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Membrane inlet mass spectrometry (M(MS) : a novel approach to the oceanic measurement of dimethylsulfide Nemcek, Nina 2007-12-31

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M E M B R A N E INLET MASS SPECTROMETRY (MIMS): A N O V E L A P P R O A C H TO THE OCEANIC M E A S U R E M E N T OF DIMETHYLSULFIDE  by NINA N E M C E K B S c , University of British Columbia, 2003  A THESIS SUBMITTED IN PARTIAL F U L F I L L M E N T OF THE REQUIREMENTS FOR THE D E G R E E OF  M A S T E R OF SCIENCE in F A C U L T Y OF G R A D U A T E STUDIES (Oceanography)  UNIVERSITY OF BRITISH C O L U M B I A March 2007 © Nina Nemcek, 2007  Abstract A n o v e l t e c h n i q u e , m e m b r a n e inlet mass spectrometry ( M I M S ) , w a s u s e d to measure d i m e t h y l s u l f i d e gas ( D M S ) a n d a l g a l d i m e t h y l s u l f o n i o p r o p i o n a t e ( D M S P p ) concentrations i n t w o different m a r i n e ecosystems o f the N E P a c i f i c . I n o c e a n i c waters a l o n g L i n e P, D M S levels had been o b s e r v e d to be u n u s u a l l y h i g h , yet particulate D M S P l e v e l s h a d not been e x t e n s i v e l y measured. D M S P p concentrations d u r i n g 3 consecutive s p r i n g cruises ranged f r o m 0.2-63.2 n M (mean 21.5 n M , s.d. 15.0 n M ) i n the upper 50 m o f the water c o l u m n , and v a r i e d s i g n i f i c a n t l y w i t h depth, across stations a n d between study years. D M S P p g e n e r a l l y decreased w i t h depth and distance f r o m the coast. D M S P p concentrations at m o s t stations i n 2 0 0 3 w e r e 2 to 3-fold higher than i n subsequent years, and w e r e s i g n i f i c a n t l y correlated to the b i o m a s s o f dinoflagellates (r = 0.46) across the s u r v e y r e g i o n . A l t h o u g h p h y t o p l a n k t o n b i o m a s s ( c h l o r o p h y l l a) also 2  d e c l i n e d i n 2 0 0 4 - 2 0 0 5 , D M S P p x h l ratios d i d as w e l l , i n d i c a t i n g a p h y s i o l o g i c a l or t a x o n o m i c change i n the p h y t o p l a n k t o n c o m m u n i t y . Surface D M S concentrations w e r e measured u n d e r w a y a l o n g w i t h pCOi, OilAx, temperature, s a l i n i t y and c h l o r o p h y l l a i n p r o d u c t i v e , coastal waters o f f B r i t i s h C o l u m b i a . A l l parameters e x h i b i t e d large ranges, (pCOi, 200-747 p p m ; D M S , < l - 2 8 . 7 n M ; c h l a, <0.1-33.2 p g L" ), h i g h l i g h t i n g the d y n a m i c nature o f the r e g i o n . A tight anti-correlation b e t w e e n pC02 and 1  6 ?Ar was o b s e r v e d across the r e g i o n (r = 0.90), w i t h the d i s t r i b u t i o n s o f these gases strongly 2  i n f l u e n c e d b y b o t h b i o l o g i c a l (photosynthesis and respiration) and p h y s i c a l ( u p w e l l i n g ) processes. In contrast, D M S l e v e l s w h i c h e x h i b i t e d r a p i d , fine-scale fluctuations irresolvable w i t h traditional m e t h o d s , w e r e unrelated to any s i n g l e v a r i a b l e . A s i g n i f i c a n t linear relationship was h o w e v e r o b s e r v e d b e t w e e n D M S and the c h l o r o p h y l l to m i x e d layer depth ratio (r = 0.83), 2  although w i t h a different s c a l i n g factor to that d e r i v e d f r o m o p e n o c e a n data. L o w e r r e s o l u t i o n  s a m p l i n g i n this r e g i o n c a n i n t r o d u c e errors as large as 4 1 % o f the m e a n concentration for D M S , e m p h a s i z i n g the u t i l i t y o f M H V I S i n d y n a m i c areas. In c o n c l u s i o n , M I M S p r o v e d to be a significant advance for D M S measurement, yet i m p r o v e m e n t s n e e d to be m a d e for it to be a v i a b l e alternative to other m e t h o d s for D M S P measurements.  Table of Contents Abstract  ii  Table o f Contents  iv  List o f Tables  vi  List o f Figures  vu  Acknowledgements  ix  Dedication  x  Co-Authorship Statement  xi  Chapter 1: Introduction  1  1.1 Dimethyl sulfide: Sources, Sinks and Climate Links  1  1.2 The C L A W Hypothesis: 20 Years Later  4  1.3 Membrane Inlet Mass Spectrometry  6  1.4 Thesis Objectives  7  1.5 References  8  Chapter 2: Springtime Variability in Particulate D M S P Concentrations along Line P, N E Pacific Ocean  11  2.1 Introduction  11  2.2 Materials and Methods  13  2.3 Results  19  2.4 Discussions  24  2.5 References  45  Chapter 3: High-Resolution Measurements o f D M S , CO2, and 0 2 / A r in Productive, Coastal Waters around Vancouver Island, Canada  49  3.1 Introduction  49  3.2 Materials and Methods  52  3.3 Results  '.  58  3.4 Discussion  67  3.5 Conclusions  78  iv  3.6 R e f e r e n c e s . . :  89  Chapter 4: C o n c l u s i o n s  93  4.1 T h e s i s O v e r v i e w  93  4.2 E v a l u a t i o n o f M U M S f o r D M S P / D M S M e a s u r e m e n t s  94  4.3 Successes and P i t f a l l s  95  4.4 F u t u r e D i r e c t i o n s  97  4.5 R e f e r e n c e s  '..  '.  99  v  List of Tables T a b l e 2.1: A n c i l l a r y o c e a n o g r a p h i c data for a l l stations s u r v e y e d d u r i n g 2 0 0 3  33  T a b l e 2.2: C o e f f i c i e n t s o f d e t e r m i n a t i o n (r ) from linear regressions b e t w e e n D M S P p levels 2  and the absolute and relative c a r b o n b i o m a s s o f different p h y t o p l a n k t o n groups for all stations d u r i n g the 2 0 0 3 survey  34  T a b l e 2.3: T h e c o n t r i b u t i o n o f D M S P p to total p h y t o p l a n k t o n c e l l c a r b o n i n 2 0 0 3  35  T a b l e 3.1: A b s o l u t e a n d relative asymptotic interpolation errors a l o n g the 6 major transects  ....79  List of Figures Figure 2.1: Map of the NE Pacific showing the 5 major stations along Line P  36  Figure 2.2: Depth profiles of DMSPp measured at the 5 major stations along Line P during 3 consecutive spring cruises 37 Figure 2.3: Depth profiles of chlorophyll a measured at the 5 major stations along Line P during 3 consecutive spring cruises  38  Figure 2.4: Depth profiles of the DMSPp :chl ratio measured at the 5 major stations along Line P during 3 consecutive spring cruises  39  Figure 2.5: Contour plots illustrating spatial and interannual variability in springtime DMSPp and chlorophyll levels along Line P 40 Figure 2.6: Relationship between chlorophyll and DMSPp concentrations along Line P for the 3 year pooled dataset  41  Figure 2.7: The relative abundance of the different phytoplankton groups enumerated at (a) 10 m depth and (b) the chlorophyll max. at all stations surveyed in 2003  42  Figure 2.8: The contribution of the different phytoplankton groups to total carbon biomass at (a) 10 m depth and (b) the chlorophyll max. at all stations surveyed in 2003 43 Figure 2.9: 50 m depth integrated DMSPpxhl ratios plotted against the mixed layer nitrate concentration  44  Figure 3.1: Map of southwestern British Columbia, Canada showing the location of underway transects 80 Figure 3.2: Surface plots of (a) temperature (°C), (b) salinity (psu), (c) chlorophyll a (ug L" ), (d) pC0 (ppm), (e) 0 /Ar (torr ratio), and (f) DMS (nM) 81 1  2  2  Figure 3.3: Detailed south-north view of all variables measured along T5  82  Figure 3.4: Detailed view of all variables measured along T7  83  Figure 3.5: The correlation across all transects between (a)pCC>2 and ( V A r (r = 0.90) with corresponding chl a concentrations overlaid (colourbar), and (b) chl a and ( V A r (r = 0.19) with corresponding temperature overlaid (colourbar)  84  Figure 3.6: The correlation across all transects between chl a and DMS (r = 0.06)  85  2  2  vn  F i g u r e 3.7: R e s u l t s o f the P C A s h o w i n g the strong separation b e t w e e n p C C » 2 a n d C V A r i n t w o - d i m e n s i o n a l space a n d the clear p a r t i t i o n i n g b e t w e e n p h y s i c a l ( T , S) and b i o l o g i c a l ( c h l a) variables  86  F i g u r e 3.8: A v e r a g e D M S concentrations p l o t t e d against C H L / M L D ratios for the A degree l  grids  87  F i g u r e 3.9: A u t o c o r r e l a t i o n f u n c t i o n s for a l l parameters m e a s u r e d a l o n g transect 5 (a); A v e r a g e D L S for a l l parameters for the entire survey (b)  88  viii  A c k n o w l e d g e m e n t s  This project would not have been possible without the tireless efforts of Mark Buckley and Robert Stannard at Hiden Analytical in the development and troubleshooting of the MIMS. My supervisor Philippe Tortell, Celine Gueguen and Chris Payne also helped tremendously over the years with method development and repairs on "Herbie". The author would like to acknowledge the Captain, crew, and science personnel of the CCGS John P. Tully  for their assistance during the Line P and Queen Charlotte Sound surveys.  The Natural Sciences and Engineering Research Council of Canada (NSERC) and the University of British Columbia funded this project in the form of an Undergraduate Student Research Award (2003), Postgraduate Scholarship and University Fellowship (2004-2006). A sincere thank you goes to the following people for their assistance with the Line P dataset: T. Peterson for performing phytoplankton identification and enumeration in 2003, M. Robert for organizing the cruises and providing access to CTD and ancillary data, W. Richardson and J. Barwell-Clarke for nutrient analysis, L. Richier for chlorophyll analysis in 2005, and M. Arychuk and CS. Wong for method intercomparisons and fruitful discussions on DMS/DMSP trends in the NE Pacific. A special thank you goes to Darren Tuele for DMSPp sample collection in 2005 and invaluable support at sea during all cruises. I am grateful to Debby Ianson for allowing me to participate in her Queen Charlotte Sound cruise, for providing ancillary CTD, nutrient, thermosalinograph and pCOi equilibrator data, and for her guidance and motivation during the writing of Chapter 3. Dave Mackas kindly provided unpublished CTD data and Marlene Jeffries helped with Matlab figures. Finally, I must thank my labmates for their camaraderie and support, and my supervisor Philippe Tortell for conceiving the application of MIMS for DMS analysis and for sending me to sea.  ix  Dedication To DT for pushing me to finish and to my parents for never pushing me.  x  Co-Authorship Statement I w i l l be the p r i m a r y author o f the manuscript "friterannual v a r i a b i l i t y i n springtime particulate D M S P concentrations a l o n g a coastal to oceanic transect i n the N E P a c i f i c " w h i c h w i l l be w r i t t e n based o n C h a p t e r 2. I p e r f o r m e d the D M S P p s a m p l e analysis i n a l l three years, integrated and a n a l y z e d the a n c i l l a r y data, a n d w i l l w r i t e the m a n u s c r i p t . N u t r i e n t , c h l o r o p h y l l and C T D data w e r e p r o v i d e d b y M a r i e R o b e r t o f the Institute o f O c e a n Sciences. T h e second author, D r . T a w n y a P e t e r s o n p e r f o r m e d p h y t o p l a n k t o n counts and p r o v i d e d c e l l d i m e n s i o n s i n 2003 f r o m w h i c h I c a l c u l a t e d c e l l b i o v o l u m e s and the b i o m a s s o f s p e c i f i c algal groups. T h e third author, m y s u p e r v i s o r D r . P h i l i p p e T o r t e l l introduced m e to the M E V I S , assisted w i t h sample analysis d u r i n g the cruise i n 2 0 0 3 , and p r o v i d e d lab space and resources. H e also p r o v i d e d guidance on the project and edited the manuscript. I w r o t e the m a n u s c r i p t " A high-resolution s u r v e y o f D M S , CO2, a n d 02/Ar distributions i n p r o d u c t i v e coastal w a t e r s " , based o n Chapter 3, w h i c h w a s s u b m i t t e d to Global Biogeochemical  Cycles. I d e v i s e d the s a m p l i n g strategy, c o l l e c t e d and processed D M S P and  c h l o r o p h y l l samples and operated the M E V I S d u r i n g the Q u e e n C h a r l o t t e S o u n d survey i n 2 0 0 4 . Post-cruise, I processed the M E V I S , PCO2 equilibrator, and t h e r m o s a l i n o g r a p h data, the latter t w o p r o v i d e d b y m y s e c o n d author D r . D e b b y Ianson o f the Institute o f O c e a n Sciences. D r . Ianson p r o v i d e d a berth o n her research cruise as w e l l as a n c i l l a r y C T D and nutrient data. T h e third author D r . P h i l i p p e T o r t e l l p e r f o r m e d the p r i n c i p a l c o m p o n e n t s analysis and calculated autocorrelation f u n c t i o n s and i n t e r p o l a t i o n errors for m y u n d e r w a y dataset. A l l other interpretations and ideas i n the d i s c u s s i o n w e r e m y o w n , h o w e v e r b o t h o f m y co-authors' edits i m p r o v e d the f i n a l m a n u s c r i p t greatly. I certify that the a b o v e statements about authorship are correct.  xi  1. Introduction 1.1 Dimethylsulfide: Sources, Sinks and Climate Links M i c r o s c o p i c algae d w e l l i n g i n surface waters o f the w o r l d ' s oceans h a v e a p r o f o u n d influence o n E a r t h ' s c l i m a t e t h r o u g h the c o n s u m p t i o n and p r o d u c t i o n o f c l i m a t o l o g i c a l l y active gases. O n e s u c h gas, d i m e t h y l s u l f i d e ( D M S ) , i s f o r m e d i n the oceans f r o m the b r e a k d o w n o f d i m e t h y l s u l f o n i o p r o p i o n a t e ( D M S P ) , a c o m p o u n d p r o d u c e d i n large quantities b y m a n y species o f p h y t o p l a n k t o n for a variety o f m e t a b o l i c functions (see S e c t i o n 2.1). In 1 9 7 2 , James L o v e l o c k observed that D M S w a s u b i q u i t o u s i n marine surface waters and postulated that the f l u x o f D M S f r o m the oceans to the atmosphere w a s s u f f i c i e n t l y large to account f o r the " m i s s i n g " sulfur i n g l o b a l transport m o d e l s [Lovelock et al, 1972]. O v e r the past t w o decades, extensive oceanic D M S measurements h a v e supported this hypothesis. D e s p i t e a g l o b a l m e a n surface water concentration o f < 3 n M [Kettle et al, 1999], D M S i s e v e r y w h e r e supersaturated i n the ocean resulting i n a steady f l u x o f s u l f u r to the atmosphere. T h i s f l u x represents the largest natural source o f atmospheric s u l f u r c o n s t i t u t i n g 20 % o f total g l o b a l e m i s s i o n s , but over 4 0 % o f the atmospheric s u l f u r b u r d e n due to the r e l a t i v e l y longer l i f e t i m e s o f D M S - d e r i v e d aerosols c o m p a r e d to anthropogenic ones [Chin and Jacob, 1996]. It w a s not u n t i l the late 1 9 8 0 ' s w h e n L o v e l o c k and c o l l e a g u e s i n t r o d u c e d the C L A W hypothesis (an a c r o n y m for the authors' names), that interest i n D M S and its potential c l i m a t i c effects s k y r o c k e t e d [Charlson et al, 1987]. T h e C L A W hypothesis states that p h y t o p l a n k t o n regulate their e n v i r o n m e n t (and b y consequence the E a r t h ' s c l i m a t e ) t h r o u g h the p r o d u c t i o n . o f D M S w h i c h affects planetary albedo. A s noted earlier b y Shaw [ 1 9 8 3 ] , D M S vented f r o m the oceans is q u i c k l y o x i d i z e d i n the atmosphere to non-sea-salt sulfate ( N S S - S O 4 ) a n d methanesulfonic a c i d ( M S A ) , species that act as c l o u d c o n d e n s a t i o n n u c l e i ( C C N ) , o r seed  1  crystals for c l o u d f o r m a t i o n . Charlson et al. [1987] argued that the D M S - d e r i v e d N S S - S O 4 aerosols are the m a i n source o f C C N over m u c h o f the E a r t h ' s surface. T h u s , a c c o r d i n g to the C L A W hypothesis, h i g h solar irradiance stimulates algal D M S p r o d u c t i o n w h i c h leads to increased planetary c l o u d c o v e r and a r e d u c t i o n i n the a m o u n t o f r a d i a t i o n r e a c h i n g the planet's surface. T h i s i n turn has a feedback effect o n the p h y t o p l a n k t o n that initiated the process through changes i n sea surface temperature a n d i n c i d e n t s u n l i g h t , thereby c l o s i n g the l o o p . H o w e v e r , the s i g n o f this feedback w a s u n c l e a r at the t i m e , since m a n y o f the factors c o n t r o l l i n g D M S p r o d u c t i o n and its dependence o n species c o m p o s i t i o n w e r e u n k n o w n . A negative feedback o n D M S p r o d u c t i o n w o u l d i m p l y a self-regulating c l i m a t e system m i t i g a t e d b y the m a r i n e b i o t a [Charlson et al, 1987]. T h e C L A W hypothesis s t i m u l a t e d the intense research efforts directed at e l u c i d a t i n g the intricacies o f the D M S c y c l e that have been o c c u r r i n g over the past t w o decades. F r o m these efforts w e have greatly i m p r o v e d o u r understanding o f the c o m p l e x f o o d w e b d y n a m i c s that drive the c y c l i n g o f D M S P and its b y p r o d u c t s . A l l D M S originates from a l g a l or particulate D M S P ( D M S P p ) . D M S P p r o d u c t i o n varies w i d e l y b y species w i t h d i n o f l a g e l l a t e s and prymnesiophytes generally b e i n g m a j o r p r o d u c e r s , and diatoms m i n o r ones [Keller et al, 1989]. H o w e v e r , even w i t h i n algal groups there is s i g n i f i c a n t v a r i a b i l i t y i n c e l l u l a r D M S P content [Keller et al, 1989], and external factors s u c h as nutrient a v a i l a b i l i t y can increase D M S P p r o d u c t i o n i n species generally c o n s i d e r e d to be l o w producers [Sunda et al, 2 0 0 2 ] . Z o o p l a n k t o n g r a z i n g [Dacey and Wakeham, 1986] and v i r a l l y s i s [Malin et al, 1998] o f p h y t o p l a n k t o n p r o m o t e the release o f D M S P into seawater w h e r e it is r a p i d l y c o n s u m e d b y various c o m p o n e n t s o f the m a r i n e f o o d w e b . H e t e r o t r o p h i c b a c t e r i a are the d o m i n a n t sink f o r d i s s o l v e d D M S P ( D M S P d ) , a l t h o u g h recent evidence suggests autotrophic cyanobacteria and  2  diatoms are also capable o f D M S P uptake and a s s i m i l a t i o n [Vila-Costa et al, 2 0 0 6 ] . B a c t e r i a possess t w o c o m p e t i n g pathways f o r the m e t a b o l i c b r e a k d o w n o f D M S P . T h e demethylation pathway converts D M S P to m e t h a n e t h i o l w h i c h is q u i c k l y i n c o r p o r a t e d into protein and bacterial b i o m a s s , thus d i v e r t i n g s u l f u r a w a y f r o m D M S [Gonzales et al, 1999]. I n contrast, the cleavage p a t h w a y u t i l i z e s D M S P - l y a s e to convert D M S P to D M S and acrylate, a l t h o u g h this is the fate o f o n l y 5-10 % o f the D M S P d m e t a b o l i z e d i n the water c o l u m n [Kiene et al, 2 0 0 0 ] . T h u s , the p r o d u c t i o n o f c l i m a t o l o g i c a l l y important D M S constitutes o n l y a s m a l l fraction o f the large f l u x o f reduced s u l f u r i n the surface ocean. S o m e p h y t o p l a n k t o n species also possess the D M S P - l y a s e e n z y m e w h i c h m i x e s w i t h its substrate d u r i n g g r a z i n g or v i r a l lysis l e a d i n g to h i g h D M S p r o d u c t i o n [Malin et al, 1998]. It i s thought that algae w i t h D M S P - l y a s e activity use this e n z y m e as an activated c h e m i c a l defence against predators, as m i c r o z o o p l a n k t o n grazers are deterred b y acrylate [Wolfe et al, 1997]. M o r e recently it has been s h o w n that acrylate m a y also deter v i r u s e s , since h i g h D M S P - l y a s e c o n t a i n i n g strains o f the p r y m n e s i o p h y t e Emiliania huxleyii appear to be i m m u n e to v i r a l attack [Evans et al, 2 0 0 6 ] . I n a d d i t i o n , p h y t o p l a n k t o n m a y use D M S P - l y a s e to trigger a p o w e r f u l antioxidant cascade, as D M S , acrylate, D M S O , and other b y p r o d u c t s o f D M S P are p o w e r f u l free radical scavengers [Sunda et al, 2 0 0 2 ] . There are three m a j o r s i n k s for D M S i n m a r i n e surface waters, (1) bacterial c o n s u m p t i o n , (2) p h o t o l y s i s and (3) v e n t i l a t i o n to the atmosphere. T h e r e l a t i v e i m p o r t a n c e o f each depends o n the depth i n t e r v a l c o n s i d e r e d and the b i o l o g i c a l , c h e m i c a l and m e t e o r o l o g i c a l c o n d i t i o n s [Kieber et al, 1996]. V e n t i l a t i o n is g e n e r a l l y considered a m i n o r s i n k because it occurs o n l y at the airsea interface, but is the d o m i n a n t loss process at h i g h w i n d speeds. P h o t o l y s i s converts D M S to non-volatile c o m p o u n d s i n c l u d i n g D M S O and occurs o v e r the depth range to w h i c h U V light can  3  penetrate [Kieber et al, 1996]. T h i s depth depends o n c l o u d c o v e r , geographic l o c a t i o n and the l o c a l o p t i c a l properties o f seawater, and can range from a f e w meters i n coastal waters to >75 m i n o p t i c a l l y clear waters under h i g h U V f l u x [i.e. Toole et al, 2 0 0 4 ] . In a d d i t i o n , p h o t o l y s i s rates are strongly i n f l u e n c e d b y the concentrations o f photosensitizers such as c h r o m o p h o r i c d i s s o l v e d organic matter ( C D O M ) [Kieber et al, 1 9 9 6 ; Toole et al, 2 0 0 4 ] . B a c t e r i a l c o n s u m p t i o n o f D M S is l i k e l y the d o m i n a n t s i n k f o r D M S o v e r m o s t o f the surface ocean because it occurs o v e r the greatest depth interval and o v e r the largest range o f conditions [Kiene et al, 1 9 9 0 ; Kieber et al, 1996]. H o w e v e r , under h i g h U V light c o n d i t i o n s , p h o t o l y s i s can exceed b i o c o n s u m p t i o n as the d o m i n a n t loss process as b a c t e r i a i n surface waters b e c o m e p h o t o i n h i b i t e d [Toole et al, 2 0 0 4 ] .  1.2 T h e C L A W H y p o t h e s i s : 20 Y e a r s L a t e r There is n o doubt that b i o g e n i c sulfur aerosols have a large i n f l u e n c e o n g l o b a l climate. T h e y are the m a i n source o f C C N over m u c h o f the remote m a r i n e atmosphere, p a r t i c u l a r l y i n the Southern H e m i s p h e r e w h e r e there are f e w terrestrial sources. M o r e o v e r , a strong coherence has been o b s e r v e d b e t w e e n seasonality i n D M S e m i s s i o n s and C C N n u m b e r s [Ayers and Gras, 1991], as w e l l as c l o u d i n e s s [Boers et al, 1994], at least i n the S o u t h e r n H e m i s p h e r e . Ice core data from V o s t o k , A n t a r c t i c a reveal concentrations o f N S S - S O 4 a n d M S A (a b y p r o d u c t e x c l u s i v e to D M S ) w e r e s i g n i f i c a n t l y higher d u r i n g g l a c i a l p e r i o d s c o m p a r e d to interglacial ones, and w e r e t i g h t l y anti-correlated to past temperature fluctuations [Legrand et al, 1991]. T h u s , marine b i o t a i n f l u e n c e c l i m a t e v i a the oceanic s u l f u r c y c l e , i n a d d i t i o n to the b i o l o g i c a l carbon p u m p . M o d e l l i n g studies reveal that 2/3 o f the present d a y c o o l i n g r e s u l t i n g f r o m b i o l o g i c a l p r o d u c t i o n i n the oceans can be attributed to D M S e m i s s i o n s , w i t h o n l y a 1/3  c o n t r i b u t i o n f r o m CO2 uptake [Watson andLiss, 1998]. M o r e recent m o d e l s estimate that r e d u c i n g present d a y D M S e m i s s i o n s b y h a l f w o u l d result i n a 1.6 ° C increase i n g l o b a l m e a n sea surface temperatures [Gunson et al,  2006].  A l t h o u g h it is clear that D M S e m i s s i o n s have a c o o l i n g effect o n g l o b a l c l i m a t e , it is still unclear w h a t i m p a c t a c h a n g i n g c l i m a t e w i l l have o n future D M S e m i s s i o n s . I m p e n d i n g changes i n solar r a d i a t i o n , surface ocean stratification, nutrient s u p p l y , and w i n d speeds w i l l a l l i n f l u e n c e the c o m p e t i n g pathways that lead to net D M S p r o d u c t i o n , s u c h that neither the magnitude nor the sign o f this effect is certain at present. Increases i n the extent a n d strength o f surface ocean stratification and i n i n c i d e n t solar radiation predicted under g l o b a l w a r m i n g scenarios c o u l d indeed lead to increased D M S p r o d u c t i o n for a n u m b e r o f reasons. F i r s t l y , stratified, nutrientdepleted waters tend to f a v o u r the g r o w t h o f h i g h D M S P p r o d u c i n g t a x a s u c h as dinoflagellates and p r y m n e s i o p h y t e s [Margalef, 1 9 7 8 ; Keller et al,  1989]. S e c o n d l y , h i g h i n c i d e n t U V radiation  promotes elevated D M S P p r o d u c t i o n i n these and other species as a p h y s i o l o g i c a l response to photo-oxidative stress [Sunda et al,  2 0 0 2 ] . F i n a l l y , strong U V p h o t o i n h i b i t i o n o f bacterial  g r o w t h i n a stratified w a t e r c o l u m n results i n less sulfur a s s i m i l a t i o n into b i o m a s s and a higher p r o p o r t i o n o f D M S P c o n v e r t e d to D M S [Simo and Pedros-Alio, 1999]. M o s t recent studies predict dramatic l a t i t u d i n a l v a r i a b i l i t y i n m o d e l l e d D M S f l u x e s under g l o b a l w a r m i n g , w i t h s m a l l to moderate increases i n net g l o b a l D M S e m i s s i o n s [Bopp et al,  2 0 0 3 ; Gabric et  al,  2 0 0 4 ] . These p r e d i c t i o n s do support the self-regulating negative feedback C L A W hypothesis, although o n l y w e a k l y and w i t h large uncertainty. A n i m p r o v e d m e c h a n i s t i c understanding o f the c o m p l e x w e b that is the oceanic D M S c y c l e is urgently needed to reduce this uncertainty.  5  1.3 M e m b r a n e Inlet M a s s Spectrometry T r a d i t i o n a l l y , o c e a n i c measurements o f D M S and related s u l f u r c o m p o u n d s have been p e r f o r m e d b y purge and trap gas c h r o m a t o g r a p h y ( P T G C ) . In this m e t h o d , discrete seawater samples are sparged w i t h an inert gas to transfer the v o l a t i l e s u l f u r analyte into a concentrating cryogenic trap. T h e trap is subsequently heated to release the analyte into a gas chromatographic c o l u m n that separates the v a r i o u s s u l p h u r c o m p o u n d s for measurement b y either a c h e m i l u m i n e s c e n t or p h o t o m e t r i c detector. A l t h o u g h this m e t h o d offers excellent sensitivity (as a result o f the c r y o g e n i c t r a p p i n g step), it i s l a b o u r intensive, a n d t i m e - c o n s u m i n g . Furthermore, it is l i m i t e d to the analysis o f discrete samples, and thus offers p o o r spatial r e s o l u t i o n capabilities. In contrast, the a p p l i c a t i o n o f m e m b r a n e inlet mass spectrometry ( M I M S ) to the measurement o f D M S i n seawater c i r c u m v e n t s m a n y o f these s h o r t c o m i n g s . M I M S i s not a n e w m e t h o d ; it was first i n t r o d u c e d I n the early 1960s [Hoch and Kok, 1963] and i s q u i c k l y g a i n i n g acceptance as an alternative to gas c h r o m a t o g r a p h y for the r a p i d analysis o f v o l a t i l e o r g a n i c c o m p o u n d s i n air and water samples [Ketola et al, 1996]. M E V I S offers the advantages o f shorter analysis t i m e and a m u c h larger linear, d y n a m i c range over P T G C m e t h o d s , w i t h c o m p a r a b l e detection l i m i t s [Ketola et al, 1 9 9 6 ] . O u r adaptation o f this technique uses a gas-permeable d i m e t h y l s i l i c o n e m e m b r a n e as the interface betw een a seawater sample and the v a c u u m o f the mass spectrometer. T h e s e n s i t i v i t y o f the system i s d i r e c t l y p r o p o r t i o n a l to the surface area o f the m e m b r a n e e x p o s e d to the s a m p l e . A n y gases d i s s o l v e d i n the aqueous s a m p l e d i f f u s e t h r o u g h the m e m b r a n e into the v a c u u m c h a m b e r , w h e r e they are i o n i z e d b y electron i m p a c t and separated i n a quadrupole filter based o n their mass-to-charge (m/z) ratios. M a j o r gases (CO2, O2, A r , N2) are detected b y a F a r a d a y cup and trace gases such as D M S b y a secondary electron m u l t i p l i e r ( S E M ) . T h e system can be u s e d w i t h a m e m b r a n e inlet  6  probe for discrete a p p l i c a t i o n s or connected d i r e c t l y to a s h i p ' s seawater intake system for  .  continuous, u n d e r w a y m o n i t o r i n g o f d i s s o l v e d gas concentrations. B y e m p l o y i n g the single i o n m o n i t o r i n g ( S I M ) m o d e o f the mass spectrometer software, and c y c l i n g t h r o u g h their respective m/z ratios, m u l t i p l e gases c a n be m o n i t o r e d i n real-time, p s e u d o - s i m u l t a n e o u s l y w i t h v e r y little effort. T h u s the m a i n advantages o f the M I M S system o v e r P T G C f o r the analysis o f D M S are its (1) s i g n i f i c a n t l y h i g h e r spatial r e s o l u t i o n capabilities and (2) r a p i d , semi-automated analysis capabilities. O n e o f the m a i n c o n t r i b u t i o n s o f this thesis is the d e v e l o p m e n t , troubleshooting and . field-testing o f a sea-going M I M S system.  1.4 T h e s i s O b j e c t i v e s T h e foremost o b j e c t i v e o f this w o r k was to determine whether M I M S c o u l d be s u c c e s s f u l l y a p p l i e d to the measurement o f trace D M S a n d D M S P concentrations i n seawater. O n c e this i n i t i a l g o a l h a d been a c h i e v e d , the research objectives w e r e (1) to measure particulate D M S P concentrations i n the N E P a c i f i c a l o n g L i n e P w h e r e s u c h measurements d i d not p r e v i o u s l y exist, (2) to e x a m i n e the spatial and interannual v a r i a b i l i t y i n particulate D M S P p levels a l o n g L i n e P i n the context o f p h y t o p l a n k t o n species c o m p o s i t i o n and nutrient levels, (3) to use the high-resolution u n d e r w a y capabilities o f the M I M S to i d e n t i f y fine-scale structure i n D M S concentrations i n d y n a m i c and p r o d u c t i v e waters o f coastal B . C . and f i n a l l y , (4) to use these high-resolution D M S data i n c o n j u n c t i o n w i t h u n d e r w a y a n c i l l a r y parameters to e x a m i n e the factors d r i v i n g the o b s e r v e d D M S distributions. 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K i e n e (2006), D i m e t h y l s u l f o n i o p r o p i o n a t e uptake b y m a r i n e p h y t o p l a n k t o n , Science, 314, 652-654. W a t s o n , A . J . and P.S. L i s s (1998), M a r i n e b i o l o g i c a l controls o n c l i m a t e v i a the c a r b o n and s u l f u r g e o c h e m i c a l c y c l e s , Phil. Trans. Roy.  Soc. Lon.  Ser. B-Biol. Sci 353,  (1365), 4 1 -  51. W o l f e , G . V . , M . S t e i n k e , and G . O . K i r s t (1997), G r a z i n g - a c t i v a t e d c h e m i c a l defense i n a u n i c e l l u l a r m a r i n e a l g a , Nature, 387, 894-897.  10  2. Springtime Variability in Particulate DMSP Concentrations along Line P, NE Pacific Ocean 1  2.1 Introduction F o r the past t w o decades, intense research efforts h a v e f o c u s e d o n d i m e t h y l s u l f i d e ( D M S ) , a v o l a t i l e degradation product o f the algal metabolite d i m e t h y l s u l f o n i o p r o p i o n a t e ( D M S P ) . T h i s research has been l a r g e l y m o t i v a t e d b y the p r o p o s a l that D M S m a y be i n v o l v e d i n g l o b a l c l i m a t e r e g u l a t i o n t h r o u g h its a b i l i t y to stimulate c l o u d f o r m a t i o n and thereby affect planetary albedo [Charlson et al, 1987]. I n contrast, interest i n its precursor, D M S P has m a i n l y been i n the r e a l m o f a l g a l c e l l p h y s i o l o g y , w i t h efforts directed l a r g e l y at i l l u m i n a t i n g ' t h e c e l l u l a r f u n c t i o n o f this c o m p o u n d [Malin and Kirst, 1 9 9 7 ; Stefels and van Leeuwe, 1998; Stefels, 2 0 0 0 ] . A l g a l D M S P p r o d u c t i o n is species s p e c i f i c , w i t h p r y m n e s i o p h y t e s and dinoflagellates b e i n g the m a i n producers [Keller et al, 1989]. I n s o m e species w i t h i n these classes, this m o l e c u l e can constitute a s i g n i f i c a n t fraction o f the c e l l u l a r s u l f u r and carbon quotas [Matrai and Keller, 1994]. Y e t despite the h i g h i n t r a c e l l u l a r concentrations o f D M S P f o u n d i n m a n y species, n o clear consensus has been reached o n its exact p h y s i o l o g i c a l role. D M S P is l i k e l y a m u l t i f a c e t e d m o l e c u l e that has been suggested to f u n c t i o n as an osmoregulant, a cryoprotectant [as r e v i e w e d b y Malin and Kirst, 1997], an o v e r f l o w m e c h a n i s m for excess c e l l energy [Stefels, 2 0 0 0 ] , and an antioxidant [Sunda et al, 2 0 0 2 ] . T h e e n z y m a t i c b r e a k d o w n o f D M S P into D M S and a c r y l i c a c i d catalyzed b y algal D M S P - l y a s e has also been suggested to f u n c t i o n as a g r a z i n g deterrent [Wolfe et al, 1997] and most recently as an anti-viral defense m e c h a n i s m [Evans et al, 2 0 0 6 ] .  ' A v e r s i o n o f this chapter w i l l be submitted as N e m c e k , N . , T . D . P e t e r s o n , and P . D . T o r t e l l . Interannual v a r i a b i l i t y i n s p r i n g t i m e particulate D M S P concentrations a l o n g a coastal to oceanic transect i n the N E P a c i f i c . Limnol.  Oceanogr.  11  In recent years, i n v e s ti g ati ons into the c y c l i n g o f D M S P b y the m i c r o b i a l f o o d w e b have revealed that this c o m p o u n d is important not o n l y to the p h y t o p l a n k t o n species that produce it, but also to other c o m p o n e n t s o f m a r i n e f o o d w e b s [as r e v i e w e d b y Kiene et al, 2 0 0 0 ] . D M S P and its degradation p r o d u c t s ( D M S , D M S O ) constitute the largest p o o l o f r e d u c e d organic s u l f u r i n the oceans, a n d as s u c h have been s h o w n to b e important g r o w t h substrates f o r m a n y species o f m a r i n e bacteria [Kiene et al, 2 0 0 0 ] . In fact, despite b e i n g present at a m i l l i o n - f o l d l o w e r concentration than sulfate, D M S P represents a n energetically f a v o u r a b l e f o r m o f sulfur and is the preferred substrate f o r heterotrophic bacteria [Kiene et al, 2 0 0 0 ] . T h u s , i n a d d i t i o n to its p h y s i o l o g i c a l role i n p h y t o p l a n k t o n , D M S P is e m e r g i n g as an integral c o m p o n e n t o f the b i o g e o c h e m i c a l s u l f u r c y c l e a n d m a y p l a y an important role i n the m a r i n e ecosystem. A s m o r e progress is m a d e towards e l u c i d a t i n g the d y n a m i c s o f o c e a n i c D M S P c y c l i n g , the e c o l o g i c a l and b i o g e o c h e m i c a l roles o f this c o m p o u n d m a y turn out to b e as s i g n i f i c a n t as the c l i m a t o l o g i c a l role o f D M S . In a n attempt to understand the c y c l i n g o f D M S / D M S P a n d the factors that regulate their p r o d u c t i o n , researchers h a v e sought to correlate concentrations o f these c o m p o u n d s w i t h various biological  [Leek et al, 1 9 9 0 ; Scarratt et al, 2 0 0 2 ; Riseman and DiTullio, 2 0 0 4 ] , c h e m i c a l  [Turner et al, 1 9 8 8 ; Leek et al, 1 9 9 0 ; Curran et al, 1 9 9 8 ; Riseman and DiTullio, 2004] and physical  [Belviso et al, 1 9 9 3 ; Simo and Pedros-Alio, 1999] variables w i t h v a r y i n g success. F o r  e x a m p l e , D M S P p concentrations i n the f i e l d tend to correlate better w i t h the presence o f s p e c i f i c algal groups s u c h as d i n o f l a g e l l a t e s o r p r y m n e s i o p h y t e s as o p p o s e d to b u l k c h l o r o p h y l l levels  [Scarratt et al, 2 0 0 2 ; Riseman and DiTullio, 2 0 0 4 ] . In a d d i t i o n , D M S P a c c u m u l a t i o n s i n natural c o m m u n i t i e s have been s h o w n to c o i n c i d e w i t h h i g h concentrations o f photoprotective p i g m e n t s , suggesting a role o f light i n D M S P p r o d u c t i o n  [Belviso et al, 1 9 9 3 ; Riseman and DiTullio,  12  2 0 0 4 ] . M o r e recently, i r o n a v a i l a b i l i t y has been suggested as a n a d d i t i o n a l factor i n f l u e n c i n g p h y t o p l a n k t o n D M S P p r o d u c t i o n as this c o m p o u n d m a y alleviate the o x i d a t i v e stress associated w i t h i r o n l i m i t a t i o n [Sunda et al, 2 0 0 2 ] . A s a result, algal D M S P concentrations i n i r o n - l i m i t e d regions are e x p e c t e d to b e r e l a t i v e l y h i g h e r than i n other areas. N u m e r o u s measurements o f particulate D M S P ( D M S P p ) h a v e been m a d e i n i r o n - l i m i t e d , h i g h nitrate, l o w c h l o r o p h y l l ( H N L C ) waters o f b o t h the S o u t h e r n O c e a n [Meyerdierks et al, 1997; Curran et al, 1998] and the equatorial P a c i f i c [Hatton et al, 1 9 9 8 ; Riseman and  DiTullio,  2 0 0 4 ] , In b o t h r e g i o n s , the data indicate that D M S P p : c h l ratios are i n d e e d h i g h e r i n iron-limited vs. iron-replete areas [Curran et al, 1 9 9 8 ; Riseman and DiTullio,  2 0 0 4 ] . B y c o m p a r i s o n , almost  no D M S P p data exist f o r the N E P a c i f i c , the t h i r d m a j o r i r o n - l i m i t e d H N L C r e g i o n . In fact, f r o m the large quantity o f r e c e n t l y c o m p i l e d D M S / D M S P measurements that have been made throughout the w o r l d ' s oceans, it i s evident that D M S P p data from the N E P a c i f i c are c o n s p i c u o u s l y absent [Kettle et al, 1999]. W e present the first extensive dataset o f particulate D M S P measurements o b t a i n e d from waters o f the N E P a c i f i c , s p a n n i n g b o t h coastal and oceanic regimes. These measurements are further u n i q u e as they represent the first a p p l i c a t i o n o f m e m b r a n e inlet mass spectrometry ( M E V I S ) for discrete D M S P measurements. T h e d i s t r i b u t i o n o f D M S P p i s e x a m i n e d i n the context o f p h y t o p l a n k t o n c o m m u n i t y c o m p o s i t i o n and nutrient concentrations, i n a n attempt to elucidate the b i o g e o c h e m i c a l regulators o f D M S P p r o d u c t i o n i n the N E P a c i f i c .  2.2 M a t e r i a l s a n d M e t h o d s Study area- T h e data presented h e r e i n w e r e obtained d u r i n g three c o n s e c u t i v e s p r i n g cruises a l o n g L i n e P i n the eastern subarctic P a c i f i c O c e a n o n b o a r d the CCGS John P. Tully. S a m p l i n g  13  was c o n d u c t e d b e t w e e n M a y 27-June 15, 2 0 0 3 , June 1-18, 2 0 0 4 a n d June 1-18, 2 0 0 5 at the 5 major stations a l o n g L i n e P, w h i c h connects O c e a n Station P a p a ( O S P or P 2 6 ) to the southern B r i t i s h C o l u m b i a coast ( F i g . 2.1). T h e L i n e P dataset represents one o f the longest r u n n i n g oceanographic t i m e series a n d m u c h is n o w k n o w n about the seasonal d y n a m i c s o f p h y t o p l a n k t o n b i o m a s s and p r o d u c t i o n i n this r e g i o n [Harrison, 2 0 0 2 ] . Stations P 4 , P I 2 are considered " c o a s t a l " stations and are t y p i c a l l y characterized b y a d i a t o m - d o m i n a t e d s p r i n g b l o o m that declines w i t h the onset o f macronutrient l i m i t a t i o n [Boyd and Harrison, 1999]. Station P 1 6 is i n the t r a n s i t i o n z o n e and stations P 2 0 and P 2 6 f a l l w i t h i n the H N L C boundary, w h e r e surface nitrate concentrations r e m a i n h i g h year-round [Whitney et al, 1998]. A t these stations, p h y t o p l a n k t o n g r o w t h is l i m i t e d b y i r o n a v a i l a b i l i t y , a n d there is little seasonality i n both b i o m a s s and p r i m a r y p r o d u c t i o n [Boyd and Harrison, 1999]. A t a l l stations i n this study, w e assume that h i g h m a c r o n u t r i e n t concentrations i n surface waters are i n d i c a t i v e o f i r o n l i m i t a t i o n [Whitney et al, 1998].  DMSPp measurements- Particulate D M S P concentrations w e r e m e a s u r e d at 5-8 depths i n the top 50 m o f the w a t e r c o l u m n d u r i n g each o f the three cruises w i t h slight m o d i f i c a t i o n s i n the m e t h o d o l o g y f r o m year to year. I n a l l cases, seawater w a s c o l l e c t e d i n 10 L N i s k i n bottles deployed o n a rosette s a m p l e r e q u i p p e d w i t h a S e a b i r d 9 1 1 + C T D . S a m p l e s w e r e d r a w n  from  N i s k i n bottles into r i n s e d 2 5 0 m l p o l y p r o p y l e n e bottles and i m m e d i a t e l y filtered. In 2 0 0 3 , duplicate 2 5 0 m l aliquots o f seawater w e r e c o l l e c t e d at each depth and filtered under l o w v a c u u m (< 5 i n . H g ) onto 25 m m G F / F filters ( n o m i n a l pore s i z e 0.7 u.m). T h e filters w e r e then transferred to 25 m l A l l t e c h glass v i a l s e q u i p p e d w i t h gas tight ' m i n i - n e r t ' closures. A 2 m l aliquot o f m e t h a n o l w a s added to each v i a l and the filters w e r e a l l o w e d to extract at -20° C for  14  several hours before 25 m l o f 1 N N a O H w a s added to the v i a l s . S e a l e d v i a l s w e r e left at r o o m temperature o v e r n i g h t p r i o r to analysis to ensure c o m p l e t e s t o i c h i o m e t r i c h y d r o l y s i s o f D M S P to D M S [Dacey and Blough, 1987]. H e a d s p a c e i n the v i a l s w a s m i n i m a l , p r o v i d i n g n e g l i g i b l e losses o f D M S into the gaseous phase o w i n g to the h i g h s o l u b i l i t y o f this gas ( K arm" ) [De Bruyn et al, 1  H  = 0.48 m o l L"  1  1995]. S a m p l e s w e r e a n a l y z e d o n b o a r d ship the f o l l o w i n g day b y M I M S  as described b e l o w . In 2 0 0 4 and 2 0 0 5 , a s i n g l e 2 5 0 m l sample w a s c o l l e c t e d at each depth and gravity filtered onto a 4 7 m m G F / F filter. T h e filters w e r e p l a c e d into 5 m l c r y o v i a l s w i t h 3 m l o f m e t h a n o l . T h e vials w e r e stored at -20° C u n t i l a n a l y z e d i n the laboratory w i t h i n 2-6 m o n t h s . D M S P stored i n this m a n n e r is k n o w n to be stable for extended p e r i o d s o f t i m e (J. D a c e y , pers. c o m m . ) . P r i o r to analysis, 2 m l aliquots o f the D M S P p extract i n m e t h a n o l w e r e transferred into 14 m l glass serum v i a l s , t o p p e d w i t h 12 m l o f I N N a O H and sealed w i t h T e f l o n - f a c e d b u t y l liners ( W h e a t o n ) and a l u m i n i u m c r i m p seals. T h e v i a l s w e r e v o r t e x e d and a l l o w e d to react o v e r n i g h t at r o o m temperature under m i n i m a l headspace. In a l l three years, D M S P p analysis w a s p e r f o r m e d 12-24 hours after base h y d r o l y s i s b y m e a s u r i n g the D M S c o n c e n t r a t i o n i n the l i q u i d phase u s i n g M I M S w i t h a m e m b r a n e inlet probe s u p p l i e d b y the m a n u f a c t u r e r ( H i d e n A n a l y t i c a l , U K ) . T h e p r o b e consists o f a 1/16" stainless steel c a p i l l a r y tube fitted w i t h a 0 . 0 0 5 " t h i c k d i m e t h y l s i l i c o n e sleeve that is i d e a l l y suited for a n a l y z i n g s m a l l - v o l u m e , discrete samples. A l t h o u g h this p r o b e has r e l a t i v e l y l o w sensitivity due to the s m a l l surface area o f the m e m b r a n e , the r e d u c e d s e n s i t i v i t y w a s not a factor as D M S P p samples w e r e concentrated b y filtration, and the resultant D M S concentrations i n the sample v i a l s ranged f r o m - 5 0 - 5 0 0 n M .  15  D u r i n g analysis, the v i a l s w e r e u n c a p p e d and the m e m b r a n e p r o b e w a s inserted d i r e c t l y into the l i q u i d sample. T h e s a m p l e w a s stirred to ensure a constant f l o w across the m e m b r a n e . D M S was m e a s u r e d b y s i n g l e i o n m o n i t o r i n g o f the m/z 62 p e a k u s i n g the r e s i d u a l gas analysis ( R G A ) m o d e o f the mass spectrometer c o n t r o l software ( M A S s o f t , H i d e n A n a l y t i c a l ) . E x p e r i m e n t s w i t h b l a n k s o l u t i o n s (2 m l m e t h a n o l + 25 m l I N N a O H ) s h o w e d that n o other ions were detected at m/z 6 2 . T h e mass spectrometer w a s r u n w i t h an i o n source e m i s s i o n current o f 1000 u A i n 2 0 0 3 w h i c h w a s r e d u c e d to 2 5 0 p A i n subsequent years f o l l o w i n g o p t i m i z a t i o n o f the quadrupole mass filter to a l o w e r mass range. D M S w a s m e a s u r e d u s i n g a secondary electron multiplier ( S E M )  set to a v o l t a g e o f 9 5 0 V w i t h detector d w e l l a n d settle times o f 3 0 0 m s .  F o r each s a m p l e measurement, the s i g n a l intensity w a s a l l o w e d to s t a b i l i z e and was then m o n i t o r e d for 2-5 m i n u t e s before subsequent samples w e r e i n t r o d u c e d . T h e s i g n a l dropped instantaneously f o l l o w i n g r e m o v a l o f the probe f r o m the s a m p l e s , i n d i c a t i n g no m e m o r y effects o n the m e m b r a n e . A l t h o u g h the v i a l s w e r e u n c a p p e d , the s i g n a l f o r i n d i v i d u a l samples was stable for the entire d u r a t i o n o f analysis t i m e , i n d i c a t i n g m i n i m a l loss o f D M S to the atmosphere. A s a further test, s o m e samples w e r e left stirring, u n c a p p e d , w i t h the p r o b e i n place for up to 30 minutes w i t h n o s i g n i f i c a n t d e c l i n e i n the D M S s i g n a l (data not s h o w n ) . T h e data stream f r o m the M I M S w a s e x p o r t e d to a spreadsheet and the m e a n m/z 62 s i g n a l intensity for each sample was determined b y a v e r a g i n g o v e r a two-minute i n t e r v a l d u r i n g the stable s i g n a l plateau. In a l l three years, a c a l i b r a t i o n c u r v e was generated for each set o f samples u s i n g fresh D M S P standards c o n s i s t i n g o f k n o w n aliquots o f sterile D M S P s o l u t i o n ( R e s e a r c h P l u s Inc.) prepared i n m e t h a n o l and I N N a O H . Standards w e r e prepared at the same t i m e , i n the same v i a l s , and i n the same v o l u m e s o f m e t h a n o l and s o d i u m h y d r o x i d e as the samples to prevent m a t r i x effects o n the  16  membrane, to ensure equivalent h y d r o l y s i s times, and to compensate f o r any s m a l l losses o f D M S to the headspace.  Ancillary oceanographic measurements- S a m p l e s for c h l o r o p h y l l a (chl a) analysis were d r a w n from the same N i s k i n bottles as the D M S P p samples and concentrations were determined u s i n g a fluorometric m e t h o d f o l l o w i n g filtration o f seawater samples onto 25 m m G F / F filters and extraction o f p i g m e n t s i n 9 0 % acetone f o r 2 4 hours [Parsons et al.,  1984]. Seawater nutrient  concentrations w e r e d e t e r m i n e d u s i n g a ship-board auto-analyzer w h i l e temperature and salinity data used to calculate potential density (O"T) were obtained f r o m the C T D . T h e l o w e r b o u n d a r y o f the m i x e d layer w a s d e f i n e d as the depth at w h i c h the v a l u e o f o r c h a n g e d b y 0.02 from that o f the surface. In 2 0 0 3 ,  1 4  C p r i m a r y p r o d u c t i v i t y a n d c a l c i f i c a t i o n rate measurements w e r e m a d e at the  c h l o r o p h y l l a m a x i m u m at each station. F o r these determinations, 2 0 0 m l o f seawater were c o l l e c t e d into acid-washed polycarbonate bottles and s p i k e d w i t h 20-50 u C , H CC>3 (50 m C i 14  m m o l " ) . B o t t l e s w e r e i n c u b a t e d i n an o n b o a r d P l e x i g l a s s f l o w - t h r o u g h i n c u b a t o r at 3 0 % surface 1  irradiance and in situ temperature f o r 2 4 hours. F o l l o w i n g i n c u b a t i o n , samples w e r e harvested onto 25 m m G F / F filters, c a r e f u l l y r i n s e d w i t h 0.2 p m filtered seawater to w a s h a w a y u n f i x e d H CC*3, and i m m e d i a t e l y frozen i n s c i n t i l l a t i o n v i a l s at - 2 0 ° C . U p o n return to the laboratory, 14  filters were a c i d i f i e d w i t h 1 m l 5 0 % p h o s p h o r i c a c i d , capped, and p l a c e d o n a shaker table overnight. Inorganic calcite c o l l e c t e d o n the G F / F w a s liberated as CO2 b y this a c i d treatment. T h i s CO2 w a s trapped i n p h e n e t h y l a m i n e (base)-soaked filters (13 m m G F / D ) stuck to the caps o f the v i a l s , w h i l e organic c a r b o n w a s left b e h i n d o n the p r i m a r y G F / F . T h e base-soaked filters were p l a c e d i n fresh v i a l s and capped, and b o t h sets o f v i a l s ( c o m p r i s i n g the inorganic and  17  organic  1 4  C fraction) w e r e c o u n t e d o n a s c i n t i l l a t i o n counter after the a d d i t i o n o f 10 m l o f  s c i n t i l l a t i o n c o c k t a i l ( S c i n t i s a f e , F i s h e r S c i e n t i f i c ) . F o u r bottle replicates w e r e r u n at each station and b o t h p r o d u c t i v i t y and c a l c i f i c a t i o n measurements w e r e corrected f o r  1 4  C uptake i n dark  bottles. In 2 0 0 3 , samples f o r p h y t o p l a n k t o n enumeration and t a x o n o m y w e r e c o l l e c t e d f r o m two depths at each station into 2 5 0 m l glass amber bottles and f i x e d w i t h hexamethylene-tetramineb u f f e r e d f o r m a l i n to a f i n a l concentration o f 0.4 % . I d e n t i f i c a t i o n and counts w e r e p e r f o r m e d o n 4 0 or 100 m l s u b s a m p l e s u s i n g inverted m i c r o s c o p y w i t h U t e r m o h l settling chambers f o l l o w i n g a 24-hour settling p e r i o d . C e l l v o l u m e s for i n d i v i d u a l p h y t o p l a n k t o n t a x a w e r e estimated u s i n g c e l l d i m e n s i o n s and b a s i c geometric f o r m u l a s (spheres, prolate spheres, c y l i n d e r s , etc.). C e l l carbon quotas w e r e d e t e r m i n e d f r o m v o l u m e estimates u s i n g the f o r m u l a s o f Montagues et al. [1994] for flagellates:  C = 0.109 * V  (1)  0 9 9 1  and Strathmann [1967] f o r d i a t o m s :  l o g C = -0.314 + 0.712 * l o g V  where C is c a r b o n content per c e l l i n p g , and V is c e l l v o l u m e i n p m  (2)  3  18  2.3 Results Application  of MIMS to DMSPp analysis- M E V I S p r o v e d to be an effective n e w t o o l for  m e a s u r i n g particulate D M S P concentrations i n seawater. S i n c e D M S P p w a s measured d i r e c t l y f r o m the l i q u i d phase, analysis t i m e w a s greatly reduced c o m p a r e d to that t y p i c a l o f purge and trap gas c h r o m a t o g r a p h y ( P T G C ) , w h i c h requires s a m p l e s p a r g i n g . I n 2 0 0 3 w h e n M E V I S was first field-tested, i n d i v i d u a l D M S P samples took an average o f 5 m i n u t e s to analyze, a l l o w i n g a c o m p l e t e depth p r o f i l e w i t h standards to be r u n i n duplicate i n under 2 hours. A l t h o u g h the m e t h o d was s i m p l e and r a p i d , it p r o d u c e d h i g h q u a l i t y data. T h e c a l i b r a t i o n curves w e r e linear over a 4 0 - f o l d c o n c e n t r a t i o n range, and sample r e p l i c a t i o n w a s g o o d ( m e a n s.d. o f duplicates = 9.5 % ) . A s a result, o n l y s i n g l e samples w e r e c o l l e c t e d at each depth i n subsequent years. In 2 0 0 4 , a s w i t c h to a thinner m e m b r a n e y i e l d e d faster response t i m e s , r e d u c i n g the analysis t i m e for each s a m p l e to 2 m i n u t e s . T h e r e d u c t i o n i n s e n s i t i v i t y ( c o m p a r e d to the large m e m b r a n e cuvette) r e s u l t i n g from the smaller surface area o f the m e m b r a n e inlet probe d i d not h i n d e r o u r a b i l i t y to accurately measure D M S P p concentrations i n any samples. A l t h o u g h detection l i m i t s w e r e not e x p l i c i t l y determined, 15 n M samples w e r e e a s i l y measurable relative to b l a n k s . T a k i n g into account sample concentration b y f i l t r a t i o n , y i e l d e d a detection l i m i t o f <1 n M in situ D M S P p . T h i s was w e l l b e l o w the m a j o r i t y o f concentrations encountered d u r i n g the surveys. D u e to the s m a l l size o f the m e m b r a n e probe, it i s p o s s i b l e to s i g n i f i c a n t l y reduce the v o l u m e o f the s a m p l e v i a l s used (i.e. f r o m 14 m l to 3 m l ) , w h i c h w o u l d reduce the v o l u m e o f the i n i t i a l seawater s a m p l e required and m i n i m i z e potential filtration artefacts [as i n Kiene and Slezak, 2 0 0 6 ] .  19  Variability in DMSPp concentration - Particulate D M S P concentrations i n the upper 50 m o f the water c o l u m n a l o n g L i n e P s h o w e d considerable v a r i a b i l i t y w i t h depth, across stations, and between study years ( F i g . 2.2). D u e to i n c l e m e n t weather, samples w e r e not c o l l e c t e d at P I 6 i n 2 0 0 5 . D M S P p levels r a n g e d f r o m a l o w o f 0.2 n M at 5 0 m depth at P 4 i n 2 0 0 4 to a h i g h o f 63.2 n M at 10 m depth at P I 2 i n 2 0 0 3 , w i t h an o v e r a l l m e a n c o n c e n t r a t i o n f o r the 3 year survey o f 21.5 n M (n = 9 4 , s.d. = 15.0 n M ) . In general, the coastal stations ( P 4 , P 1 2 ) h a d higher m a x i m u m D M S P p concentrations than the offshore stations (P16-P26), i n c o n j u n c t i o n w i t h higher levels o f c h l o r o p h y l l a (Figsf 2.2, 2.3). T h e one notable e x c e p t i o n w a s P 1 2 i n 2 0 0 5 w h e n b o t h the D M S P p and c h l o r o p h y l l concentrations w e r e u n i f o r m l y l o w . C h l o r o p h y l l levels a l o n g L i n e P were u s u a l l y less than 1 p g L" , except for at P 4 i n 2 0 0 4 , w h e r e a p r o m i n e n t subsurface c h l o r o p h y l l 1  m a x i m u m o f 2.2 p g L" w a s encountered ( F i g . 2.3). 1  B o t h the D M S P p and c h l o r o p h y l l a depth p r o f i l e s at P 4 and P 1 2 w e r e characterized b y subsurface m a x i m a and m o r e v e r t i c a l v a r i a b i l i t y than f o u n d o f f s h o r e ( F i g s . 2.2, 2.3). A t these stations, the D M S P p m a x i m u m o c c u r r e d near the base o f the rather s h a l l o w m i x e d layer (<20 m ) , w h i l e the c h l o r o p h y l l a m a x i m u m generally o c c u r r e d j u s t b e l o w this depth. In contrast, stations P 1 6 - P 2 6 h a d deeper m i x e d layers (30-40 m ) and r e l a t i v e l y u n i f o r m depth p r o f i l e s w i t h v e r y little v e r t i c a l structure i n either D M S P p or c h l o r o p h y l l a concentrations i n the upper 50 m o f the water c o l u m n ( F i g s . 2.2, 2.3). T h e progressive loss o f v e r t i c a l structure i n the p r o f i l e s m o v i n g o f f s h o r e f r o m P 4 to P 2 6 c o i n c i d e d w i t h the p r o g r e s s i v e d e e p e n i n g o f the m i x e d layer and l i k e l y also to a d e e p e n i n g o f the euphotic z o n e due to decreased p h y t o p l a n k t o n b i o m a s s (as estimated b y c h l o r o p h y l l a). V a r i a b i l i t y i n D M S P p concentrations a l o n g L i n e P b e t w e e n survey years was h i g h ( F i g . 2.2). T h i s v a r i a b i l i t y w a s m o s t p r o n o u n c e d at P 4 and P I 2 , and appeared to be d r i v e n at least i n  20  part b y h i g h v a r i a b i l i t y i n c h l o r o p h y l l a levels ( F i g . 2.3). T h e l o n e e x c e p t i o n w a s station P 2 0 w h i c h had r e l a t i v e l y constant D M S P p and c h l o r o p h y l l a concentrations i n a l l three years. In general, D M S P p concentrations across the survey r e g i o n w e r e highest i n 2 0 0 3 and decreased s i g n i f i c a n t l y i n the f o l l o w i n g years b y i n m a n y cases m o r e than 50 % ( F i g . 2.2). T h e m e a n D M S P p concentrations across a l l stations i n 2 0 0 4 (14.6 n M ) and 2 0 0 5 (16.6 n M ) w e r e less than h a l f the m e a n c o n c e n t r a t i o n i n 2 0 0 3 (33.6 n M ) . A l t h o u g h this decrease i n D M S P p levels appeared to o c c u r i n c o n j u n c t i o n w i t h a decrease i n c h l o r o p h y l l a concentrations ( F i g . 2.3), D M S P p x h l ratios also d r o p p e d i n subsequent years, i n d i c a t i n g that the d e c l i n e i n D M S P p levels was due to m o r e than just a drop i n o v e r a l l p h y t o p l a n k t o n b i o m a s s ( F i g . 2.4). D M S P p x h l ratios a l o n g L i n e P ranged f r o m 0.67 n m o l u g " at 50 m at P 4 i n 2 0 0 4 to 103 n m o l u.g"' at 2 0 m at P 2 6 1  i n 2003 ( F i g . 2.4). T h e m e a n ratio across a l l stations i n 2 0 0 3 (61.1 n m o l pg" ) w a s almost t w i c e 1  the m e a n ratio o b s e r v e d i n 2004. (33.6 n m o l p g " ) and 2 0 0 5 (37.0 n m o l p.g"'). C o n t o u r plots 1  illustrating the interannual and spatial v a r i a b i l i t y o f b o t h D M S P p and c h l o r o p h y l l a a l o n g L i n e P are presented i n F i g u r e 2.5.  DMSPp in relation to other variables- W h i l e D M S P p concentrations v a r i e d to s o m e extent w i t h total p h y t o p l a n k t o n b i o m a s s , other factors i n c l u d i n g the p h y s i o l o g i c a l status and species c o m p o s i t i o n o f the p h y t o p l a n k t o n c o m m u n i t y also l i k e l y affected the observed distributions. A l o n g L i n e P, the depth p r o f i l e s o f D M S P p generally r e s e m b l e d those o f c h l o r o p h y l l at i n d i v i d u a l stations, but o v e r a l l there was o n l y a w e a k linear c o r r e l a t i o n b e t w e e n the t w o variables for the 3-year p o o l e d dataset (r  2  = 0.20,  n = 9 4 , p < 0 . 0 0 0 1 ; F i g . 2.6). T w o significant  outliers characterized b y h i g h c h l o r o p h y l l concentrations c o n f o u n d e d this relationship (P4, 2 0 and 25 m depth i n 2 0 0 4 ) . W i t h these outliers r e m o v e d the p o s i t i v e l i n e a r c o r r e l a t i o n between  21  D M S P p and c h l o r o p h y l l a w a s s i g n i f i c a n t l y strengthened (r  2  = 0 . 4 5 , n - 92,/?<0.0001), although  a g o o d deal o f scatter r e m a i n e d . T o e x a m i n e other factors p o t e n t i a l l y i n f l u e n c i n g D M S P p v a r i a b i l i t y , a n c i l l a r y measurements i n c l u d i n g p r i m a r y p r o d u c t i v i t y and c a l c i f i c a t i o n rates, as w e l l as detailed p h y t o p l a n k t o n species counts w e r e m a d e d u r i n g the 2003 survey. F o r interannual c o m p a r i s o n purposes, o n l y D M S P p data c o l l e c t e d at the 5 major time-series stations a l o n g L i n e P are presented, h o w e v e r , measurements w e r e also obtained at a d d i t i o n a l s u r r o u n d i n g stations each year. In 2 0 0 3 , D M S P p concentrations and supporting parameters w e r e also measured at stations A 3 , A 4 , and A 6 (see F i g . 2.1 f o r station locations) and are i n c l u d e d i n the f o l l o w i n g analyses to increase the s a m p l e s i z e ( T a b l e 2.1). D e t a i l e d p h y t o p l a n k t o n c o m m u n i t y c o m p o s i t i o n data w e r e c o l l e c t e d at 10 m depth i n the m i x e d layer a n d at the c h l o r o p h y l l m a x i m u m at a l l stations d u r i n g 2 0 0 3 . V a r i o u s s m a l l , u n i d e n t i f i e d flagellates w e r e n u m e r i c a l l y d o m i n a n t at the m o r e coastal stations (P4, P I 2 , P I 6 , A 6 ; F i g . 2.7). A l t h o u g h these flagellates w e r e s t i l l abundant o f f s h o r e ( P 2 0 , P 2 6 , A 3 , A 4 ) , s m a l l diatoms and p r y m n e s i o p h y t e s increased i n abundance at these o c e a n i c stations ( F i g . 2.7). D M S P p concentrations w e r e not l i n e a r l y correlated to the abundance o f any p h y t o p l a n k t o n group. H o w e v e r , w h e n p h y t o p l a n k t o n abundance w a s c o n v e r t e d to c a r b o n b i o m a s s u s i n g the appropriate c o n v e r s i o n factors (see methods), the relative c o n t r i b u t i o n o f each group to the total c o m m u n i t y c h a n g e d d r a m a t i c a l l y ( F i g . 2.8). D e s p i t e c o n t r i b u t i n g less than 10 % to total c e l l abundance, d i n o f l a g e l l a t e s d o m i n a t e d c a r b o n b i o m a s s at P 4 and P I 2 , and m a d e a significant c o n t r i b u t i o n at the other stations ( F i g . 2.8). A s i g n i f i c a n t , p o s i t i v e r e l a t i o n s h i p w a s observed between D M S P p concentrations and the relative c a r b o n b i o m a s s o f dinoflagellates across the survey r e g i o n ( r = 0.46, n = \6,pO.05, 2  T a b l e 2.2). S m a l l , pennate d i a t o m s also contributed  22  s i g n i f i c a n t l y to c a r b o n b i o m a s s at m o s t stations p a r t i c u l a r l y at the c h l o r o p h y l l m a x . at stations P 2 0 , A 4 and A 6 ( F i g . 2.8b). H o w e v e r , d i a t o m b i o m a s s w a s l o w e s t at P 4 , P 1 2 and A 3 , the 3 stations w i t h the highest D M S P p concentrations r e s u l t i n g i n a s i g n i f i c a n t inverse relationship between d i a t o m b i o m a s s and D M S P p levels across all stations (r  2  = 0.33, n = 16,p<0.05, Table  2.2). G i v e n the estimates o f p h y t o p l a n k t o n c a r b o n b i o m a s s , the percent c e l l c a r b o n as D M S P i n each sample w a s c a l c u l a t e d u s i n g a C : S ratio o f 5:1 for D M S P ( T a b l e 2.3). P r i m a r y p r o d u c t i v i t y rates measured at the c h l o r o p h y l l a m a x i m u m i n 2 0 0 3 w e r e ~3 times higher at P 4 than at any other station and thus appeared to be unrelated to D M S P p levels (Table 2.1). In contrast, c a l c i f i c a t i o n rates expressed as a percentage o f p r i m a r y p r o d u c t i v i t y rates were highest i n o f f s h o r e H N L C waters ( P 2 0 , P 2 6 , A 3 and A 4 ; T a b l e 2.1), i n accordance w i t h an increased p r o p o r t i o n o f p r y m n e s i o p h y t e s (Figs. 2.7b, 2.8b). T h e r e w a s a significant linear relationship b e tw e e n relative c a l c i f i c a t i o n rates and the relative c a r b o n b i o m a s s o f p r y m n e s i o p h y t e s ( o f w h i c h c o c c o l i t h o p h o r e s are a p r o m i n e n t subgroup) across the survey r e g i o n (r  2  = 0.69, n = S,p  = 0 . 0 1 , data not shown). H o w e v e r , neither absolute n o r relative c a l c i f i c a t i o n  rates w e r e correlated to D M S P p o r D M S P p x h l levels. T h e effect o f v a r i a b l e nutrient concentrations o n the cross-transect and inter-cruise v a r i a b i l i t y i n D M S P p l e v e l s w a s e x a m i n e d . D e p t h integrated D M S P p x h l ratios are plotted against the m i x e d layer nitrate concentration at each station i n F i g u r e 2.9. A l t h o u g h there was some i n d i c a t i o n that h i g h e r D M S P p x h l ratios w e r e associated w i t h h i g h nitrate, l o w i r o n waters i n 2 0 0 3 , this trend d i d not h o l d i n subsequent years ( F i g . 2.9). F u r t h e r m o r e , v a r i a b i l i t y i n D M S P p x h l ratios b e t w e e n study years appeared unrelated to surface nitrate levels.  23  2.4 Discussion T h e stations o f the N E P a c i f i c connected b y L i n e P boast one o f the longest and most c o m p r e h e n s i v e o p e n o c e a n t i m e series, and m u c h oceanographic data has b e e n c o l l e c t e d here over the past 50 years, p a r t i c u l a r l y at O c e a n Station P a p a (P26) [see r e v i e w Harrison,  2002].  D e s p i t e a l l the i n f o r m a t i o n amassed o n the d y n a m i c s o f 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 and b i o m a s s [Booth et al, 1 9 9 3 ; Boyd and Harrison,  1999], there have been v i r t u a l l y n o D M S P  measurements m a d e i n this r e g i o n [see Kettle et al, 1999]. P r i o r to the recent p u b l i c a t i o n o f D M S P p measurements o b t a i n e d d u r i n g the S E R I E S i r o n - f e r t i l i z a t i o n e x p e r i m e n t conducted near Station P a p a [Levasseur et al, 2 0 0 6 ] , the o n l y D M S P data for the entire N E P a c i f i c consisted o f a single depth p r o f i l e taken o f f the coast o f W a s h i n g t o n state [Bates et al, 1994]. T h i s i s surprising g i v e n the n u m e r o u s measurements o f surface water D M S concentrations [ Watanabe et al, 1995; Aranami et al, 2 0 0 1 ; Wong et al, 2005] that have b e e n m a d e i n the area. A recently p u b l i s h e d 6-year t i m e series o f D M S concentrations a l o n g L i n e P [Wong et al, 2005] c o n f i r m s earlier observations that this r e g i o n hosts some o f the highest spring/summer D M S levels i n the w o r l d [Kettle etal,1999],  and m a y thus be an important source o f this gas to the atmosphere.  S i n c e the p r o d u c t i o n o f D M S u l t i m a t e l y depends o n the algal p r o d u c t i o n o f D M S P , measurements o f this p o o l constitute the first step towards u n d e r s t a n d i n g the factors leading to elevated D M S levels i n the N E P a c i f i c . T h e data presented h e r e i n c o m p r i s e the first extensive set o f D M S P p measurements m a d e i n this r e g i o n . These data reveal that concentrations o f D M S P p exhibit s i g n i f i c a n t inter-cruise and spatial v a r i a b i l i t y , b o t h w i t h depth and across the survey area.  Spatial Variability  in DMSPp  levels- H i g h spatial v a r i a b i l i t y i n D M S P p levels was observed i n  each survey year w i t h a 2-3 f o l d range i n m a x i m u m concentrations a l o n g L i n e P. In general,  24  D M S P p concentrations decreased offshore, t h o u g h D M S P p x h l ratios s h o w e d no consistent trend between coastal a n d H N L C waters. A t stations P 4 and P 1 2 w h e r e p r o m i n e n t depth m a x i m a i n b o t h c h l o r o p h y l l a a n d D M S P p w e r e observed (Figs. 2.2, 2.3), D M S P p m a x i m a were always s h a l l o w e r than c h l o r o p h y l l a m a x i m a . T h i s is a c o m m o n l y reported p h e n o m e n o n [Turner et 1988; Belviso et al,  1 9 9 3 ; Dacey et al,  al,  1998], and is l i k e l y due to the o p p o s i n g effects o f  nutrients and light o n the synthesis o f c h l o r o p h y l l and D M S P . In surface waters, c o n d i t i o n s that promote o x i d a t i v e stress s u c h as h i g h light [Sunda et al,  2 0 0 2 ; Slezak and Herndl, 2003] and  l o w nutrients [Bucciarelli and Sunda, 2003] m a y p r o m o t e increased a l g a l p r o d u c t i o n o f D M S P . In contrast, deeper i n the water c o l u m n p h y t o p l a n k t o n p h o t o a c c l i m a t e i n response to higher nutrient and l o w l i g h t c o n d i t i o n s b y i n c r e a s i n g their i n t r a c e l l u l a r c h l o r o p h y l l a concentrations [Falkowski andLaRoche, 1991]. A l t h o u g h it has been suggested that d e p t h differences i n D M S P p and c h l o r o p h y l l a m a x i m a m a y s i m p l y reflect changes i n species c o m p o s i t i o n w i t h depth, [Dacey et al,  1 9 9 8 ] , w e d i d not observe any dramatic shifts i n p h y t o p l a n k t o n c o m m u n i t y  c o m p o s i t i o n b e t w e e n surface and deep waters (Figs. 2.7, 2.8). In fact, the b i o m a s s o f p r o m i n e n t D M S P - p r o d u c e r s s u c h as dinoflagellates [Keller et al,  1999], w a s h i g h e r at the c h l o r o p h y l l m a x .  than at 10 m depth i n the m i x e d layer suggesting a p h y s i o l o g i c a l d r i v e r f o r the depth structure o f D M S P p levels ( F i g . 2.8).  DMSP in relation to other variables- Consistent w i t h the results o f m a n y p r e v i o u s studies [Belviso et al,  1 9 9 3 ; Townsend and Keller, 1996; Dacey et al,  1 9 9 8 ] , o n l y a w e a k correlation  between particulate D M S P and c h l o r o p h y l l a concentrations w a s o b s e r v e d over the 3-year survey p e r i o d a l o n g L i n e P ( F i g . 2.6). S i n c e significant D M S P p r o d u c t i o n is c o n f i n e d to a few classes o f p h y t o p l a n k t o n [Keller et al,  1989], D M S P p concentrations s h o u l d be m o r e strongly  25  correlated to the presence o f certain t a x o n o m i c groups than to b u l k c h l o r o p h y l l concentrations, as has been o b s e r v e d p r e v i o u s l y [Scarratt et al,  2 0 0 2 ] . In 2 0 0 3 , o n l y a w e a k linear correlation  was observed b e t w e e n D M S P p levels and b u l k c h l o r o p h y l l at the 8 stations surveyed (r  2  = 0.32,  n = 4 6 , p< 0.001). Instead, D M S P p w a s m o r e strongly correlated to the relative c a r b o n b i o m a s s o f dinoflagellates (r  2  [Keller et al,  = 0.46, n = 16, £><0.05, T a b l e 2.2), a g r o u p k n o w n f o r h i g h D M S P quotas  1989]. Interestingly, D M S P p concentrations also s h o w e d a s i g n i f i c a n t , inverse  relationship to the relative c a r b o n b i o m a s s o f d i a t o m s (r  2  w h i c h generally p r o d u c e little D M S P [Keller et al,  = 0 . 3 3 , n = 16,/?<0.05, T a b l e 2.2),  1989]. In contrast, there was no linear  correlation b e tw e e n D M S P p concentrations and the b i o m a s s o f p r y m n e s i o p h y t e s (Table 2.2), another group o f p r o m i n e n t D M S P - p r o d u c e r s , despite the fact that this g r o u p increased i n abundance i n F f N L C waters i n c o n j u n c t i o n w i t h elevated D M S P p : c h l ratios and h i g h e r relative c a l c i f i c a t i o n rates ( T a b l e 2.1). T h i s is not entirely s u r p r i s i n g as i n a d d i t i o n to t a x o n o m i c effects, D M S P p r o d u c t i o n is i n f l u e n c e d b y p h y s i c o c h e m i c a l factors i n c l u d i n g nutrient a v a i l a b i l i t y [Stefels and van Leeuwe, 1 9 9 8 ; Sunda et al, [Belviso et al,  2 0 0 2 ; Bucciarelli and Sunda, 2003] and both v i s i b l e  1 9 9 3 ; Stefels and van Leeuwe, 1998] and U V l i g h t [Slezak and Herndl, 2 0 0 3 ] .  These factors v a r y w i d e l y i n the m a r i n e environment and m a y obfuscate attempts to l i n k the algal D M S P p o o l w i t h any s p e c i f i c p h y t o p l a n k t o n group.  Contribution of DMSP to carbon biomass- B a s e d o n measured D M S P p concentrations and the estimates o f c a r b o n b i o m a s s d e t e r m i n e d f r o m p h y t o p l a n k t o n c e l l counts, the relative c o n t r i b u t i o n o f D M S P to total autotrophic c e l l c a r b o n w a s determined f o r each s a m p l e i n 2 0 0 3 (Table 2.3). A p a r t f r o m at stations P 4 a n d P I 2, the estimates o f percent c e l l c a r b o n as D M S P i n this study were m u c h h i g h e r ( m e a n 20.3 ± 1 5 % ) than those reported i n p r e v i o u s f i e l d studies (1-10 % ) ,  26  despite the fact that the c o r r e s p o n d i n g D M S P p x h l ratios f e l l w i t h i n the range o f those i n the literature [see r e v i e w b y Kiene et al,  2 0 0 0 ; Simo et al,  2 0 0 2 ] . A l t h o u g h m a n y o f these p r e v i o u s  studies d i d not estimate c a r b o n b i o m a s s d i r e c t l y and s i m p l y a s s u m e d a C x h l ratio o f 50 g g" [i.e. 1  Simo et al,  2 0 0 2 ] , it is p o s s i b l e that some o f the C x h l ratios c a l c u l a t e d h e r e i n are underestimates  (i.e. 6.5 g g"' at 10 m P 1 6 , T a b l e 2.3). C a r b o n - t o - c h l o r o p h y l l ratios can range from 20-160 g g" and v a r y w i t h depth, latitude, 1  season [Taylor et al,  1 9 9 7 ] , and p r o x i m i t y to the coast [Chang et al,  2 0 0 3 ] . W i t h o n l y a few  exceptions (P4, P 1 2 ) , C x h l ratios i n this study w e r e a l l less than 3 0 g g " , values m o r e t y p i c a l o f 1  nutrient-replete, l i g h t - l i m i t e d deep waters [Taylor et al,  1 9 9 7 ] , rather than i r o n - l i m i t e d surface  waters. T h e C x h l ratio estimated f o r the i r o n - l i m i t e d station P 2 6 (20-30 g g" ) is about h a l f the 1  50 g g" spring/summer average d e t e r m i n e d p r e v i o u s l y for this station [Booth et al, 1  1993].  Stations P I 6 and A 6 w h e r e the lowest C x h l ratios w e r e m e a s u r e d w e r e d o m i n a t e d b y s m a l l u n i d e n t i f i a b l e flagellates ( F i g . 2.7), w h o s e exact n u m b e r s , d i m e n s i o n s and thus c a r b o n content were prone to larger errors. In a d d i t i o n , preserved c e l l s have a t e n d e n c y to shrink, l e a d i n g to reduced v o l u m e estimates and hence underestimated c a r b o n quotas. F u r t h e r m o r e w i t h the m i c r o s c o p i c m e t h o d u s e d , it w a s not p o s s i b l e to enumerate c y a n o b a c t e r i a s u c h as Synechococcus spp. w h i c h are k n o w n to be abundant at the oceanic stations a l o n g L i n e P [Booth et al,  1993],  and l i k e l y m a d e a s i g n i f i c a n t c o n t r i b u t i o n to total autotrophic c a r b o n b i o m a s s . T h i s uncertainty i n the b i o m a s s estimates m a y also have c o n f o u n d e d attempts to relate D M S P levels to the c a r b o n b i o m a s s o f i n d i v i d u a l p h y t o p l a n k t o n groups across the survey r e g i o n . A p p l y i n g a constant C x h l ratio o f 50 g g" to a l l stations y i e l d s a percent c o n t r i b u t i o n o f D M S P to c e l l c a r b o n o f 2.5-18.8 % 1  (mean 8.4 % ) , m o r e i n l i n e w i t h other oceanic f i e l d studies [Kiene et al,  2 0 0 0 ; Simo et  al,  2002].  27  Inter-cruise Variability in DMSPp along Line P- V a r i a b i l i t y i n s p r i n g t i m e D M S P p concentrations a l o n g L i n e P b e t w e e n survey years was h i g h , and w a s m o s t p r o n o u n c e d at the coastal stations P 4 and P 1 2 ( F i g s . 2.2, 2.5). Stations P 1 6 and P 2 0 s h o w e d m u c h less v a r i a b i l i t y i n D M S P p l e v e l s , w h i l e at P 2 6 a dramatic (>3-fold) d e c l i n e i n D M S P p levels o c c u r r e d after 2 0 0 3 , w i t h concentrations r e m a i n i n g r e l a t i v e l y constant i n the f o l l o w i n g t w o years (Figs. 2.2, 2.5). A c r o s s the survey r e g i o n , D M S P p concentrations (and D M S P p : c h l rations) w e r e generally highest i n 2 0 0 3 . T h e surveys i n a l l 3 years o c c u r r e d d u r i n g a l m o s t the same three-week p e r i o d , however, the t i m i n g o f the s p r i n g b l o o m at the coastal stations m a y have d i f f e r e d l e a d i n g to the h i g h v a r i a b i l i t y i n b o t h D M S P p ( F i g . 2.2) and c h l o r o p h y l l a l e v e l s ( F i g . 2.3) observed at P 4 and P I 2. A s a result o f the r e l a t i v e l y s h a l l o w w i n t e r m i x e d layer, tight c o u p l i n g betw een autotrophs and grazers, and persistent i r o n - l i m i t a t i o n , stations P 2 0 - P 2 6 e x h i b i t m u c h less seasonality i n p h y t o p l a n k t o n b i o m a s s [Boyd and Harrison, 1999], such that the t i m i n g o f the cruises w o u l d have less i m p a c t o n the o b s e r v e d v a r i a b i l i t y . D u e to a l a c k o f k n o w l e d g e o f the seasonal v a r i a b i l i t y i n D M S P p concentrations i n this r e g i o n , it is d i f f i c u l t to speculate whether the inter-cruise differences represent true inter-annual v a r i a b i l i t y or different phases o f the seasonal cycle. M e t h o d o l o g i c a l differences i n D M S P p analysis also exist b e t w e e n study years, as samples w e r e a n a l y z e d at sea i n 2 0 0 3 , and several months after s a m p l e c o l l e c t i o n i n 2 0 0 4 and 2 0 0 5 . H o w e v e r , this is l i k e l y not a significant factor i n d e t e r m i n i n g inter-cruise v a r i a b i l i t y , since samples w e r e a l w a y s m e a s u r e d relative to fresh standards and D M S P p is k n o w n to be stable i n m e t h a n o l for m o n t h s (J. D a c e y , pers. c o m m . ) . D i f f e r e n c e s i n sea surface temperature greater than 2 ° C w e r e o b s e r v e d b e t w e e n study years at P 2 6 , and these a l o n g w i t h changes i n nutrient s u p p l y and c h l o r o p h y l l b i o m a s s support the changes i n D M S P p l e v e l s , a r g u i n g against a m e t h o d o l o g i c a l cause o f v a r i a b i l i t y .  28  T h e differences i n D M S P p x h l ratios at i n d i v i d u a l stations b e t w e e n study years ( F i g . 2.4), c o u p l e d w i t h the w e a k o v e r a l l c o r r e l a t i o n between D M S P p and c h l o r o p h y l l a ( F i g . 2.6) point to a t a x o n o m i c or p h y s i o l o g i c a l d r i v e r o f D M S P p v a r i a b i l i t y . In 2 0 0 3 , D M S P - r i c h dinoflagellates made a s i g n i f i c a n t c o n t r i b u t i o n to total autotrophic c a r b o n at a l l stations ( F i g . 2.8). A l t h o u g h p h y t o p l a n k t o n c o m m u n i t y c o m p o s i t i o n data w e r e not c o l l e c t e d i n subsequent years, it is p o s s i b l e that a d e c l i n e i n dinoflagellates c o u l d account for the drop i n D M S P p levels and D M S P p x h l ratios after 2 0 0 3 . D r a m a t i c shifts i n the b i o m a s s o f dinoflagellates and p r y m n e s i o p h y t e s (up to 2 orders o f magnitude) h a v e b e e n s h o w n to o c c u r i n c o n s e c u t i v e years at P 2 6 [Wong et al, 2 0 0 6 ] . N u t r i e n t c o n d i t i o n s c a n also i n f l u e n c e D M S P p v a r i a b i l i t y . S i n c e D M S P and its byproducts have been s h o w n to f u n c t i o n as p o w e r f u l antioxidants [Sunda et al, 2 0 0 2 ] , algal D M S P p r o d u c t i o n is e x p e c t e d to increase under o x i d a t i v e stressors s u c h as i r o n - l i m i t a t i o n . In 2 0 0 3 , there w a s s o m e e v i d e n c e to this effect as D M S P p x h l ratios at i n d i v i d u a l stations increased i n c o n j u n c t i o n w i t h h i g h e r surface nitrate levels, an i n d i c a t o r o f i r o n - l i m i t a t i o n i n this region ( F i g . 2.9). E v i d e n c e for a p o s i t i v e relationship between D M S P p x h l ratios and nitrate concentrations also c o m e s from H N L C waters o f the S o u t h e r n O c e a n [Curran et al, 1998; Jones et al, 1998]. In this latter study, the highest D M S P p concentrations c o i n c i d e d w i t h l o w c h l o r o p h y l l levels and h i g h nitrate waters [Curran et al, 1 9 9 8 ; Jones et al, 1988]. T h e authors d i d not d i r e c t l y relate this o b s e r v a t i o n to i r o n concentrations, but i r o n a v a i l a b i l i t y is k n o w n to l i m i t p h y t o p l a n k t o n g r o w t h i n these waters [Curran et al, 1998]. Perhaps the strongest f i e l d evidence for the effect o f i r o n o n D M S P p r o d u c t i o n c o m e s from the i r o n - l i m i t e d waters o f the equatorial P a c i f i c w h e r e D M S P p x h l ratios w e r e 2-6 h i g h e r i n i r o n depleted offshore waters than i n iron-rich coastal waters [Riseman and DiTullio, 2 0 0 4 ] .  29  T h e p o s i t i v e trend o b s e r v e d betw een D M S P p x h l ratios and nitrate levels i n 2003 d i d not o c c u r i n subsequent years ( F i g . 2.9). In 2 0 0 4 , a large d e c l i n e i n the D M S P p x h l i n v e n t o r y o c c u r r e d at station P 2 6 despite a r e l a t i v e l y constant nitrate c o n c e n t r a t i o n . Interestingly i n 2 0 0 5 , the s p r i n g surface nitrate c o n c e n t r a t i o n at P 2 6 p l u m m e t e d to about h a l f the u s u a l 14 u M value. In a l l 3 years, the p r e v i o u s w i n t e r ' s surface nitrate concentration at P 2 6 w a s a constant ~ 1 4 p M , i n d i c a t i n g that s i g n i f i c a n t nitrate d r a w d o w n h a d o c c u r r e d at this station i n 2 0 0 5 , p o s s i b l y as a result o f an i r o n i n j e c t i o n . A l t h o u g h b o t h c h l o r o p h y l l ( F i g . 2.3) a n d D M S P p levels ( F i g . 2.2) s h o w e d a slight increase o y e r 2 0 0 4 levels i n c o n j u n c t i o n w i t h this d r a w d o w n , D M S P p x h l ratios r e m a i n e d l a r g e l y u n c h a n g e d f r o m the p r e v i o u s year ( F i g s . 2.4, 2.9). W i t h o u t t a x o n o m i c data for 2 0 0 4 and 2005 it is d i f f i c u l t to determine whether the interannual v a r i a b i l i t y i n D M S P levels a l o n g L i n e P was due to changes i n p h y t o p l a n k t o n c o m m u n i t y c o m p o s i t i o n or p h y s i o l o g y . E v e n w i t h this data it is d i f f i c u l t to tease apart these two factors i n the f i e l d since changes i n nutrient concentrations tend to be l i n k e d to f l o r i s t i c shifts i n the p h y t o p l a n k t o n c o m m u n i t y . T h i s w a s the case i n the equatorial P a c i f i c w h e r e an increase i n D M S P p x h l ratios u n d e r i r o n - l i m i t a t i o n w a s also associated w i t h a shift f r o m d i a t o m s to cryptophytes and p r y m n e s i o p h y t e s [Riseman and DiTullio, 2 0 0 4 ] . S m a l l c e l l s s u c h as flagellates that are p r o m i n e n t D M S P - p r o d u c e r s [Keller et al.,  1989] tend to d o m i n a t e w h e n nutrient  concentrations are l o w . W h e t h e r this f l o r i s t i c shift occurs p r e c i s e l y because these c e l l s have a c o m p e t i t i v e advantage under l o w nutrient c o n d i t i o n s not o n l y because o f their greater surface area to v o l u m e ratio, but also because o f their D M S P p r o d u c t i o n c a p a b i l i t i e s , has yet to be shown.  30  DMSPp variability in relation to DMS  concentrations- T h e o r i g i n a l impetus for this w o r k was to  determine i f the u n u s u a l l y h i g h spring/summer D M S levels characteristic o f the H N L C region around Station P a p a [Kettle et al.,  1 9 9 9 ; Wong et al,  2005] w e r e associated w i t h h i g h D M S P p  concentrations. D M S levels up to 25 n M have been reported d u r i n g the spring/summer months at stations a l o n g L i n e P [Wong et al,  2 0 0 5 ] . T h e s e values are e x c e p t i o n a l f o r o p e n ocean areas  where D M S rarely exceeds 5 n M [Kettle et al,  1999], and are m o r e t y p i c a l o f p r o d u c t i v e , coastal  regions. D e s p i t e the u n u s u a l l y h i g h D M S levels at Station P a p a , the D M S P p p o o l had p r e v i o u s l y not been measured i n this r e g i o n . G i v e n that D M S P p is the u l t i m a t e source o f a l l D M S , its measurement p r o v i d e s important insight into the factors d r i v i n g h i g h D M S concentrations. U n f o r t u n a t e l y , D M S concentrations w e r e u n c h a r a c t e r i s t i c a l l y l o w i n June d u r i n g our 3year survey p e r i o d , as d e t e r m i n e d b y M I M S i n 2003 [Tortell, 2 0 0 5 ] and i n d e p e n d e n t l y b y P T G C ( C S . W o n g , u n p u b l i s h e d data). A t the oceanic stations b e y o n d P 1 2 , D M S concentrations were less than 5 n M i n a l l three years ( C S . W o n g , u n p u b l i s h e d data). T h i s trend is consistent w i t h the general d e c l i n e i n s p r i n g a n d s u m m e r D M S levels that o c c u r r e d a l o n g L i n e P f o l l o w i n g the 1998-1999 transition f r o m an E l N i n o to a L a N i n a event [Wong et al,  2 0 0 5 ] . T h e >10-fold  decrease i n D M S levels at P 2 6 between 1998 and 1999 o c c u r r e d i n c o n j u n c t i o n w i t h a dramatic drop i n the b i o m a s s o f dinoflagellates and p r y m n e s i o p h y t e s [Wong et al,  2 0 0 6 ] , and thus  p r e s u m a b l y , D M S P p . T h e Wong et al. [2005] t i m e series o n l y presents data to 2 0 0 1 ; interestingly, d u r i n g the S E R I E S i r o n f e r t i l i z a t i o n e x p e r i m e n t c o n d u c t e d near Station P a p a i n J u l y o f 2 0 0 2 , D M S l e v e l s w e r e o n c e again h i g h at >15 n M i n the c o n t r o l waters outside the ironenriched patch [Levasseur et al,  2 0 0 6 ] . Perhaps m o r e s i g n i f i c a n t l y , c o r r e s p o n d i n g D M S P p  concentrations w e r e b e t w e e n 98-130 n M and p r y m n e s i o p h y t e s w e r e v e r y abundant [Levasseur et al,  2 0 0 6 ] . A s noted b y Levasseur et al. [2006], b o t h the D M S and D M S P p  concentrations  31  measured p r i o r to S E R I E S w e r e 2-6 times h i g h e r than those m e a s u r e d at the onset o f previous i r o n f e r t i l i z a t i o n e x p e r i m e n t s i n H N L C waters o f the equatorial P a c i f i c and Southern O c e a n . M o r e o v e r , D M S and D M S P p levels i n J u l y 2 0 0 2 w e r e at least 2-3 f o l d h i g h e r than any measured at the oceanic stations d u r i n g the J u n e 2003-2005 surveys. T h u s , the h i g h D M S levels observed p r e v i o u s l y i n the N E P a c i f i c H N L C r e g i o n appear to o c c u r i n c o n j u n c t i o n w i t h h i g h D M S P p concentrations. In the absence o f s u f f i c i e n t evidence l i n k i n g D M S P p v a r i a b i l i t y to variations i n nutrient s u p p l y d u r i n g the present study, w e c o n c l u d e that changes i n D M S P p concentrations are l i k e l y due to t a x o n o m i c shifts i n the p h y t o p l a n k t o n c o m m u n i t y , and d r i v e the v a r i a b i l i t y i n D M S levels o b s e r v e d i n recent years i n this H N L C r e g i o n .  32  T a b l e 2.1: A n c i l l a r y oceanographic data f o r a l l stations surveyed d u r i n g 2 0 0 3 . M i x e d layer d e p t h ( M L D ) is d e f i n e d as a change i n sigma-t (density) o f 0.02 from that o f the surface. P r i m a r y p r o d u c t i v i t y ( P P ) and c a l c i f i c a t i o n rates ( C a l c . ) were determined at the c h l a m a x . at each station. D M S P p x h l ratios are 5 0 m depth integrated values.  STATION  MLD  ^o'"]  P  P  C a l c  -  Calc./PP  DMSPpxhl  ()  (MM)  (ug C L-' d-')  (ug C V d' )  (%)  P4  10  0.1  31.8  0.42  1.33  58.4 ±3.8  P12  25  3.3  11.6  0.29  2.48  62.9 ± 1.5  P16  40  6.1  7.3  0.01  0.16  55.4 ± 1.7  P20  40  8.0  8.3  0.35  4.25  56.8 ± 1.1  P26  35  13.9  9.3  0.57  6.15  79.2 ±2.4  A3  <10  15.4  9.5  0.26  2.69  118.2+1.6  A4  <10  9.9  7.5  0.31  4.12  81.8 ± 1.8  A6  <10  3.0  10.5  0.12.  1.16  49.5 ± 0.9  m  1  1  (umol rug  1  ni ) 2  Table 2.2:  C o e f f i c i e n t s o f determination (r ) f r o m linear regressions b e t w e e n D M S P p levels and 2  the absolute and relative c a r b o n b i o m a s s o f different p h y t o p l a n k t o n groups for a l l stations d u r i n g the 2003 survey; n = 16, * denotes statistically significant r e l a t i o n s h i p s w i t h p < 0.05, j d e n o t e s inverse relationship.  Group misc. flagellates  DMSPp vs. C L"  DMSPp vs. % C contribution  0.13  0.016  1  Prymnesiophytes  4.1xl0"  Dinoflagellates  0.33*  0.46*  Prasinophytes  0.070  0.044  Diatoms  8.0xl0"  Cryptophytes  0.12  6  3  9.3xl0"  3  0.33*t 0.048  34  Table 2.3: T h e c o n t r i b u t i o n o f D M S P p to total p h y t o p l a n k t o n c e l l carbon i n 2 0 0 3 . P h y t o p l a n k t o n b i o v o l u m e was determined from m i c r o s c o p i c c e l l counts and the appropriate geometric f o r m u l a s at two depths at each station. B i o v o l u m e was converted to c a r b o n b i o m a s s u s i n g the formulas o f Strathmann [1967] for d i a t o m s and Montagues et al. [1994] f o r flagellates.  Station  P4  P12  P16  P20  P26  A3  A4  A6  depth  chl  a  (m)  C biomass  DMSPp  (HSL" )  (nM)  1  C:chl ratio  (  us" ) 1  DMSPp:chl  % cell C as  (nmol jig" )  DMSP  1  10  0.66  65.8  60.3  99.1  90.8  5.5  20  0.90  88.5  59.7  98.5  66.4  4.0  10  0.76  21.3  63.2  27.9  63.2  7.9  30  0.96  22.6  56.8  23.6  56.8  6.2  10  0.44  2.8  26.6  6.53  60.5  55.6  40  0.62  10.8  29.2  17.5  47.5  16.3  10  0.38  9.8  22.6  26.2  60.1  13.8  50  0.43  31.2  21.2  72.1  48.9  4.1  10  0.42  12.6  37.3  30.1  88.8  17.7  50  0.58  14.9  27.0  20.5  46.5  13.6  10  0.39  8.2  59.1  21.1  152  43.1  25  0.51  11.3  56.7  22.1  111  30.2  10  0.37  7.9  37.0  21.5  101  28.1  40  0.44  5.8  12.5  13.3  28.7  12.9  10  0.59  7.1  40.2  12.0  68.0  34.0  25  0.62  6.3  33.5  10.1  54.0  32.1  Figure 2.1: M a p o f the N E Pacific showing the 5 major stations along Line P. A l s o shown are supplemental stations where measurements were made i n different years ( A 3 A 4 A 6 in 2003W 3 , W 6 , W 8 in 2005).  36  Figure 2.2: D e p t h profiles o f D M S P p m e a s u r e d at the 5 major stations a l o n g L i n e P d u r i n g 3 consecutive s p r i n g cruises. E r r o r bars i n 2 0 0 3 represent standard deviations o f d u p l i c a t e samples.  F i g u r e 2 . 3 : D e p t h profiles o f c h l o r o p h y l l a measured at the 5 m a j o r stations a l o n g L i n e P d u r i n g 3 consecutive s p r i n g cruises. N o t e the d i f f e r e n t scale for station P4.  F i g u r e 2.4: D e p t h profiles o f the D M S P p x h l ratio m e a s u r e d at the 5 major stations a l o n g L i n e P d u r i n g 3 consecutive s p r i n g cruises. N o t e that the ratios were g e n e r a l l y highest i n 2 0 0 3 .  Figure 2.5: Contour plots illustrating spatial and interannual variability i n springtime D M S P p and chlorophyll levels along Line P.  Figure 2.6:  R e l a t i o n s h i p b e t w e e n c h l o r o p h y l l and D M S P p concentrations a l o n g L i n e P for the 3  year p o o l e d dataset (r  2  = 0.20, n = 94, /?<0.0001). L i n e a r r e g r e s s i o n s h o w n is w i t h the t w o  outliers i n brackets e x c l u d e d (r  2  = 0.45, n = 92,/?<0.0001).  41  100 80 -) "35  O  60 H  "TO  o o  40 20 0 P4  P12 P16 P20 P26 A3  A4  A6  "o5  O  ro o  60  P20 P26 Station  Diatoms f V \ ^ l Dinoflagellates mm Prymnesiophytes i i Misc. flagellates P W J Prasinophytes i i i i i n i Cryptophytes I444444I Microcystis sp. V7777Z\  Figure 2.7: T h e relative abundance (percent o f total cells) o f the different p h y t o p l a n k t o n groups enumerated at (a) 10 m d e p t h a n d (b) the c h l o r o p h y l l m a x . at a l l stations surveyed i n 2 0 0 3 . See  T a b l e 2.3 f o r depths o f c h l o r o p h y l l m a x .  42  c o ro O ro o K<4—  o  P4 . P12 P16 P20 P26 A3  A4 A6  P4  A4 A6  P12 P16 P20 P26 A3 Station  Diatoms CV\X] Dinoflagellates Rasaaa Prymnesiophytes ' ' Misc. flagellates i w v i Prasinophytes rrrrmi Cryptophytes 11111 [ i Microcystis sp.  F i g u r e 2.8: The contribution o f the different phytoplankton groups to total carbon biomass at (a) 10 m depth and (b) the chlorophyll max. at all stations surveyed i n 2003. See Table 2.3 for depths o f chlorophyll max.  43  140  •  - •  120  100  T  o  2003 2004 2005  A3 •  H A4  E  "o  80  E  ±  P12  P4  Z: o  60 -t.  c/3  40  cL Q_  •  P4  •  P12  o  o  •  P20  P26  P26  P20  P12  •  P20  P16 • W6  A6  •  i  20  P26  •  W3  W8  P16  P4  —i—  6  8  10  12  —i—  14  16  MLD N 0 3 " (uM)  Figure 2.9:  5 0 m depth integrated D M S P p x h l ratios p l o t t e d against the m i x e d layer nitrate  concentration. In 2 0 0 3 , stations A 3 , A 4 and A 6 are i n c l u d e d ; i n 2 0 0 5 , stations W 3 , W 6 and W 8 are i n c l u d e d (see F i g . 2.1 f o r locations). N o t e that i n 2 0 0 3 , elevated D M S P p x h l ratios were f o u n d i n h i g h nitrate waters, but not i n subsequent years.  44  2.5 References: A x a n a m i , K., S. W a t a n a b e , S. T s u n o g a i , M . H a y a s h i , K . F u r u y a , and T . 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High-Resolution Measurements of DMS, C 0 and 0 / A r in Productive Coastal Waters around Vancouver Island, Canada 2  2  2  3.1 Introduction Rising concern about global climate change, driven mainly by high anthropogenic CO2 emissions [Denman et al, 1996], has led to increased efforts to quantify the ocean's role as a source or sink of climatologically-active gases. These include greenhouse gases such as CO2 that lead to a rise in global temperatures, as well as gases such as dimethylsulfide (DMS), which can potentially cool Earth's climate through the formation of cloud condensation nuclei that promote cloud formation and scatter incoming radiation [Charlson et al, 1987]. Decades of oceanographic gas surveys have culminated in the synthesis of thousands of measurements of /?C0 and D M S into monthly global climatologies for both gases [Takahashi etal, 2002 and 2  Kettle et al, 1999, respectively]. Although the number of measurements continues to steadily increase, data from continental shelf waters remain sparse. As a result, these regions suffer from low spatial and temporal resolution and are thus poorly represented in global gas climatologies [Takahashi et al, 2002; Kettle et al, 1999]. This is a significant limitation since coastal regions, despite their small areal extent, play a disproportionately large role in air-sea gas exchange due to their high productivity and dynamic physics. Coastal waters are particularly large sources of trace biogenic gases such as D M S . Neglecting to include these areas in global climatologies may thus impart significant errors on D M S flux estimates. Moreover, because D M S has a short atmospheric lifetime [Chin and Jacob, 1996], it is especially important to identify its local sources, particularly in coastal areas where biogenic sulfur sources affect the relative importance of anthropogenic ones [Jones et al, 2001].  A version of this chapter has been submitted for publication. Nemcek, N . , D. Ianson, and P.D. Tortell. A high-resolution survey of D M S , C 0 , and 0 / A r distributions in productive coastal waters. Global Biogeochem. Cycles 2  2  2  49  C o m p o u n d i n g the p r o b l e m o f l o w measurement r e s o l u t i o n is o u r p o o r understanding o f the u n d e r l y i n g processes d r i v i n g the observed gas d i s t r i b u t i o n s . F o r C O 2 ,  the relative importance  o f the s o l u b i l i t y p u m p versus the b i o l o g i c a l p u m p i n oceanic c a r b o n c y c l i n g needs to be evaluated [Volk andHoffert, 1985]. A l t h o u g h b i o l o g i c a l processes s u c h as photosynthesis, respiration and c a l c i f i c a t i o n are b e l i e v e d to d o m i n a t e the seasonal v a r i a t i o n s i n pCOi over most o f the surface ocean [Takahashi et al, 2 0 0 2 ] , at h i g h latitudes temperature changes l i k e l y p l a y a larger role i n d r i v i n g air-sea C O 2 f l u x e s [Murata and Takizawa, 2 0 0 3 ] . I n o u r study r e g i o n , strong b i o l o g i c a l C O 2 d r a w d o w n occurs i n s u m m e r w h i l e out-gassing dominates i n winter, yet net annual C O 2 f l u x e s are s t i l l p o o r l y constrained [Ianson and Alien, 2 0 0 2 ] . T h e D M S c y c l e is m o r e c o m p l e x than that o f C 0  2  and w e are s t i l l far f r o m a mechanistic  understanding o f the factors d r i v i n g D M S p r o d u c t i o n [see Simo, 2 0 0 4 , and references therein]. A l t h o u g h it is clear that D M S originates f r o m the algal metabolite d i m e t h y l s u l f o n i o p r o p i o n a t e ( D M S P ) , there are species-specific differences i n D M S P p r o d u c t i o n [Keller et al, 1989], w h i c h i n turn are affected b y the nutrient status o f the cells [Sunda et al, 2 0 0 2 ; Bucciarelli and Sunda, 2 0 0 3 ] . M o r e o v e r , the process b y w h i c h D M S P is released f r o m cells and converted to v o l a t i l e D M S i n v o l v e s the entire p l a n k t o n c o m m u n i t y ( f r o m viruses to grazers) [Simo, 2 0 0 1 ] , w h i c h i t s e l f is p r o f o u n d l y affected b y the p h y s i c o c h e m i c a l e n v i r o n m e n t [Simo and Pedros-Alio, 1999]. A s a result, attempts to relate D M S levels to total c h l o r o p h y l l , p h y t o p l a n k t o n species c o m p o s i t i o n , or e v e n D M S P concentrations have p r o v e n l a r g e l y u n s u c c e s s f u l [Leek et al, 1 9 9 0 ; Kettle et al, 1999]. R e c e n t efforts have focussed o n f i n d i n g e m p i r i c a l relationships between D M S levels and c o m b i n a t i o n s o f relevant b i o l o g i c a l , p h y s i c a l , and c h e m i c a l variables that w o u l d obviate the need f o r a f u l l m e c h a n i s t i c understanding o f the processes i n v o l v e d . T h i s approach has led to the c o n s t r u c t i o n o f a l g o r i t h m s a i m e d at p r e d i c t i n g D M S concentrations f r o m remotely  50  sensed data s u c h as ocean c o l o u r , and w e l l - c o n s t r a i n e d , or e a s i l y m e a s u r a b l e e n v i r o n m e n t a l parameters [i.e. Anderson et al,  2 0 0 1 ; Simo andDachs, 2 0 0 2 ; Belviso et al., 2 0 0 4 a ] . T h e  a l g o r i t h m o f Simo and Dachs [2002] (hereafter referred to as S&D2002) is p a r t i c u l a r l y appealing as it has s h o w n success at s i m u l a t i n g oceanic D M S based s o l e l y o n the ratio between S e a W i F S surface c h l o r o p h y l l and a c l i m a t o l o g i c a l m i x e d layer depth. T h e r e are currently no c o m p a r a b l e algorithms for use i n coastal waters as most are either based o n open-ocean c l i m a t o l o g i e s [Anderson et al., 2 0 0 1 ; Simo and Dachs, 2 0 0 2 ] , and/or h a v e d i f f i c u l t y s i m u l a t i n g D M S levels i n diatom-dominated coastal waters [Belviso et al,  2004a].  U l t i m a t e l y , m o r e i n f o r m a t i o n is needed o n the spatial and seasonal distributions o f oceanic gases i n c o n j u n c t i o n w i t h process studies a i m e d at e l u c i d a t i n g the factors d r i v i n g the observed d i s t r i b u t i o n s . T h i s g o a l has been h a m p e r e d b y l i m i t a t i o n s o f current analytical methods w h i c h often l a c k the s a m p l i n g r e s o l u t i o n to capture fine-scale v a r i a b i l i t y and measure i n d i v i d u a l gases i n i s o l a t i o n . I n s u f f i c i e n t s a m p l i n g r e s o l u t i o n is p a r t i c u l a r l y p r o b l e m a t i c f o r D M S w h i c h is at best m e a s u r e d a f e w t i m e s an h o u r w i t h purge-and-trap gas c h r o m a t o g r a p h y . A s e m p h a s i z e d b y Simo and Dachs [ 2 0 0 2 ] , " i t is not e n o u g h to q u a n t i f y the a n n u a l D M S e m i s s i o n f l u x f r o m the g l o b a l o c e a n ; rather it is necessary to resolve its spatial and t e m p o r a l v a r i a b i l i t y . . . w h i c h exceeds our c a p a b i l i t y f o r s a m p l i n g and m e a s u r i n g i n the f i e l d . " R e c e n t w o r k has demonstrated that m e m b r a n e inlet mass spectrometry ( M I M S ) p r o v i d e s the c a p a b i l i t y to resolve spatial v a r i a b i l i t y i n D M S and other gases [Tortell, 2 0 0 5 a , 2 0 0 5 b ] . T h i s m e t h o d is i d e a l l y suited f o r u n d e r w a y surveys since a suite o f b o t h m a j o r and trace gases can be measured s i m u l t a n e o u s l y at a rate o f t w i c e per m i n u t e , p r o v i d i n g a dramatic increase i n spatial resolution over current m e t h o d s , p a r t i c u l a r l y for D M S . H e r e i n , w e present the first dedicated a p p l i c a t i o n o f this m e t h o d to d y n a m i c , p r o d u c t i v e coastal waters o f f B r i t i s h C o l u m b i a , C a n a d a .  51  T h e emphasis o f the present study w a s to e x a m i n e the c o v a r i a n c e o f the gas and hydrographic data to relate o b s e r v e d D M S l e v e l s to other e n v i r o n m e n t a l v a r i a b l e s a n d to test the a p p l i c a b i l i t y o f the p r e d i c t i v e S&D2002  D M S a l g o r i t h m for use i n coastal regions. O u r f i n d i n g s suggest that  D M S concentrations i n h i g h l y p r o d u c t i v e areas c a n b e related to the C H L / M L D ratio suggested b y these authors, a l t h o u g h b y a different s c a l i n g factor. O u r results also e m p h a s i z e the u t i l i t y o f multi-gas measurements and the need t o s a m p l e gases at appropriate spatial scales, w h i c h are especially short i n coastal waters.  3.2 Materials and Methods Study Area and Sampling- D i s s o l v e d gas, temperature, s a l i n i t y a n d c h l o r o p h y l l a measurements were made u n d e r w a y a l o n g several transects o f f the west coast o f B r i t i s h C o l u m b i a , C a n a d a between A u g . 11-19, 2 0 0 4 o n b o a r d the CCGSJohn P. Tully (see F i g . 3.1 for transect locations). T h e cruise track c r o s s e d m a n y d y n a m i c oceanographic features i n c l u d i n g nearshore straits i n f l u e n c e d b y s t r o n g t i d a l m i x i n g (e.g. T l a , T 8 , F i g . 3.1), as w e l l as o p e n s h e l f areas that are sites o f u p w e l l i n g and the f o r m a t i o n o f b o t h c y c l o n i c and a n t i c y c l o n i c eddies. T h e northern s h e l f transects ( T l b - 5 , F i g . 3.1) w e r e m a d e i n Q u e e n C h a r l o t t e ( Q C ) S o u n d , a n area that serves as the source r e g i o n for a n t i c y c l o n i c ( d o w n w e l l i n g ) eddies that carry coastal waters r i c h i n i r o n offshore to the G u l f o f A l a s k a [Johnson et al., 2 0 0 5 ] . D u r i n g s u m m e r , t h i s r e g i o n is thought to experience r e l a x a t i o n from strong w i n t e r d o w n w e l l i n g , w i t h v e r y little actual u p w e l l i n g . Transect 5 crosses from this r e l a x a t i o n r e g i o n into an area w h e r e s p o r a d i c s u m m e r u p w e l l i n g i s expected o f f V a n c o u v e r Island. N e a r the southern end o f V a n c o u v e r Island, transects 6-7 enter the J u a n de F u c a eddy, a persistent, l o c a l i z e d s u m m e r t i m e feature. T h i s c y c l o n i c ( u p w e l l i n g ) eddy supports a h i g h l y p r o d u c t i v e , diatom-dominated c o m m u n i t y [Marchetti et al, 2 0 0 4 ] .  52  Gas Measurements- M e m b r a n e inlet mass spectrometry ( M I M S ) w a s u s e d to measure d i s s o l v e d gases u n d e r w a y ( D M S , O2, A r , and CO2)  as recently d e s c r i b e d i n d e t a i l i n Tortell [2005a].  B r i e f l y , seawater f r o m the s h i p ' s u n d e r w a y intake system (4.5 m depth) w a s p u m p e d through p o l y p r o p y l e n e t u b i n g into a s a m p l i n g cuvette connected to the mass spectrometer. F l o w rates through the cuvette w e r e c o n t r o l l e d w i t h a gear p u m p and w e r e kept constant at - 2 0 0 m l m i n " . 1  A gas permeable d i m e t h y l s i l i c o n e m e m b r a n e i n s i d e the cuvette acted as the interface between the water s a m p l e and the v a c u u m o f the mass spectrometer. A f t e r d i f f u s i o n through the m e m b r a n e , gases w e r e m e a s u r e d i n the v a c u u m c h a m b e r b y s i n g l e i o n m o n i t o r i n g ( S I M ) o f the signal intensities at the relevant m/z (mass to charge) ratios (32, 4 0 , 4 4 a n d 62 for O2, A r ,  CO2,  and D M S , respectively). W e use a H i d e n A n a l y t i c a l H A L 2 0 t r i p l e filter q u a d r u p o l e mass spectrometer w i t h an electron i m p a c t i o n source set at a 5 0 0 p A e m i s s i o n current. A l l gases w e r e measured at a f r e q u e n c y o f t w i c e per m i n u t e , or a p p r o x i m a t e l y e v e r y 160 m at the t y p i c a l vessel c r u i s i n g speed o f - 1 0 k n o t s . T h e D M S s i g n a l output f r o m the M I M S w a s calibrated u s i n g standards prepared b y h y d r o l y s i s o f sterile D M S P stock solutions ( R e s e a r c h P l u s Inc.) i n 1 N N a O H . A l i q u o t s o f the l i q u i d D M S standard w e r e added to 500 m l v o l u m e s o f D M S - f r e e seawater ( > 1 0 0 0 m depth) and w e r e a l w a y s d i l u t e d 1 0 - 1 0 - f o l d to keep the p H o f the f i n a l D M S standard constant, and prevent 5  6  m a t r i x effects o n the m e m b r a n e . Standards w e r e a n a l y z e d o n the M I M S b y r e c i r c u l a t i n g the l i q u i d through the s a m p l i n g cuvette u s i n g a gear p u m p connected to a m a n u a l s a m p l i n g valve. T h e detection l i m i t based o n a 3:1 signal-to-noise ratio o f the b l a n k s w a s 1 n M . O n the last day o f s a m p l i n g w e e x p e r i e n c e d a p o w e r failure w h i c h caused the instrument to shut d o w n . T h i s resulted i n a m a l f u n c t i o n i n the secondary electron m u l t i p l i e r ( S E M ) , the detector used to measure D M S . T h u s , D M S data w e r e not available a l o n g T 8 ( F i g . 3.2f).  53  G a s standards w e r e not available for C O 2 , O2 or A r . H o w e v e r , independent measurements o f pC0  2  obtained f r o m an u n d e r w a y E R . p C 0 e q u i l i b r a t o r ( L I - C O R LI-6262) 2  were used to calibrate the M E V I S C O 2 signal. E a c h day o f s a m p l i n g w a s calibrated separately to compensate for shifts i n the m/z 4 4 baseline o f the mass spectrometer. C o e f f i c i e n t s o f determination f o r the pC0  c a l i b r a t i o n were as f o l l o w s : r = 0.98 f o r T I , r = 0.60 for T 2 , T 3 , 2  2  and T 4 , r = 0.87 f o r T 5 , r = 0.96 f o r T 6 , and r == 0.93 f o r T 7 and T 8 . T h e p o o r correlation 2  2  between the M E V I S C O 2 s i g n a l and the equilibrator data for T2-4 is expected because the range ofpC0  2  values encountered a l o n g these transects (304-335 p p m ) w a s s m a l l , s u c h that any noise  i n the M E V I S C O 2 s i g n a l and any offsets i n the time stamps o f the t w o instruments were a m p l i f i e d . H o w e v e r , despite the p o o r c a l i b r a t i o n curve f o r these transects, the average absolute difference between c o r r e s p o n d i n g measurements f r o m the t w o instruments w a s ~5 p p m (data not shown). T h e o x y g e n a n d argon data w e r e not calibrated; h o w e v e r the O2 s i g n a l w a s n o r m a l i z e d to A r to y i e l d a b i o l o g i c a l l y relevant parameter representing the balance between photosynthesis and respiration. T h i s is p o s s i b l e because o x y g e n and argon have s i m i l a r s o l u b i l i t i e s i n seawater and argon concentrations are unaffected b y b i o l o g i c a l processes [Craig and Hayward, 1987]. T h u s , the ( V A r ratio i s a strict measure o f b i o l o g i c a l o x y g e n w i t h p h y s i c a l processes that affect gas concentrations, s u c h as b u b b l e injections and temperature and s a l i n i t y changes, r e m o v e d .  DMSP measurements- P e r i o d i c a l l y d u r i n g the u n d e r w a y gas analysis, discrete samples were collected for particulate D M S P ( D M S P p ) and d i s s o l v e d D M S P ( D M S P d ) analysis f r o m the u n d e r w a y intake line. T h e s e consisted o f 2 5 0 m l aliquots o f seawater w h i c h w e r e i m m e d i a t e l y gravity filtered onto 47 m m G F / F filters to separate the particulate a n d d i s s o l v e d D M S P fractions. T h e filters c o n t a i n i n g D M S P p were p l a c e d i n 5 m l c r y o v i a l s to w h i c h 3 m l o f m e t h a n o l  54  were added. These s a m p l e s w e r e stored at - 2 0 ° C u n t i l a n a l y z e d b y M I M S i n the laboratory several m o n t h s later (see b e l o w ) . D M S P stored i n m e t h a n o l is k n o w n to be stable for extended periods o f t i m e (J. D a c e y , pers. c o m m . ) . T h e filtrates c o n t a i n i n g D M S P d w e r e transferred to 2 5 0 m l ground-glass stoppered bottles, a c i d i f i e d w i t h 5 0 0 p i o f 1 2 N H C 1 to prevent m i c r o b i a l degradation o f D M S P d , and b u b b l e d w i t h a i r f o r 3 0 m i n u t e s to r e m o v e D M S . A 5 m l a l i q u o t o f 1 0 N N a O H w a s then added to the samples to h y d r o l y z e the D M S P to D M S . T h e bottles w e r e stoppered and left to react overnight under m i n i m a l headspace. P r i o r to analysis o n the f o l l o w i n g day, 3.6 m l o f 1 2 N H C 1 were added to the samples to l o w e r the p H to 9.5, the tolerable range f o r the s a m p l i n g m e m b r a n e . S a m p l e s w e r e then p u m p e d into the cuvette v i a the m a n u a l s a m p l i n g v a l v e and a n a l y z e d o n the M I M S b y s i n g l e i o n m o n i t o r i n g o f the m/z 62 s i g n a l w i t h d w e l l and settle times o f 3 0 0 m s . C a l i b r a t i o n standards w e r e m a d e b y a d d i n g aliquots o f sterile D M S P to 2 5 0 m l v o l u m e s o f D M S / D M S P - f r e e deep seawater and w e r e treated the same as the samples. In the laboratory, D M S P p samples w e r e a n a l y z e d o n the M I M S u s i n g a m e m b r a n e inlet probe s p e c i f i c a l l y d e s i g n e d for s m a l l v o l u m e , discrete samples. T h i s p r o b e consists o f a 1/16" stainless steel c a p i l l a r y w i t h s m a l l holes at one end fitted w i t h a 0 . 0 0 5 " t h i c k d i m e t h y l s i l i c o n e sleeve. P r i o r to a n a l y s i s , 2 m l aliquots o f the D M S P extract i n m e t h a n o l w e r e p l a c e d into 1 4 m l serum v i a l s to w h i c h 12 m l o f 1 N N a O H w a s added, c o m p l e t e l y f i l l i n g the v i a l s . T h e v i a l s were i m m e d i a t e l y sealed w i t h gas-tight a l u m i n u m c r i m p seals w i t h T e f l o n - f a c e d b u t y l rubber liners and left to react o v e r n i g h t to ensure c o m p l e t e c o n v e r s i o n o f D M S P to D M S . D M S P concentrations w e r e m e a s u r e d as D M S i n the l i q u i d phase b y M I M S b y i ns erti ng the m e m b r a n e inlet probe d i r e c t l y into the v i a l s . Standards were prepared b y a d d i n g aliquots o f sterile D M S P to 2 m l o f m e t h a n o l and h y d r o l y z i n g overnight w i t h 12 m l o f 1 N N a O H as above.  55  W e are aware that o u r D M S P data m a y be subject to recently i d e n t i f i e d filtration artefacts [Kiene and Slezak, 2 0 0 6 ] . A l t h o u g h w e used gravity f i l t r a t i o n , o u r d i s s o l v e d D M S P numbers m a y be overestimated and o u r particulate D M S P underestimated as a result o f the relatively large v o l u m e s o f seawater f i l t e r e d [Kiene and Slezak, 2 0 0 6 ] . H o w e v e r , since the t w o fractions came f r o m the same w a t e r s a m p l e , total D M S P concentrations ( D M S P d + D M S P p ) s h o u l d be accurate. F o r the p u rp o s e o f this study w e are m o r e interested i n the c o v a r i a n c e o f the D M S P and D M S data, rather than absolute D M S P v a l u e s , and w e use D M S P as an a n c i l l a r y parameter to investigate differences i n D M S levels betw een sites. T h e m a g n i t u d e o f the f i l t r a t i o n artefacts is l i k e l y dependent o n the health and 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 c o m m u n i t i e s , and m a y be smaller i n coastal u p w e l l i n g systems than i n s o m e o f the e n v i r o n m e n t s studied b y Kiene and Slezak [2006].  Hydrographic Measurements- Surface temperature, s a l i n i t y and c h l o r o p h y l l a fluorescence data were c o l l e c t e d i n c o n j u n c t i o n w i t h the d i s s o l v e d gas measurements u s i n g a S e a B i r d S B E - 2 5 C T D c o n t i n u o u s l y s a m p l i n g from the same seawater intake system as the M I M S . D u r i n g parts o f transects 1 and 3, the S B E - 2 5 w a s not l o g g i n g data due to loss o f battery p o w e r , thus gaps exist i n the s a l i n i t y and c h l o r o p h y l l data a l o n g these transects ( F i g . 3.2b-c). F o r t u n a t e l y , the temperature data w e r e a v a i l a b l e f o r these areas from the u n d e r w a y pCOi equilibrator. T h e fluorescence s i g n a l w a s calibrated w i t h discrete c h l o r o p h y l l a measurements {r = 2  0.97, n = 18)  determined f l u o r o m e t r i c a l l y f o l l o w i n g filtration o f seawater onto G F / F filters and extraction i n 9 0 % acetone f o r 2 4 hours [Parsons et al.,  1984]. A t a n u m b e r o f l o c a t i o n s a l o n g the cruise track,  C T D p r o f i l e s w e r e also o b t a i n e d u s i n g a S e a B i r d S B E 9 1 1 + C T D attached to a rosette sampler.  56  Data Binning Procedures- In order to test the S&D2002 a l g o r i t h m , w e b i n n e d a l l the u n d e r w a y D M S and c h l o r o p h y l l a data onto A x A degree surface grids a n d c a l c u l a t e d an average value for l  l  each parameter f o r each square. F o l l o w i n g the b i n n i n g procedure, the data w e r e reduced - 1 0 0 f o l d to 2 9 i n d i v i d u a l squares each c o n s i s t i n g o f a single C H L and D M S v a l u e . A c o r r e s p o n d i n g m i x e d layer depth ( M L D ) w a s assigned to each data pair. In m o s t cases, M L D s w e r e determined f r o m C T D density p r o f i l e s and w e r e d e f i n e d as a 0.125 cr change f r o m the surface value i n t  accordance w i t h the c r i t e r i a o f the a l g o r i t h m . In Q C S o u n d ( T l b - 5 ; F i g . 3.1) w h e r e w e had g o o d C T D coverage, linear i n t e r p o l a t i o n w a s used to assign an M L D v a l u e to each g r i d based o n n e i g h b o u r i n g C T D p r o f i l e s . D u e to the n a r r o w range o f M L D s i n this r e g i o n (6-12 m ) , any error introduced b y this i n t e r p o l a t i o n w a s s m a l l . C T D data w e r e not a v a i l a b l e d u r i n g our cruise i n the v i c i n i t y o f T l a i n Q C Strait, or for T 6 and T 7 o f f B a r k l e y S o u n d . S t r o n g t i d a l f o r c i n g and turbulent f l o w s , rather than w i n d or b u o y a n c y f l u x e s c o n t r o l M L D s at the head o f Q C Strait ( T l a ) , s u c h that the upper m i x e d layer is consistently deep (40-80 m , P. C u m m i n s , pers. c o m m . ) . W e used h o r i z o n t a l gradients i n surface temperature and pCC>2 to i d e n t i f y these t i d a l l y m i x e d r e g i o n s , a s s u m i n g that h i g h e r p C C h and c o l d e r temperatures w e r e i n d i c a t i v e o f deeper m i x i n g . T h i s estimate y i e l d e d a h o r i z o n t a l gradient i n M L D s d e c r e a s i n g f r o m 8 0 m at the head o f Q C Strait to 5 0 m t o w a r d s the m o u t h . F o r T 6 and T 7 o f f B a r k l e y S o u n d , M L D s w e r e estimated based o n 26 C T D p r o f i l e s taken i n the same area three w e e k s later ( D . M a c k a s , u n p u b l i s h e d data). M i x e d layer depths d u r i n g the latter survey ranged f r o m 8-24 m , and f e l l w i t h i n the 9 5 % C.I. o f average s u m m e r t i m e M L D s determined f r o m several decades w o r t h o f C T D data c o l l e c t e d i n that area [Thomson and Fine, 2 0 0 3 ] . G i v e n that w i n d s w e r e w e a k to moderate p r i o r to, and f o l l o w i n g our o c c u p a t i o n , M L D s estimated for T 6 and T 7 f r o m the latter s u r v e y are l i k e l y accurate. O n c e a m i x e d layer depth w a s assigned to  57  each Vi x Vi degree square, w e c a l c u l a t e d the expected D M S c o n c e n t r a t i o n u s i n g the C H L / M L D ratio and the appropriate equation from S&D2002 (see results).  Spatial and Statistical Analysis- W e e x a m i n e d the spatial v a r i a b i l i t y o f the gas and h y d r o g r a p h i c data u s i n g t w o different statistical approaches. L a g g e d a u t o c o r r e l a t i o n f u n c t i o n s w e r e c o m p u t e d to estimate d e c o r r e l a t i o n l e n g t h scales ( D L S ) for the v a r i o u s parameters as described b y Murphy et al. [2001]. T h e s e f u n c t i o n s c o m p u t e the c o r r e l a t i o n o f p o i n t measurements at steadily increasing s a m p l i n g intervals, or lags. A s the s a m p l i n g i n t e r v a l increases, the p r o b a b i l i t y that t w o points separated b y that i n t e r v a l are related approaches zero. T h e D L S thus gives an i n d i c a t i o n o f the spatial length scale at w h i c h measurements b e c o m e independent o f each other. T o estimate the errors that c o u l d result f r o m u n d e r s a m p l i n g h i g h l y v a r i a b l e surface data, w e calculated interpolation errors f o l l o w i n g Hales and Takahashi [2004]. E s s e n t i a l l y , this approach calculates the average error o b t a i n e d b y r e s a m p l i n g h i g h r e s o l u t i o n data sets w i t h i n c r e a s i n g l y l o w e r resolution. P r i n c i p l e c o m p o n e n t s analysis ( P C A ) w a s also used to e x a m i n e the covariance o f the gas distributions and the h y d r o g r a p h i c measurements [Shaw, 2 0 0 3 ] .  3.3 Results Surface Gas and Hydrographic  Distributions-  T h e surface d i s t r i b u t i o n s o f temperature, salinity  and c h l o r o p h y l l a are s h o w n alongside pC0 , 0 /Ar and D M S data from the M I M S i n Figures 2  2  3.2a-f, respectively. A l l parameters e x h i b i t e d large ranges h i g h l i g h t i n g the d y n a m i c nature o f the study r e g i o n . S u r f a c e temperatures ranged f r o m 10.0-18.6 ° C (avg. 16.2 ° C ; F i g . 3.2a), salinity ranged from 24.2- 32.3 p s u (avg. 3 1 . 4 ; F i g . 3.2b), w h i l e c h l o r o p h y l l a concentrations ranged f r o m <0.1-33.2 p g L" (avg. 2.6 p g L" ; F i g . 3.2c). 1  1  58  G a s d i s t r i b u t i o n s o v e r the study r e g i o n w e r e also h i g h l y v a r i a b l e . T h e partial pressure o f CO2 d u r i n g this survey ranged f r o m undersaturated values as l o w as 201 p p m to h i g h l y supersaturated at 7 4 7 p p m w i t h an average o f 362 p p m . T h e surface m a p s s h o w spatial associations b e t w e e n the d i s t r i b u t i o n o f pCOi and that o f several p h y s i c a l and b i o l o g i c a l variables ( F i g . 3.2a-d). A l t h o u g h C O 2 concentrations w e r e b e l o w atmospheric e q u i l i b r i u m values over most o f the study r e g i o n , they w e r e strongly undersaturated i n areas that had h i g h c h l o r o p h y l l concentrations, i n d i c a t i v e o f h i g h p h y t o p l a n k t o n b i o m a s s , and p r e s u m a b l y a strong b i o l o g i c a l CO2 s i n k ( F i g . 3.2c-d). These lowpCOi areas are evident i n F i g . 3.2d a l o n g T l a - b i n Q C Strait, and a l o n g T 6 and T 7 i n w h a t is l i k e l y the J d F eddy. In contrast, regions o f h i g h / ? C 0  2  o c c u r r e d i n J d F Strait (T8) and at the head o f Q C Strait ( T l a ) , areas i n f l u e n c e d b y strong tidal m i x i n g w h i c h b r i n g s deep waters e n r i c h e d i n respired C O 2 to the surface. A l o n g T l a i n Q C Strait w e observed a dramatic t r a n s i t i o n f r o m supersaturated to undersaturated c o n d i t i o n s characterized b y an almost 5 0 0 p p m drop  inpCOi levels (747 p p m to 255 p p m ) i n the span o f 26 minutes or a  distance o f 8 k m ( F i g . 3.2d). S m a l l e r regions o f C O 2 supersaturation also o c c u r r e d to a lesser extent w h e n the cruise track crossed the s h e l f break i n areas o f l o c a l i z e d u p w e l l i n g (e.g. T 5 , T 7 ; F i g . 3.2d). T h e p h y s i c a l l y i n d u c e d changes  inpCOi are corroborated b y the c o r r e s p o n d i n g l o w  temperatures i n these regions w h i c h reflect deep waters m i x i n g to the surface ( F i g . 3.2a). T h e average v a l u e o f p C C » 2 (362 p p m ) indicates that the survey area o n the w h o l e w a s near, or s l i g h t l y b e l o w e q u i l i b r i u m w i t h respect to the atmosphere, but w i t h l o c a l areas o f large disequilibria. C V A r ratios expressed as a ratio o f the m/z 32/40 i o n currents f r o m the M I M S ranged f r o m 7.9-23.5 w i t h an average o f 15.1 ( F i g . 3.2e). These data are u n c a l i b r a t e d and thus strictly qualitative; h o w e v e r , they do p r o v i d e an i n d i c a t i o n o f the degree o f b i o l o g i c a l o x y g e n saturation  59  [Craig and Hayward, 1987]. P r e v i o u s laboratory and f i e l d studies [i.e. Tortell, 2 0 0 5 b ] u s i n g airequilibrated water s a m p l e s h a v e s h o w n that C V A r ratios o f 12-15 represent an o x y g e n saturation o f 1 0 0 % o v e r a large range o f temperature and s a l i n i t y c o n d i t i o n s . T h e range i n the measured saturation ratio represents changes i n i o n source p e r f o r m a n c e o v e r m u l t i p l e cruises and v a r y i n g operating c o n d i t i o n s , as o p p o s e d to temperature and s a l i n i t y effects o n the ratio per se. T h u s , although w e cannot p i n p o i n t an exact value for 100 % O2 saturation, C V A r values greater than 15 characterize supersaturated waters, w h i l e values less than 12 represent undersaturation. H i g h C V A r ratios i n o u r s u r v e y r e g i o n (>16) c p i n c i d e d w i t h areas o f elevated c h l o r o p h y l l concentrations and l o w pC0  2  i n d i c a t i n g apparent b i o l o g i c a l l y i n d u c e d o x y g e n supersaturation  ( T I , T 5 , T 7 ; F i g . 3.2c-e). A large area o f o x y g e n undersaturation o c c u r r e d i n J d F Strait (T8) concurrent w i t h the elevated pC0  2  levels o f this t i d a l l y m i x e d z o n e ( F i g . 3.2d-e).  D i m e t h y l s u l f i d e concentrations ranged from undetectable (<1 n M ) to 28.7 n M w i t h an average o f 5.8 n M ( F i g . 3.2f). A r e a s o f h i g h D M S levels w e r e c o n f i n e d to QC S o u n d and corresponded to r e g i o n s o f moderate c h l o r o p h y l l a levels ( F i g s . 3.2c, 3.2f). In contrast to the other gases, D M S concentrations c o u l d not be related i n a general sense to the p h y s i c a l or b i o l o g i c a l e n v i r o n m e n t . C o n c e n t r a t i o n s o f this gas w e r e l o w i n r e g i o n s o f b o t h very h i g h and l o w c h l o r o p h y l l b i o m a s s and i n b o t h w a r m and c o l d waters. D i s s o l v e d , particulate and total D M S P levels were unrelated to the o b s e r v e d D M S concentrations w i t h c o e f f i c i e n t s o f determination (r ) 2  for all parameters <0.02 (data not s h o w n ) . D M S P p concentrations ranged f r o m 8.7-167 n M (mean 45 n M ) w i t h the highest levels o c c u r r i n g i n areas o f h i g h b i o m a s s o f f B a r k l e y S o u n d (T7) and i n QC Strait ( T l a ) , w h e r e D M S levels w e r e r e l a t i v e l y l o w (<10 n M ; F i g . 3.2f). In contrast, the r e g i o n o f h i g h D M S i n QC S o u n d c o i n c i d e d w i t h r e l a t i v e l y l o w D M S P p concentrations (30-  60  40 n M ) . D i s s o l v e d D M S P concentrations ranged from 5.4-135 n M (mean 27 n M ) w i t h most values f a l l i n g t o w a r d the l o w e r e n d o f the spectrum. A l t h o u g h the m a p s o f near-surface gas distributions s h o w n i n F i g . 3.2 p r o v i d e a general o v e r v i e w o f the b i o l o g i c a l a n d h y d r o g r a p h i c properties o f the s a m p l i n g area, they obscure m u c h o f the fine-scale structure revealed b y the h i g h r e s o l u t i o n M I M S data; F i g u r e s 3.3 and 3.4 s h o w expanded v i e w s o f t w o transects o f f the west coast o f V a n c o u v e r Island ( T 5 and T 7 ) that h i g h l i g h t the c o v a r i a n c e o f the gas and hydrographic data i n greater detail. F i g u r e 3.3 shows an 8 hour transect ( T 5 ) w h e r e the highest D M S concentrations w e r e encountered. Interesting p h y s i c a l features are apparent i n this high-resolution v i e w , i n c l u d i n g a r e g i o n that appears to be i n f l u e n c e d b y l o c a l i z e d u p w e l l i n g j u s t south o f 50.8 ° N . T h i s area is illustrated b y a large and sudden drop i n temperature, a slight increase i n salinity, a dramatic s p i k e i n pCOi and an associated decrease i n the 02/Ar ratio. A n a n o m a l o u s l y h i g h l e v e l o f D M S P d (135 n M ) w a s also associated w i t h this strong temperature front ( F i g 3.3a, 3.3c). M o v i n g n o r t h w a r d from this u p w e l l i n g z o n e , b o t h D M S concentrations and c h l o r o p h y l l levels i n c r e a s e d (in c o n j u n c t i o n w i t h increasing temperatures) u n t i l the t w o variables became u n c o u p l e d at 51.0 ° N ( F i g . 3.3a). F r o m this p o i n t , D M S concentrations c o n t i n u e d to increase despite a d r o p i n c h l o r o p h y l l levels and relatively constant D M S P p concentrations. A n o t h e r sharp temperature front was encountered at the northernmost s e c t i o n o f the transect, w h e r e a sudden increase i n temperature w a s associated w i t h a r a p i d , large drop i n D M S concentrations. T h i s greater than 2 0 n M d e c l i n e i n D M S levels o c c u r r e d i n under 2 0 m i n u t e s , over a short distance (~5.5 k m ) , b u t w a s represented b y ~ 4 0 i n d i v i d u a l data p o i n t s ( F i g . 3.3a). T h i s type o f r e s o l u t i o n c o u l d not have b e e n achieved w i t h other s a m p l i n g techniques. T h e high-resolution data r e i n f o r c e the observation-that D M S seems to v a r y independently o f the other parameters, but changes d r a m a t i c a l l y at frontal regions w h e r e '  61  two water masses converge. In contrast, pC0 and ( V A r showed a striking anti-correlation along 2  the sampling transect, reflecting the coupling of these gases through photosynthesis and respiration (r = 0.89,/KO.OOOl, Fig 3.3b). 2  Figure 3.4 shows a second, 3 hour cross-shelf transect (T7) which captured the peak in chlorophyll levels encountered during our survey. In contrast to T5 (Fig. 3.3), the high levels of chlorophyll along T7 were associated with relatively low and constant DMS concentrations despite the presence of a large gradient in DMSPp levels (45-167 nM, Fig. 3.4a). Similar to the observations of T5, however, was the poor correlation between DMS and other measured biological and physical variables. The most striking feature of this transect is the strong correlation between chlorophyll concentrations and the CVAr ratio (r = 0.86,p<0.001, Fig. 3.4a2  b). This is particularly remarkable considering that the measurements came from independent instruments measuring different parameters: chlorophyll a, a measure of phytoplankton biomass, and the CVAr ratio, a proxy for net community productivity [Kaiser et al., 2005]. The CVAr and pC0 distributions were once again anti-correlated, although not as strongly as along transect 5 2  (r = 0.64,p< 0.0001, Fig. 3.4b). 2  Covariance of Gas and Hydrographic Data- Figure 3.5 illustrates the correlation between pairs  of variables for the entire dataset. The strong anti-correlation between pC0 and CVAr that was 2  obvious for T5 and T7 (Figs. 3.3b, 3.4b) is also apparent for the pooled dataset (r = 0.90, 2  p<0.0001; Fig. 3.5a). Overlaid on this scatterplot is the chlorophyll concentration. The highest chlorophyll is associated with the highest 02/Ar values and the lowest pC0 values, while low 2  chlorophyll occurs over a larger range of 02/Ar andpC0 values (Fig. 3.5a). A trend is also 2  evident between 02/Ar and chlorophyll although there is considerably more scatter around this  62  relationship (r  2  = 0.19,/?<0.0001; F i g . 3.5b). F r o m the temperature data o v e r l a i d o n this plot it is  clear that the m a i n d e v i a t i o n s f r o m linearity o c c u r at c o l d surface temperatures that represent water masses f r o m the t i d a l l y m i x e d J u a n de F u c a and Q C Straits. T h e s e u p w e l l e d waters b r i n g w i t h t h e m the l o w C V A r signatures o f deep water that has been subject to o x y g e n loss due to respiration. D a t a f r o m these areas thus appear as a negative a n o m a l y o n the O2/A1  vs.  c h l o r o p h y l l p l o t ( F i g . 3.5b). E x c l u s i o n o f a l l the data at temperatures b e l o w 13.0 ° C yields a m u c h tighter p o s i t i v e r e l a t i o n s h i p between ( V A r and c h l o r o p h y l l l e v e l s i n these " a g e d " surface waters (r  2  = 0.73, p<0.0001;  see d i s c u s s i o n ) . D M S is plotted against c h l o r o p h y l l concentration i n  F i g u r e 3.6. A s illustrated i n F i g u r e s 3.2-3.4 and evident f r o m this p l o t , D M S concentrations were not correlated to c h l o r o p h y l l levels (r  2  = 0.06; F i g . 3.6), n o r to any other single variable.  A l t h o u g h the data i n F i g . 3.6 appear to f a l l into clusters, these clusters d i d not c o i n c i d e w i t h geographical l o c a t i o n , n o r w e r e they related to temperature, s a l i n i t y , /7CO2 or C V A r levels. W e used p r i n c i p a l c o m p o n e n t s analysis ( P C A ) to e x a m i n e the u n d e r l y i n g associations between the variables i n our m u l t i d i m e n s i o n a l data set. T h i s m e t h o d is p a r t i c u l a r l y useful w h e n m a n y variables are inter-correlated w i t h each other as is the case f o r o u r gas and hydrographic measurements. U s e o f this technique y i e l d e d t w o p r e d i c t i v e factors (linear c o m b i n a t i o n s o f the o r i g i n a l variables) w h i c h e x p l a i n e d 7 4 % o f the total variance represented b y the s i x u n d e r l y i n g parameters. T h e results o f the analysis revealed several clear patterns i n the covariance o f the data set. T h e m o s t evident o f these was the distinct separation o f /7CO2 and 02/Ar i n twod i m e n s i o n a l space, i n d i c a t i v e o f the strong anti-correlation b e t w e e n these variables a l o n g a l l s a m p l i n g transects ( F i g . 3.7). T h e second noticeable feature o f the P C A w a s the separation between b i o l o g i c a l variables (i.e. c h l o r o p h y l l ) and p h y s i c a l ones (temperature and salinity). T h e l o c a t i o n o f _ p C 0 and 02/Ar o n the p l o t suggests that these parameters w e r e i n f l u e n c e d almost 2  63  e q u a l l y b y p h y s i c a l and b i o l o g i c a l d r i v i n g forces. In contrast, D M S clustered m o s t t i g h t l y w i t h c h l o r o p h y l l i n d i c a t i v e o f its b i o l o g i c a l o r i g i n .  A Test of a Predictive DMS  Algorithm- N o t s u r p r i s i n g l y , none o f the correlative analyses w e  a p p l i e d w e r e capable o f e x p l a i n i n g the d i s t r i b u t i o n o f D M S i n o u r survey. O f a l l the variables, c h l o r o p h y l l clustered m o s t t i g h t l y w i t h D M S i n the P C A ( F i g . 3.7), and co-varied w i t h D M S a l o n g parts o f s o m e transects ( F i g . 3.3a), but w a s a p o o r p r e d i c t o r o f D M S i n general ( F i g . 3.6). W e also o b s e r v e d areas w h e r e D M S changed sharply w i t h temperature and s a l i n i t y across fronts ( F i g . 3.3a, 3.3c), s u g g e s t i n g a direct, o r indirect, p h y s i c a l i n f l u e n c e s u c h as m i x e d layer depth, o n D M S concentrations. V a r i a b i l i t y i n the M L D leads to v a r i a b l e nutrient a n d l i g h t levels, w h i c h i n f l u e n c e b o t h p h y t o p l a n k t o n and bacterial g r o w t h rates and species c o m p o s i t i o n s , and thereby D M S P p r o d u c t i o n and its subsequent b r e a k d o w n [Simo and Pedros-Alio, 1999]. F u r t h e r m o r e , surface D M S levels m a y be related to the thickness o f the m i x e d layer b y a s i m p l e d i l u t i o n m o d e l , s u c h that D M S l e v e l s are h i g h w h e r e M L D s are s h a l l o w and v i c e versa [Aranami and Tsunogai, 2 0 0 4 ] . Simo and Dachs [2002] h a d s u c c e s s f u l l y used the C H L / M L D ratio as a c o m b i n e d b i o l o g i c a l / p h y s i c a l p r e d i c t o r variable for D M S i n their a l g o r i t h m . C a p i t a l i z i n g on our abundant u n d e r w a y measurements o f b o t h c h l o r o p h y l l and D M S , w e w e r e able to test the a p p l i c a b i l i t y o f this p r e d i c t i v e a l g o r i t h m i n the coastal waters o f o u r survey. F o l l o w i n g the b i n n i n g procedure (see methods), w e a p p l i e d the appropriate equations to calculate the p r e d i c t e d D M S concentration based o n the m a g n i t u d e o f the C H L / M L D ratio. In a l l cases, this ratio w a s greater than 0.02 and w e thus used equation 2 o f S&D2002 that linearly relates D M S to the C H L / M L D ratio a c c o r d i n g to:  64  D M S = 55.8 * ( C H L / M L D ) + 0.6.  (1)  In all but t w o cases, the S&D2002 equation overestimated the D M S concentration b y a factor o f at least 2, r e s u l t i n g i n a p o o r fit to the data ( F i g . 3.8, dashed line). H o w e v e r , w h e n the actual D M S concentrations w e r e plotted against their c o r r e s p o n d i n g C H L / M L D ratios, w e observed a g o o d linear fit b e t w e e n the t w o variables (r = 0 . 8 3 , n = 27,/><0.0001; F i g . 3.8), after 2  e x c l u d i n g t w o s i g n i f i c a n t outliers f r o m the regression. There w a s s t i l l n o correlation between b i n n e d D M S and c h l o r o p h y l l levels, i n d i c a t i n g that the linear trend w a s not the result o f the b i n n i n g and a v e r a g i n g process itself. T h e r e l a t i o n s h i p between D M S a n d C H L / M L D exists despite the fact that m a n y o f the c h l o r o p h y l l values exceeded the 15 p g L"  1  c u t o f f o f the o r i g i n a l  S&D2002 a l g o r i t h m , and m i x e d layer depths for some o f the p o i n t s w e r e estimated u s i n g data f r o m other cruises (open s y m b o l s i n F i g . 3.8; see methods). W h e n o n l y data f r o m Q C S o u n d were used (where M L D s w e r e measured e x p l i c i t l y ) , the c o e f f i c i e n t o f d e t e r m i n a t i o n for the linear relationship i m p r o v e d to r - 0.96 w i t h o u t m u c h effect o n the slope (closed s y m b o l s i n 2  F i g . 3.8). T h e resultant fit to o u r data i s :  D M S - 21.0 * ( C H L / M L D ) - 0 . 1 .  (2)  T h i s slope is less than h a l f that o f the o r i g i n a l S&D2002 f o r m u l a t i o n . O u r results suggest that the C H L / M L D ratio m a y be a u s e f u l p r o x y for s i m u l a t i n g D M S concentrations even i n p r o d u c t i v e coastal areas, a l t h o u g h w i t h a s i g n i f i c a n t l y different slope.  65  Spatial Analyses- In a d d i t i o n to e x a m i n i n g the covariance o f our m e a s u r e d parameters, w e c a p i t a l i z e d o n the h i g h r e s o l u t i o n nature o f the dataset to q u a n t i f y the length scales o f v a r i a b i l i t y o f the gas and h y d r o g r a p h i c data. W e c o m p u t e d lagged autocorrelation f u n c t i o n s ( A C F ) a l o n g i n d i v i d u a l transects f r o m w h i c h the decorrelation length scales ( D L S ) w e r e d e f i n e d as the first zero c r o s s i n g o f the f u n c t i o n . A d i a g r a m o f the A C F for the s i x measured variables a l o n g T 5 is s h o w n i n F i g u r e 3.9a. A l o n g this transect, D M S h a d the shortest D L S o f 10.5 k m i n d i c a t i n g that its d i s t r i b u t i o n v a r i e d o v e r the shortest distance. In contrast, the p h y s i c a l parameters, temperature and salinity s h o w e d l o n g e r length scales o f v a r i a b i l i t y w i t h D L S o f 23-28 k m . T h e s t r i k i n g feature o f this figure is the strong s i m i l a r i t y between the f u n c t i o n s o f c h l o r o p h y l l a, pCOz and 02/Ar. A l l three h a d r o u g h l y the same shape and a D L S o f - 1 7 k m , f a l l i n g between that o f D M S and the p h y s i c a l parameters. T h i s trend illustrates the tight c o u p l i n g b e t w e e n these three parameters w i t h changes i n p h y t o p l a n k t o n b i o m a s s l i k e l y d r i v i n g the v a r i a b i l i t y i n the J9CO2 and 0 / A r distributions. T h e D L S a l o n g the six major transects a n a l y z e d ( T l and T 3 w e r e e x c l u d e d 2  due to gaps i n the h y d r o g r a p h i c data, see methods) ranged f r o m 2.5-32.2 k m w i t h m e a n values for the six measured parameters r a n g i n g from 7-14 k m ( F i g . 3.9b). A l t h o u g h the gases appeared to v a r y o n shorter spatial scales than the h y d r o g r a p h i c data, the differences w e r e not statistically significant (see d i s c u s s i o n ) . A n a d d i t i o n a l analysis w a s used to estimate the errors that c o u l d result f r o m l o w e r frequency s a m p l i n g . F o l l o w i n g the w o r k o f Hales and frequency  Takahashi [ 2 0 0 4 ] , w e resampled our h i g h  data at ever coarser r e s o l u t i o n , and calculated the average error r e s u l t i n g f r o m linear  interpolations b e t w e e n the c o a r s e l y s a m p l e d data. A s the s a m p l i n g frequency decreases, the interpolation error increases to an asymptotic value. B e y o n d this p o i n t , coarser s a m p l i n g has  66  little to n o effect o n the m a g n i t u d e o f the i n t e r p o l a t i o n error [see Hales and  Takahashi, 2 0 0 4 ,  F i g . 19]. T a b l e 3.1 s u m m a r i z e s the asymptotic i n t e r p o l a t i o n errors c a l c u l a t e d for each o f the six parameters a l o n g the s i x m a j o r transects surveyed. F r o n t a l regions e v i d e n t l y h a d significant effects o n the m a g n i t u d e o f the asymptotic i n t e r p o l a t i o n error f o r a l l parameters. T 5 crossed a l o c a l u p w e l l i n g z o n e and w a s characterized b y t w o sharp temperature fronts ( F i g . 3.2a, F i g . 3.3c). C o n s e q u e n t l y , a l o n g this transect the i n t e r p o l a t i o n errors for m o s t parameters were large, p a r t i c u l a r l y for D M S w h i c h c h a n g e d d r a m a t i c a l l y at frontal z o n e s ( F i g . 3.3a). T h e asymptotic interpolation error for D M S i n this case w a s almost 100 % o f the 8.6 n M m e a n concentration a l o n g this transect ( T a b l e 3.1). In contrast, T 7 and T 8 w e r e characterized b y large, sharp gradients i n c h l o r o p h y l l concentrations (Figs. 3.2c, 3.4a). T h e s e large changes i n p h y t o p l a n k t o n b i o m a s s d r o v e h i g h v a r i a b i l i t y i n the pC02 and 02/Ar levels w h i c h c o u p l e d w i t h the m i x i n g o f deep waters to the surface a l o n g other parts o f these transects resulted i n large ranges i n the PCO2 (~450 p p m a l o n g T 8 ) , and 02/Ar measurements. A s a result, the absolute errors for c h l o r o p h y l l , pC02 and 02/Ar w e r e largest a l o n g transects 7 and 8 ( T a b l e 3.1).  3.4 D i s c u s s i o n Spatial Scales of Variability- B i o g e o c h e m i c a l v a r i a b i l i t y i n this coastal z o n e w a s observed b o t h i n the large range o f values measured and i n the short distances o v e r w h i c h gas and surface water hydrography v a r i e d . B o t h the autocorrelation functions and the i n t e r p o l a t i o n error analyses s h o w e d that, o n average, the m a j o r i t y o f the v a r i a b i l i t y i n the gas, temperature, salinity and c h l o r o p h y l l d i s t r i b u t i o n s o c c u r r e d o n spatial scales o f less than 2 0 k m ( F i g . 3.9b, T a b l e 3.1), consistent w i t h the expected R o s s b y radius (the c o r r e l a t i o n length f o r p h y s i c a l properties) i n this  67  region. A l t h o u g h the m e a n D L S seemed to be shorter for the gases (<10 k m ) , than for the h y d r o g r a p h i c parameters (12-14 k m ) , the differences w e r e not s t a t i s t i c a l l y s i g n i f i c a n t ( F i g . 3.9b) o w i n g to large v a r i a b i l i t y i n the D L S . H o w e v e r , m u c h o f this v a r i a b i l i t y appears to result f r o m an apparent artefact o f the analysis i n w h i c h the D L S f o r any g i v e n parameter increases w i t h i n c r e a s i n g transect length. T h u s , part o f the v a r i a b i l i t y i n the m e a n D L S f o r each parameter is due to v a r i a b i l i t y i n the transect length, m a k i n g c o m p a r i s o n o f D L S f o r a g i v e n parameter d i f f i c u l t b e t w e e n transects and e v e n between studies [i.e. Murphy et al, 2 0 0 1 ; Hales and Takahashi, 2 0 0 4 ; Tortell, 2 0 0 5 b ] . N o n e t h e l e s s , differences i n D L S b e t w e e n parameters a l o n g any single transect are m e a n i n g f u l ( F i g . 3.9a), and the o v e r a l l m e a n values are p r o b a b l y representative o f inherent differences between these parameters ( F i g . 3.9b). T h u s it appears that the gases do v a r y o n shorter length scales than temperature, s a l i n i t y and c h l o r o p h y l l , w i t h D M S p o s s i b l y e x h i b i t i n g the shortest D L S as suggested b y p r e v i o u s data [Tortell, 2 0 0 5 b ] . M o r e robust statistical approaches m a y be needed to q u a n t i f y the relevant spatial scales o f v a r i a b i l i t y o f our dataset, but the results c l e a r l y indicate that gases e x h i b i t v a r i a b i l i t y o v e r distances that are not s u f f i c i e n t l y r e s o l v e d i n m a n y field studies. A s y m p t o t i c i n t e r p o l a t i o n errors were c a l c u l a t e d to q u a n t i f y the m a g n i t u d e o f the errors resulting f r o m l o w r e s o l u t i o n s a m p l i n g . O u r c a l c u l a t i o n s s h o w that f o r D M S and c h l o r o p h y l l i n particular, i n s u f f i c i e n t s a m p l i n g r e s o l u t i o n c a n i n t r o d u c e errors a p p r o a c h i n g 100 % o f the m e a n concentration. L a r g e errors (-20 % ) are also associated w i t h pCOi and 0 / A r measurements 2  ( T a b l e 3.1). A l t h o u g h the relative error for pCOi and 0 / A r is s m a l l e r than that for D M S or 2  c h l o r o p h y l l a, a 4 6 p p m average absolute error i n the m e a n p C 0  2  c o n c e n t r a t i o n w o u l d be h i g h l y  significant i f accurate estimates o f r e g i o n a l air-sea f l u x e s w e r e r e q u i r e d , and c o u l d even change the d i r e c t i o n o f the f l u x . T h e same is true i n the case o f 0 2 / A r , w h e r e a u n i t change i n the ratio  68  can represent a 5-10 % change i n the saturation state o f O2. T h i s w o u l d have large i m p l i c a t i o n s for estimates o f net c o m m u n i t y p r o d u c t i v i t y from ( V A r ratios [as i n Kaiser et al, 2 0 0 5 ] . W h i l e the a s y m p t o t i c i n t e r p o l a t i o n errors represent m a x i m u m uncertainty associated w i t h l o w r e s o l u t i o n s a m p l i n g , e.g. hydrostations w i t h separations o f 20-60 k m , s i g n i f i c a n t errors can also o c c u r d u r i n g u n d e r w a y s a m p l i n g at i n s u f f i c i e n t r e s o l u t i o n . A m o r e m e a n i n g f u l a p p l i c a t i o n o f the analysis is to calculate actual s a m p l i n g errors for u n d e r w a y measurements w i t h a g i v e n s a m p l i n g frequency. W e c a l c u l a t e d i n t e r p o l a t i o n errors for o u r h i g h - r e s o l u t i o n pC0 and D M S 2  measurements r e s a m p l e d at the frequency o f our pC0 e q u i l i b r a t o r (5 m i n . ) , and a t y p i c a l 2  u n d e r w a y D M S s a m p l i n g f r e q u e n c y o f 30 minutes. F o r the pC0 e q u i l i b r a t o r , the interpolation 2  errors a l o n g i n d i v i d u a l transects ranged from 1.4-8.3 p p m , equivalent to a m a x i m u m error o f 1.5 % relative to the m e a n . T h i s r e l a t i v e l y s m a l l error w o u l d not be p a r t i c u l a r l y significant for most b i o g e o c h e m i c a l studies, and it thus appears that the s a m p l i n g r e s o l u t i o n o f the pC0  2  equilibrator is s u f f i c i e n t to capture nearly a l l o f the u n d e r l y i n g v a r i a b i l i t y . In contrast, a 30 m i n u t e s a m p l i n g r e g i m e f o r D M S i n t r o d u c e d errors o f b e t w e e n 0.7-2.8 n M , o r as h i g h as 41 % o f the m e a n a l o n g a g i v e n transect. T h i s error i n the m e a n D M S c o n c e n t r a t i o n translates to an equivalent error i n the D M S f l u x , an estimate already prone to large errors due to the uncertainty i n the gas transfer c o e f f i c i e n t [Nightingale et al, 2 0 0 0 ] .  pCC>2 and 02/Ar Distributions- T h e distributions oi~pC0 and 0 2 / A r d u r i n g this survey e x h i b i t e d 2  considerable patchiness w i t h regions o f strong undersaturation and supersaturation i n close p r o x i m i t y to each other (e.g. 5 0 0 p p m change in/7CO2 o v e r 8 k m , T l a , F i g . 3.2d). T h e range o f /7CO2 concentrations encountered (201-747 p p m ) was s i m i l a r to other s u m m e r t i m e values reported for the southwest coast o f V a n c o u v e r Island [Ianson et al, 2 0 0 3 ] , and the O r e g o n  69  coastal u p w e l l i n g system [Hales et al, 2 0 0 5 ] . In a l l three surveys, r e g i o n s o f intense C O 2 supersaturation w e r e c o n f i n e d to n a r r o w nearshore strips w i t h the m a j o r i t y o f the outer s h e l f areas undersaturated as a result o f b i o l o g i c a l d r a w d o w n . T h e large regions o f  CO2  undersaturation f o u n d a l o n g the s h e l f ( T 6 , T 7 ) and i n Q C S o u n d ( T l b - 5 ) l i k e l y compensated for the l o c a l l y intense C O 2 sources i d e n t i f i e d i n the straits ( T l a , T 8 ) . W e suggest that this r e g i o n i s still an o v e r a l l C O 2 s i n k d u r i n g the t i m e o f the survey, a l t h o u g h s i g n i f i c a n t l y s m a l l e r due to the consistent t i d a l sources. It is important to r e c o g n i z e that these t i d a l l y - i n f l u e n c e d regions o f persistently h i g h C O 2 m u s t h a v e a large i m p a c t o n net annual c a r b o n budgets, whereas l o c a l i z e d u p w e l l i n g a l o n g the s h e l f leads to transiently h i g h C O 2 concentrations that are u s u a l l y q u i c k l y d r a w n d o w n b y b i o l o g i c a l p r o d u c t i o n . A l t h o u g h there is s t i l l c o n s i d e r a b l e debate as to whether coastal u p w e l l i n g z o n e s are net sources o r s i n k s o f C 0  2  o v e r an a n n u a l c y c l e [Ianson and Allen,  2 0 0 2 ; Hales et al, 2 0 0 5 ] , Hales et al. [2005] suggest that C O 2 uptake i n the u p w e l l i n g m a r g i n s a l o n g the west coast o f N o r t h A m e r i c a is equivalent to 5 0 % o f the entire s u m m e r t i m e , oceanic N o r t h P a c i f i c C O 2 sink. T h e s e f i n d i n g s c l e a r l y demonstrate the d i s p r o p o r t i o n a t e l y large i n f l u e n c e o f coastal m a r g i n s i n the oceanic c a r b o n c y c l e and the n e e d to incorporate these areas into g l o b a l C O 2 c l i m a t o l o g i e s . There is m u c h current interest i n i d e n t i f y i n g the relative i m p o r t a n c e o f b i o l o g i c a l processes versus p h y s i c a l ones i n d r i v i n g the d i s t r i b u t i o n (and hence air-sea f l u x ) o f  CO2  [Sarmiento et al, 2 0 0 0 ] . O u r results demonstrate that v a r i a b i l i t y i n surface pCOi was largely i n f l u e n c e d b y the o p p o s i n g b i o l o g i c a l processes o f photosynthesis and respiration i n this p r o d u c t i v e , coastal m a r g i n . T h i s is evident f r o m the tight anti-correlation b e t w e e n pCOi and O2/AX (a strictly b i o l o g i c a l parameter) a l o n g a l l s a m p l i n g transects e v e n i n the finest resolution (Figs. 3.3b, 3.4b, a n d 3.5a), a n d the occurrence o f l o w  pC0 levels i n r e g i o n s o f h i g h b i o m a s s 2  70  a l o n g certain transects (e.g. T i b , T 7 , F i g . 3.2c-d). H o w e v e r , it w a s the p h y s i c s o f this region ( u p w e l l i n g , t i d a l m i x i n g ) that d r o v e the large-scale pC0  2  large range o f pC0  2  d i s t r i b u t i o n s a n d accounted for the  levels b y p r o v i d i n g a m e c h a n i s m f o r deep waters e n r i c h e d i n r e m i n e r a l i z e d  CO2 to reach the surface. A t m o s p h e r i c exchange also affects the d i s t r i b u t i o n o f pC0  2  i n surface waters although its  effects are not as p r o n o u n c e d as those o f m i x i n g and b i o l o g i c a l d r a w d o w n . T h i s is due i n part to the s l o w rate o f CO2 e x c h a n g e w h i c h occurs o n timescales o f days to w e e k s and is about 10 times s l o w e r than the rate o f O2 exchange [Broecker and Peng, 1982]. T h e s e differentia] gas exchange rates m a y e x p l a i n the divergent slopes o f the pC0  v s . 02/Ar relationship o n the left  2  side o f the p l o t i n F i g . 3.5a. H e r e , / ? C 0 2 concentrations o f - 2 0 0 p p m are associated w i t h t w o very different levels o f c h l o r o p h y l l a n d 02/Ar ratios. A t the top o f the p l o t , the highest c h l o r o p h y l l l e v e l s (red area, F i g . 3.5a) c o r r e s p o n d to the highest 0 IAx ratios a n d lowest pC0 2  values m e a s u r e d , yet l o w e r d o w n o n the y-axis, the same pC0  2  2  levels are associated w i t h m u c h  l o w e r c h l o r o p h y l l a n d 02/Ar levels ( F i g . 3.5a). T h u s it appears that the O2 concentrations i n these latter waters w e r e able to re-equilibrate to the s u r r o u n d i n g b i o m a s s m u c h faster than CO2 levels. D u e to the longer " h i s t o r y " o f the pC0  2  signature, a strong anti-correlation between pC0  2  levels and c h l o r o p h y l l concentrations w a s o n l y evident i n areas w h e r e b i o m a s s w a s h i g h , p r e s u m a b l y w h e r e p r o d u c t i v i t y rates w e r e at their peak ( F i g . 3.4a-b). T h e 02/Ar ratio has been used as a p r o x y f o r the net c o m m u n i t y p r o d u c t i o n o f a water mass integrated o v e r a t i m e scale that depends o n the gas transfer v e l o c i t y and the depth o f the m i x e d layer [Kaiser et al, 2 0 0 5 ] . W e observed a strong linear relationship between 02/Ar and c h l o r o p h y l l levels i n surface waters i n d i c a t i n g i n c r e a s i n g p r i m a r y p r o d u c t i v i t y w i t h increasing p h y t o p l a n k t o n b i o m a s s ( F i g . 3.5b). H o w e v e r , several factors c o m p l i c a t e the relationship between  71  chlorophyll and 02/Ar. O n the one hand, deep mixing brings up cold subsurface waters which carry the low 02/Ar values characteristic o f oxygen loss due to respiration. This creates an initial oxygen deficit in surface waters which persists despite the growth o f phytoplankton and the addition o f photosynthetically derived oxygen. This scenario may explain the negative anomaly in the 02/Ar vs. chlorophyll relationship (blue regions in Fig. 3.5b), and highlights the difficulty of estimating net community productivity from 02/Ar ratios in regions with significant upwelling or vertical mixing [Kaiser et al, 2005]. O n the other hand, high 02/Ar ratios may persist in the mixed layer despite the presence o f low surface chlorophyll levels (red regions in Fig. 3.5b). These waters may have warmed and stratified to the extent o f cutting off the supply o f nutrients from below the mixed layer, thus either creating a subsurface chlorophyll maximum, or causing the phytoplankton to be removed from the surface before the O2/AJ productivity signature could be reset by atmospheric exchange. Thus, knowledge o f the relevant timescales o f both gas exchange and phytoplankton turnover rates is critical when inferring production rates from gas distributions.  Factors Driving DMS Distributions- The range o f D M S concentrations encountered during our survey was large and variable, from undetectable (<1 n M ) to almost 30 n M . The upper end o f this range is at least an order o f magnitude higher than current estimates o f the global mean D M S concentration [Belviso et al, 2004b], and comparable to data obtained in various other coastal regions [Leek etal, 1990; Townsend and Keller, 1996; Locarnini et al, 1998; Tortell, 2005b]. However, our measurements provide much higher spatial resolution than previous surveys and thus offer new insight into the small-scale patchiness o f D M S distributions in coastal waters (Fig.3.2f).  72  W e c o m p a r e d o u r D M S data to measurements taken i n the same general area (coastal B r i t i s h C o l u m b i a b e t w e e n 48-55° N ) , d u r i n g the same w e e k , the p r e v i o u s year b y automated purge-and-trap gas c h r o m a t o g r a p h y s a m p l i n g at a frequency o f o n c e e v e r y 3 0 minutes (J.E. J o h n s o n , u n p u b l i s h e d data a v a i l a b l e at http://saRa.pmel.noaa.gov/dms/'). T h e range o f D M S concentrations encountered i n 2 0 0 3 (0.7-17.6 n M ) w a s s m a l l e r than that o b s e r v e d d u r i n g our 2004 survey (<1- 28.7 n M ) , but the m e a n D M S concentrations w e r e r e m a r k a b l y s i m i l a r (5.3 n M vs. our value o f 5.8 n M ) . It is p o s s i b l e that the range o f concentrations d u r i n g 2003 w a s l o w e r because J o h n s o n ' s survey m a y have been unable to resolve the true v a r i a b i l i t y o f the D M S distributions (as i n d i c a t e d b y our 3 0 m i n . i n t e r p o l a t i o n error analysis). A s o b s e r v e d p r e v i o u s l y  [Hales and Takahashi, 2 0 0 4 ; Tortell, 2 0 0 5 b ] , w e note that the m e a s u r e d range o f a g i v e n parameter is m u c h m o r e prone to s a m p l i n g errors than the r e g i o n a l m e a n . It is thus not s u r p r i s i n g that the o v e r a l l m e a n o f J o h n s o n ' s survey w a s s i m i l a r to that o f the present survey, despite a m u c h s m a l l e r range o f reported D M S concentrations. H o w e v e r , m a n y e n v i r o n m e n t a l factors c o u l d account f o r the interannual v a r i a b i l i t y i n D M S concentrations. V a r i a t i o n s i n w i n d v e l o c i t y c o u l d be r e s p o n s i b l e for the difference i n m a x i m u m D M S levels b e t w e e n the t w o years. W i n d s were v e r y light d u r i n g our entire cruise (rarely e x c e e d i n g 5 m s" ), c o m p a r e d to those reported i n 1  2003 ( m a x 13 m s " , m e a n 5 . 1 m s" ). A s a result, a larger gas transfer v e l o c i t y i n 2 0 0 3 c o u l d 1  1  have created a larger D M S loss t e r m i n surface waters, acting against the a c c u m u l a t i o n o f h i g h D M S concentrations i n the m i x e d layer. It s h o u l d be noted, h o w e v e r , that v e n t i l a t i o n to the atmosphere is o n l y one s i n k for D M S and often o n l y a m i n o r one Photolysis  [Kiene and Bates, 1990].  [Kieber et al, 1 9 9 6 ; Toole et al, 2004] and bacterial c o n s u m p t i o n [Kiene and Bates,  1990] can be m u c h larger s i n k s for D M S , and these a l o n g w i t h u n d e r l y i n g differences i n p h y t o p l a n k t o n species c o m p o s i t i o n and b i o m a s s , z o o p l a n k t o n g r a z i n g rates, nutrient s u p p l y and  73  light intensity c o u l d a l l account f o r interannual v a r i a b i l i t y i n D M S concentrations. D e s p i t e these potential differences, w e o b s e r v e d a s t r i k i n g s i m i l a r i t y i n the spatial d i s t r i b u t i o n o f D M S between the t w o years. B o t h the 2 0 0 3 survey and our 2 0 0 4 survey reported peak D M S levels i n the r e g i o n north o f V a n c o u v e r Island between 51-52 ° N . T h e area o f s u m m e r r e l a x a t i o n over the broad s h e l f i n Q C S o u n d m a y thus be a r e g i o n o f persistently h i g h D M S d u r i n g A u g u s t . T h e h i g h D M S concentrations (>10 n M ) observed i n Q C S o u n d d u r i n g our survey were associated w i t h moderate levels o f c h l o r o p h y l l ( T i b , T 4 , T 5 ; F i g s . 3.2c, 3.2f). In contrast, D M S concentrations w e r e l o w i n areas w h e r e c h l o r o p h y l l levels exceeded 15 p g L"  1  ( T l a , T 7 ; Figs.  3.2c, 3.2f). S i n c e D M S P p r o d u c t i o n b y p h y t o p l a n k t o n is k n o w n to be h i g h l y species-specific [Keller et al, 1989], v a r i a b i l i t y i n p h y t o p l a n k t o n c o m m u n i t y c o m p o s i t i o n m a y account for differences i n D M S levels b e t w e e n regions. T h i s t a x o n o m i c effect is p o t e n t i a l l y c o n f o u n d e d , however, b y d i f f e r i n g e n v i r o n m e n t a l c o n d i t i o n s across o u r s a m p l i n g r e g i o n . S i n c e D M S P and its byproducts are p o w e r f u l a n t i o x i d a n t s , cells increase their D M S P (and D M S ) p r o d u c t i o n w h e n exposed to o x i d a t i v e stressors s u c h as l o w nutrients and h i g h U V light [Sunda et al, 2 0 0 2 ] . These c o n d i t i o n s g e n e r a l l y o c c u r i n later stage b l o o m s w h e n surface waters have stratified, cutting o f f the s u p p l y o f nutrients f r o m b e l o w the t h e r m o c l i n e and e x p o s i n g cells to higher levels o f U V light. Stratified waters tend to f a v o u r flagellate groups s u c h as p r y m n e s i o p h y t e s that have adapted to l i v i n g under l o w nutrient, h i g h light c o n d i t i o n s [Margalef, 1978] and are perhaps nonc o i n c i d e n t a l l y the same groups that are p r o m i n e n t D M S P - p r o d u c e r s [Keller et al, 1989]. Furthermore, nutrient l i m i t a t i o n can i n d u c e D M S P p r o d u c t i o n i n groups s u c h as diatoms [Sunda et al, 2 0 0 2 ; Bucciarelli and Sunda, 2 0 0 3 ] , that have t r a d i t i o n a l l y b e e n c o n s i d e r e d l o w D M S P producers [Keller et al, 1989]. T h e h i g h D M S concentrations i n Q C S o u n d o c c u r r e d where m i x e d layer depths w e r e less than 12 m and surface nitrate was depleted, suggesting an o x i d a t i v e  74  stress effect o n D M S P / D M S p r o d u c t i o n . In contrast, the waters i n QC Strait and i n the J d F eddy ( T l a , T 7 ) w e r e r e c e i v i n g a steady nutrient s u p p l y t h r o u g h l o c a l i z e d u p w e l l i n g or deep m i x i n g resulting i n h i g h (or measurable) surface nitrate and h i g h p h y t o p l a n k t o n b i o m a s s , w i t h little D M S p r o d u c t i o n . T h e o c c u r r e n c e o f l o w D M S l e v e l s i n recently u p w e l l e d waters has been observed p r e v i o u s l y [Belviso et ai, 2 0 0 3 ] . H o w e v e r , since changes i n p h y t o p l a n k t o n species c o m p o s i t i o n generally o c c u r i n c o n j u n c t i o n w i t h c h a n g i n g e n v i r o n m e n t a l c o n d i t i o n s , it is d i f f i c u l t to d i s t i n g u i s h b e t w e e n species c o m p o s i t i o n effects and nutrient/light effects i n d e t e r m i n i n g D M S levels. A s i n p r e v i o u s studies [Locarnini et al.,  1 9 9 8 ; Tortell, 2 0 0 5 b ] , D M S concentrations  d u r i n g this survey c h a n g e d d r a m a t i c a l l y at fronts, regions w h e r e abrupt gradients i n nutrient concentrations, l i g h t r e g i m e s , p r o d u c t i v i t y and p l a n k t o n c o m m u n i t y c o m p o s i t i o n are c o m m o n . T h i s trend reflects the c o m p l e x interplay o f p h y s i c s and b i o l o g y that characterizes the oceanic D M S c y c l e [see Simo, 2 0 0 4 ] . T h e a n o m a l o u s l y h i g h l e v e l o f D M S P d (135 n M ) measured o f f the northwest coast o f V a n c o u v e r I s l a n d ( F i g . 3.3a) also o c c u r r e d at a sharp temperature front. T h e e x c e p t i o n a l l y h i g h abundance o f z o o p l a n k t o n encountered at this site (R.  El-Sabaawi,  u n p u b l i s h e d data) m a y have been responsible for m a s s i v e grazer-mediated release o f D M S P [Dacey and Wakeham, 1986].  Towards Global DMS Prediction- Intense efforts have been a i m e d at understanding the factors c o n t r o l l i n g D M S p r o d u c t i o n i n the oceans, and q u a n t i f y i n g its a t m o s p h e r i c f l u x i n order to evaluate the f e a s i b i l i t y o f the h y p o t h e s i z e d b i o l o g i c a l l y - m e d i a t e d homeostasis ( C L A W hypothesis) [Charlson et al., 1987]. It has p r o v e d d i f f i c u l t to project future oceanic D M S emissions i n a c h a n g i n g c l i m a t e due to an i n c o m p l e t e m e c h a n i s t i c u n d e r s t a n d i n g o f the D M S  75  c y c l e and uncertainties i n the g l o b a l and seasonal d i s t r i b u t i o n s o f this gas. T h e s e uncertainties c o u p l e d w i t h uncertainties i n the gas transfer c o e f f i c i e n t [Nightingale et al,  2000] hamper the  a b i l i t y o f a t m o s p h e r i c s u l f u r m o d e l s to evaluate the m o d u l a t i n g effect o f this gas o n g l o b a l climate. T w o c o m p l e m e n t a r y approaches are needed to better c o n s t r a i n the g l o b a l d i s t r i b u t i o n o f D M S . A c c u r a t e , s p a t i a l l y r e s o l v e d g l o b a l D M S measurements w i t h g o o d seasonal coverage are the first step, and M I M S c a n greatly facilitate this endeavour. E v e n w i t h an automated M I M S system, it w o u l d s t i l l be u n f e a s i b l e to m a p the entire ocean at s u f f i c i e n t t e m p o r a l r e s o l u t i o n . H e n c e , the s e c o n d a p p r o a c h i n v o l v e s d e v e l o p i n g p r e d i c t i v e a l g o r i t h m s that simulate D M S levels based o n w e l l c o n s t r a i n e d b i o g e o c h e m i c a l parameters. A recent c o m p a r i s o n o f several such algorithms has s h o w n that they h a v e various strengths and weaknesses and d i f f e r i n their a b i l i t y to accurately replicate D M S levels b o t h seasonally and r e g i o n a l l y [Belviso et al,  2004b].  W e chose to evaluate the a l g o r i t h m o f Simo and Dachs [2002] w i t h our coastal dataset because it is one o f the f e w w i t h o u t a m o d e l e d term that relies o n t w o c o m m o n l y measured parameters ( c h l o r o p h y l l and M L D ) . A s noted above, the m i x e d layer depth encompasses a n u m b e r o f factors s u c h as nutrient a v a i l a b i l i t y , l i g h t a v a i l a b i l i t y a n d p h y t o p l a n k t o n s u c c e s s i o n , ( w h i c h appeared to e x p l a i n m a n y o f the r e g i o n a l differences i n o b s e r v e d D M S levels), into a single variable. A l t h o u g h the o r i g i n a l Simo and Dachs [2002] a l g o r i t h m f a i l e d to re-create the D M S levels f o u n d d u r i n g o u r survey, w i t h a different slope the C H L / M L D ratio was a g o o d predictor o f surface D M S concentrations (r  2  = 0 . 8 3 ; F i g . 3.8). T h i s is p a r t i c u l a r l y i m p r e s s i v e  c o n s i d e r i n g the c o m p l e t e l a c k o f a relationship between D M S and c h l o r o p h y l l alone (r  2  - 0.06;  F i g . 3.6).  76  O u r slope (21.0) is less than h a l f that o f the o r i g i n a l a l g o r i t h m (55.8) w h i c h m a y result from the h i g h e r relative p r o p o r t i o n o f diatoms ( w i t h l o w e r D M S P content per unit c h l o r o p h y l l ) [Keller et al, 1989] i n coastal regions c o m p a r e d to oceanic waters. O u r coastal data encompassed a m u c h larger range o f C H L / M L D ratios (0.03-2.26) than o r i g i n a l l y used to formulate the a l g o r i t h m (<0.20), a n d w e o b s e r v e d a g o o d fit to the D M S data u p to a C H L / M L D ratio o f 1.0. T h e t w o p r o m i n e n t outliers that w e r e e x c l u d e d from the r e g r e s s i o n h a d C H L / M L D ratios greater than 1.0 and c a m e from the r e g i o n o f h i g h b i o m a s s o f f B a r k l e y S o u n d . A l t h o u g h w e d i d not e x p l i c i t l y measure M L D s outside the area o f Q C S o u n d , w e chose to incorporate the w h o l e survey r e g i o n into the a l g o r i t h m to e x p a n d the ranges o f M L D , D M S , and c h l o r o p h y l l concentrations used. T h u s w e r e l i e d o n p r e v i o u s k n o w l e d g e o f the area, and data f r o m other cruises to estimate M L D s outside o f Q C S o u n d (open s y m b o l s , F i g . 3.8). A s a result, there is potential error i n the M L D s for the t w o outliers from B a r k l e y S o u n d . H o w e v e r , based o n the measured D M S and c h l o r o p h y l l concentrations, M L D s f o r the t w o outliers w o u l d have had to have been u n r e a s o n a b l y deep (75-100 m ) i n order to fit the curve. It i s p o s s i b l e that the linear relationship s i m p l y does not h o l d b e y o n d C H L / M L D ratios greater than 1.0 w h i c h w o u l d characterize rare regions o f v e r y h i g h b i o m a s s and r e l a t i v e l y s h a l l o w m i x e d layers. T h e area o f f B a r k l e y S o u n d w h e r e these h i g h C H L / M L D ratios w e r e observed w a s l i k e l y part o f the Juan de F u c a eddy, w h e r e e x c e e d i n g l y h i g h p h y t o p l a n k t o n b i o m a s s i s sustained throughout the s u m m e r through the constant i n j e c t i o n o f nutrients into the e d d y core [Marchetti S&D2002  et al, 2 0 0 4 ] . T h e  a l g o r i t h m uses b o t h a linear and a negative l o g a r i t h m i c equation to m o d e l D M S  concentrations based o n the m a g n i t u d e o f the C H L / M L D ratio. O u r results indicate a linear equation w o r k s i n the C H L / M L D range o f 0.02-1.0 but not b e y o n d . E i t h e r n o relationship exists between D M S and C H L / M L D b e y o n d C H L / M L D > 1 o r a different equation applies. M o r e data  77  i n p r o d u c t i v e regions w h e r e these h i g h C H L / M L D ratios are l i k e l y to be f o u n d is needed to test this p o s s i b i l i t y .  3.5 C o n c l u s i o n s O u r f i n d i n g s extend the u t i l i t y o f the C H L / M L D ratio as a p r e d i c t o r o f D M S levels into h i g h l y p r o d u c t i v e coastal waters. A p p l i c a t i o n o f s i m i l a r analyses i n v a r i o u s coastal waters is needed to test the general a p p l i c a b i l i t y o f our d e r i v e d a l g o r i t h m . T h i s w o u l d a i d i n d e t e r m i n i n g whether the r e l a t i o n s h i p is s p e c i f i c to this particular r e g i o n , or season, or m o r e b r o a d l y applicable to distinct coastal systems (i.e. temperate). O u r results also e m p h a s i z e the utility o f membrane inlet mass spectrometry f o r r e s o l v i n g spatial v a r i a b i l i t y i n gas d i s t r i b u t i o n s , and the need for high-frequency s a m p l i n g i n coastal waters i f the a i m is to accurately q u a n t i f y f l u x e s o f b o t h major and trace gases i n these important regions. T h e a p p l i c a t i o n o f M I M S (or other comparable a n a l y t i c a l tools) is thus l i k e l y to s i g n i f i c a n t l y increase o u r understanding o f oceanic gas distributions o v e r the c o m i n g decade. A s m o r e i n f o r m a t i o n b e c o m e s a v a i l a b l e o n the concentration and v a r i a b i l i t y o f gases i n d y n a m i c coastal r e g i o n s , n e w insight m a y be gained into the b i o l o g i c a l and p h y s i c a l c o n t r o l s o n gas distributions. In the case o f the D M S c y c l e , a better mechanistic u n d e r s t a n d i n g is r e q u i r e d o f the u n d e r l y i n g p r o d u c t i o n and c o n s u m p t i o n processes, and this m a y be obtained b y c o u p l i n g f o c u s e d process studies w i t h real-time u n d e r w a y surveys. U l t i m a t e l y , this i n f o r m a t i o n w i l l a i d i n the d e v e l o p m e n t o f better p r e d i c t i v e algorithms w h i c h are needed to understand the p o t e n t i a l i m p a c t o f D M S e m i s s i o n s o n future c l i m a t e , and conversely, the impact o f c l i m a t e change o n the oceanic c y c l e o f D M S .  78  Table 3.1: A b s o l u t e and relative asymptotic interpolation errors a l o n g the 6 m a j o r transects  Temp. (°C) Transect  *abs. error  frel. error  (%)  Sal. (psu) abs. error  rel. error  (%)  Chl a (ug L" ) 1  abs. error  rel. error  (%)  02/Ar (torr ratio) abs. error  rel. error  (%)  />C0 (ppm)  DMS  2  abs. error  rel. error  (%)  (nM)  abs. error  rel. error  (%)  2  0.45  2.7  0.06  0.19  0.15  22  0.15  1.0  12  3.7  1.25  48  4  0.28  1.6  0.17  0.54  0.80  65  0.28  1.8  8  2.5  6.0  90  5  2.25  15  0.23  0.71  3.50  97  1.30  8.2  45  13.9  8.0  93  6  0.18  1.1  0.15  0.47  0.80  97  0.70  4.3  30  9.9  1.5  52  7  0.90  6.1  0.15  0.47  7.50  62  2.00  11.5  55  18.0  1.0  24  8  0.35  3.1  0.10  0.32  2.75  55  1.75  UJ  125  23.4  mean  0.74  5.0  0.14  0.45  2.6  66  1.0  6.9  46  12  —  3.6  ~  61  * absolute a s y m p t o t i c interpolation error represents the m a x i m u m error i n the m e a n ( i n the respective units o f each parameter), o b t a i n e d w h e n the actual data a l o n g each transect is re-sampled at ever coarser r e s o l u t i o n . T h e asymptotic error was reached at a n average s a m p l i n g distance o f 3 4 k m . T h e characteristic error length scale (analogous to the D L S ) is c o m p u t e d as 2/3 the distance to the asymptote (-20 k m ) [Hales and Takahashi, 2 0 0 4 ] . t relative asymptotic i n t e r p o l a t i o n error is the percentage o f the m e a n v a l u e a l o n g e a c h transect that the absolute error represents  A  52°N  ^  British Columbia  Queen \ 2 Charlotte Sound  51 °N  yQueen Charlotte Strait la  50°N  Vancouver Island  49°N  48°N  132°W  130°W  J  Strait of Juan de FUG  128°W  .  i  126°W  124°W  122°W  Figure 3.1: M a p o f southwestern British Columbia, Canada showing the location o f underway  transects.  J  80  52 N  49 N  o 48 N  0  51 N  o  49 N  10  12  14  16  18  20  10  15  20  25  F i g u r e 3.2: S u r f a c e plots o f (a) temperature ( ° C ) , (b) s a l i n i t y (psu), (c) c h l o r o p h y l l a (u.g L" ), 1  pC0  2  ( p p m ) , (e) 0 / A r (torr ratio), a n d (f) D M S ( n M ) . See F i g . 3.1 f o r transect labels. See  (d)  2  methods for e x p l a n a t i o n o f gaps i n s a l i n i t y , c h l o r o p h y l l a n d D M S p l o t s .  81  1 O  0  L  r 450  - 350  (ppm)  - 400  CM  - 300 O  o  - 250  Q.  - 200  r 18  Latitude (degrees N)  F i g u r e 3.3:  D e t a i l e d south-north v i e w o f a l l variables measured a l o n g T 5 (see F i g . 3.1 for  location); (a) D M S ( _ ) , salinity ( _ ) ,  c h l a (—), D M S P d (•), D M S P p ( A ) ; (b) 0 / A r ( _ ) , pC0 (—); 2  2  (c)  and temperature (—). N o t e the h i g h D M S P d that c o i n c i d e s w i t h the sharp  temperature front s o u t h o f 50.8 ° N . L o c a l i z e d u p w e l l i n g is evident j u s t south o f 50.8 ° N .  82  0  10  20  30  40  50  60  Distance (km)  Figure 3.4: Detailed view o f all variables measured along T7 (see Fig. 3.1 for location); (a) D M S ( _ ) , chl a (—), D M S P p . ( A ) ; (b) 0 /Ar ( - ) , p C 0 (—); (c) salinity ( _ ) , and temperature (—). Note the strong correspondence o f the chl a and 0 / A r traces and the mirror images of the pC0 and 0 / A r data. D M S P d data were not available for this transect due to the malfunction o f the S E M detector near the end o f the cruise. 2  2  2  2  2  83  pC02 [ppm]  chl a [ug/L]  Figure 3.5: The correlation across all transects between (a) pC0  and 0 / A r (r = 0.90) with corresponding chl a concentrations overlaid (colourbar), and (b) chl a and 0 / A r (r = 0.19) with corresponding temperature overlaid (colourbar). Deviations from linearity in (b) mainly occur at cold surface temperature representing deep-mixed water masses, exclusion o f these data from the regression improves the correlation to r = 0.73. 2  2  2  2  2  2  Chl a  (jug  L")1  F i g u r e 3.6: T h e c o r r e l a t i o n across a l l transects b e t w e e n c h l a and D M S (r = 0.06). N o t e that h i g h D M S was g e n e r a l l y associated w i t h l o w to moderate c h l a levels (<10 u g L" ) w h i l e regions w i t h h i g h c h l a h a d r e l a t i v e l y l o w D M S concentrations. 2  1  85  1.0 Temp  .•j  _o  J  —  1  — i —  -1.0  1  — • —  1  —  1  — i —  -0.5  1  —  1  —  1  —  1  —  1  —  1  —  1  —  0.0  1  —  1  —  1  —  0.5  1.0  Factor 1  Figure 3.7: Results of the PCA showing the strong separation between pC0 and 0 /Ar in twodimensional space and the clear partitioning between physical (T, S) and biological (chl a) variables. 2  2  86  CHL/MLD (mg m" ) 4  F i g u r e 3 . 8 : A v e r a g e D M S concentrations p l o t t e d against C H L / M L D ratios f o r the !/ degree 4  grids. T h e dashed l i n e represents the p r e d i c t e d D M S c o n c e n t r a t i o n based o n equation 2 o f S&D2002 ( D M S = 55.8 * ( C H L / M L D ) + 0.6). T h e s o l i d l i n e is the linear regression o f the actual data ( D M S = 2 1 . 0 * ( C H L / M L D ) - 0 . 1 , r = 0.83, n = 27,/?<0.0001); Y error bars are standard errors o f the D M S m e a n . F i l l e d s y m b o l s represent data p o i n t s w i t h m e a s u r e d M L D s , o p e n s y m b o l s represent data p o i n t s w i t h estimated M L D s (see m e t h o d s ) . T h e t w o outliers o n the far right w e r e e x c l u d e d from the regression and represent data from transect 7.  87  0  5  10  15  20  Decorrelation Length Scale (Km) F i g u r e 3.9: A u t o c o r r e l a t i o n functions for a l l parameters m e a s u r e d a l o n g transect 5 (a); the D L S  is the first zero c r o s s i n g o f the f u n c t i o n ; average D L S for a l l parameters for the entire survey (b).  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H o f f e r t , (1985) O c e a n c a r b o n p u m p s : analysis o f relative strengths and e f f i c i e n c i e s i n ocean-driven atmospheric atmospheric C0 : 2  pC02 changes, i n The carbon cycle and  Natural variations, archean to present, Geophys. Monogr. Ser.,  edited  b y E . D . S u n d q u i s t and W . S . B r o e c k e r , p p . 99-110, A G U , W a s h i n g t o n , D . C .  92  Chapter 4: Conclusions 4.1 Thesis Overview T h e i m p o r t a n c e o f c o n s t r a i n i n g g l o b a l D M S fluxes and u n r a v e l l i n g the c o m p l e x oceanic sulfur c y c l e is b e c o m i n g i n c r e a s i n g l y clear as w e face a c h a n g i n g g l o b a l c l i m a t e . B i o g e n i c s u l f u r e m i s s i o n s from the w o r l d ' s oceans have a large impact o n the E a r t h ' s c l i m a t e and m a y help to counteract the effects o f i n c r e a s i n g greenhouse gas e m i s s i o n s [Watson and Liss, 1998; Gunson et al, 2 0 0 6 ] . H o w e v e r , to evaluate the m a g n i t u d e o f this i m p a c t w e need (1) a better m e c h a n i s t i c understanding o f the D M S c y c l e , and (2) m o r e advanced m e t h o d o l o g y to facilitate measurement o f D M S i n the oceans. T h e p r e c e d i n g t w o chapters that f o r m e d the b o d y o f this w o r k t a c k l e d these t w o goals. E a c h f o c u s e d o n a different a p p l i c a t i o n o f m e m b r a n e inlet mass spectrometry to the study o f b i o g e n i c s u l f u r c o m p o u n d s i n the oceans. These chapters i n t r o d u c e d n e w m e t h o d o l o g y for m e a s u r i n g b o t h d i s s o l v e d and particulate D M S P concentrations as w e l l as v o l a t i l e D M S , and p r o v i d e d n e w insights into the distributions o f these important c o m p o u n d s . T h e focus o f Chapter 2 was o n discrete, v e r t i c a l measurements o f springtime D M S P p i n oceanic waters a l o n g L i n e P over a three year t i m e p e r i o d . T h i s survey attempted to relate D M S P p concentrations to the c o m p o s i t i o n o f the p h y t o p l a n k t o n c o m m u n i t y and offered insight into the v e r t i c a l v a r i a b i l i t y o f this c o m p o u n d o n an interannual timescale i n an ocean b a s i n w h e r e s u c h measurements d i d not p r e v i o u s l y exist. In contrast, C h a p t e r 3 f o c u s e d o n c o n t i n u o u s , real-time, surface measurements o f D M S i n coastal surface waters around Q u e e n Charlotte S o u n d o v e r a w e e k - l o n g p e r i o d i n summer. T h i s study e x a m i n e d the high-resolution co-variance o f m u l t i p l e parameters i n relation to D M S d i s t r i b u t i o n s i n a h i g h l y d y n a m i c and p r o d u c t i v e r e g i o n .  93  4.2 Evaluation of MIMS for DMSP/DMS Measurements I n i t i a l l y , o u r results from the L i n e P a n d Q u e e n C h a r l o t t e S o u n d surveys i n d i c a t e d that M I M S w a s a p r o m i s i n g n e w technique f o r m e a s u r i n g o c e a n i c D M S P p ( a n d D M S P d ) concentrations. M e a s u r e m e n t p r e c i s i o n w a s g o o d , c a l i b r a t i o n c u r v e s w e r e linear o v e r a large range and analysis t i m e s w e r e short (see S e c t i o n 2.3). H o w e v e r , s o m e recently p u b l i s h e d data as w e l l as s o m e o f o u r o w n f i n d i n g s indicate that there are p o t e n t i a l p r o b l e m s w i t h this m e t h o d . F i r s t l y , w e d i s c o v e r e d that p r o l o n g e d exposure o f the d i m e t h y l s i l i c o n e m e m b r a n e to strong base (such as that used to h y d r o l y z e D M S P samples) leads to a b r e a k d o w n o f the m e m b r a n e and subsequent s i l i c o n e c o n t a m i n a t i o n o f the mass spectrometer i o n source. T h i s results i n a drastic loss o f s e n s i t i v i t y w h i c h c a n o n l y be r e c t i f i e d b y c o s t l y replacements o f the entire i o n source. T h i s p r o b l e m does not affect the q u a l i t y o f the data, but leads to frequent, i m p r a c t i c a l and expensive repairs. D i s s o l v e d D M S P samples are also t y p i c a l l y m e a s u r e d at h i g h p H . W e attempted to c i r c u m v e n t the a b o v e p r o b l e m f o r D M S P d analysis d u r i n g o u r Q u e e n C h a r l o t t e S o u n d survey b y l o w e r i n g the p H o f the seawater s a m p l e to 9.5 f o l l o w i n g the 2 4 h o u r h y d r o l y s i s p e r i o d (see S e c t i o n 3.2). T h i s r e d u c e d p H p r o l o n g e d the l i f e o f the m e m b r a n e w i t h o u t a f f e c t i n g the D M S i n the s a m p l e . H o w e v e r , this m e t h o d d i d n o t p r o d u c e r e l i a b l e results w h e n tested w i t h extracted D M S P p s a m p l e s w h i c h l a c k e d the b u f f e r i n g capacity o f seawater. E v e n w h e n exact additions o f a c i d and base w e r e m a d e , slight v a r i a b i l i t y i n the p H betw een samples affected the p e r m e a b i l i t y o f the m e m b r a n e w h i c h i n turn l e d to v a r i a b i l i t y i n the D M S signals f o r a g i v e n concentration. T h u s , D M S P p samples h a d to be measured at h i g h p H u s i n g the small-area p r o b e w i t h the risk o f d a m a g i n g the system.  94  T h e s e c o n d recently i d e n t i f i e d p r o b l e m that m a k e s M I M S not i d e a l f o r d i s s o l v e d and particulate D M S P analysis is the r e l a t i v e l y large sample v o l u m e s r e q u i r e d . In the case o f D M S P p , w e fdtered 2 5 0 m l o f seawater to compensate f o r the r e d u c e d s e n s i t i v i t y o f the smallarea m e m b r a n e inlet p r o b e u s e d . In h i n d s i g h t , this v o l u m e c o u l d h a v e b e e n r e d u c e d to 50 m l i f a 3 m l v i a l had been u s e d d u r i n g analysis w i t h o u t a f f e c t i n g the ~1 n M detection l i m i t o f  in situ  D M S P p (see S e c t i o n 2.3). H o w e v e r , l o w ambient D M S P d concentrations need to be measured o n the m o r e sensitive large-area m e m b r a n e cuvette that requires a r e l a t i v e l y large v o l u m e o f r e c i r c u l a t i n g s a m p l e (>200 m l ) . It has o n l y recently c o m e to l i g h t that f i l t e r i n g large v o l u m e s o f water for D M S P d analysis (even b y gravity) leads to elevated D M S P d levels due to ruptured phytoplankton cells  [Kiene and Slezak,  2 0 0 6 ] . T h e s e f i l t r a t i o n artifacts c a n be quite significant  and increase w i t h the v o l u m e o f water filtered  [Kiene and Slezak,  2 0 0 6 ] . It is n o w r e c o m m e n d e d  that o n l y the first 3.5 m l o f filtrate c o l l e c t e d b y g r a v i t y f i l t r a t i o n be u s e d to determine D M S P d concentrations, w h i l e D M S P p concentrations are best c a l c u l a t e d f r o m total D M S P ( D M S P t = D M S P d + D M S P p ) d e t e r m i n e d f r o m w h o l e seawater samples  [Kiene and Slezak,  2006].  A l t h o u g h the s e n s i t i v i t y o f the M I M S is sufficient to measure ambient D M S P t concentrations i n most regions o f the w o r l d ' s oceans, an unconcentrated 3.5 m l D M S P d s a m p l e c o u l d not be measured w i t h either the s m a l l or large-area m e m b r a n e inlets w i t h the current M I M S c o n f i g u r a t i o n g i v e n that the g l o b a l average D M S P d c o n c e n t r a t i o n is estimated at < 3 n M [Kiene  and Slezak,  2006].  4.3 Successes a n d P i t f a l l s D u r i n g the L i n e P surveys w e w e r e able to achieve o u r o r i g i n a l g o a l o f adapting M I M S for oceanic D M S P p me as u re me n ts , but the data m a y h a v e u n d e r e s t i m a t e d the true D M S P p  95  concentrations i n this r e g i o n because o f the f i l t r a t i o n m e t h o d u s e d . H o w e v e r , the filtrationi n d u c e d artifact is l i k e l y not as severe for D M S P p , w h i c h is present at 10-100-fold higher concentrations than D M S P d [Kiene and Slezak, 2 0 0 6 ] . F u r t h e r m o r e , the m a j o r i t y o f investigators have c o l l e c t e d D M S P p s a m p l e s i n m u c h the same w a y i n the past, w i t h m a n y u s i n g m o r e d a m a g i n g v a c u u m filtration [i.e. Ledyard and Dacey, 1 9 9 6 ; Matrai and filtering up to 1 L v o l u m e s o f seawater [Dacey et al,  Vernet, 1997] and others  1998]. T h e r e f o r e , a l t h o u g h the true D M S P p  concentrations i n the N E P a c i f i c m a y i n d e e d be s l i g h t l y h i g h e r than w e m e a s u r e d , our data are still u s e f u l for c o m p a r i s o n w i t h other studies i n other regions o f the w o r l d ' s oceans. In 2 0 0 3 , w e used l o w v a c u u m f i l t r a t i o n , whereas i n subsequent years w e s w i t c h e d to the m o r e gentle m e t h o d o f gravity  filtration.  W e w o u l d thus expect that 2003 D M S P p concentrations w e r e p o t e n t i a l l y  m o r e underestimated than i n later years. T h i s i m p l i e s that the true d e c l i n e i n D M S P p levels a l o n g L i n e P be tw e e n 2 0 0 3 and 2 0 0 4 m a y be e v e n larger than o u r results suggest (see S e c t i o n 2.3). Perhaps the biggest l i m i t a t i o n o f the L i n e P D M S P p s u r v e y lies i n the p h y t o p l a n k t o n data. T h e m i c r o s c o p i c m e t h o d u s e d to count i n d i v i d u a l c e l l s appeared to underestimate the true autotrophic b i o m a s s as evident from the resultant l o w C x h l ratios and the h i g h estimates o f c e l l u l a r D M S P quotas (see S e c t i o n 2.4). T h i s l i k e l y h a m p e r e d attempts to relate D M S P p levels to the b i o m a s s o f s p e c i f i c p h y t o p l a n k t o n groups i n 2 0 0 3 . T h e biggest s h o r t f a l l o f the 3-year dataset h o w e v e r , is the l a c k o f s u p p o r t i n g p h y t o p l a n k t o n data for 2 0 0 4 and 2 0 0 5 . A s a result, w e were unable to d i s t i n g u i s h b e t w e e n the roles o f p h y t o p l a n k t o n t a x o n o m y and p h y s i o l o g y i n d r i v i n g cross-station or i n t e r a n n u a l D M S P p v a r i a b i l i t y . N o n e t h e l e s s , o u r study p r o v i d e d the first D M S P p measurements m a d e i n this i mp o rta nt and well-studied H N L C r e g i o n that p r o v i d e a reference p o i n t for future measurements i n this area.  96  T h e Q u e e n C h a r l o t t e S o u n d survey o n the other hand w a s a r e s o u n d i n g success and w e w e r e able to meet o r e x c e e d a l l o f o u r research goals. W e w e r e able to s h o w that D M S levels e x h i b i t e d r a p i d , large fluctuations o v e r spatial scales that c o u l d not be r e s o l v e d w i t h traditional G C methods. M o r e i m p o r t a n t l y , w e w e r e able to c o n v i n c i n g l y relate surface D M S levels to the ratio o f c h l o r o p h y l l / m i x e d layer depth, s o m e t h i n g that h a d p r e v i o u s l y not been a c h i e v e d i n h i g h p r o d u c t i v i t y , coastal r e g i o n s . F u r t h e r m o r e , w e obtained interesting results o n the strong covariance o f c h l o r o p h y l l concentrations, 0 2 / A r levels and P C O 2 l e v e l s . T h e s e data h i g h l i g h t e d the p o w e r o f M I M S for m e a s u r i n g m u l t i p l e gases at high-resolution s i m u l t a n e o u s l y , and thereby p r o v i d i n g v a l u a b l e a n c i l l a r y i n f o r m a t i o n u s e f u l for interpreting D M S d i s t r i b u t i o n s . T h i s dataset c o u l d , h o w e v e r , have been i m p r o v e d b y c a l i b r a t i n g the O2/AJ s i g n a l to g i v e m o r e m e a n i n g f u l % ' O2 saturation values. T h i s w a s not p o s s i b l e d u r i n g this cruise due to t i m e and man-power constraints. In the future, m o r e frequent s a m p l i n g o f total D M S P w o u l d be u s e f u l to e x a m i n e its co-variance w i t h D M S . S a m p l i n g f o r total D M S P w o u l d require less effort than separating the d i s s o l v e d and particulate fractions, and w o u l d bypass some o f the issues associated w i t h these measurements as m e n t i o n e d above.  4.4 F u t u r e D i r e c t i o n s W e h a v e s h o w n that M I M S has capabilities that far e x c e e d m a n y o f those o f P T G C f o r the measurement o f o c e a n i c D M S concentrations, and have thus m o v e d c l o s e r to a c c o m p l i s h i n g the s e c o n d g o a l o u t l i n e d above: the d e v e l o p m e n t o f a d v a n c e d m e t h o d o l o g i e s that s i m p l i f y oceanic D M S measurements. T h e one area w h e r e M I M S c u r r e n t l y falls short i s i n sensitivity. D u e to a lack o f a c o n c e n t r a t i n g step for D M S , the current detection l i m i t o f - 1 n M is not sufficient to accurately measure this gas i n m a n y oceanic r e g i o n s , p a r t i c u l a r l y d u r i n g the w i n t e r  97  months. S e n s i t i v i t y c a n be i m p r o v e d i n the future b y i n c r e a s i n g the surface area o f the m e m b r a n e ; h o w e v e r , this w o u l d l i k e l y necessitate the e m p l o y m e n t o f a w a t e r trap to counteract the negative i m p a c t o f an increase i n water v a p o u r i n the v a c u u m o f the mass spectrometer. A s noted above, the M I M S system i n its present c o n f i g u r a t i o n is not u s e f u l f o r d i s s o l v e d D M S P measurements, and thus w i l l not replace P T G C for s m a l l v o l u m e , h i g h s e n s i t i v i t y applications. In order to p r e d i c t the i m p a c t o f D M S e m i s s i o n s o n future c l i m a t e and the i m p a c t o f a c h a n g i n g c l i m a t e o n the s u l f u r c y c l e , w e need to achieve the p r i m a r y g o a l as stated above: a mechanistic u n d e r s t a n d i n g o f the u n d e r l y i n g processes that d r i v e the p r o d u c t i o n and c o n s u m p t i o n o f D M S P , D M S and related c o m p o u n d s . R e c e n t progress i n this area has been steady, w i t h "S tracer studies m a p p i n g the fate o f D M S P a s s i m i l a t i o n and destruction pathways 35  [Kiene and Linn, 2 0 0 0 ; Vila et al, al,  2004] and p r o v i d i n g estimates o f s u l f u r c y c l i n g rates [Toole et  2 0 0 4 ] . Future a p p l i c a t i o n s o f M I M S s h o u l d a i m to c o u p l e its c o n t i n u o u s s a m p l i n g  capabilities w i t h d e t a i l e d process studies. F o r e x a m p l e , rates o f m i c r o b i a l D M S / D M S P decay c o u l d be m o n i t o r e d i n real-time w i t h o u t the need for r a d i o i s o t o p e s or destructive, interval s a m p l i n g . F u r t h e r m o r e , c a p i t a l i z i n g o n the real-time nature o f the M I M S output i n the  field  w o u l d a l l o w for i m m e d i a t e , targeted s a m p l i n g of a n c i l l a r y parameters at D M S " f r o n t s " such as those observed d u r i n g the Q u e e n C h a r l o t t e S o u n d survey (see S e c t i o n 3.3). S u c h studies w i l l further enhance o u r u n d e r s t a n d i n g o f the intricacies o f the m a r i n e s u l f u r c y c l e and lead us closer to p r o v i n g or r e f u t i n g the C L A W hypothesis.  98  4.5 References D a c e y , J . W . H . , F . A . H o w s e , A . F . M i c h a e l s , and S . G . W a k e h a m ( 1 9 9 8 ) , T e m p o r a l v a r i a b i l i t y o f d i m e t h y l s u l f i d e and d i m e t h y l s u l f o n i o p r o p i o n a t e i n the Sargasso Sea, Deep-Sea Res. I 45, 2085-2104. G u n s o n , J.R., S . A . S p a l l , T . R . A n d e r s o n , A . Jones, I.J. T o t t e r d e l l , a n d M . J . W o o d a g e (2006), C l i m a t e s e n s i t i v i t y to ocean d i m e t h y l s u l p h i d e e m i s s i o n s , Geophys. Res. Lett., 33, L07701,doi:10.1029/2005GL024982. K i e n e , R.P., a n d L.J. L i n n (2000), D i s t r i b u t i o n and turnover o f d i s s o l v e d D M S P and its r e l a t i o n s h i p w i t h bacterial p r o d u c t i o n i n the G u l f o f M e x i c o , 861.  Limnol. Oceanogr., 45, 849-  K i e n e , R.P. and D . S l e z a k (2006), L o w d i s s o l v e d D M S P concentrations i n seawater revealed b y s m a l l - v o l u m e g r a v i t y f i l t r a t i o n and d i a l y s i s s a m p l i n g , Limnol. Oceanogr.: Methods, 80-95.  4,  L e d y a r d , K . M . and J . W . H . D a c e y (1996), M i c r o b i a l c y c l i n g o f D M S P and D M S i n coastal and o l i g o t r o p h i c seawater, Limnol. Oceanogr., 41, 33-40. M a t r a i , P. A . and M . V e r n e t (1997), D y n a m i c s o f the v e r n a l b l o o m i n the m a r g i n a l ice zone o f the Barents Sea: d i m e t h y l s u l f i d e and d i m e t h y l s u l f o n i o p r o p i o n a t e budgets, J. Geophys. Res., 102, 2 2 9 6 5 - 2 2 9 7 9 . T o o l e , D . A . , D.J. K i e b e r , R.P. K i e n e , E . M . W h i t e , J . B i s g r o v e , D . A . d e l V a l l e , and D. S l e z a k ( 2 0 0 4 ) , H i g h d i m e t h y l s u l f i d e p h o t o l y s i s rates i n nitrate-rich A n t a r c t i c waters, Geophys. Res. Lett., 31, L I 1307, d o i : 1 0 . 1 0 2 9 / 2 0 0 4 G L 0 1 9 8 6 3 . V i l a , M . , R. S i m o , R.P. K i e n e , J . P i n h a s s i , J . A . G o n z a l e z , M . A . M o r a n , and C . P e d r o s - A l i o (2004), U s e o f m i c r o a u t o r a d i o g r a p h y c o m b i n e d w i t h f l u o r e s c e n c e i n situ h y b r i d i z a t i o n to determine d i m e t h y l s u l f o n i o p r o p i o n a t e i n c o r p o r a t i o n b y m a r i n e b a c t e r i o p l a n k t o n taxa, Appl. Environ. Microbiol., 70 (8), 4648-4657. W a t s o n , A . J . and P.S. L i s s (1998), M a r i n e b i o l o g i c a l controls o n c l i m a t e v i a the carbon and sulfur geochemical cycles, 51.  Phil. Trans. Roy. Soc. Lon. Ser. B-Biol. Sci 353, (1365), 41-  99  

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