e p , a c o m p u t a t i o n a l p r o b l e m t h a t w o u l d e f f e c t i v e l y generate b i o m a s s b y  simply moving  it o n a grid. A s  a result, I decreased the t i m e step  for  a d v e c t i o n to 4 hours (alternatively, one c o u l d increase the spatial g r i d size) w h i c h e l i m i n a t e d the computational  problem.  In  order  to  minimize  computational  errors  in  the  simulations  that  i n c l u d e d advection, advection and e c o l o g i c a l processes were run i n subsequent order rather than s i m u l t a n e o u s l y , i.e. e a c h d a y b i o m a s s w a s first a d v e c t e d b y  surface currents then  biological  processes o c c u r r e d . W h e n c a l c u l a t e d o n a 4 8 6 - 6 6 m i c r o p r o c e s s o r , c o m p u t a t i o n o f the s p a t i a l l y e x p l i c i t s i m u l a t i o n o f the 4 - t r o p h i c l e v e l s m o d e l i n c l u d i n g a d v e c t i o n takes a p p r o x i m a t e l y  two  weeks.  162  Advection M o v i n g concentrations i n space is not a trivial p r o b l e m e s p e c i a l l y i f the p h y s i c a l sizes o f the spatial dimensions another.  o f the different  F o r example,  fields i n a spatially-explicit simulation  the latitude o f each  1° x 1° f i e l d d e t e r m i n e s  differ  from  one  the area i t spans,  and  n e i g h b o r i n g f i e l d s h a v e d i f f e r e n t m i x e d l a y e r d e p t h s as c a l c u l a t e d f r o m s e a s u r f a c e and  scalar  wind  speed  (Subsection  3.2.2.). A d v e c t i o n  o f concentrations  temperature  was computed b y  d e f i n i n g i m p o r t as p o s i t i v e f r o m t h e w e s t a n d s o u t h , a n d e x p o r t as p o s i t i v e t o the east a n d n o r t h f o r e a c h f i e l d . D a i l y i m p o r t a n d e x p o r t v e c t o r s w e r e t a k e n as the m e a n v a l u e s o f the r e s p e c t i v e d a i l y u - a n d v - v e c t o r s that w e r e c a l c u l a t e d f o r e a c h l o n g i t u d e a n d l a t i t u d e f r o m m o n t h l y vectors  o f the  Ocean  Surface  Current  Simulation  (OSCURS;  f o r spatio-temporal  mean data  interpolation see S u b s e c t i o n 3.2.1.). W h e n s o m e f i e l d 1 w i t h a b i o m a s s c o n c e n t r a t i o n Cj a n d a m i x e d l a y e r d e p t h ZMLI w a t e r m a s s e s i n t o a n a d j a c e n t ( e a s t e r n ) f i e l d 2 w i t h a d e e p e r m i x e d l a y e r ( i . e . ZMLI > ZMU,  exports ZML i n  n e g a t i v e v a l u e s as m i x e d l a y e r d e p t h w a s r e g a r d e d as a d e p t h c o o r d i n a t e ) t o t a l e x p o r t e d b i o m a s s ABj p e r t i m e s t e p At i s g i v e n b y :  A/J, —  = -u Ayz C 12  ML  1  (Eq.4.1a)  w h e r e un i s t h e ( e a s t w a r d ) c u r r e n t v e c t o r f r o m f i e l d 1 t o f i e l d 2 , a n d Ay (= Ayi = Ay2) r e p r e s e n t s the f i e l d ' s length i n y - d i m e n s i o n .  Biomass concentration i n field 1 changes by:  AC,  «„  w h e r e Ax (= Axi = AX2) r e p r e s e n t s t h e f i e l d ' s l e n g t h i n x - d i m e n s i o n , w h i c h i s a f u n c t i o n o f latitude. D u e to the deeper m i x e d layer depth b i o m a s s concentration i n f i e l d 2 changes by:  163  (Eq. 4.1c)  W h e n n o w field 2 with a biomass concentration C  2  a n d a m i x e d l a y e r d e p t h ZMLI  exports  w a t e r m a s s e s i n t o a n a d j a c e n t ( e a s t e r n ) f i e l d 3 w i t h a s h a l l o w e r m i x e d l a y e r ( i . e . ZMLI < ZMU',  ZML  i n negative values as m i x e d layer depth w a s regarded as a depth coordinate) total exported b i o m a s s AB  2  p e r t i m e s t e p At i s g i v e n b y :  AB  2  =  (Eq. 4.2a)  -u Ayz C 2i  ML)  2  w h e r e u 3 i s the (eastward) current v e c t o r f r o m f i e l d 2 t o f i e l d 3 . N o t e that here w a t e r m a s s e s 2  w e r e m o v e d b e t w e e n f i e l d s c o n s i d e r i n g t h e s h a l l o w e r m i x e d l a y e r ZMU- T h e r a t i o n a l e f o r t h i s i s the f o l l o w i n g : T h e transition at depth b e t w e e n t w o adjacent f i e l d s w i t h different m i x e d l a y e r depths i s a step f u n c t i o n . (Ideally o n e s h o u l d a p p l y a g r a d u a l t r a n s i t i o n . H o w e v e r , that  would  require a h i g h e r spatial resolution, w h i c h i n turn affords a shorter t i m e step, a n d w h o s e c o m b i n e d effect is a nonlinear increase i n computation time.) A biomass f l o w f r o m f i e l d 2 into 3 c o u l d be written as:  AB  2  Note  =  -u Ayz C  that i n E q . 4 . 2 b w a t e r m a s s e s are m o v e d  23  MLi  (Eq. 4.2b)  2  considering the deeper  mixed  layer  depth.  H o w e v e r , this w o u l d m e a n that parts o f the w a t e r c o l u m n o f f i e l d 2 e n d u p b e l o w the m i x e d l a y e r d e p t h o f f i e l d 3 , i . e . a n e t b i o m a s s l o s s d u e t o t r a n s p o r t . It h a s b e e n p o i n t e d o u t t h a t i n b i o l o g i c a l s i m u l a t i o n s o n e has t o b e c a r e f u l not t o create (e.g. d u e t o a t o o large timestep) o r d e s t r o y o r g a n i s m s w h e n m o v i n g t h e m o n a g r i d (Walters 1986) thus E q . 4.2b s e e m e d unrealistic. T o a v o i d the p r o b l e m o f destroying biomass w h e n m o v i n g i t one c o u l d increase the total b i o m a s s i n f i e l d 3 b y the amount calculated i n E q . 4.2b. T h i s , h o w e v e r ,  w o u l d represent a  164  c o n c e n t r a t i o n p r o c e s s w h e r e b y the total e x p o r t e d b i o m a s s f o r m the f i e l d w i t h the deeper  mixed  l a y e r i s c o m p r e s s e d i n t o a v o l u m e w i t h a s h a l l o w e r m i x e d l a y e r . T h i s s c e n a r i o as w e l l s e e m e d unrealistic. Eq.  4 . 2 a thus represents the m o s t realistic representation  o f advection o f concentrations  e s p e c i a l l y w h e n c o n s i d e r i n g that O S C U R S c u r r e n t v e c t o r s w h e r e c a l c u l a t e d f r o m C O A D S wind  data  velocities  (see at the  S e c t i o n 3.2.) surface, and  and  wind-induced  which  currents  i n nature  are d e c r e a s i n g t o w a r d s  depth  produce within  vector  highest the  current  mixed  layer.  F o l l o w i n g f r o m E q . 4.2a then:  AC At  u z C Ax z ML.2  2  2i  ML3  (Eq. 4.2c)  2  AC, —  At  \JU, = T C 1  Ax  (Eq.4.2d)  2  T h e s a m e rules a p p l y to transport i n north-south direction  Spatial Closure Spatially-explicit  ecosystem  simulations  were  restricted t o the open  Northeast  e x c l u d i n g processes i n the o p e n P a c i f i c west o f 180°W a n d south o f 35 °N,  Pacific,  a n d the coastal  r e g i o n s . H o w e v e r , b e c a u s e the N o r t h e a s t P a c i f i c cannot b e c o n s i d e r e d a c l o s e d s y s t e m  when  advection i s included, assumptions  Three  different  boundary  conditions  have  to b e made  f o r biomass  about  concentrations  the b o u n d a r y c o n d i t i o n s . were  tested i n simulations  were  b i o m a s s c o n c e n t r a t i o n s ( i n i t i a l b i o m a s s c o n c e n t r a t i o n f o r a l l o p e n o c e a n f i e l d s : 1 m g C m~ )  were  3  s u b j e c t e d to a d v e c t i o n b u t not to b i o l o g i c a l p r o c e s s e s , i.e. g r o w t h a n d d e a t h . ( N o t e that f o r the m i x e d layer depth zero-gradient boundary conditions had to be adopted, otherwise gross export f r o m the N o r t h e a s t P a c i f i c w o u l d b e z e r o (see E q . 4.2c).):  165  (1) Z e r o B o u n d a r y C o n d i t i o n s : A l l b o u n d a r y f i e l d s t o w a r d s the o p e n P a c i f i c a n d t o w a r d s the coast have a biomass concentration of zero. (2)  Zero-Gradient  Boundary  Conditions:  A  boundary  field towards  the  open  Pacific  t o w a r d s t h e c o a s t h a s t h e s a m e b i o m a s s c o n c e n t r a t i o n as t h e a d j a c e n t ( i n x - o r y - d i r e c t i o n )  or  open  o c e a n f i e l d that is i n c l u d e d i n the s i m u l a t i o n . (3) C o m b i n e d Z e r o / Z e r o - G r a d i e n t B o u n d a r y  Conditions: A l l boundary fields towards  the  coast h a v e b i o m a s s concentrations o f zero. A b o u n d a r y f i e l d t o w a r d s the o p e n P a c i f i c has the same biomass  c o n c e n t r a t i o n as t h e  adjacent  (in  x-  or y-direction)  open  o c e a n f i e l d that  is  i n c l u d e d i n the s i m u l a t i o n . As  e x p e c t e d f r o m general c i r c u l a t i o n patterns  c o n d i t i o n s , i.e. n o  gross import  ( F i g . 2.1)  simulations with zero  of biomass, accumulate biomass  N o r t h e a s t P a c i f i c i n the c o u r s e o f one year  boundary  o n the eastern side o f  the  ( F i g . 4.1). T h e r e i s also a slight a c c u m u l a t i o n  of  b i o m a s s s o u t h o f the A l e u t i a n Islands, the r e g i o n w h e r e B r i s t o l B a y s o c k e y e s a l m o n enter the o p e n o c e a n r e a l m ( B u r g n e r 1991). W h i l e the h i g h b i o m a s s density s o u t h o f the A l e u t i a n Islands m u s t b e a c o n s e q u e n c e o f r e d u c e d e x p o r t f r o m that r e g i o n i n s i m u l a t i o n s w i t h z e r o  boundary  c o n d i t i o n s , the e v e n h i g h e r c o n c e n t r a t i o n s i n that area i n s i m u l a t i o n s w i t h z e r o g r a d i e n t b o u n d a r y c o n d i t i o n s ( F i g . 4.2)  are a c o n s e q u e n c e o f the b o u n d a r y c o n d i t i o n s t h e m s e l v e s .  In  order  to  m i n i m i z e the a c c u m u l a t i o n effects o f zero-gradient b o u n d a r y c o n d i t i o n s i n the c o a s t a l r e g i o n s o n the b i o m a s s c o n c e n t r a t i o n south o f the A l e u t i a n Islands a n d still h a v e the realistic o p e n b o u n d a r y conditions  with  respect  to  gross  import  and  export,  combined  zero  /  166  Fig.  4.1:  S i m u l a t e d b i o m a s s c o n c e n t r a t i o n [ m g C m" ] f o r z e r o b o u n d a r y c o n d i t i o n s f o r J a n u a r y , 3  A p r i l , J u l y and October 1951. S i m u l a t i o n i n c l u d e d advection but no b i o l o g i c a l processes. Initial (30 D e c  1950) b i o m a s s concentration for all open ocean fields was 1 m g C  m " . A l l m a p s are 3  d e p i c t e d as d i s p l a y e d b y the N o r t h e a s t P a c i f i c M a p V i e w e r , a m a p p i n g p r o g r a m that I w r o t e i n 1 9 9 6 . D i s p l a y e d o n t h e b o t t o m o f e a c h p a n e l ( f r o m l e f t t o r i g h t ) : l o n g i t u d e a n d l a t i t u d e o f the c u r s o r p o s i t i o n , m o n t h a n d year. P o o r text quality is a c o n s e q u e n c e o f b i t m a p size reductions.  167  F i g . 4.1: C o n t i n u e d  Fig.  4.2:  January,  S i m u l a t e d b i o m a s s c o n c e n t r a t i o n [ m g C m" ] f o r z e r o - g r a d i e n t b o u n d a r y c o n d i t i o n s f o r 3  April,  July  and  October  1951.  Simulation  included  advection  but  no  biological  p r o c e s s e s . I n i t i a l ( 3 0 D e c 1 9 5 0 ) b i o m a s s c o n c e n t r a t i o n f o r a l l o p e n o c e a n f i e l d s w a s 1 m g C m" .  169  F i g . 4.2: C o n t i n u e d  170  z e r o - g r a d i e n t b o u n d a r y c o n d i t i o n s ( F i g . 4.3) w e r e a p p l i e d f o r the w h o l e - e c o s y s t e m s i m u l a t i o n . T h e severity o f a c c u m u l a t i o n effects due to zero-gradient b o u n d a r y c o n d i t i o n s b e c o m e s o b v i o u s w h e n l o o k i n g at l o n g e r t i m e s c a l e s : F i g . 4 . 4 c o m p a r e s t h e b i o m a s s c o n c e n t r a t i o n s i n G u l f A l a s k a w i t h zero-gradient boundary conditions and combined zero / zero-gradient  of  boundary  c o n d i t i o n s after a p e r i o d o f 10 years: I also tested f o r the effects o f i n i t i a l c o n d i t i o n s o n the b i o m a s s d i s t r i b u t i o n b y r u n n i n g the b i o m a s s a d v e c t i o n s i m u l a t i o n (again w i t h o u t b i o l o g y ) f r o m 1951 to 1960. E a c h f i e l d w a s g i v e n a u n i f o r m l y - d i s t r i b u t e d r a n d o m i n i t i a l b i o m a s s c o n c e n t r a t i o n b e t w e e n 0 a n d 1. F o r r e p e a t e d r u n s b i o m a s s concentration patterns l o o k e d very s i m i l a r w i t h i n less than a year ( F i g . 4.5). T h i s is not s u r p r i s i n g as c i r c u l a t i o n p a t t e r n s a r e s i m i l a r o v e r l a r g e a r e a s a n d r a n d o m i n i t i a l c o n d i t i o n s s h o u l d a v e r a g e out o v e r l a r g e areas. In  summary,  (OSCURS)  show  tests u s i n g that f o r  advection results f r o m  a spatial resolution of  1°  the  Ocean  longitude  Surface x  1°  Current  latitude, the  Simulation maximum  t o l e r a b l e t i m e step i n o r d e r not to generate b i o m a s s b y s i m p l y m o v i n g it o n the g r i d i s 12 h o u r s . To  allow for  smoother  biomass  advection, this m a x i m u m  time  step i n the  advection  c o m p o n e n t o f the w h o l e e c o s y s t e m simulations w a s r e d u c e d to 4 hours. F u r t h e r m o r e ,  sub-  combined  z e r o b o u n d a r y c o n d i t i o n s (towards the coast) / zero-gradient b o u n d a r y c o n d i t i o n s (towards o p e n ocean) w e r e a p p l i e d i n order to a v o i d e x c e s s i v e b i o m a s s a c c u m u l a t i o n s i n the G u l f  the of  Alaska.  171  » NC-Pocific HonViewcf IM. Daumonr, 1'XJGI  Fig.  4.3:  Simulated  biomass  concentration  P!Fi  [mg C  m" ] 3  for combined  b o u n d a r y c o n d i t i o n s (see text) f o r January, A p r i l , J u l y a n d O c t o b e r  I  -  zero  /  zero-gradient  1951. Simulation  included  advection but n o b i o l o g i c a l processes. Initial (30 D e c 1950) biomass concentration for a l l open o c e a n f i e l d s w a s 1 m g C m" .  172  « NC-Pocific HooVicwcf IH. D .sumanr, T » G |  ffalE3  F i g . 4.3: C o n t i n u e d  173  Fig. 4.4:  Simulated biomass concentration [mg  simulation). Upper panel: Zero-gradient  C  m" ] 3  for October  1960  (after  10 years  boundary conditions. L o w e r panel: C o m b i n e d zero  of /  zero-gradient boundary conditions. S i m u l a t i o n i n c l u d e d advection but no b i o l o g i c a l processes. Initial (30 D e c 1950) b i o m a s s c o n c e n t r a t i o n for a l l o p e n o c e a n f i e l d s w a s 1 m g C  m" . 3  174  Fig.  4.5:  Simulated  biomass  concentration  [mg  C  m' ] 3  from two  runs w i t h r a n d o m  initial  c o n d i t i o n s . S i m u l a t i o n s i n c l u d e d a d v e c t i o n (but n o b i o l o g i c a l processes) w i t h c o m b i n e d z e r o  /  z e r o - g r a d i e n t b o u n d a r y c o n d i t i o n s (see text). F i r s t t w o p a n e l s s h o w J a n u a r y , s e c o n d t w o p a n e l s October biomass concentrations.  175  176  4.2. Simulation Results and Analysis While  m y models  a n d s i m u l a t i o n s have the potential t o address m a n y s p e c i f i c questions  about the e c o s y s t e m o f the N o r t h e a s t P a c i f i c , the e v a l u a t i o n o f outputs has v a r i o u s  interpretive  p i t f a l l s s t a r t i n g at t h e l e v e l o f q u a l i t y , a n d s p a t i a l a n d t e m p o r a l r e s o l u t i o n o f t h e i n p u t d a t a t o t h e m e c h a n i c s r e p r e s e n t e d i n t h e m o d e l s . I set t h e f o l l o w i n g m i n i m u m r e q u i r e m e n t s  for model  validation: (1) E m p i r i c a l V a l i d a t i o n : M o d e l s m u s t b e c o n s i s t e n t w i t h incorporating requirement  justifiable  mechanisms  at t h e c o r r e c t  l a c k i n g i n s o m e o f the 1 - d i m e n s i o n a l  Ocean funded by  spatial  relevant  observational data, thus  a n d temporal  scales  (a basic  e c o s y s t e m m o d e l s o f the N o r t h e a s t  Pacific  GLOBEC).  (2) O p e r a t i o n a l V a l i d a t i o n : T h e s i m u l a t i o n s m u s t b e a b l e t o p r o v i d e a n s w e r s t o the q u e s t i o n f o r w h i c h I h a v e d e s i g n e d the m o d e l s .  4.2.1. Empirical Validation O n l y a f e w s p a t i a l l y - e x p l i c i t datasets are a v a i l a b l e f o r e m p i r i c a l v a l i d a t i o n o f m o d e l results. In cases w h e r e n o observational data were available I a n a l y z e d the m o d e l  intuitively  most  realistic m o d e l  representation  starting w i t h  the  o f natural processes i n the e c o s y s t e m o f the  N o r t h e a s t P a c i f i c , i.e. t h e s i m u l a t i o n o f the 4 - t r o p h i c l e v e l s m o d e l  including advection, a n d  w o r k i n g m y w a y b a c k w a r d s v i a the 3 - t r o p h i c l e v e l s m o d e l i n c l u d i n g a d v e c t i o n , a n d the 4 - t r o p h i c levels  a n d 3-trophic  levels  models  without  advection.  T h e effort  w a s directed  towards  plausible(!) explanations o f discrepancies between simulation results and current k n o w l e d g e i n  177  the l i g h t o f the s i m p l i f i c a t i o n p r o c e s s that n e c e s s a r i l y m u s t o c c u r w i t h m o d e l d e s i g n (see C h a p t e r 7 i n H o l l i n g 1978). N o n e t h e l e s s , let m e e m p h a s i z e  that e c o l o g i c a l p r o c e s s e s o c c u r at v a r i o u s  organizational,  spatial a n d t e m p o r a l scales, w h i c h generate characteristic patterns i n the natural (e.g. f o o d w e b s , b i o g e o g r a p h i c a l d i s t r i b u t i o n ) . U n f o r t u n a t e l y ,  environment  similar processes d o not  always  result i n s i m i l a r patterns, a n d s i m i l a r patterns c a n often be e x p l a i n e d b y a v a r i e t y o f processes. C o n s e q u e n t l y , c a u s a t i o n i n e c o s y s t e m s i s a r e l a t i v e l y s i m p l e t a s k a n d o n e m u s t b e c a u t i o u s (as a n a n a l y s t a s w e l l as a r e a d e r ) w h e n i d e n t i f y i n g t h e ' t r u e ' m e c h a n i s m .  Seasonal and Interannual Variability: Primary Productivity T h e spatial e v o l u t i o n o f s i m u l a t e d d a i l y p r i m a r y p r o d u c t i v i t y ( F i g . 4.6) f o l l o w s q u a l i t a t i v e l y t h a t w h i c h h a s b e e n i n f e r r e d f r o m e s t i m a t i o n s o f t h e c r i t i c a l d e p t h a n d t h e m i x e d l a y e r d e p t h , as w e l l as f r o m zooplankton However,  d a t a ( F i g . 2 . 4 ; P a r s o n s et al.  the development  o f the horseshoe-shaped  p r o d u c t i v i t y i n s p r i n g o c c u r s about three m o n t h s  1966; pattern  Parsons & LeBrasseur o f increased  daily  1968). primary  later i n simulations than i n Parsons et al.'s  results. T h i s time l a g has t w o potential explanations: (1) S i m u l a t i o n r e s u l t s ( F i g . 4.6) a n d o b s e r v a t i o n s ( F i g . 2.4) d o n o t c o v e r the s a m e y e a r s . T h i s i s t r u e , as P a r s o n s et al. ( 1 9 6 6 ) u s e d c o m p o s i t e d a t a f r o m v a r i o u s s o u r c e s f o r t h e y e a r s 1 9 4 7 t o 1963 w h i l e I present 1982 and 1983 simulation results. H o w e v e r , model  o f the average  spatio-temporal  their result i s a conceptual  onset o f the " s p r i n g b l o o m " ,  i.e. w h e n  critical  depth  b e c o m e s s h a l l o w e r than m i x e d layer depth, rather than actual data. A l s o , the r e a s o n w h y I have m a p p e d the s p a t i o - t e m p o r a l e v o l u t i o n o f d a i l y p r i m a r y  production for  1982  and  1983 i s that  these t w o years represent t w o very different oceanographic c o n d i t i o n s i n the Northeast  Pacific  178  [mg Cm-3 d-1] • 0-10 • 10-20 • 20-30 • 30-10 • 10-50 •  F i g . 4.6:  Simulated daily primary productivity  [mg  C  m"  3  d" ] 1  >50  for January, A p r i l , July  October of 1982 and 1983.  179  and  NC-Pocific MogViews* IM. Pauimrwi 1 JC'CI  F i g . 4.6:  mm  Continued  180  181  F i g . 4.6: C o n t i n u e d  182  ( B r o d e u r & P e a r c y 1992): rather l o w temperatures c a u s e d b y the before  1982/83  E l Nino-Southern  in 1982; and very w a r m conditions i n  O s c i l l a t i o n ( E N S O ) event, the strongest o n  1983, record  1997.  (2) T h e t i m e l a g b e t w e e n s i m u l a t e d a n d o b s e r v e d p r i m a r y p r o d u c t i v i t y i n c r e a s e i s c a u s e d b y the w a y p r i m a r y p r o d u c t i v i t y i s m o d e l e d (see E q . 3 . 1 3 g ) . P r i m a r y p r o d u c t i v i t y i s a function of various environmental  v a r i a b l e s (i.e. m i x e d l a y e r d e p t h , s e a s u r f a c e  nonlinear  temperature,  d a i l y p h o t o s y n t h e t i c a l l y a c t i v e r a d i a t i o n , a n d the c a r b o n - t o - c h l o r o p h y l l - a ratio) as w e l l as the c h l o r o p h y l l - a concentration, w h i c h has been  defined  constant throughout the year,  and  the  p h o t o s y n t h e t i c e f f i c i e n c y , w h i c h represents the i n i t i a l slope o f the p h o t o s y n t h e s i s v s . i r r a d i a n c e curve. E x c e p t for sea-surface temperature, w h i c h is an observed variable (Subsection 3.2.1), each o f the e n v i r o n m e n t a l variables is itself a nonlinear f u n c t i o n o f one or m o r e other  environmental  v a r i a b l e s (e.g. m i x e d l a y e r d e p t h is a f u n c t i o n o f sea surface temperature a n d s c a l a r w i n d s p e e d ( S u b s e c t i o n 3.2.2)). C o n s e q u e n t l y , any o f the f u n c t i o n a l relationships r e l a t i n g these v a r i a b l e s or p a r a m e t e r s to p r i m a r y p r o d u c t i o n has the p o t e n t i a l to c a u s e this t i m e l a g . A s f o r i n t e r a n n u a l v a r i a b i l i t y , s i m u l a t i o n results s h o w that w h i l e J a n u a r y c o n d i t i o n s i n a n d 1 9 8 3 are v e r y s i m i l a r ( F i g . 4.6), the i n c r e a s e i n p r i m a r y p r o d u c t i v i t y p r o g r e s s e s  1982  northward  faster i n spring 1983 although w i t h less o f a latitudinal gradient. In J u l y a n d A u g u s t o f b o t h years the front o f i n c r e a s e d p r i m a r y p r o d u c t i o n reaches its northernmost p o i n t w i t h a h o r s e s h o e - s h a p e d coastal m a x i m u m . In 1983 the p r i m a r y p r o d u c t i o n front reaches further n o r t h w a r d a n d has h i g h e r coastal m a x i m a than i n 1982. B y f a l l 1983 the effects o f the 1 9 8 2 / 8 3 E N S O - e v e n t h a v e c o m p l e t e l y d i s a p p e a r e d w i t h respect to p r i m a r y  almost  productivity.  183  Seasonal and Interannual Variability: Herbivores (Microzooplankton) No  o b s e r v a t i o n a l data o n the seasonal, annual o r d e c a d a l s p a t i o - t e m p o r a l d i s t r i b u t i o n o f  m i c r o z o o p l a n k t o n i n the N o r t h e a s t P a c i f i c are a v a i l a b l e . R e s u l t s f r o m the 4 - t r o p h i c  level simulation with advection show  that  microzooplankton  b i o m a s s c o n c e n t r a t i o n s ( F i g . 4.7) e x h i b i t s p a t i o - t e m p o r a l patterns that are s i m i l a r t o those o f p r i m a r y p r o d u c t i v i t y ( F i g . 4.6). T h i s is not s u r p r i s i n g because: (1) d a i l y h e r b i v o r e a s s i m i l a t i o n w a s d e f i n e d to b e e q u a l t o d a i l y p r i m a r y p r o d u c t i o n ( E q .  3.8),  and (2)  mesozooplankton  combination  o f lack  density i s strongly r e d u c e d i n the w i n t e r m o n t h ( F i g . 4.8),  o f primary  production  a n d high  predation  pressure  due t o a  from  a  large  m a c r o z o o p l a n k t o n standing stock (Fig. 4.9). T h u s m i c r o z o o p l a n k t o n is effectively uncontrolled i n early spring and accumulates q u i c k l y , e v e n p r e c e d i n g the front o f i n c r e a s e d p r i m a r y p r o d u c t i v i t y ( c o m p a r e F i g s . 4.7 a n d 4.6). O n  the  other h a n d results o b t a i n e d f r o m the 3 - t r o p h i c l e v e l s i m u l a t i o n w i t h a d v e c t i o n s h o w m u c h less s i m i l a r i t y i n patterns b e t w e e n m i c r o z o o p l a n k t o n b i o m a s s concentrations ( F i g . 4.10)  and  daily  p r i m a r y p r o d u c t i v i t y ( F i g . 4.7). T h i s is b e c a u s e , i n this case, m e s o z o o p l a n k t o n c o n c e n t r a t i o n s are not being regulated b y a higher trophic level, mesozooplankton c a n b u i l d up b i o m a s s q u i c k l y i n spring and thus control m i c r o z o o p l a n k t o n standing stock effectively.  El  F u r t h e r , as c a n b e s e e n f r o m t h e 4 - t r o p h i c l e v e l s s i m u l a t i o n s t h e c o n s e q u e n c e s o f t h e  1982/83  Nino  primary  d o not  manifest  themselves  i n the  microzooplankton  standing  stock  and  p r o d u c t i o n i n the N o r t h e a s t P a c i f i c u n t i l late s p r i n g 1983, a n d h a v e d i s a p p e a r e d b y f a l l (Fig.4.7). A  c o m p a r i s o n o f s i m u l a t i o n results w i t h a n d w i t h o u t a d v e c t i o n s h o w s that a d v e c t i o n h a d  only  m i n o r effects o n m i c r o z o o p l a n k t o n spatio-temporal distribution patterns i n the 4 - t r o p h i c levels  184  F i g . 4.7: October period:  Simulated microzooplankton concentration [mg of  1982  and  1983.  Simulation: 4-trophic  C  m" ] 3  levels models  for January, A p r i l , July with advection.  and  Simulation  1951-1990.  185  F i g . 4.7: C o n t i n u e d  186  187  « N[-Pacific MopVicwct iW. Dauitam 13061  [mg C m-3] • 0-15 ED 1 5 - 3 0 • 30-15 • 15-60 • > 60  F i g . 4.7: C o n t i n u e d  188  Nt-Pocific -. . ..  F i g . 4.8: October period:  MapVieww \U.  ,. ...... . . „.„...._..  ,. .....  D-ninonr  • ............  VKGI ... ....,  Simulated mesozooplankton  concentration [ m g C m" ] f o r January, A p r i l , J u l y a n d  o f 1982 a n d 1983. Simulation: 4-trophic  3  levels models  with advection.  Simulation  1951-1990.  189  190  NC-Pocific f . i : V *-i IM [i.njmarw, 1'JOGI r  [mg C m-3] | • 0-0.5 • 0.5-1.0 • 1.0-1.5 • 1.5-2.0 • >2.0  F i g . 4.8: C o n t i n u e d  191  , N [-Pacific WapVierrn (M. I-juBiom 1'BCI -tfc> c w v  [mg C m-3] • • • • •  0-0.5 0.5-1.0 1.0-1.5 1.5-2.0 >2.0  (*t 40  F i g . 4.8: C o n t i n u e d  192  F i g . 4.9: October period:  Simulated macrozooplankton of  1982  and  1983.  concentration [mg  Simulation: 4-trophic  C  nf ] 3  levels models  for January, A p r i l , July with advection.  and  Simulation  1951-1990.  193  F i g . 4.9: C o n t i n u e d  194  J v NC-Pocific WopV  U I -».. .1 »  r lil  [mg C m-3]  •  0-3  I I 3~ 6 NC-Pacific MopVicw^r (U. [i-au«ionri 130CI  F i g . 4.9:  • •  6- 9 9-12  •  >12  Continued  196  m o d e l i n both years (compare Figs. 4 . 7 and 4.11), w h i l e i n the 3-trophic levels simulations a d v e c t i o n h a d a m u c h larger effect o n the s p a t i o - t e m p o r a l d i s t r i b u t i o n o f m i c r o z o o p l a n k t o n i n 1982 than i n the 1983-E1 N i n o year (compare F i g s . 4.10 and 4.12).  Seasonal and Interannual Variability Carnivores 1 (Mesozooplankton) T h e s p a t i o - t e m p o r a l d i s t r i b u t i o n o f m e s o z o o p l a n k t o n i s o f g r e a t e s t i n t e r e s t t o t h i s s t u d y as i t is the s i z e c l a s s that represents the m o s t i m p o r t a n t f o o d s o u r c e f o r o c e a n g o i n g j u v e n i l e s o c k e y e s a l m o n ( s e e C o n j e c t u r e i n S e c t i o n 1.4; s e e a l s o S e c t i o n 2 . 1 . a n d S u b s e c t i o n 4 . 2 . 2 ) . A  c o m p a r i s o n o f s i m u l a t i o n results w i t h data s h o w s that t h e s p a t i o - t e m p o r a l i n c r e a s e i n  m e s o z o o p l a n k t o n c o n c e n t r a t i o n i n the 4 - t r o p h i c l e v e l s i m u l a t i o n ( F i g . 4 . 8 ) o c c u r s about  three  m o n t h s l a t e r t h a n i n d i c a t e d i n t h e d a t a ( F i g . 2 . 4 ; P a r s o n s et al. 1 9 6 6 ) . T h i s d i s c r e p a n c y c a n b e e x p l a i n e d b y the f o l l o w i n g : ( 1 ) C o p e p o d s p e c i e s c o l l e c t e d b y P a r s o n s et al. ( 1 9 6 6 ) a r e n o t r e p r e s e n t e d i n t h e m o d e l . A s p o i n t e d out i n 3.4. M e a n F i e l d S i m u l a t i o n s , there i s n o i n f o r m a t i o n a v a i l a b l e o n w h a t d e t e r m i n e s the t i m e o f ascent a n d descent i n the ontogenetic m i g r a t i o n o f copepods  i n the Northeast  Neocalanus  Pacific;  Neocalanus  see S e c t i o n 2.2.), n o r w h a t  s p e c i e s (the  determines  dominant  the survival o f  s p e c i e s at d e p t h . C o n s e q u e n t l y , I h a d n o o t h e r c h o i c e t h a n e x c l u d i n g t h e m f r o m m y  simulations. ( 2 ) I t m a y w e l l b e t h a t c o p e p o d s t h a t h a v e b e e n c o l l e c t e d at n e a r - c o a s t l o c a t i o n s w e r e a c t u a l l y p r o d u c e d i n c o a s t a l e c o s y s t e m s a n d w e r e then transported into the o p e n o c e a n r e g i o n s b y surface currents  (a possibility implicit i n the modeling  Resolution A n d Advection:  Spatial Closure).  results described i n 4 . 1 . S p a t i o - T e m p o r a l  H o w e v e r , the c o a s t a l e c o s y s t e m s h a v e not  been  m o d e l e d at a l l ( s e e S e c t i o n 3 . 1 . ) .  197  [mg C m-3] • 0-10 El 10-20 • 20-30 •  30-40  •  >.40  F i g . 4 . 1 0 : S i m u l a t e d m i c r o z o o p l a n k t o n c o n c e n t r a t i o n [ m g C m" ] f o r J u l y o f 1 9 8 2 ( u p p e r p a n e l ) 3  and 1983 (lower panel). S i m u l a t i o n : 3-trophic levels models w i t h advection. S i m u l a t i o n period: 1 9 5 1 - 1 9 9 0 . N o t e the different s c a l i n g o f b i o m a s s concentrations c o m p a r e d to F i g . 4.6.  198  F i g . 4.11:  Simulated microzooplankton concentration [mg C m ]  and  (lower  3  1983  period:  panel).  Simulation: 4-trophic  levels models  for July o f 1982 (upper panel) without  advection.  Simulation  1951-1990.  199  [mg C m-3] • 0-10 E3 10-20 • 20-30 • 30 - 40 • >40  F i g . 4 . 1 2 : S i m u l a t e d m i c r o z o o p l a n k t o n c o n c e n t r a t i o n [ m g C m" ] f o r J u l y o f 1 9 8 2 ( u p p e r p a n e l ) 3  and  1983  period:  (lower  panel).  Simulation: 3-trophic  levels models  without  advection.  Simulation  1951-1990.  200  D e s p i t e these t w o s h o r t c o m i n g s the s i m u l a t e d m e s o z o o p l a n k t o n horseshoe-shaped  structure i n 1983 but not i n 1982  ( F i g . 4.8).  c o n c e n t r a t i o n attains the  N e v e r t h e l e s s , the  horseshoe-  shaped spatial distribution of mesozooplankton manifests itself i n simulations o f 1962 and  1963,  the years w h e r e the data h a v e b e e n c o l l e c t e d ( F i g . 4.13). W h i l e s i m u l a t i o n s u s i n g the 4 - t r o p h i c l e v e l s m o d e l p r o d u c e m e s o z o o p l a n k t o n c o n c e n t r a t i o n s t h a t a r e w i t h i n t h e o b s e r v e d r a n g e ( f o r S t a t i o n P: 0 . 5 - 3 m g C m " )  simulated  mesozooplankton  densities f r o m the 3 - t r o p h i c l e v e l s are m u c h t o o h i g h (in c e r t a i n areas b y 2 orders o f see d i s c u s s i o n i n 3.4. M e a n  Field Simulations); however,  magnitude;  the e s t a b l i s h m e n t o f a h o r s e s h o e -  s h a p e d h i g h - d e n s i t y belt f o r m e s o z o o p l a n k t o n a r o u n d the edge o f the G u l f o f A l a s k a c a n still be o b s e r v e d i n s u m m e r ( F i g . 4 . 1 4 ) . T h i s p a t t e r n i s c l e a r e r i n 1 9 8 3 t h a n i n 1 9 8 2 , j u s t as i n t h e 4 trophic levels simulation with advection. R e g a r d i n g the effects o f surface currents, a c o m p a r i s o n o f the results o f the 4 - t r o p h i c l e v e l s s i m u l a t i o n w i t h ( F i g . 4.8) a n d w i t h o u t a d v e c t i o n ( F i g . 4.15) s h o w s that a c c u m u l a t i o n effects are a g a i n (as f o r m i c r o z o o p l a n k t o n ) g e n e r a l l y s m a l l b u t a r e l a r g e r i n s u m m e r .  Seasonal and Interannual Variability: Carnivores 2 (Macrozooplankton) No  o b s e r v a t i o n a l data o n the seasonal, annual o r d e c a d a l s p a t i o - t e m p o r a l  distribution o f  m a c r o z o o p l a n k t o n i n the N o r t h e a s t P a c i f i c are a v a i l a b l e . S i m u l a t e d m a c r o z o o p l a n k t o n concentrations (Fig. 4.9) appear to b e m o r e p a t c h i l y distributed (on the large  1° l o n g i t u d e x 1° l a t i t u d e s c a l e ) t h a n c o n c e n t r a t i o n s o f o r g a n i s m s  from  lower  trophic levels (Figs. 4.7 a n d 4.8). A c o m p a r i s o n o f results f r o m s i m u l a t i o n s w i t h ( F i g . 4.9)  and  w i t h o u t a d v e c t i o n ( F i g . 4.16) r e v e a l s that the p a t c h i n e s s is a n e f f e c t o f currents. F u r t h e r m o r e ,  201  -. NC-Pacific MopVicwcf [tt. D-suitann 1 HG|  [mg C m-3] • 0-0.5 0 0.5-1.0 • 1.0-1.5 • 1.5-2.0 • >2.0  F i g . 4.13: Simulated mesozooplankton concentration [ m g C m" ] f o r J u l y o f 1962 (upper panel) 3  and 1963 ( l o w e r panel). S i m u l a t i o n : 4-trophic levels m o d e l s w i t h a d v e c t i o n . S i m u l a t i o n period: 1951-1990.  202  « N [-Pacific ManViewcf lU. D a u m m 1 '»CI  EJ  F i g . 4.14: Simulated mesozooplankton concentration [mg C m" ] for July o f 1982 (upper panel) 3  and 1983 (lower panel). S i m u l a t i o n : 3-trophic levels models w i t h advection. S i m u l a t i o n period: 1951-1990.  203  F i g . 4 . 1 5 : S i m u l a t e d m e s o z o o p l a n k t o n c o n c e n t r a t i o n [ m g C m" ] 3  (first t w o panels) a n d  for July and October of  1983 (second two panels). Simulation: 4-trophic levels models  advection. Simulation period:  1982  without  1951-1990.  204  205  F i g . 4 . 1 6 : S i m u l a t e d m a c r o z o o p l a n k t o n c o n c e n t r a t i o n [ m g C m" ] f o r J u l y a n d O c t o b e r o f 3  (first t w o panels) a n d 1983 ( s e c o n d t w o panels). S i m u l a t i o n : 4 - t r o p h i c l e v e l s m o d e l s advection. Simulation period:  1982  without  1951-1990.  206  NC-Pocific ManViewcf IM. [i.njmonr, T » C |  F i g . 4.16: Continued  ~~r J~ l  these currents l e a d to the v e r y h i g h concentrations o f s i m u l a t e d m a c r o z o o p l a n k t o n a l o n g the coastal f r i n g e o f the G u l f o f A l a s k a . S i m u l a t i o n results f r o m the 4 - t r o p h i c l e v e l s m o d e l w i t h a d v e c t i o n s h o w that s p a t i o - t e m p o r a l b i o m a s s c o n c e n t r a t i o n s o f m a c r o z o o p l a n k t o n ( F i g . 4.9) and m e s o z o o p l a n k t o n ( F i g . 4.8)  appear  to b e i n v e r s e l y related (except f o r the n o r t h w e s t e r n part o f the study area w h e r e b o t h b i o m a s s concentrations  are  low  in  general).  However,  in  summer  the  simulated  macrozooplankton  d i s t r i b u t i o n seems to f o l l o w the d i s t r i b u t i o n o f m i c r o z o o p l a n k t o n (and thus p r i m a r y p r o d u c t i v i t y ; F i g s . 4 . 7 a n d 4 . 6 ) , a r e s u l t c o n s i s t e n t w i t h t h e c o n c e p t o f t r o p h i c c a s c a d i n g ( C a r p e n t e r et 1 9 8 5 ; C a r p e n t e r et al.  al.  1987).  W h i l e it has b e e n s h o w n e m p i r i c a l l y that s p e c i e s a s s e m b l a g e s a n d t r o p h i c r e l a t i o n s h i p s o f f t h e N o r t h A m e r i c a n w e s t c o a s t c h a n g e as a c o n s e q u e n c e o f a n E N S O e v e n t ( B r o d e u r &  Pearcy  1 9 9 2 ) , m y s i m u l a t i o n results s h o w that the i n t e r a n n u a l v a r i a b i l i t y i n m a c r o z o o p l a n k t o n b i o m a s s c o n c e n t r a t i o n s i n the s o u t h e r n part o f the s t u d y a r e a are o n l y s m a l l . H o w e v e r , it i s i n the s o u t h e r n r e g i o n s w h e r e the m a n y a s s u m p t i o n s m a d e d u r i n g m o d e l a n d s i m u l a t i o n d e s i g n (see C h a p t e r 3) m i g h t not hold.  208  Interdecadal Variability: Carnivores 1 (Mesozooplankton) Mean 1980  simulated mesozooplankton  to 1989  periods (Fig.  (Fig. 4.17) 1.7; B r o d e u r  concentrations f o r the m o n t h o f J u l y  almost completely contradict observations & Ware  1992 their Figure  made  1956 t o 1959  and  i n the same  time  1.). N o t o n l y d o t h e s i m u l a t e d r e s u l t s  suggest a decrease i n o v e r a l l m e s o z o o p l a n k t o n concentrations b e t w e e n the late 1950s and 1980s but this decrease also o c c u r s i n l o c a t i o n s w h e r e the strongest i n c r e a s e has b e e n  the  measured  ( o n the n o r t h e r n f r i n g e o f the G u l f o f A l a s k a ) . T h i s d i s c r e p a n c y c a n b e e x p l a i n e d b y the fact that the m o d e l e d m e s o z o o p l a n k t o n  species d o not represent the s a m p l e d m e s o z o o p l a n k t o n  species  r e p o r t e d b y B r o d e u r & W a r e ( 1 9 9 2 ) ; see a l s o 3.4. M e a n F i e l d S i m u l a t i o n s ) . It i s i m p o r t a n t t o note h o w o n e ' s p e r c e p t i o n o f the interdecadal v a r i a b i l i t y i n z o o p l a n k t o n s t a n d i n g s t o c k changes w h e n y o u l o o k at t h e s i m u l a t i o n r e s u l t s f o r t h e m o n t h o f A u g u s t f o r b o t h p e r i o d s o f t i m e ( F i g . 4.18). In  summary,  understanding  model  and simulation design (Chapter  3) f o l l o w e d the current  o f processes i n the e c o s y s t e m N o r t h e a s t P a c i f i c  (Chapter  mechanistic  2). Spatio-temporal  scales a n d r e s o l u t i o n o f the s i m u l a t i o n s w e r e a c o m p r o m i s e b e t w e e n the r e s o l u t i o n o f the input data, the a s s u m e d relevant scales o f b i o l o g i c a l processes, and c o m p u t a t i o n t i m e requirements. for observations: W h i l e  As  m a n y b i o l o g i c a l v a r i a b l e s h a v e b e e n c o l l e c t e d a t S t a t i o n P, m o s t o f  w h i c h are i r r e l e v a n t f o r m y s i m u l a t i o n s (e.g. nutrients, c h l o r o p h y l l - a , o n t o g e n e t i c a l l y  migrating  zooplankton  Since a l l  species;  see  Chapter  3), spatially-explicit biological  data  a r e rare.  o b s e r v a t i o n a l data w e r e t a k e n f r o m the p u b l i s h e d literature a n d w e r e therefore not d e s i g n e d t o test a n y p a r t i c u l a r h y p o t h e s i s o f the m o d e l s , e m p i r i c a l m o d e l v a l i d a t i o n i s a d i f f i c u l t t a s k (see C h a p t e r 5). H o w e v e r , i t m u s t b e e m p h a s i z e d that the s i m u l a t i o n s o f the 4 - t r o p h i c l e v e l s m o d e l  209  F i g . 4 . 1 7 : M e a n s i m u l a t e d m e s o z o o p l a n k t o n c o n c e n t r a t i o n s [ m g C m" ] 3  f o r the m o n t h o f J u l y  1956 to 1959 (upper panel) a n d 1980 to 1989 (lower panel). S i m u l a t i o n : 4 - t r o p h i c l e v e l s m o d e l s with advection. Simulation period:  1951-1990.  210  [mg C m-3] • 0-0.5 B 0.5-1.0 • 1.0-1.5 • 1.5-2.0 • >2.0  F i g . 4 . 1 8 : M e a n s i m u l a t e d m e s o z o o p l a n k t o n c o n c e n t r a t i o n s [ m g C m" ] f o r the m o n t h o f A u g u s t 3  1956 to 1959 ( u p p e r p a n e l ) a n d 1980 to 1989 ( l o w e r panel). S i m u l a t i o n : 4 - t r o p h i c l e v e l s m o d e l s with advection. Simulation period:  1951-1990.  211  including  advection  produced  seasonal  spatio-temporal  biomass  concentrations  that  are i n  general agreement w i t h intuitive expectations ( w h i c h , o f course, is h a r d l y a measure o f validity).  4.2.2. Operational Validation W e m u s t n o w see w h e t h e r the s i m u l a t i o n s p r o v i d e a n s w e r s t o the q u e s t i o n s f o r w h i c h I h a v e d e s i g n e d t h e m o d e l s ( s e e C o n j e c t u r e i n S e c t i o n 1.4.): H o w , i f at a l l , d o e s t h e a v a i l a b i l i t y o f p r e y o r g a n i s m s , w h o s e d e n s i t y at a p a r t i c u l a r l o c a t i o n a n d t i m e i s the result o f b i o l o g i c a l p r o d u c t i o n p r o c e s s e s as w e l l as a d v e c t i o n , a f f e c t s o c k e y e s a l m o n c o h o r t s u r v i v a l ? To  answer  this  question  I  have  looked  at t h e spatial  progression  of  simulated  m e s o z o o p l a n k t o n density f r o m J u l y t o the f o l l o w i n g F e b r u a r y f o r years w i t h the l o w e s t a n d the highest cohort survival o fc o m b i n e d Fraser R i v e r stocks and c o m b i n e d B r i s t o l B a y river systems, r e s p e c t i v e l y ( s e e F i g . 1.4). A l l s i m u l a t i o n r e s u l t s a r e f r o m t h e 4 - t r o p h i c l e v e l s m o d e l  including  advection.  Fraser River Stocks After emergence f r o m gravel Fraser R i v e r sockeye salmon spend one winter i n freshwater a n d t w o w i n t e r s i n the o c e a n ( B u r g n e r 1991 h i s T a b l e 2). I first s c a n n e d f o r c o h o r t s w i t h l o w a n d h i g h s u r v i v a l r a t e s , r e s p e c t i v e l y , a n d t h e n l o o k e d at t h e s i m u l a t e d s p a t i o - t e m p o r a l d i s t r i b u t i o n o f m e s o z o o p l a n k t o n i n the year the respective y e a r - c l a s s entered the o c e a n . B r o o d years w i t h the r e s p e c t i v e l o w e s t a n d highest s u r v i v a l rate o f c o m b i n e d F r a s e r R i v e r s t o c k s are 1958 a n d  1955  ( s e e F i g . 1.4). H o w e v e r , i n o r d e r t o a v o i d s p a t i a l i n i t i a l c o n d i t i o n e f f e c t s , 1 9 5 5 i s c l o s e t o t h e s i m u l a t i o n starting p o i n t i n 1 9 5 1 , 1 c o n s i d e r e d the c o h o r t w i t h the s e c o n d h i g h e s t s u r v i v a l rate, i.e. the b r o o d y e a r class 1981. T h u s m o n t h l y m a p s h a v e b e e n p l o t t e d f o r J u l y 1 9 6 0 t o F e b r u a r y  212  1961 (1960: o c e a n entry o f l o w s u r v i v a l cohort; F i g . 4.19), a n d for J u l y 1983 to F e b r u a r y  1984  (1983: ocean entry of h i g h survival cohort; F i g . 4.20). A c o m p a r i s o n o f the respective m o n t h s o f the l o w ( F i g . 4.19) a n d h i g h s u r v i v a l years 4.20)  s h o w s f o r J u l y , i.e. the m o n t h w h e n j u v e n i l e s o c k e y e s a l m o n l e a v e the S t r a i t o f  t h r o u g h J o h n s t o n e S t r a i t a n d are c o n f r o n t e d w i t h the o p e n o c e a n f o r the f i r s t t i m e  (Fig.  Georgia (Burgner  1991), s i m u l a t e d m e s o z o o p l a n k t o n m a x i m a were both higher and larger i n extent i n the eastern part o f the G u l f o f A l a s k a i n the h i g h s u r v i v a l year. H o w e v e r , i n A u g u s t the s i t u a t i o n reversed, a n d i n the h i g h - s u r v i v a l year the h o r s e s h o e - s h a p e d h i g h density r i d g e w a s far offshore.  Simulated  p r e y a v a i l a b i l i t y i n September (without c o n s i d e r i n g r i s k o f predation o n part o f s o c k e y e s a l m o n ) a c t u a l l y suggests that s o c k e y e s a l m o n s h o u l d fare better i n the l o w s u r v i v a l year. T h e  spatio-  t e m p o r a l d i s t r i b u t i o n o f s i m u l a t e d m e s o z o o p l a n k t o n i n O c t o b e r o f the h i g h s u r v i v a l year s h o w s a h i g h concentration about 7 0 0 k m south o f K o d i a k Island, too far offshore for j u v e n i l e sockeye w h i c h at t h i s t i m e s t i l l l i v e c l o s e t o o r e v e n i n c o a s t a l w a t e r s . T h e  spatial distributions  simulated mesozooplankton densities f r o m N o v e m b e r  to F e b r u a r y l o o k s i m i l a r , a n d i f  suggest  produced  that  the  low  survival  year  should  have  high  cohort  of  anything  survival  rates.  213  F i g . 4.19:  Monthly  simulated mesozooplankton  concentrations [mg  C  m" ] 3  for July  1960  to  F e b r u a r y 1961 ( l o w s u r v i v a l year for F r a s e r R i v e r s o c k e y e s a l m o n ) . N o t e the c h a n g e i n scale for November period:  to F e b r u a r y  maps. Simulation: 4-trophic levels models with advection.  Simulation  1951-1990.  214  •a  NC-r«cific MooViewcf IM.  C-sLmiinri  TBCI  Fig. 4.19: Continued  215  216  217  Fig.  Monthly  4.20:  February  1984  November period:  simulated mesozooplankton  (high survival year  to February  concentrations [mg  for Fraser River  salmon).  Note  C  m" ] f o r J u l y  the  change  in  maps. Simulation: 4-trophic levels models with advection.  1983 scale  to for  Simulation  1951-1990.  218  220  Fig. 4.20: Continued  221  Bristol Bay River Systems B r i s t o l B a y s o c k e y e s a l m o n spend one or t w o winters i n freshwater a n d t w o or three winters i n the o c e a n (Burgner scanned  the  data  1991 T a b l e 2). T o a c c o u n t f o r the v a r i a b i l i t y i n s e a w a r d m i g r a t i o n I first  for  pairs  of  successive brood  years  with  low  and  high  survival  rates,  r e s p e c t i v e l y , a n d t h e n l o o k e d at t h e s p a t i o - t e m p o r a l d i s t r i b u t i o n o f m e s o z o o p l a n k t o n t w o y e a r s a f t e r t h a t l a t e r y e a r , i . e . t h e t i m e w h e n a 1.x f i s h t h a t w a s s p a w n e d i n t h a t l a t e r y e a r  would  m i g r a t e to sea. T h e pairs o f b r o o d years w i t h l o w a n d h i g h s u r v i v a l are 1 9 6 8 / 1 9 6 9 a n d  1976/77  ( s e e F i g . 1.4). M o n t h l y m a p s h a v e b e e n p l o t t e d f o r J u l y 1 9 7 1 t o F e b r u a r y  survival  1972 (low  y e a r ; F i g . 4 . 2 1 ) , a n d f o r J u l y 1979 to F e b r u a r y 1980 ( h i g h s u r v i v a l y e a r ; F i g . 4 . 2 2 ) . C o m p a r i n g F i g s . 4.21 ( l o w s u r v i v a l year) a n d 4 . 2 2 ( h i g h s u r v i v a l year) s h o w s that the h i g h density m e s o z o o p l a n k t o n front progresses w e s t w a r d m u c h faster a n d reaches further w e s t i n the h i g h s u r v i v a l year. B y A u g u s t , i.e. the t i m e w h e n j u v e n i l e B r i s t o l B a y s o c k e y e s a l m o n m i g r a t e s o u t h w a r d i n t o the G u l f o f A l a s k a , a r e g i o n w i t h h i g h m e s o z o o p l a n k t o n d e n s i t y has e s t a b l i s h e d itself for  the h i g h  survival year  (Fig.  4.22),  while for  the l o w  density year  the  westward  m o v e m e n t o f the front has stagnated ( F i g . 4.21). T h e S e p t e m b e r m a p s h o w s a n a g g r e g a t i o n  of  s i m u l a t e d m e s o z o o p l a n k t o n j u s t south o f the A l a s k a P e n i n s u l a i n the h i g h s u r v i v a l year, traces o f which  still  can  be  found  in  October  (Fig.  4.22).  The  spatial  distribution  of  simulated  mesozooplankton concentrations in N o v e m b e r and December indicates a relatively high standing s t o c k o n the southeastern fringe o f the G u l f o f A l a s k a f o r the l o w s u r v i v a l year a n d a n offshore w e s t e r n a c c u m u l a t i o n south o f the A l e u t i a n Islands f o r the h i g h s u r v i v a l year. J a n u a r y  and  F e b r u a r y m a p s f o r b o t h years s h o w the s a m e s p a t i a l d i s t r i b u t i o n s t h a n i n the r e s p e c t i v e p r e v i o u s months only w i t h n o w lower concentrations.  222  •a NC-Pacific ManViewcf I.W. I.njiiarr,  Fig.  4.21:  Monthly  I'KGI  simulated mesozooplankton  concentrations [ m g C m" ] f o r July  1971 t o  February 1972 ( l o w s u r v i v a l year for B r i s t o l B a y sockeye salmon). N o t e the change i n scale f o r November period:  to February  maps. Simulation: 4-trophic levels models with advection.  Simulation  1951-1990.  223  « NC-Pocific WaoViewo IM. D auvarn 1 » G I  [mg C m-3] • 0-0.5 • 0.5-1.0 1 1.0-1.5 I 1.5-2.0 I >2.0  Fig. 4.21: Continued  224  Fig. 4.21: C o n t i n u e d  225  Fig. 4.21: Continued  226  F i g . 4.22:  Monthly  February November period:  simulated mesozooplankton  concentrations [mg  C  m" ] f o r J u l y 1 9 7 9  to  1980 ( h i g h s u r v i v a l year f o r B r i s t o l B a y s o c k e y e s a l m o n ) . N o t e the c h a n g e i n scale f o r to F e b r u a r y  maps. Simulation: 4-trophic levels models w i t h advection.  Simulation  1951-1990.  227  NE-PaciFic  MooVicrro . IW .  [Lniiowi  1  \KiG|  Fig. 4.22: C o n t i n u e d  228  F i g . 4.22: C o n t i n u e d  229  F i g . 4.22: C o n t i n u e d  230  In s u m m a r y , effects o f the spatio-temporal distribution o f prey for j u v e n i l e s o c k e y e s a l m o n , i.e. m e s o z o o p l a n k t o n plausible  for  (see S e c t i o n 2.1.), o n the s u r v i v a l rates o f c o m b i n e d  Bristol Bay  sockeye salmon  than  for  combined  Fraser  River  s t o c k s are stocks.  c o n s i s t e n t w i t h the fact that s u r v i v a l rates o f the B r i s t o l B a y r i v e r s y s t e m s are m o r e  more  This  is  frequently  c r o s s - c o r r e l a t e d w i t h e a c h other than F r a s e r R i v e r s t o c k s are w i t h e a c h other (see F i g .  1.3).  H o w e v e r , k n o w i n g better t h a n a n y b o d y e l s e the s h o r t c o m i n g s o f the i n p u t d a t a as w e l l as a l l the a s s u m p t i o n s that w e n t i n t o the p o p u l a t i o n m o d e l s a n d e c o s y s t e m s i m u l a t i o n s , m y  interpretation  o f the result is rather devastating: S i m u l a t i o n results o f m y s p a t i a l l y - e x p l i c i t s i m u l a t i o n s d o suggest a c l e a r l i n k a g e b e t w e e n prey density i n the o c e a n i c e n v i r o n m e n t  and sockeye  not  salmon  c o h o r t s u r v i v a l (see a l s o C h a p t e r 5).  231  5.  CONCLUSIONS  "The primary value of models is heuristic." N . O r e s k e s et a l . ( 1 9 9 4 )  "But many of our pictures are incarnations of concepts masquerading as neutral descriptions of nature. These are the most potent sources of conformity ..." S.J. G o u l d (1989) W o n d e r f u l L i f e  T h e c o n c l u s i o n s to m y r e s e a r c h are stated u n d e r the a s s u m p t i o n that i n p u t d a t a ( S e c t i o n as w e l l a s d a t a u s e d t o v a l i d a t e s i m u l a t i o n r e s u l t s ( S e c t i o n 4 . 2 . )  somehow  reflect the  3.2.)  natural  w o r l d . T h i s a s s u m p t i o n is p r o b a b l y reasonable e v e n t h o u g h it has been q u e s t i o n e d i n p r i n c i p l e ; clearly, w i t h o u t it any interpretation is possible. Furthermore, I w i l l abstain f r o m suggesting lists o f i m p r o v e d input data, and critical data (variables, locations, time) for m o d e l and  simulation  v a l i d a t i o n , a s w e l l as f r o m s u g g e s t i o n s f o r m o d e l i m p r o v e m e n t s . A l t h o u g h s u c h p r o p o s i t i o n s a r e s t a n d a r d p r a c t i c e , t h e y a r e e i t h e r o b v i o u s ( e . g . I f c r u c i a l d a t a o f a s p e c i f i c k i n d at a p a r t i c u l a r l o c a t i o n f o r a certain p e r i o d o f t i m e h a v e not yet b e e n c o l l e c t e d , they s h o u l d be c o l l e c t e d i n the future.) o r i m p l i c i t i n the m o d e l a n d s i m u l a t i o n d e v e l o p m e n t  ( C h a p t e r 3 a n d S e c t i o n 4.1.)  and  s i m u l a t i o n results ( S e c t i o n s 3.4. a n d 4.2.).  C o n c l u s i o n : I h a v e tried to d e s i g n the best m o d e l s w i t h i n reason u t i l i z i n g the best on  environmental  information  f o r c i n g s a n d b i o l o g i c a l p r o c e s s e s a v a i l a b l e at t h e t i m e . N e v e r t h e l e s s ,  results d o not suggest a clear l i n k a g e b e t w e e n prey density i n the o c e a n i c e n v i r o n m e n t  my and  sockeye salmon cohort survival.  232  C o r o l l a r y #1:  W h i l e l i f e h i s t o r y strategies are a n u i s a n c e to t r o p h o d y n a m i c m o d e l i n g , t h e y are  the essence o f life.  Sockeye Salmon T h e e n v i r o n m e n t t h a t h a s b e e n s i m u l a t e d r e p r e s e n t s at b e s t a f r a c t i o n o f t h e s p a c e a n d t i m e o f s o c k e y e s a l m o n l i f e t i m e habitat (i.e. the i n t e g r a l o f a b i o t i c a n d b i o t i c f a c t o r s that a f f e c t s o c k e y e s a l m o n i n c e r t a i n l o c a t i o n s at c e r t a i n t i m e s ) : A f t e r e m e r g e n c e a j u v e n i l e s o c k e y e s a l m o n s p e n d s o n e o r t w o w i n t e r s i n a l a k e , f o l l o w e d b y a m i g r a t i o n to the sea w h e r e its spends another one to three w i n t e r s ( B u r g n e r 1991). In e a c h o f the e n c o u n t e r e d habitats (i.e. c r e e k , r i v e r , l a k e , r i v e r , estuary,  coastal ocean, open  ocean, coastal ocean, estuary,  river,  creek)  a sockeye  salmon  interacts w i t h l o c a l populations b y foraging u p o n prey, outwitting intraspecific and interspecific competitors, and a v o i d i n g predators, all before a b a c k g r o u n d o f abiotic environmental  conditions  (e.g. temperature a n d salinity), w i t h the s i m p l e g o a l to s u r v i v e a n d r e p r o d u c e . A s a n i n d i v i d u a l enters e a c h o f these habitats it w i l l h a v e to m a k e b e h a v i o r a l d e c i s i o n s (e.g. w h e n to forage, h i d e , emigrate)  depending  on  its b o d y  size  (a  function  of  previous  habitats  and  thus  historical  contingent), predation risk and growth potential (both c o m p l e x functions of biotic and abiotic c o m p o n e n t s o f t h e h a b i t a t i t i s i n ) . W h a t ' s m o r e , a n i n d i v i d u a l w i l l a d a p t t o t h e s i t u a t i o n at h a n d w i t h i n the l a r g e r c o n t e x t o f the average b e h a v i o r (i.e. l i f e h i s t o r y strategy). A l t h o u g h the s o c k e y e s a l m o n life c y c l e is relatively simple (semelparous, constant life c y c l e w i t h clearly defined life h i s t o r y stages i n different habitats) the c o m p l e x i t y o f the s p e c i f i c s i s c l e a r l y o v e r w h e l m i n g . W h i l e i t i s a s s u m e d that e a r l i e r l i f e h i s t o r y stages h a v e h i g h e r s p e c i f i c m o r t a l i t y rates as w e l l as h i g h e r v a r i a b i l i t y i n s p e c i f i c m o r t a l i t y r a t e s t h a n l a t e r o n e s , i t i s n o t k n o w n  whether  one  p a r t i c u l a r l i f e h i s t o r y stage determines year class s u r v i v o r s h i p o f s o c k e y e s a l m o n n o r w h e t h e r it  233  i s t h e s a m e f o r e v e r y c o h o r t o f e v e r y s t o c k ( s e e A s s u m p t i o n #1 i n S e c t i o n 1.4.). Y e t , i t m i g h t w e l l b e that f o r s o m e s t o c k s i n s o m e years c o h o r t s u r v i v a l is d e t e r m i n e d e a r l y i n m a r i n e  life.  H o w e v e r , e v e n i f early m a r i n e l i f e determines year class s u r v i v o r s h i p , h o w l i k e l y i s it that I w i l l see s i m i l a r t e m p o r a l patterns i n s o c k e y e c o h o r t s u r v i v a l a n d s p a t i o - t e m p o r a l d i s t r i b u t i o n o f p r e y d e n s i t y i n the northeastern G u l f o f A l a s k a ? N o t very, because o f the f o l l o w i n g : (1) T o p - d o w n a r g u m e n t : J u v e n i l e f i s h d o n ' t u s u a l l y starve t o d e a t h b u t are r a t h e r p r e y e d u p o n b y p r e d a t o r s . H o w e v e r , n e i t h e r f i s h n o r p r e d a t o r s are i n c l u d e d i n m y m o d e l s (see S e c t i o n 3.1.), and  even  if they  were,  current  computational  limitations do  not  allow  implementation  of  b e h a v i o r a l r e s p o n s e s at t h e c o r r e c t s p a t i o - t e m p o r a l s c a l e s ( s e e A s s u m p t i o n s #2 a n d #4 i n S e c t i o n 1.4.). ( 2 ) B o t t o m - u p a r g u m e n t : It h a s b e e n s h o w n t h a t i n t h e f i r s t m o n t h s at s e a ( J u l y t o F e b r u a r y ) j u v e n i l e B r i t i s h C o l u m b i a s o c k e y e s a l m o n m i g r a t e w i t h the m a i n currents (see F i g . 2.1) a l o n g the coastal regions o f the G u l f o f A l a s k a (J. S c a n d o l 1996 pers. c o m m . (simulations); D. W e l c h pers.  comm.  (data)).  However,  insufficient information  on  the  ecosystem  of  the  1998  Coastal  D o w n w e l l i n g D o m a i n ( S u b s e c t i o n 2.2.2.), the l a c k o f a c o a s t a l a d v e c t i o n m o d e l , a n d the s p a t i o t e m p o r a l r e s o l u t i o n o f the i n p u t d a t a ( S e c t i o n 3.2.) f o r c e d m e e x c l u d e the c o a s t a l r e g i o n f r o m m y s i m u l a t i o n s . F u r t h e r , w h i l e i n nature o n e sees a t r a n s i t i o n f r o m o p e n o c e a n to c o a s t a l e c o s y s t e m s w i t h a variable seasonal a n d interannual gradient (steepness, space, time) i n species distributions ( D . M a c k a s 1 9 9 6 p e r s . c o m m . ) , a l l h a b i t a t s w e r e s i m u l a t e d as o p e n o c e a n e c o s y s t e m s , n o t a b l y with  no  consideration  for  the  seasonal  variability  in  chlorophyll-a  and  macronutrient  c o n c e n t r a t i o n s ( S e c t i o n 3.1.).  (3)  Argument  of  spatio-temporal  scales:  Let's  for  a  moment  assume  that  mesozooplankton densities determine year-class survival i n sockeye salmon. H o w w e l l do  the the  234  p r e y d i s t r i b u t i o n s d e p i c t e d i n the m a p s i n S e c t i o n 4.2. reflect the a v a i l a b i l i t y o f p r e y to s o c k e y e s a l m o n i n the natural e n v i r o n m e n t (even i n the absence o f predators)? T h e m a p s  show  monthly  m e a n c o n c e n t r a t i o n s w i t h a s p a t i a l r e s o l u t i o n o f 1° l o n g i t u d e x 1° l a t i t u d e ( a p p r o x i m a t e l y 1 0 0 x 100  km).  Consequently,  natural patchiness b e l o w  that r e s o l u t i o n i s not  represented  i n the  s i m u l a t i o n s . T h u s , a s c h o o l o f j u v e n i l e s o c k e y e s a l m o n that enters the s i m u l a t e d o c e a n habitat i n J u l y w i l l f i n d a c o m p l e t e l y u n i f o r m 100 x 100 k m patch w h i c h it w i l l cross w i t h i n four days o r so. T h e s c h o o l w i l l then enter the next c o m p l e t e l y u n i f o r m 100 x 100 k m p a t c h , a n d so o n . W h i l e the o u t m i g r a t i o n mismatch  t i m i n g has been d e e m e d  hypothesis)  the spatio-temporal  important scales  ( c u l m i n a t i n g i n the c o n c e p t u a l m a t c h -  o f the  ecological processes involved are  e x t r e m e l y d i f f i c u l t t o a s s e s s . It i s u n l i k e l y t h a t a c o a r s e 1 0 0 x 1 0 0 k m g r i d d o e s p r o v i d e  the  c o r r e c t s p a t i a l a n d t e m p o r a l scales t o a c c o u n t f o r p o p u l a t i o n l e v e l e c o l o g i c a l p r o c e s s e s (see A s s u m p t i o n #4 i n S e c t i o n 1.4.).  Zooplankton The  obviously important  survival, important  and  i f so, what  question is: Does  determines  aspect o f the o p e n  increased zooplankton  zooplankton  abundance,  how,  abundance  when  o c e a n e c o s y s t e m o f the N o r t h e a s t P a c i f i c  and  are  affect  where?  fish One  ontogenetically  migrating c o p e p o d species. Unfortunately, until n o w only descriptive studies covering l i m i t e d s p a t i o - t e m p o r a l d o m a i n s (the p e r i o d w h e n c e r t a i n d e v e l o p m e n t a l stages i n h a b i t the s u r f a c e ) h a v e been conducted (R. Goldblatt  1998  pers. c o m m . ) ,  mostly due  t o the l o g i s t i c d i f f i c u l t i e s o f  e x p l o r i n g a m e s o p e l a g i c ecosystem. Important questions o n the l i f e history o f ontogenetically m i g r a t i n g z o o p l a n k t o n s p e c i e s are:  235  (1) W h a t d e t e r m i n e s t h e i r t i m e o f a s c e n t a n d d e s c e n t i n the o n t o g e n e t i c m i g r a t i o n ? ( 2 ) W h a t d e t e r m i n e s t h e i r s u r v i v a l at d e p t h ? (3) H o w d o s u r f a c e a n d d e e p - w a t e r c u r r e n t s a f f e c t t h e i r d i s t r i b u t i o n ? W h i l e l i t t l e i s k n o w n a b o u t t h e a g e n t s i n t h e l i f e h i s t o r y o f m e s o z o o p l a n k t o n ( i . e . Neocalanus spp.) e v e n less i n f o r m a t i o n is a v a i l a b l e o n m a c r o z o o p l a n k t o n . N o t o n l y are certain gelatinous a n d fast  swimming  groups  of  macrozooplankton  undersampled  by  standard  sampling  devices  (Parsons & L a l l i 1988) but the l a c k o f k n o w l e d g e about the b i o l o g y o f the o r g a n i s m s o f this s i z e c l a s s a l s o c o m p e l l e d m e t o m o d e l m a c r o z o o p l a n k t o n m o r t a l i t y ( i n t h e 4 - t r o p h i c l e v e l s m o d e l ) as a d e n s i t y - i n d e p e n d e n t f u n c t i o n o f adult b o d y s i z e and temperature ( S e c t i o n 3.4.). A l t h o u g h it is p o s s i b l e i n p r i n c i p l e , m o r t a l i t y i s n o t l i k e l y t o b e d e n s i t y - i n d e p e n d e n t at a l l l i f e - h i s t o r y s t a g e s . S i n c e l i f e - h i s t o r y s t r a t e g i e s at a l l t r o p h i c l e v e l s h a v e t h e p o t e n t i a l o f a l t e r i n g s i m u l a t i o n results s i g n i f i c a n t l y future m o d e l i n g a n d s i m u l a t i o n excercises w i l l u l t i m a t e l y h a v e to address them.  C o r o l l a r y #2: T r o p h o d y n a m i c s i m u l a t i o n s are i n a d e q u a t e to p r e d i c t e f f e c t s o f e c o s y s t e m s o n the dynamics of a particular population. T r o p h o d y n a m i c m o d e l s a n d s i m u l a t i o n s are o f the t y p e d e v e l o p e d i n t h i s t h e s i s w h e r e v a r i o u s g r o u p s o f o r g a n i s m s are aggregated i n t o h y p o t h e t i c a l t r o p h i c l e v e l s w h i c h t h r o u g h c o n s u m p t i o n a n d p r o d u c t i o n p r o c e s s energy (in the f r o m o f r e d u c e d c a r b o n c o m p o u n d s ) . D u r i n g m o d e l d e s i g n I a s s u m e d that f o r p l a n k t o n o r g a n i s m different s i z e classes d o represent different t r o p h i c l e v e l s ( s e e A s s u m p t i o n #3 trophodynamic  in Chapter  1). H o w e v e r , t h e r e a r e s e v e r a l p r o b l e m s a s s o c i a t e d w i t h t h i s  a p p r o a c h (see a l s o C o u s i n s  1987; Peters  1977, and for a synthesis  Oksanen  1991):  236  (1)  P a r t i c u l a r s p e c i e s c a n n o t b e c a t a l o g u e d to a p a r t i c u l a r i n t e g e r t r o p h i c l e v e l , i.e.  s p e c i e s h a v e a m i x e d diet. S o m e authors (e.g. P a u l y & C h r i s t e n s e n 1 9 9 5 a ; P a u l y & 1 9 9 5 b ; W u l f f et al.  most  Christensen  1989) have tried to go a r o u n d this p r o b l e m b y a l l o c a t i n g o r g a n i s m s to partial  trophic levels (TL),  o r e f f e c t i v e t r o p h i c p o s i t i o n s ( F i e l d et  al.  1989), f o l l o w i n g  the  simple  formula:  TL  =  1  +X  (weight o f f o o d i t e m i i n stomach contents) •(trophic l e v e l o f f o o d i t e m i) (total w e i g h t o f s t o m a c h contents)  w h e r e the s u m represents the m e a n t r o p h i c l e v e l o f the p r e y o r g a n i s m s . H o w e v e r , w e i g h t (wet, dry, carbon?)  one c o u l d use v o l u m e  or energy  content ( L i n d e m a n n  i n s t e a d o f s t o m a c h c o n t e n t s , i.e. i n g e s t e d f o o d , o n e c o u l d use a s s i m i l a t e d f o o d .  (Eq.5.1)  instead 1942),  of and  Determining  t r o p h i c l e v e l s o f a l l the f o o d i t e m s i n a f o o d w e b is thus not o n l y tedious but a l s o a m b i g u o u s .  See  also the s i x different definitions for "trophic l e v e l " g i v e n i n Y o d z i s (1989). (2)  While  knowledge  the  about  aggregation  of  the  (Rice  system  biospecies into 1995)  and  trophic will  thus  levels is generally result  in  unequal  a  function  of  resolution  of  a g g r e g a t i o n , a g g r e g a t i o n b y s i z e c l a s s as d o n e i n m y m o d e l s s e e m s l e s s a r b i t r a r y t h a n a n y o t h e r c a t e g o r i z a t i o n i n m a r i n e s y s t e m s . H o w e v e r , i f w e l o o k at s o c k e y e s a l m o n w e f i n d t h a t  some  o r g a n i s m s are p r e y f o r s o c k e y e s a l m o n o r f o o d f o r its prey, a n d so o n . O t h e r s are c o m p e t i t o r s ( r e s i d i n g i n t h e ' f i s h ' b o x i n F i g . 1.9)  or predators o f s o c k e y e s a l m o n , yet others prey  sockeye competitors or predators thus i m p r o v i n g  survival of sockeye salmon, while  upon  sockeye  s a l m o n itself is predator, competitor, prey, or cause o f indirect positive or negative effects o n other populations. ( A n y indirect effects w h i c h non-adjacent trophic levels have onto each other a r e c a l l e d t r o p h i c c a s c a d i n g ( C a r p e n t e r et  al.  1 9 8 5 ; C a r p e n t e r et  al.  1987).) S o , for e x a m p l e it  237  m i g h t w e l l b e that m e s o z o o p l a n k t o n p r o d u c t i o n a n d a v a i l a b i l i t y i n c r e a s e s w h i l e s o c k e y e s a l m o n survival decreases. T h u s , m o r e i m p o r t a n t than the c o n c e p t o f trophic l e v e l i t s e l f is the heterogeneity (diversity i n s p e c i e s a n d l i f e h i s t o r y strategies) that o n e w i l l i n c o r p o r a t e i n t o w h a t e v e r a g g r e g a t i o n o n e is g o i n g to c h o o s e . A s I h a v e t r i e d to d e m o n s t r a t e l i f e h i s t o r y strategies are v e r y c o m p l e x c o n c e p t s (Corollary  #1),  a n d c o n s e q u e n t l y it is not guaranteed  that the " a v e r a g e "  simulated  o r g a n i s m r e p r e s e n t i n g a c e r t a i n s i z e class w i l l r e s p o n d to a b i o t i c (e.g. temperature)  plankton and biotic  ( e . g . p r e y d e n s i t y ) i n t h e s a m e w a y as t h e d i v e r s i t y o f s p e c i e s i n t h e n a t u r a l s y s t e m ( s e e B r o w n  &  R o t h e r y (1993) their S e c t i o n 8.15). (3) A  p a r t i c u l a r s p e c i e s w i l l n o t o c c u p y t h e s a m e t r o p h i c l e v e l at d i f f e r e n t l o c a t i o n s , t i m e s  a n d l i f e h i s t o r y s t a g e s ( s e e a l s o C o r o l l a r y #1).  A n o b v i o u s e x a m p l e is the c h a n g e i n diet that  f o l l o w s o n t o g e n e t i c g r o w t h a n d d e v e l o p m e n t o f an i n d i v i d u a l , i.e. m e t a p h o e t e s i s .  A s shown in  E q . 3.6 ( C h a p t e r 3) a n y i n c r e a s e i n b i o m a s s o f a p a r t i c u l a r s i z e c l a s s i s d u e to a s s i m i l a t i o n o f f o o d (i.e. i n g e s t i o n - (egestion + respiration)), r e c r u i t m e n t or i m p o r t . H o w e v e r , l a c k i n g data o n r e c r u i t m e n t (e.g. b i r t h s , a n d m o l d i n g a n d b o d y g r o w t h p r o c e s s e s ) , b i o m a s s c h a n g e s h a d to b e restricted to  feeding  (Eqs.  immediately  assumes  the  particular size class.  (A  3.8-3.10). foraging  This  means  that  specific abilities of  similar problem  a unit an  of  average  assimilated prey predator  can be found in simulations where  biomass  organism  of  organisms  expressed in units o f numbers o f individuals and where a n e w l y b o r n i n d i v i d u a l  a are  immediately  assumes the abilities o f an adult o r g a n i s m . In fact, s i m u l a t i o n i n units o f n u m b e r s a n d b i o m a s s should run  simultaneously.)  (4)  food  Detritus  chains and  microbial loops  do  not  fit the t r o p h o d y n a m i c  concept  of  u n i d i r e c t i o n a l energy transfers. C o n s e q u e n t l y , the r o l e o f m i c r o z o o p l a n k t o n , w h i c h represents a  238  c r u c i a l p a r t o f t h e m i c r o b i a l l o o p ( s e e F i g . 1.9), f o r t h e d y n a m i c s o f h i g h e r t r o p h i c l e v e l s i s n o t adequately addressed in m y simulations. F u r t h e r m o r e , it has b e e n s h o w n that s i m p l e e x p e r i m e n t a l s y s t e m s w i t h m o r e t h a n o n e s p e c i e s p e r c l e a r l y d e f i n a b l e t r o p h i c l e v e l e x h i b i t f a i r l y c o m p l e x , a n d n o t at a l l i n t u i t i v e , (Leibold & Wilbur  dynamics  1992; P i m m 1992). O b v i o u s l y , the l i n e a r a n d f a i r l y tractable t r o p h o d y n a m i c  a p p r o a c h to e c o s y s t e m research is inadequate, or to quote Jake R i c e (1995): " A l t h o u g h w e m a y w i s h f o r s y s t e m s that are m o r e t r a c t a b l e , it m a y b e n e c e s s a r y t o a c c e p t the l i m i t s o f p r e d i c t a b i l i t y o f m a r i n e e c o s y s t e m s . ... W h e n w e t r y t o p r e d i c t t r o p h i c c o n s e q u e n c e s o f the e n v i r o n m e n t a l l y d r i v e n changes i n abundances, s c i e n c e q u i c k l y b e c o m e s f i c t i o n . " In m y o p i n i o n , the categorization o f e c o s y s t e m c o m p o n e n t s into trophic levels is one o f the w o r s t a g g r e g a t i o n e r r o r s i n e c o l o g y , o n e t h a t i m p l i c i t l y i n c l u d e s e r r o r s o f h i e r a r c h i c a l o r g a n i z a t i o n as w e l l as o f s p a t i o - t e m p o r a l s t a b i l i t y . C o n s e q u e n t l y , t h e d e v e l o p m e n t  of a new  trophodynamic  t h e o r y w i l l b e n e c e s s a r y , o n e that r e f l e c t s l i f e h i s t o r y strategies o f m a n y v e r y d i f f e r e n t i n t e r a c t i n g s p e c i e s m o r e a p p r o p r i a t e l y ( s e e a l s o C o r o l l a r i e s #5 a n d #1). H o w e v e r , t h i s d e v e l o p m e n t w i l l b e c l o s e l y l i n k e d t o b i o d i v e r s i t y r e s e a r c h , a f i e l d that has b e c o m e s c i e n t i f i c a l l y l o c k e d i n f o r d e c a d e s a n d w h i c h is not l i k e l y to m a k e m a j o r advances i n the near future (compare the classic d o g m a b y H u t c h i n s o n (1961) w i t h the little k n o w n p u b l i c a t i o n b y G h i l a r o v (1984)).  239  Corollary  'What i f  #3:  q u e s t i o n s are irrelevant w h e n i m p o r t a n t  v a r i a b l e s a n d p r o c e s s e s are  unresolved in models and simulations. A g a i n , s i m u l a t i o n results do not suggest a clear l i n k a g e b e t w e e n prey density i n the o c e a n i c environment  and sockeye salmon cohort survival.. So, I could proceed by m o d i f y i n g  several  aspects o f m y m o d e l ( s ) a n d s i m u l a t i o n s : e.g. i n c l u d e nutrient d y n a m i c s ; a d d another s i z e class o f phytoplankton,  or  another  trophic  level,  or  a whole  coastal ecosystem model;  n u m e r i c a l v a l u e s o f b i o l o g i c a l parameters or the f u n c t i o n a l r e l a t i o n s h i p s b e t w e e n forcings  and  resolution;  dependent include  macrozooplankton,  physical  a  f i s h ...  forced  or  biological  ontogenetic  variables; vertical  increase migration  or  . A t w h a t p o i n t w o u l d I d e c i d e that further  the  environmental  decrease  of  change  the  spatial  mesozooplankton, modifications of  the  m o d e l ( s ) o r s i m u l a t i o n s are n o l o n g e r n e c e s s a r y o r j u s t i f i a b l e ? W h e n the s i m u l a t i o n r e s u l t s fit the o b s e r v a t i o n s ? T h e r e are t w o p r o b l e m s w i t h this: F i r s t , " w h a t w e c a l l data are  inference-laden  s i g n i f i e r s o f n a t u r a l p h e n o m e n a t o w h i c h w e h a v e i n c o m p l e t e a c c e s s . " ( O r e s k e s et al. a l s o C o r o l l a r y #4).  1994; see  I r e a l i z e that this is a rather d e s t r u c t i v e a r g u m e n t f o r the c a u s e o f s c i e n c e i n  general but o b v i o u s l y f o r any natural e c o s y s t e m (and f o r the N o r t h e a s t P a c i f i c i n particular) there w i l l n e v e r be e n o u g h data to e x c l u d e a variety o f alternative e x p l a n a t i o n s f o r any  observed  p h e n o m e n o n (a c h a r a c t e r i s t i c o f a l l o p e n systems). A n d s e c o n d , m o d e l a n d s i m u l a t i o n results are n o n - u n i q u e , i.e. p o s s i b l y m a n y other m o d e l s r e p r e s e n t i n g v e r y d i f f e r e n t m e c h a n i s m s w i l l e x h i b i t the same result. The  futility of  simulation  experiments  becomes  even  more  clear w h e n  considering  the  f o l l o w i n g : S u p p o s e that o n e w o u l d l i k e i n v e s t i g a t e the e f f e c t s o f a 1 0 % t e m p e r a t u r e i n c r e a s e i n the surface l a y e r o f the N o r t h e a s t P a c i f i c onto the m e s o z o o p l a n k t o n s p a t i o - t e m p o r a l d i s t r i b u t i o n u s i n g a c o m p l e t e l y v e r i f i e d m o d e l ( a l t h o u g h I agree that v e r i f i c a t i o n is n o t p o s s i b l e i n p r i n c i p l e ;  240  s e e O r e s k e s et know  if  the  al.  1994). C o n s i d e r i n g the m a n y n o n - l i n e a r i t i e s i n the natural w o r l d , w e c a n n o t  natural  system  transititions i n c o m m u n i t y  would  not  undergo  major  fundamental  changes  (e.g.  structure, species life histories) under n e w e n v i r o n m e n t a l  phase  forcings,  c h a n g e s t h a t c o u l d n o t h a v e b e e n a n t i c i p a t e d at t h e t i m e o f o r i g i n a l m o d e l d e s i g n . S i m u l a t i o n e x p e r i m e n t s w i l l thus a l w a y s p u s h a m o d e l b e y o n d its d o m a i n o f i n f e r e n c e (but i f t h e y w o u l d n ' t , w h y c o n d u c t a s i m u l a t i o n e x p e r i m e n t i n t h e first p l a c e ? ) . It h a s a l s o b e e n s u g g e s t e d t h a t s t o c h a s t i c d y n a m i c m o d e l s ( B r o w n & R o t h e r y 1 9 9 3 ; L e v i n al.  1997; Steele 1985; Steele &  phenomena  because  applicable." (Steele  "... 1985)  Henderson  deterministic  1994) m i g h t be a better representation o f  ecosystem  concepts  and  models  are  A s s u m i n g that t h i s i s true (see a l s o C o r o l l a r y #4),  not  et  natural  so  does it  e c o s y s t e m r e s e a r c h less a m b i g u o u s ? T h e first q u e s t i o n that arises i s : W h e r e i n the m o d e l  easily make should  w e a d d r a n d o m c o m p o n e n t s ? T o one (many, all?) e n v i r o n m e n t a l input variables, to one or m o r e p o p u l a t i o n p a r a m e t e r s , t o p o p u l a t i o n p r o c e s s e s ( s u c h as b i r t h s a n d d e a t h s ) ? A n d i f s o h o w l a r g e a r a n d o m d i s t u r b a n c e , w h e r e a n d at w h a t t i m e ? U n f o r t u n a t e l y , s i m u l a t i o n r e s u l t s o f e v e n s i m p l e ( s i n g l e p o p u l a t i o n a n d predator-prey) m o d e l s are o f t e n c o n t r a d i c t o r y a n d d e p e n d c r i t i c a l l y o n the terms (variable, parameter) to w h i c h stochastic noise has been added ( P i m m 1982).  L o o k i n g at C o r o l l a r i e s #2 a n d #3, o n e m i g h t s u g g e s t t h a t t r o p h o d y n a m i c least b e  improved  dominating  to  the  extent  as t o  ecosystem aggregations  (e.g.  correctly predict fish  in Fig.  1.9).  (or  hindcast)  While  the  it c a n b e  m o d e l s c o u l d at behavior  of  the  a r g u e d that this  p r o p o s i t i o n is false i n p r i n c i p l e (since w e d o not k n o w the i m p o r t a n c e o f the less  apparent  s p e c i e s to the f u n c t i o n i n g o f the e c o l o g i c a l c o m m u n i t y ) , e m p i r i c a l e v i d e n c e suggests that w e are still far a w a y f r o m such predictions even for systems w i t h , for all practical purposes, u n l i m i t e d  241  research funding  and data: E.g. T r y  to predict (or hindcast) the b e h a v i o r  o f the  Dow-Jones  Industrial Index ( w h i c h s u m m a r i z e s the b e h a v i o r o f 3 0 (agreed u p o n ! ) representative i n d u s t r i a l s t o c k s ) ; o r , f o r t h e s a k e o f d i s p r o v i n g C o r o l l a r y #2, t h e b e h a v i o r o f a n y p a r t i c u l a r s t o c k f r o m t h e behavior o f Standard and P o o r ' s 500 Stock Index.  C o r o l l a r y #4:  S y s t e m c o m p l e x i t y and h u m a n nature m a k e it i m p o s s i b l e to predict the b e h a v i o r  of ecosystem components  by  all practical standards. F a l s e predictions c a n a l w a y s easily  be  e x p l a i n e d b y a variety o f components and processes w h o s e effects have not been considered i n an ecosystem analysis and synthesis. C o r o l l a r y #4 i s t h e c o n s e q u e n c e o f a m i s t a k e I o r i g i n a l l y m a d e w h e n p r e p a r i n g t h e m a p s F i g . 4 . 1 7 : Instead o f p l o t t i n g the m e a n s i m u l a t e d m e s o z o o p l a n k t o n c o n c e n t r a t i o n s f o r the of July  1956 to 1959, and  1980 to  in  month  1989  as a r e s u l t o f t h e 4 - t r o p h i c  levels simulation  with  a d v e c t i o n ( F i g . 4.17), I p l o t t e d the m e a n  simulated mesozooplankton  concentrations for  the  m o n t h o f J u l y 1 9 5 6 t o 1 9 5 9 as a r e s u l t o f t h e 4 - t r o p h i c l e v e l s s i m u l a t i o n w i t h o u t a d v e c t i o n ( F i g . 5 . 1 . , u p p e r p a n e l ) , a n d f o r the s a m e m o n t h f o r the years 1 9 8 0 to 1 9 8 9 as a r e s u l t o f the 4 - t r o p h i c levels  simulation  with  advection  (Fig.  5.1,  lower  panel).  Fig.  5.1  resembles  very  much  o b s e r v a t i o n a l d a t a ( s e e F i g . 1.7). S o , n a t u r a l l y w h e n I f o u n d t h e m i s t a k e f o r a s e c o n d o r s o I w i s h e d I h a d n o t r e c h e c k e d t h e m a p s , a n d l e s s t h a n t w o h o u r s l a t e r I c o u l d c o m e u p w i t h at l e a s t a d o z e n e x p l a n a t i o n s w h y F i g s . 4 . 1 7 a n d 1.7 d o n ' t r e s e m b l e e a c h o t h e r . E c o s y s t e m research confronts scientists very q u i c k l y w i t h an o v e r w h e l m i n g amount o f detail a n d i n f o r m a t i o n (e.g. see T a b l e 1 i n B r i a n d 1 9 8 3 , T a b l e U i n P a r s o n s & L a l l i 1 9 8 8 , T a b l e s 3 a n d 4 i n H e a l e y 1991). In order to process the w e a l t h o f i n f o r m a t i o n about a n e c o l o g i c a l , o r any other c o m p l e x adaptive, system i n a 'meaningful'  way, simplifications have to be made.  It i s  not  242  F i g . 5.1:  Mean  month of July  simulated mesozooplankton  concentrations  1 9 5 6 t o 1 9 5 9 as a r e s u l t o f t h e 4 - t r o p h i c  [mg  C  m" ]. 3  Upper  panel: For  the  levels simulation without  advection.  L o w e r p a n e l : F o r t h e m o n t h o f J u l y 1 9 8 0 t o 1 9 8 9 as a r e s u l t o f t h e 4 - t r o p h i c l e v e l s  simulation  with advection.  243  u n r e a s o n a b l e to a s s u m e that the s i m p l i f i c a t i o n (or c o g n i t i v e m o d e l b u i l d i n g ) p r o c e s s that o c c u r s i n o u r b r a i n is the result o f natural s e l e c t i o n a n d thus reflects a b i l i t i e s that w e r e r e l e v a n t f o r o u r s u r v i v a l (but not n e c e s s a r i l y relevant f o r science). E v o l u t i o n a r y e p i s t e m o l o g i s t s h a v e s t u d i e d this p r o b l e m and have developed four theorems about h u m a n  cognitive behavior (Riedl  1984,  R.  R i e d l 1989 pers. c o m m . ) . T h e h u m a n analytical/logical-deductiveapparatus behaves: (1) as i f the m o s t l i k e l y e x p l a n a t i o n is true ( H y p o t h e s i s o f A p p a r e n t T r u t h ) . (2) i n o r d e r to m a g n i f y s i m i l a r i t i e s a n d i g n o r e d i f f e r e n c e s ( H y p o t h e s i s o f the C o m p a r a b l e ) . (3) as i f s i m i l a r c o n s e q u e n c e s h a v e s i m i l a r c a u s e s ( H y p o t h e s i s o f the F i r s t C a u s e ) . (4) as i f s i m i l a r c a u s e s h a v e s i m i l a r c o n s e q u e n c e s ( H y p o t h e s i s o f the P u r p o s e f u l ) . ( A s m a n y o f y o u w i l l note, the first t h e o r e m p r o v i d e s an e x p l a n a t i o n f o r the m a n y s c h o o l s o f t h o u g h t i n t h e s c i e n t i f i c c o m m u n i t y as ' m o s t l i k e l y ' i s a c o n s e q u e n c e o f t h e i n t e r p r e t a t i o n  of  i n c o m p l e t e d a t a ( s e e C o r o l l a r y 3 ) . ) It t h u s f o l l o w s f r o m t h e c o m p l e x i t y o f t h e n a t u r a l e c o s y s t e m a n d the a r c h i t e c t u r e o f the h u m a n m i n d that: ( 1 ) a m o d e l e r ' s k n o w l e d g e a b o u t t h e m o d e l e d s y s t e m as w e l l as a b o u t t h e s i m p l i f i c a t i o n s t h a t went  into  the  model  will  (hopefully)  always  enable  her  or  him  to  identify  alternative  explanations, and (2) h e o r s h e w i l l a c t u a l l y ' b e l i e v e ' i n these e x p l a n a t i o n s .  Corollary #5:  T h e d e v e l o p m e n t o f n e w a n a l y t i c a l a n d synthetic m e t h o d o l o g i e s is c r u c i a l f o r the  study o f c o m p l e x systems. W h a t do m y c o n c l u s i o n s then m e a n i n practical terms for e c o s y s t e m research? W e have little predictive  capacity  about  spatio-temporal  changes  in  physical  forcings.  We  have  little  u n d e r s t a n d i n g a b o u t b i o l o g i c a l o r g a n i z a t i o n a l a d a p t a t i o n as w e l l as s p a t i o - t e m p o r a l e c o l o g i c a l  244  patterns  and  p r o c e s s e s that o c c u r i n e v e n  constant environments.  And  we  have  k n o w l e d g e about the effects o f p h y s i c a l f o r c i n g s o n e c o l o g i c a l p r o c e s s e s (e.g. distribution,  competition,  f o r c i n g s ( D a v i s et al.  predator-prey  relationships),  not  to  mention  even  less  spatio-temporal  changes  in  physical  1998). W o r s t o f a l l b e c a u s e e c o s y s t e m s are so c o m p l e x a n d w e h a v e  l i m i t e d access to data ( w h i c h  are l i k e l y to d o c u m e n t  discussion  May  see D u r l a u f  1997;  1976a;  Rice  only  1995)  interesting events  we  will  believe  in  anyway; any  only for  a  reasonable  e x p l a n a t i o n set b y t h e e v o l u t i o n a r y c o n s t r a i n t s o f o u r m i n d . While  w e humans have certainly acquired cognitive capabilities during  our  phylogenetic  d e v e l o p m e n t that a l l o w us to m a k e p r e d i c t i o n s n e c e s s a r y f o r s u r v i v a l a n d r e p r o d u c t i o n  (Survive  a n d r e p r o d u c e are f a i r l y s i m p l e r u l e s ! ) , the true nature o f c o m p l e x s y s t e m s m a y w e l l l i e b e y o n d the scope o f our understanding, and our s i m p l i f i c a t i o n apparatus m a y s i m p l y be not adapted to d e a l w i t h c o m p l e x s y s t e m s s u c h as e c o s y s t e m s o r s t o c k m a r k e t s . W h a t s o l u t i o n s d o I s u g g e s t ? (1)  W i t h respect to understanding  the w o r k i n g o f e c o s y s t e m s , future  ecological research  s h o u l d f o c u s o n the f u l l c o m p l e x i t y o f e c o s y s t e m s a n d try to i m p l e m e n t i n t o c o m p u t e r s synthetic systems w i t h a large number of components  that are able to adapt  (i.e. A r t i f i c i a l L i f e ) .  As  d e m o n s t r a t e d i n this thesis s i m p l e t r o p h o d y n a m i c m o d e l s are s i m p l y t o o v a g u e a n d a s s u m p t i o n l a d e n as to c o n t r i b u t e to a d e e p e r u n d e r s t a n d i n g o f e c o l o g i c a l s y s t e m s . (2) A n d w i t h r e s p e c t to p r e d i c t i o n o f c o m p l e x s y s t e m s b e h a v i o r : W h i l e w e h u m a n s d o  admit  to the fact that a l g o r i t h m i c c o m p u t i n g o u t p e r f o r m s h u m a n a r i t h m e t i c c a p a b i l i t i e s i n d e f i n i t e l y f o r all  practical purposes,  the  idea  that  intelligent  computational  platforms  perform  complex  a n a l y t i c a l tasks ( a s s i m i l a t i o n o f u n c e r t a i n data a n d a d v a n c e d l o g i c a l operation) that l i e  beyond  human comprehension,  started  may  w e l l be unsettling for m a n y  (as it w a s f o r  me  when  I  245  d e v e l o p i n g ideas to that end). H o w e v e r , q u a l i t a t i v e l y n e w tasks w i l l r e q u i r e q u a l i t a t i v e l y n e w m e t h o d o l o g y . W e s h o u l d c o n s i d e r the p o s s i b i l i t i e s o f our o w n l i m i t a t i o n s .  T h e p u r p o s e o f s i m u l a t i o n s i s to test h y p o t h e s e s , a n d the c o m p l e x h y p o t h e s i s that I  have  t e s t e d i n m y s i m u l a t i o n s c a n b e s t a t e d a s : Is i t e n o u g h t o c o n s i d e r l o w e r t r o p h i c l e v e l d y n a m i c s i n the o c e a n i c e n v i r o n m e n t  i n order to e x p l a i n the v a r i a b i l i t y i n s o c k e y e s a l m o n c o h o r t  S i n c e m y results d o not suggest a clear l i n k a g e b e t w e e n prey density i n the o c e a n i c  survival?  environment  a n d s o c k e y e s a l m o n s u r v i v a l , o b v i o u s l y other f a c t o r s (e.g. s o c k e y e s a l m o n , its c o m p e t i t o r s predators,  i n v a r i o u s habitats, a n d p o s s i b l y w i t h the w h o l e  behaviors)  h a v e to be i n c l u d e d i n the c o n c e p t u a l (or otherwise) m o d e l s i n order to p r o v i d e  satisfactory  explanation  for  sockeye  salmon  cohort  spectrum of individual  and  survival.  Future  sampling  complex a  programs,  e x p e r i m e n t s a n d c o m p u t e r s i m u l a t i o n s s h o u l d take the n e x t step a n d i n v e s t i g a t e e c o s y s t e m s f r o m the  viewpoint  their  components'  life  history  strategies  rather  than  trophic  relationships.  C o n s i d e r i n g h o w little i n f o r m a t i o n is a v a i l a b l e o n e v e n w e l l - s t u d i e d o r g a n i s m s (e.g. N e o c a l a n u s sp., S u b s e c t i o n 2.2.1.), this is not an easy task. S i m p l e (even abstract m o d e l s ) s h o u l d enable us to at l e a s t a s s e s s h o w  s u c c e s s f u l t h i s a p p r o a c h m i g h t b e a n d w h a t k i n d s o f d a t a at w h a t s p a t i o -  temporal resolution w i l l be necessary. O n l y then w e should m a k e c h o i c e s o n future  research  topics.  246  References A b r a m s , P . A . 1 9 9 4 S h o u l d P r e y O v e r e s t i m a t e t h e R i s k o f P r e d a t i o n ? Am. 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