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Interannual and interdecadal patterns in timing and abundance of phytoplankton and zooplankton in the… Bornhold, Elizabeth Anne 2000

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INTERANNUAL AND INTERDECADAL PATTERNS IN TIMING AND ABUNDANCE OF PHYTOPLANKTON AND ZOOPLANKTON IN THE CENTRAL STRAIT OF GEORGIA, BC: WITH SPECIAL REFERENCE TO NEOCALANUSPLUMCHRUS by Elizabeth Anne Bornhold B.Sc . M c G i l l University, 1996 A THESIS S U B M I T T E D I N P A R T I A L F U L F I L M E N T O F T H E R E Q U I R E M E N T S F O R T H E D E G R E E O F M A S T E R O F S C I E N C E in T H E F A C U L T Y O F G R A D U A T E S T U D I E S D E P A R T M E N T O F E A R T H A N D O C E A N S C I E N C E S We accept this thesis as conforming to the required standard T H E U N I V E R S I T Y O F B R I T I S H C O L U M B I A F E B R U A R Y 2000 © Elizabeth A . Bornhold, 2000 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of hn^rrt* <3c*™ Sr-ie^coA The University of British Columbia Vancouver, Canada Date DE-6 (2/88) ii Abstract Aspects o f the biological oceanography of the Strait o f Georgia have been studied periodically since the 1930s, although very few studies have measured nutrient concentrations, the timing and magnitude of the spring phytoplankton bloom, and the timing and magnitude o f the spring zooplankton peak biomass. This thesis reports nutrient concentrations and phytoplankton and zooplankton biomass during spring of 1996-1998 in the central Strait o f Georgia. Sampling was conducted biweekly from February to June, for nitrate, phosphate, ammonium, and urea concentrations, chlorophyll concentrations, zooplankton dry weight, and Neocalanus plumchrus stage composition. Nitrate and phosphate concentrations were high in late winter but began to decline in late March reaching a minimum in June. Ammonium and urea concentrations were low in late winter and increased in late March reaching a maximum in A p r i l . Concentrations then declined and reached a minimum in June. Two large chlorophyll peaks were observed in the Strait in March and June, bracketing the large peak in zooplankton biomass that occurred in late Apr i l . These results were then compared with results from previous studies in an effort to determine i f the timing and the magnitude o f spring bloom are controlling the timing and magnitude o f the zooplankton peak biomass. Since the mid-1960s there have been nine years where sampling took place in the spring for both chlorophyll and zooplankton biomass. These data revealed in years when there was tight coupling between phytoplankton and zooplankton, zooplankton biomass was not always greater than years with poor coupling. This suggests that this relationship is more complex and that there are other factors that determine the magnitude of the spring peak in zooplankton biomass. Ill The observed peak in zooplankton biomass in the 1990s occurred approximately one month earlier than had previously been reported for the Strait in the 1960s and 1970s. Because N. plumchrus is the dominant zooplankton species in these waters at this time, it was proposed that this shift in timing was due to a change in the developmental timing of this copepod. When historical N. plumchrus stage composition data were collected it was determined that there has been a shift in developmental timing on the order o f 25 days. Three hypotheses were proposed to explain this change. Presently, the favoured hypothesis is that a gradual warming o f the surface layer in the Strait over the past 30 years has induced this change, possibly due to a shortening of each copepodite developmental stage. IV Table of Contents Abstract i i List of Tables v i List of Figures v i i Acknowledgements ix General Introduction Study Area 1 The ecological importance of the Strait o f Georgia 3 Physiological, feeding, and life history characteristics o f the calanoid copepod Neocalanus plumchrus 8 Study Objectives 12 1. Timing and abundance of nutrients, phytoplankton, and zooplankton in the Strait of Georgia, BC in spring and early summer 1.1 Introduction 14 1.2 Materials and Methods 15 1.2.1. Nutrients 16 1.2.2. Chlorophyll-a analysis 16 1.2.3. Zooplankton sampling 16 1.2.4. Physical data 19 1.2.5. Selection of sampling stations 19 1.3 Results 20 1.4 Discussion 44 2. Interannual variation in the coupling of the spring bloom and zooplankton biomass in the Strait of Georgia, BC 2.1 Introduction 48 2.1.1 Controls on timing and abundance of phytoplankton and zooplankton biomass in the Strait o f Georgia 51 2.2 Materials and Methods 52 V 2.3 Results and Discussion 2.3.1 Interannual variation in biological coupling of phytoplankton and zooplankton 53 2.3.2 Interdecadal variation in phytoplankton and zooplankton timing 60 2.3.3 Conclusions 63 3. Interdecadal variation in the timing and development of the copepod Neocalanusplumchrus in the Strait of Georgia, BC. 3.1 Introduction 65 3.2 Materials and Methods 71 3.2.1 Zooplankton data: sources and sampling methods 71 3.2.2 Estimation of developmental timing ' 7 2 3.2.3 Environmental data ' 74 3.3 Results 75 3.3.1 Interdecadal shifts in N. plumchrus stage composition 75 3.3.2 Changes in environmental parameters i 82 ...... 3.4 Discussion 82 . 3.4.1 Possible environmental controls for developmental timing 82 3.4.2 Proposed hypotheses 87 3.4.3 Potential impact on predator populations 88 3.5 Conclusions 90 General Discussion and Future Research 91 Literature Cited 94 Appendices Appendix A 101 Appendix B 111 Appendix C 112 Appendix D 115 Appendix E 116 List of Tables Table 2.1 Summary of match-mismatch data, and date, magnitude and duration o f peak phytoplankton and zooplankton biomass 59 Table 3.1 Stage durations o f naupliar and copepodite stages o f N. plumchrus 70 Table 3.2 Differences in life cycle timing o f N. plumchrus among oceanographic regions 84 List of Figures Figure 1. Map of the Strait o f Georgia 4 Figure 2. Seasonal changes in biomass of dominant zooplankton species in the Strait o f Georgia 7 Figure 3. Life history diagram of the calanoid copepod N. plumchrus in the Strait o f Georgia, B C 11 Figure 1.1 Depth integrated chlorophyll-a for all three stations, 1996 25 Figure 1.2 Vertical profiles of chlorophyll- a for all 3 stations, 1996 26 Figure 1.3 Vertical profiles o f nitrate for all 3 stations, 1996 27 Figure 1.4 Vertical profiles of phosphate for all 3 stations, 1996 28 Figure 1.5 Vertical profiles o f ammonium for all 3 stations, 1996 29 Figure 1.6 Vertical profiles o f urea for all 3 stations, 1996 30 Figure 1.7 Size fractionated zooplankton dry weights, 1996 31 Figure 1.8 Depth integrated chlorophyll-a for all three stations, 1996 ^ 32 Figure 1.9 Vertical profiles o f chlorophyll- a for all 3 stations, 1997 33 Figure 1.10 Vertical profiles o f nitrate for all 3 stations, 1997 34 Figure 1.11 Vertical profiles o f phosphate for all 3 stations, 1997 c 35 Figure 1.12 Vertical profiles o f ammonium for all 3 stations, 1997 < 36 Figure 1.13 Vertical profiles o f urea for all 3 stations, 1997 37 Figure 1.14 Size fractionated zooplankton dry weights, 1997 38 Figure 1.15 Depth integrated chlorophyll-a for all three stations, 1998 39 Figure 1.16 Vertical profiles o f chlorophyll- a for all 3 stations, 1998 40 Figure 1.17 Size fractionated zooplankton dry weights, 1998 41 Figure 1.18 Time series o f chlorophyll concentration from February to June, 1996-1998 42 Figure 1.19 Time series o f zooplankton dry weight from February to June, 1996-1998 43 Figure 2.1 Schematic representation of the match-mismatch hypothesis, as it applies to the Strait o f Georgia, B C 50 Figure 2.2 Timing of the spring phytoplankton biomass and spring zooplankton biomass in the Strait o f Georgia, B C , 966-68, 1975-77, 1996-98 58 Figure 2.3 Mean monthly chlorophyll-a and zooplankton biomass estimates viii 1966-68, 1975-77, 1996-98, Strait o f Georgia, B C 62 Figure 3.1 Comparison of the life history timing of iV. plumchrus between the open ocean 69 Figure 3.2 Modelled stage composition and biomass vs. date 73 Figure 3.3 Interannual comparison o f TY. plumchrus stage composition versus date, Strait o f Georgia, B C 76 Figure 3.4 Regression of date of maximum biomass and year estimated from both biomass data and from stage composition data 77 Figure 3.5 . 0-5 m temperature anomalies for the Strait o f Georgia, 1968-1998 78 Figure 3.6 150 m temperature anomalies for the Strait o f Georgia, 1968-1998 79 Figure 3.7 >350 m temperature anomalies for the Strait o f Georgia, 1968-1998 80 Figure 3.8 N. plumchrus developmental timing versus anomalies o f March to M a y SST in the Strait o f Georgia 8 1 . Figure 3.9 Date o f maximum zooplankton biomass at OSP from 1954-1998 85 Figure 3.10 mterannual comparison o f N. plumchrus stage composition versus date at OSP 86 ix Acknowledgements I am indebted to Dr. Paul J. Harrison for his encouragement and financial support throughout my studies at U B C . His patience was most appreciated, especially while I was writing. I thank all o f the students and members of the Paul J. Harrison Lab and A l Lewis Lab for their help, support and constructive criticism. I especially thank Melissa Evanson, Martha Haro-Gray, and Brad Beaith for going out sampling with me regardless o f the weather, and for helping me process all o f the samples. Thanks go to Kedong Y i n as wel l for processing all o f the nutrient samples. A l l o f the 1996 sampling was conducted by Deb Mugg l i and for that I am very grateful. I extend my most sincere thanks to Ray Scarsbrook, the skipper o f the Clupea, who not only drove the boat but who also helped me with all o f my sampling, and to Hugh McLean who regularly helped with sampling and equipment upkeep. Special thanks go to Dr. R. Beamish, from the Department o f Fisheries and Oceans, who provided the funding for this project. In addition, he secured the use o f the Clupea and ensured that a skipper was always available. This project would really not have been possible without his help. Dave Mackas spent many hours helping develop ideas and offering very constructive criticism and took the time to read my drafts arid for this I am very grateful. I also thank my supervisors, Paul Harrison, A l Lewis and John Dower for their support and for taking the time to read numerous drafts. I thank all o f the officers and crew o f the CCGSs JP Tully, Vector and R.B. Young, their help ! and patience was most appreciated. I thank my father, Brian Bornhold, for reading numerous initial drafts of my thesis and of papers, and for his insight and introduction into the world of science. Finally, I extend my deepest gratitude to Michael Meredith who supported me throughout my entire degree and who always held faith in my abilities. This emotional support was invaluable and w i l l always be appreciated. General Introduction Semi-enclosed, coastal waters are some o f the most productive, and ecologically and economically important areas on Earth. They are enriched by nutrients from land drainage, precipitation, and recycling; they are efficient at retaining these nutrients via water circulation processes. The food webs in these areas are complex and, with few exceptions, poorly understood. Recent studies have shown that significant oceanographic changes are occurring in these coastal waters which are leading to decreases in both diversity and abundance of ecologically and commercially important species (Levy et al. 1996). The purpose of this study was to investigate phytoplankton-zooplankton interactions during spring in the Strait o f Georgia, British Columbia, the most important semi-enclosed coastal marine area in western Canada. Study A r e a The Strait o f Georgia is a partially enclosed basin located between mainland British Columbia and Vancouver Island. The physical oceanography of the Strait has been extensively reviewed by Tul ly and Dodimead (1957), Waldichuck (1957), Thomson (1981) and LeBlond (1983). Therefore I w i l l only mention those characteristics that are important to the biological oceanography of this region. The Strait is approximately 222 k m long, 20-40 km wide, and covers an area o f 6800 km , while occupying a volume o f 1050 m . The mean depth of the Strait is 155 m, and the maximum depth reaches 420 m. The mean sea surface temperature in summer is 15°C, and in winter 7°C. The average salinity is 31. 2 Circulation in the Strait is controlled by three main factors: 1) water flow through the shallow and narrow passages at the north and south ends of the Strait; 2) Fraser River runoff; and, 3) wind. Water exchange with the outside waters occurs through Johnstone Strait in the north and through Haro Strait and Rosario Strait in the south. Because Johnstone Strait is so narrow and shallow, most of the exchange occurs through the southern Straits. Haro and Rosario Straits, are much wider than Johnstone Strait, however they are also quite shallow, preventing deep-water exchange. Semi-diurnal tidal pulses also pass through these, channels partially driving circulation in the Strait. Fraser River runoff significantly dilutes surface waters in the southern portion o f the Strait throughout the year as it accounts for more than 80% of the freshwater runoff with a mean annual flow of3630 m 3 s"1. Peak discharge is >7000 m 3 s*1 in June (Canada 1991). This dilution influences surface currents in the southern portion o f the Strait, as wel l as contributing to the stability of the Strait. The wind blows mainly from the 'southeast most of the winter and from the northwest in summer. Overall, circulation in the Strait is generally counter-clockwise, with the strongest currents along the mainland coast (Thomson 1981). The stability o f the Strait varies with location and season. The northern portion o f the Strait is stratified in summer due to heating o f the surface water, and is wel l mixed in winter due to wind mixing. The stability of the southern Strait is influenced primarily by the Fraser River input. In June, when the discharge is highest, a large freshwater plume extends into the Strait creating more stable conditions (LeBlond 1983). The Strait is often divided into three zones: the Northern, the Central and the Southern Strait (Fig. 1) (Watson 1998). A l l o f the sampling for this study was conducted along an East-West transect in the Central Strait, hereafter referred to as the Strait (Fig. 1). The ecological importance of the Strait of Georgia The Strait o f Georgia marine ecosystem is one o f the most important salmon producing areas in the Pacific Ocean (Levy et al. 1996). The Fraser River is the largest river entering the Strait and many o f the salmon spawn within the confines o f its drainage basin. The Strait then serves as a valuable nursery and rearing ground for these ecologically and commercially important Pacific salmon species. Ecologically these species are important,: as they are part of the food web in freshwaters^ marine coastal waters, and deep-sea waters during the various stages o f their life cycle. In each o f these environments the salmon are important predators, as wel l as . important prey species. Economically, Pacific salmon contribute significantly to the major commercial and sport fisheries in the northeast Pacific Ocean. The past several decades have shown a decrease in the stocks o f .these valuable species (Beamish and Boui l lon 1993) and therefore efforts have been made to try to understand the causes of their decline. The following have been.proposed as responsible for the decline: 1) over exploitation, 2) climate change, 3) habitat degradation, and 4) changes in food availability (Beamish and Boui l lon 1993; Hinch et al. 1995; Levy et al. 1996). The focus o f this study is on the lower trophic level interactions, in an attempt to gain a further understanding o f the food available for higher trophic levels such as juvenile salmon, herring, and hake. Although the Strait has been studied extensively since the early 1900s, much o f the biological oceanography of the Strait remains poorly understood. The focus o f many earlier studies, such as those conducted by Campbell (1929; 1934), Stephens et al. (1969) and Parsons et al. (1969 a; 1969 b), was on the identification o f species present in surface waters, and on the enumeration of individual species present there. There has been little historical research on trophic level interactions in the Strait. Figure 1.1 Map of the Strait of Georgia, BC with station locations (l-3)and geographical areas. The biological oceanography o f the Strait has been extensively reviewed by Harrison et al. (1983). I w i l l only mention, as background, those facts that are relevant to the present study. Generally there are two large seasonal phytoplankton blooms in this region. The first, referred to as the spring bloom, occurs in March or early A p r i l , and the second occurs in late June or July (Stockner et al. 1979). The phytoplankton of the Strait o f Georgia are typical o f cold, temperate coastal waters (Harrison et al. 1983). During the spring, the community is composed primarily o f large chain-forming diatoms, including Skeletonema costatum, Thalassiosira spp., and Chaetoceros spp. (Legare 1957; Stockner et al. 1979; Harrison et al. 1983). B y early summer, the blooms are composed primarily of Chaetoceros spp. and dinoflagellates (Parsons et al. 1970; Stockner et al. 1979). These two peaks in phytoplankton biomass bracket the zooplankton biomass maximum. The timing and magnitude of these blooms is variable. A balance between "bottom-up" control by nutrient limitation and "top-down" control by grazing, controls the magnitude o f these phytoplankton blooms. The timing o f the onset o f the spring bloom is controlled by highly variable parameters such as light, nutrients, temperature, wind mixing, and grazing. In the winter when wind mixing or turbulence is intense and irradiance is low, phytoplankton growth is very slow. Some degree of mixing is required to bring nutrients to the surface, however i f this mixing is too great, phytoplankton are transported below the euphotic zone, inhibiting production. In the spring when solar heating increases, the water column becomes more stable, or stratified trapping phytoplankton in the nutrient rich surface waters. Phytoplankton growth w i l l initially be high, however growth may become limited i f highly stable conditions persist, due to the depletion of nutrients from these surface waters. O f the marine zooplankton the taxa Copepoda is by far the most ubiquitous, often comprising at least 70% of the zooplankton biomass (Raymont 1983; Parsons et al. 1984 a; Shih 1986). The dominant zooplankton in the Strait o f Georgia include three copepod species Neocalanus plumchrus, Pseudocalanus minutus, Calanus marshallae, and one euphausiid species Euphausia pacifica (Fig. 2) (Harrison et al. 1983). During the spring, the dominant zooplankter in surface waters is the calanoid copepod Neocalanus plumchrus. Its biomass accounts for more than 75% of the zooplankton biomass at this time (LeBrasseur et al, 1969; Stephens et al. 1969; Parsons et al: 1969 b; Fulton 1973; Harrison et alS 1983); A detailed description o f this zooplankton species is given below. .• . • , The pacific salmon species that are o f great importance in B C waters include .Oncorhynchus kisutch (coho), Oncorhynchus tshawytscha (chinook), Oncorhynchus nerka (sockeye), Oncorhynchus gorbuscha (pink) and Oncorhynchus keta (chum):; In this thesis they w i l l be referred to by their common names. They enter the Strait in the spring, generally between March and May, as juveniles and spend anywhere from several weeks (all five species) to their entire marine life stage (coho and chinook) in the Strait. During their time in the Strait, copepods make up a significant portion o f the diets o f juvenile coho, sockeye, and chum (Healey 1980). 7 2 T co Winter Spring Summer Fall Figure 2. Seasonal changes in biomass o f dominant zooplankton species inhabiting the upper 20-50 m of the water column in the Strait o f Georgia, (redrawn from Harrison etal. 1983) Physiological, feeding, and life history characteristics of the calanoid copepod Neocalanus plumchrus (Marukawa) Calanoid copepods range in size from several hundred micrometers to tens o f millimetres. Their bodies are divided into two parts, the fore-body or prosome and the much narrower hind-body or urosome. The prosome consists of a head fused to the first of several thoracic segments. Each thoracic segment has a pair of swimming legs. The major body articulation lies behind the 5 t h pair of swimming legs, and separates the prosome from the urosome. The urosome is also divided into segments; the last of which has caudal rami. Calanoids have long antennae which help maintain buoyancy. The number o f segments in the urosomes, the length o f the caudal rami, and the length of the antennae vary with species (Dudley 1986). ; Adult N. plumchrus are large and range in size from 4.1 to 5.5 mm (Mil ler 1988). They have five thoracic and four urosome segments. The caudal rami are long and plumose. Their antennae are the same length as their total body length (excluding setae o f the caudal rami) (Marshall and Orr 1972; Gardner and Szabo 1982; M i l l e r 1988). l Calanoid copepods feed either raptorially or by filter-feeding. Raptorial feeding involves active hunting and capture o f prey; filter-feeding is passive. In the Strait o f Georgia N. plumchrusis a filter feeder which feeds primarily on diatoms. A t Station P, N. plumchrus is carnivorous and feeds primarily on protists and other microzooplankton (Frost et al. 1983; Wen 1995; Mackas et al. 1998). Its maxillae and maxillipeds are structured to facilitate the flow of water past its mouthparts. The maxillary setae are long and finely spaced to maximise the trapping o f particulate matter. The different feeding characteristics suggest that these zooplankton are grazing on an abundant food source that is within the size fraction that they are able to filter and consume, regardless of food type. 9 The life cycle of TV. plumchrus is similar to that o f many o f its cogeners (N. flemingeri, TV cristatus, and TV. tonsus). It is an ontogenetic migrator with a strong seasonal vertical migration closely linked to its developmental and reproductive cycle, (Fulton 1973; Mi l l e r et al. 1984; M i l l e r and demons 1988; Mackas et al. 1998). Copepod development generally includes an embryonic stage, a series o f six naupliar stages (N1-N6), and a series of six copepodite stages (C1-C6), where C6 is the final, sexually mature, adult stage. Copepod life histories are all extremely variable among species or between geographical locations. However, throughout their oceanographic range, the developmental cycle o f TV. plumchrus fits a seasonal pattern (Mil ler and Terazaki 1989). The TV. plumchrus population spends the late summer through autumn at depths ranging from 250-1000 m, as C5 copepodites in a non-feeding, reduced metabolic state1. Between autumn and early winter, they molt into a non-feeding adult, mate and spawn, and remain at depth until they die. The young migrate up to the surface as non-feeding nauplii, and reach the surface as a last stage nauplius (N6) or as a first stage copepodite ( C l ) . A t the surface they molt from the first to the fifth copepodite stage (C1-C5) while feeding. They remain in surface waters until they are ready to migrate down to their overwintering depth (250-1000 m). In the Strait o f Georgia, TV. plumchrus adults overwinter at depths greater than 300 m. Spawning begins in early January at depths of 250-400 m. Females are capable o f laying multiple broods, and continue to lay eggs until early February. B y late January all the males have died; spent females die shortly after laying their eggs. Once eggs are released, laboratory studies have shown that they have neutral or slightly negative buoyancy, and quickly hatch into the first naupliar stage ( N l ) (Evanson et al. 1999; Beaith 1999; Saito and Tsuda pers. comm.). It is 1 In this document the terms "non-feeding, reduced metabolic state" and "diapause" w i l l be used interchangeably when referring to the calanoid copepod N. plumchrus (French 1988). 10 extremely difficult to differentiate the naupliar stages o f N. plumchrus (Mil ler and Clemons 1988). Therefore it is not known exactly how many days are spent in each naupliar stage; however, they undergo a series of molts (N1-N6), while migrating upwards to the surface. This upward migration has been reported to take on the order o f 30 days (Fulton 1973). While at the surface they molt from C l to C5 , feed and accumulate lipids. Their diet includes large diatoms, and as C4s and C5s they are capable o f exerting significant grazing pressure on the spring phytoplankton bloom (Parsons et al. 1969b). During feeding, they lay. down lipid reserves to both sustain themselves during diapause and to reproduce (Falk-Petersen etal. 1987;Evanson et al. 1999). The duration o f molting through the five copepodite stages ( C l to prermigrant C5) takes approximately 70-100 days. It is during this C5 stage that N . plumchrus. begins its downward migration to their overwintering depth; the trigger for this /descent is not known, although it is possible that it is related to the retention o f lipids during feeding. This downward migration occurs in late spring or early summer. Once they have completed their downward migration to the overwintering depth, they enter a stage which has ,been called diapause, remaining at depth as C5s, until late fall when they molt to the adult (diapause) stage (Fig. 3). r WINTER SPRING SUMMER FALL Figure 3. Life history diagram of the calanoid copepod N. plumchrus in the Strait of Georgia, B C (reprinted from Mackas et al. 1998) 12 Study Objectives While the biological oceanography of the Strait o f Georgia has been studied previously, most sampling occurred either over a relatively short period o f time (Mackas et al. 1980; Harrison et al. 1991; Y i n et al. 1996; Y i n et al. 1997), or on a monthly basis (Legare 1957; Stephens et al. 1969; Parsons et al. 1969 a; Stockner et al. 1979). Both types of sampling are not sufficient to resolve the dynamics o f phytoplankton - zooplankton coupling. Therefore the primary goal o f this thesis was to investigate the phytoplankton - zooplankton (mainly N. plumchrus) interactions in the Strait o f Georgia on a bi-monthly basis throughout the spring. In order to meet this primary objective the study was divided into three sub-objectives. The first objective (Chapter 1) was to estimate and report the current biological oceanographic conditions in the Strait o f Georgia (1996-1998). In the 1960s Parsons et al. (1969 a; 1969 b) measured both phytoplankton and zooplankton biomass during the spring bloom; since then only one other group, Stockner et al. (1979) has measured both o f these parameters throughout the spring and early summer. The goal o f this chapter was to document the nutrient concentrations, phytoplankton biomass and species, and zooplankton biomass and species during this period. The second objective (Chapter 2) was to examine the interannual and interdecadal variability in the biological coupling o f the spring phytoplankton bloom and peak zooplankton biomass in the Strait. Various studies o f phytoplankton and zooplankton have taken place in the Strait since the mid-1960s, however these data have not been compiled and examined for interannual or interdecadal trends. The goal was to compile these data and to determine in which years there was tight or weak coupling between the spring phytoplankton bloom and peak zooplankton biomass. 13 The third objective (Chapter 3) was to investigate the interannual and interdecadal variations in the developmental timing of the copepod N. plumchrus in the Strait. Earlier developmental timing of this copepod was reported by Mackas et al. (1998) for the northeast subarctic Pacific Ocean. The goal o f this study was to determine whether earlier developmental timing of this copepod was also occurring in coastal waters o f British Columbia. 14 Chapter 1 Timing and Abundance of Nutrients, Phytoplankton, and Zooplankton in the Strait of Georgia, BC in Spring and Early Summer 1.1 Introduction The sampling associated with previous studies in the Strait was either conducted intensively over a short period of time, or was conducted less frequently over a longer period. In the late 1960s (1966-1968) a sampling program was in place as part o f Canada's contribution to the International Biological Program, which sampled seasonal levels o f nutrients (NO3 and PO4), primary production, and zooplankton (Stevens 1969, Parsons 1970). Samples were collected from 15 stations approximately 5 times per year. From these data the approximate time o f the spring bloom and the peak in zooplankton biomass were estimated, and this was the first research to address phytoplankton-zooplankton interactions in the Strait. Several years later Stockner et al. (1979) conducted a similar study over a three year time period (1975-1977), however the sampling frequency was even less than in the 1960s. In addition, most o f Stockner's stations were either situated in waters that would be influenced by the Fraser River plume or in sheltered inlets. He was able to provide very general estimates of the timing o f the spring phytoplankton bloom, however sampling occurred so infrequently that it is possible that the peak o f spring production was missed. There was little work done on phytoplankton-zooplankton interactions in the Strait during the early 1980s and it was not until 1988 that Y i n et al. (1997) began sampling in the Strait. Their interest was mainly in nutrients, and the effects of zooplankton grazing on the spring 15 bloom. Sampling was conducted in three short (approx. two weeks) yet intensive periods in 1988, 1992 and 1993. The earliest of these cruises was A p r i l 6, while the latest did not begin until M a y 31. Again sampling although more frequent, occurred over such a short period o f time that it is also possible that the initiation and/or the degradation o f the spring bloom was missed. Bi-monthly sampling for this study was proposed since there have been no recent studies in which the sampling program was designed to resolve the development and degradation of the spring bloom and the effects of grazing on that bloom, in addition to the associated changes in nutrient concentrations. In this study, chlorophyll was used as a proxy for phytoplankton standing stock and zooplankton dry weight was used to estimate zooplankton standing stock, since these measurements were comparable with past data. The primary goal o f this chapter was to determine the nutrient concentrations, the phytoplankton standing stock and the zooplankton biomass during spring in the Strait o f Georgia, over a three year period. 1.2 Mater ia ls and Methods Samples for nutrients, chlorophyll-a, phytoplankton species, zooplankton species, zooplankton biomass, and temperature and salinity, were collected from three stations ( D F O l , 49° 15.0' N , 123° 44.9' W ; D F O 2, 49° 19.7' N , 123° 44.1' W ; and D F 0 3 , 49° 24.2' N , 123° 37.0' W ) in the central Strait o f Georgia (Fig. 1.1). On all occasions, sampling took place between 0800 and 1300; a summary of dates, stations sampled, and samples collected is presented in Appendix A . Water samples for nutrients, and chlorophyll were collected from six pre-determined depths: 0, 5, 10, 15, 20, and 50 m. Water samples were collected using 5 L P V C 16 Nisk in bottles equipped with silicone rubber springs. Dr. D . Muggl i conducted all 1996 sampling, and used exactly the same techniques as described in this thesis. 1.2.1 Nutrients Sub-samples were drawn from all depths using an acid-washed syringe. They were filtered through combusted (460°C for 4 h) G F / F filters, which were mounted in 25 mm Mil l ipore Swinex® filter holders. The filtrate was collected in 0.1 N HC1 acid-washed 30 m l polyethylene bottles. Samples were then placed in a cooler containing ice, and stored in the freezer upon return to the laboratory where they remained until analysis. A l l nutrient concentrations were determined using a Technicon AutoAnalyser® n. Nitrate plus nitrite, ammonium, urea, and phosphate samples were analyzed following the procedures o f Wood et al. (1967), Slawyk and Maclsaac (1972), Price and Harrison (1987), and Hager et al. (1968) respectively. 1.2.2 Chlorophyll-a analysis Sub-samples were drawn from all six depths for chlorophyll-a analysis. The water samples were placed in a cooler containing ice and filtered upon return to the laboratory. Once in the laboratory, the samples were filtered onto 25 m m G F / F filters. Chlorophyll-a was extracted by placing the filters in 15 m l screw-top test tubes and adding 10 m l of 90% acetone. The test tubes were then shaken vigorously for 10 min and placed in the freezer for 18-24 h. Chlorophyll-a was determined by in vitro fluorometry using a Turner Designs (Model 10-AU) fluorometer (Parsons etal. 1984b). 1.2.3 Zooplankton 17 Vertical net hauls, from 50 m to the surface, were conducted at all three stations using a bongo net with a mouth diameter o f 0.52 m, and 202 urn mesh netting. The total volume filtered depends on the area o f the mouth, the depth sampled, and the filtration efficiency o f the net. This assumes that the wire angle is kept constant and that there is no change in water flow due to clogging o f the net. To ensure that the tow was vertical through the water column, the nets were ballasted with a 25 kg weight. In order to determine the filtration efficiency of the net, the net was hauled vertically to the surface from 200 m with a calibrated flow meter placed halfway between the r im and the centre of the net opening. The flow meter revolutions were compared to identical hauls of the flow meter and the net frame without the nets attached. A filtration efficiency of 95% was calculated for this particular set o f nets during spring conditions. Other studies, such as that by Tranter and Smith (1968), calculated filtration efficiencies of >85%, for a similar net apparatus and mesh size. Using these values the volume filtered could then be estimated: Volume filtered (m3) = A depth x 7tr2 x filtration efficiency (Eqn. 1.1) Two tows, from 50 m to the surface, were completed at all three stations. A t D F O l additional tows were conducted from 200 m to the surface, and from 400 m to the surface. On the boat, the sample from each cod-end was transferred into a 1 L glass jar and was brought back to the laboratory, unpreserved and unsplit. Upon return to the laboratory, the sample was split using a Fulsom Plankton Splitter (McEwen et al. 1954). One split was preserved for species identification, and the other was frozen for size-fractionated biomass measurements. Side A of the splitter was always used for 18 size fractionated biomass, and concurrently side B was always used for species identification. The precision o f this splitter is 51% for side A , and 49% for side B . Stage identification After the original sample was split, half was poured back into the labeled 1 L glass jar. Seawater was added to the sample to increase the volume to approximately 1 L . Concentrated buffered formalin was added to the jar to yield a final concentration o f 5% formalin i f the samples were not very dense, and 10% formalin i f there was a large amount o f organic matter in the sample (Parsons et al. 1984 b). For identification, the sample was once again split until the fraction to be counted and identified contained approximately 200 large calanoid copepods. Samples were rinsed with filtered seawater and transferred to a gridded Pyrex container. Neocalanus plumchrus individuals were identified to stage following the keys in Gardner and Szabo (1982) and Mi l l e r (1988). Samples for Size-fractionated biomass The remaining half o f the sample was filtered onto Nitex netting and frozen until size fractionation could be completed. To do this, Nitex netting (202 pm mesh size) was cut into squares which fit into a P V C filtering apparatus that was especially made for this purpose. The filtering apparatus was connected to a vacuum pump. A cascade o f three sieves was used for the size fractionated biomass measurements, the mesh sizes were 1000 pm, 475 pm, and 202 pm. The sample was placed on the top of the filtration cascade and rinsed gently from the Nitex 19 mesh. After rinsing, each size-fractionated sample was transferred into a beaker and filtered onto pre-weighed Nitex filters o f mesh size 202 pm. The filters and the samples were then placed in a pre-weighed aluminium weighing boat and dried at 56°C for approximately 48 h. Once the drying was completed, the samples (with filters and boats) were reweighed. The dry weight per cubic meter was then calculated using the water volume filtered which was calculated above using Eqn. 1. 1.2.4 Physical Data Physical data were collected using vertically profiling C T D s (conductivity, temperature, depth). A n internally recording InterOcean S4 current meter that collected vertical profiles o f conductivity, temperature and depth was used when sampling was conducted from the smaller vessels (Caligus and Clupea) (see Appendix A ) . The C T D was lowered at a rate o f 1 m s"1, and readings were recorded every 2 s. When sampling was conducted from the larger vessels (CCGS Vector and CCGS Tully) (see Appendix A ) a SeaBird C T D with a transmissometer and fluorometer was used. The C T D was lowered at a rate o f 1 m s"1, and readings were recorded every 0.5 s. 1.2.5 Selection of sampling stations The three stations were selected because they are sites in the Strait that have been sampled previously. D F O l , also called G E O l by other researchers, has been the site o f numerous sampling programs since the 1960s. Legare (1957), Parsons (1969 a; 1969 b), Stephens (1969), Gardner (1972), Fulton (1973), Stockner (1979), Black (1984) and Y i n (1997) all sampled at this station. In addition, X B T (Expendable Bathy Thermographs) and C T D 20 (Conductivity Temperature Depth) casts have been routinely conducted at this station since the late 1960s by researchers in the Department of Oceanography at the University o f British Columbia and by researchers at the Nanoose Bay Underwater Weapons Test Range. Wi th a depth o f 400 m, D F O l is located in the deepest portion of the Strait, and is the site where a large N. plumchrus population overwinters. D F 0 2 is located approximately mid-way between the two other stations (Fig. 1.1), and is on Halibut Bank, another frequently sampled region o f the Strait. It is shallower than D F O l , yet deeper than D F 0 3 . D F 0 3 is close to the eastern shore o f the Strait (Fig.1.1), and is much shallower than the two other stations. Many of the above researchers conducted sampling along N - S transects within the Strait. I however wanted to examine the progression o f the spring phytoplankton bloom and the peak in zooplankton biomass along an E - W transect within the Strait. 1.3 Results The 1996 and 1998 time series o f nutrients, cholorphyll-a and zooplankton biomass are quite similar, however the 1997 results differ markedly from the other 2 years. In 1996, the integrated chlorophyll values revealed two periods of very high phytoplankton standing stock (Fig. 1.1 and F ig . 1.18). The first was in late March (March 26) and the second spanned the month of M a y (May 8-23). This relationship held true for all three stations. Prior to the first bloom, the chlorophyll concentrations were very low (5-19 mg m") and the nitrate and phosphate concentrations relatively high (25 and 2 u M respectively), representative o f winter conditions (Figs. 1.2 and 1.3). B y late March (March 26), the spring bloom had commenced and the chlorophyll-a profile was similar for all three stations. 21 Chlorophyll was low (2-5 pg l"1) in the top 5-10 m, while a deep chlorophyll maximum was present at 15-20 m (Fig. 1.4). The nitrate (NO3) and phosphate (PO4) profiles were very similar for all three stations with relatively high concentrations o f NO3 and PO4, 20-30 p M and 1.5-2.5 p M respectively (Figs. 1.2 and 1.3). The ammonium ( N H 4 ) and urea concentrations were low (<0.5 p M ) at both D F 0 2 and D F 0 3 , and only slightly higher at D F O l (Figs. 1.5 and 1.6). The nutrient data indicated that the spring bloom was in the initial stage since there was no apparent depression in nutrients at this time. Two weeks later, on A p r i l 8 t h , the bloom waned and the chlorophyll concentrations returned to pre-bloom values, 0-2 pg-L" 1. However the NO3 and PO4 concentrations remained high (10-25 and 1-2 p M respectively) throughout the water column at all three stations. The N H 4 and urea concentrations were substantially higher, 1-4 p M , than the previous month at all three stations, suggesting that it was grazing and not nutrient limitation that was responsible for the observed decrease in phytoplankton. B y late A p r i l (Apr i l 26), the chlorophyll concentrations were still low (< 2 pg L" 1 ) . The NO3 and PO4 concentrations decreased slightly while the NH4 and urea concentrations increased substantially, further supporting the fact that grazing was controlling the phytoplankton bloom at this time. B y early M a y (May 8) the phytoplankton bloom resumed with chlorophyll concentrations reaching 20 pg-L" 1. It appears that this bloom began in the eastern portion of the Strait and progressed westward. There is a higher zooplankton biomass at D F O l in early M a y which may account for the lower observed chlorophyll at this station. This suggestion of higher grazing pressure at D F O l is further supported by high concentrations of NH4 and urea at D F O l . Unl ike the earlier phytoplankton bloom, this bloom was concentrated at the surface (0-5 m). NO3 and 22 PO4 concentrations were also low at the surface, although they increased with depth, indicating that the phytoplankton assimilated these nutrients for growth. The chlorophyll concentrations remained high throughout M a y and into June, and surface nutrients were continuously depleted, although deeper nutrient concentrations remained high. In February and March the >1000 ^im zooplankton fraction was mostly present in the deep tow (400-0 m) (Fig. 1.10). However by early A p r i l (Apr i l 8), the >1000 um size fraction increased markedly in surface tows (50-0 m) and reached a maximum in late A p r i l (Apr i l 26), the time o f the lowest chlorophyll. B y early M a y (May 8) the zooplankton biomass had decreased substantially in surface waters, presumably relieving the grazing pressure on the phytoplankton, allowing another phytoplankton bloom to develop. The highest chlorophyll concentrations in M a y were at the eastern-most stations. The zooplankton data also support the notion that grazing was higher at D F O l than at the other two stations since the zooplankton biomass at D F O l was approximately 15 times greater. B y late M a y and early June, the biomass o f the smaller zooplankton had increased, consistent with the report by Harrison et al. (1983) that N. plumchrus (>1000 um size fraction) is the dominant zooplankton species in surface waters in spring, but by summer the smaller copepods, such as Pseudocalanus minutus, dominate. The 1997 observations were very different from those in 1996 and 1998 (Fig 1.18). Integrated chlorophyll values indicate that there were two large blooms in 1997. The first was observed only at D F O l in early A p r i l (Apr i l 8) and the second at all three stations in late M a y (May 22) (Fig. 1.8 and Fig . 1.18). A t this time the NO3 and PO4 concentrations were still quite high (20 and 2 u M respectively) at the surface (Figs. 1.9 and 1.10). Unl ike 1996, the majority of the 1997 phytoplankton biomass was at the surface. B y A p r i l 15 the bloom was still present at 23 D F O l . The N 0 3 and P 0 4 concentrations at D F O l and D F 0 2 were still high, although they declined slightly at the surface. The NH4 and the urea concentrations at all stations sampled were not as high as those sampled in 1996 (Figs. 1.11 and 1.12 respectively) suggesting that grazing had not commenced by mid-Apr i l . In early May , the chlorophyll concentrations were low (> 7 pg L" 1 ) at all three stations and the concentrations of NO3 and PO4 decreased (10 and 1 p M respectively) at the surface. Ammonium concentrations increased (2 p M ) indicating that grazing had commenced. B y late May, chlorophyll was higher than in early A p r i l . A s in 1996, chlorophyll was the lowest at D F O l , and NH4 and urea concentrations were the highest. Chlorophyll concentrations remained high into June and NO3 and PO4 were depleted from the surface waters. The trends in zooplankton were also very different in 1997. There was low biomass of all three zooplankton size fractions at the surface in January, however by February the biomass o f the largest size fraction increased (Fig. 1.13). The surface zooplankton biomass remained low (> 50 mg m"3) into late M a y (May 22). It was not until June that an increase irt the surface zooplankton biomass was observed. In 1997 zooplankton biomass remained substantially lower overall compared with values in 1996 and 1998. It is interesting to note that this low zooplankton biomass year occurred in the same year that the spring bloom was delayed (or missed). In 1998, sampling was less intense and chlorophyll and zooplankton samples were collected primarily at D F O l . The integrated chlorophyll concentration began to increase in mid-March and remained high (>225 mg m"2) throughout the remainder o f March (Fig 1.14 and 1.18). The chlorophyll was concentrated in the surface waters and there was no deep chlorophyll maximum during spring (Fig. 1.15). The chlorophyll concentration dropped substantially in A p r i l 24 (> 40 m g m ' 2 ) and remained low into early M a y (Fig. 1.14). B y the initiation o f the second bloom in late M a y and early June there was a deep chlorophyll maximum at 15 m at D F O l and D F 0 3 (Fig. 1.15). The zooplankton biomass in 1998 was low in January, February and mid-March (Fig 1.16 and 1.19). Unfortunately samples for March 25, 1998 were lost. Samples from March 31, 1998 contained large amounts of phytoplankton in all three size fractions substantially altering the .zooplankton biomass (probably due to the net clogging) and therefore they were not included. ,By late A p r i l , the surface zooplankton biomass was high (> 300 mg m"3). The surface zooplankton biomass remained high until early May. Coincident with this period o f high zooplankton biomass was a period, o f low phytoplankton biomass. B y late M a y the zooplankton biomass was low for all size fractions at all depths (Fig 1.17). However by early June, the zooplankton biomass in the 200-0 m tow and in the 400-0 m tow increased substantially. The greatest increase was seen in the >1000 urn size fraction suggesting that this increase was a reflection o f the downward migration of/V*. plumchrus. 25 4 0 0 f 3 0 0 + E O) E ro i s: O 2 0 0 1 0 0 0 5.0 DF01 19.3 f'. " I DF02 Feb 12,1996 17.7 DF03 § ro • IE o 4 0 0 3 0 0 2 0 0 1 0 0 0 Mar 26,1996 232.8 DF01 304.2 • DF02 259.3 DF03 4 0 0 < T 3 0 0 E o> E . 200 ro 5 100 o Apr 26,1996 19.2 ItttttittM 20.4 13.3 I IS I i 111 DF01 DF02 DF03 DF01 DF02 DF03 4 0 0 < T 3 0 0 4 E o> E 2 0 0 § 1 0 0 j 0 4 0 0 < T 3 0 0 E O) £ 2 0 0 ro • O 1 0 0 DF01 119.4 DF02 110.7 134.0 I IIIII: May 8,1996 138.7 DF03 Jun 11,1996 75.8 DF01 DF02 DF03 4 0 0 • T 3 0 0 E O) E . 2 0 0 2 g 1 0 0 279.8 4 0 0 •r 300 E o> E . 200 2 S 100 DF01 27.2 DF01 286.8 May 23,1996 i S H i i IIIII v v \ v : • 11 ±3 i 98.8 DF02 DF03 Jul 3,1996 39.4 64.7 Mil DF02 DF03 Figure 1.1 Depth integrated (0-50 m) chlorophyll-a (mg m"3) for all three stations, 1996. 27 28 30 DFOl, 50 200 DF02, 50 DF03, 50 DF01, 50 DF02, 50 DF03, 50 Figure 1.7 Size fractionated (>1000 um, 500-1000 um, and 202-500 um) mean zooplankton dry weights during 1996. Station number and vertical tow depth (eg. D F O l , 400 m). 32 400 j E 300 g 200 -1 s: 100 O 0 -DF01 DF02 DF03 400 "E 300 O) £ 200 n 2 100 o 0 Feb 10,1997 23.5 mrni i 14.5 12.5 DFOl DF02 DF03 400 j CM E 300 O) E, 200 re Z 100 o DF01 DF02 DF03 400 E 300 o> E. 200 re ± 100 o Mar 25,1997 21.2 38.2 30.8 i rnwn DF01 DF02 DF03 CM E 300 at E, 200 re • 100 o DF01 DF02 DF03 400 CM E 300 O) E. 200 re "E 100 o 0 -DF01 DF02 DF03 E 300 | a E 200 20.8 DFOl 49.1 May 2,1997 72.7 DF02 DF03 82.3 132.9 • Jun 10,1997 128.5 DF01 DF02 DF03 400 g 300 O) E. 200 ± 100 o 0 May 22,1997 3 2 0 6 113.6 DFOl DF02 244.8 DF03 400 E 300 O) E. 200 re ± 100 o 0 Jul 9,1997 162.5 N/A N/A DFOl DF02 DF03 Figure 1.8 Depth-integrated (0-50 m) chlorophyll-a (mg m"2) for all three stations during 1997. 33 35 37 38 200 E 150 at E £ 100 t 50 a >1000um 500-1000 • 200-500 Jan 21,1997 = rrn 200 DFO1.400 DFO1.200 DFO1.50 DF02, 50 DF03, 50 DF01.400 DFO1.200 DFO1.50 DFO2.50 DFO3.50 200 DF01.400 DFO1.200 DFO1.50 DFO2.50 DFO3.50 14DF01.400 April 15,1997 14DF01, 50 15DF01, 50 200 200 DFO1.200 DFO1.50 DFO1.50 DF03,50 DFO1.200 DFO1.50 DF01,400 DFO1.50 DF02,50 DF03,50 DF01.400 DFO1.200 DFO1.50 Figure 1.14 Size fractionated (>1000 pm, 500-1000 pm, and 202-500 pm) mean zooplankton dry weights during 1997. Station number and vertical tow depth (eg. D F O l , 400 m). 39 < T 4 0 0 E 300 o> E, 200 2 100 .e O 0 Jan 20,1998 28.76 N/A N/A DF01 DF02 DF03 *r* 400 DF01 DF02 DF03 c r 4 0 0 E 300 o> E. 200 2 100 O 0 ~ 400 Feb 3,1998 21.37 N/A N/A DF01 DF02 DF03 DF01 DF02 DF03 « T 4 0 0 E 300 E 200 rs • 100 0 231.13 Mar 25,1998 -N/A N/A — i DF01 DF02 DF03 — 400 E 300 E. 200 2 100 387.46 DF01 Mar 31,1998 3 7 8 g 5 333.83 • DF02 DF03 «r 4 0 0 E 300 a E. 200 2 100 O 0 Apr 22,1998 36.52 N/A N/A DF01 DF02 DF03 _ 400 E 300 o> E, 200 2 100 May 8,1998 25.29 N/A N/A ItUUUUU DF01 DF02 DF03 — 400 DF01 DF02 DF03 _ 400 CN E 300 -| o> E, 200 2 100 5 o 100.22 DF01 42.90 DF02 Jun4,1998 157.87 DF03 Figure 1.15 Depth-integrated (0-50 m) chlorophyll-a (mg m") for all three stations during 1998. 40 41 s >1000 um s 500-1000 ii 200-500 DFO1.200 Jan 20,1998 DFO1.50 200 DFO1.400 DFO1.200 321 DFO1.400 DFO1.200 DFO1.50 DFO1.400 DFO1.200 DFO1.50 DFO2.50 DFO3.50 Figure 1.17 Size fractionated (>1000 urn, 500-1000 um, and 202-500 (am) mean zooplankton dry weights during 1998. Station number and tow depth (eg. D F O l , 400 m). Figure 1.18 Time series of average (Stn 1-3) integrated (0-50 m) chlorophyll concentration (ragm" 2) from January to June 1996-1998. Error bars represent ± 1 S D , n=3 for all points with error bars, and n=l for all points without error bars. Figure 1.19 Time series of mean (Stn 1-3) surface (0-50 m) zooplankton dry weight (mg m"3) from January to June 1996-1998. Error bars represent ± 1 S D , n=3 for all points with error bars, and n=l for all points without error bars. 44 1.4 Discussion The rapid increase in phytoplankton biomass during spring has been well documented for the Strait o f Georgia and the adjacent inlets and fjords (Stephens et al. 1969; Parsons et al. 1969 b; Shim 1977; Stockner et al. 1979). This bloom is often observed in late-March or early A p r i l , and forms in response to a stabilization of the water column and an increase in nutrient availability following winter mixing (Harrison et al. 1983). The decline of the spring bloom may be in response to several factors including depletion of essential nutrients, intense mixing, advection, and grazing. In the Strait o f Georgia, from 1996-1998, the data suggest that there was the initiation o f a spring bloom in late March or early A p r i l , that the bloom was cut short in late A p r i l by grazing, and that there was a 'second bloom' , or the continuation o f the spring bloom, in early June. In the Strait, nitrate has been reported to be limiting to phytoplankton growth at concentrations less than 1 p M (Harrison etal. 1983; Shim 1977). From 1996 to 1998, pre-bloom nutrient concentrations were never low enough to limit growth, however, post-bloom nutrients did reach concentrations low enough to limit growth. Throughout the three sampling seasons concentrations of nitrate remained > 1 p M from February through A p r i l . B y early M a y however concentrations in surface waters had decreased to below 1 p M , and remained low through June. In general, the nutrient (specifically N 0 3 and P 0 4 ) concentrations for 1996-1998 were similar to those reported by Parsons et al. (1969 a), Shim (1977), Stockner et al. (1979), and Harrison et al. (1991). Cullen (1982) first discussed the presence of a deep chlorophyll maximum in temperate coastal waters. It is during June that a deep chlorophyll maximum is expected, when concentrations of nutrients are depleted in surface waters, yet abundant in deeper waters (Cullen 45 1982; Cochlan et al. 1990). In late M a y and June of both 1997 and 1998, a deep chlorophyll maximum was observed at 15-20 m. In 1996 however, a deep chlorophyll maximum was present in March, which was surprising since it is often associated with a nutricline, which was not pronounced at this time. In June, during the second large phytoplankton biomass peak, the chlorophyll was concentrated in the upper 10 m. Stephens et al. (1969) reported that a zooplankton peak, with values ranging from 400-800 mg m" 3 (total dry weight), occurred in M a y in the Strait. However, Stockner et al. (1979), reported values that were significantly lower (5-200 mg m" , total dry weight), although he still reported a peak for May. In the 1990s though, the peak in zooplankton biomass was in A p r i l and the values were similar in magnitude to those reported by Stephens et al. (1969). A n examination of the size fractionated data showed that the >1000 pm size fraction contributed the greatest percent o f biomass during spring. It was not until June that the smaller size fractions (202-475 pm and 475-1000 pm) made up a significant percentage o f the biomass. Throughout all five copepodite stages N. plumchrus is greater than 1000 pm, and therefore it is assumed that the >1000 pm size fraction is composed primarily o f N. plumchrus, with the greatest biomass occurring when the population is comprised o f the later stage copepodites. During A p r i l when N. plumchrus is present in the surface layer, chlorophyll values were very low, however adequate nutrient concentrations were present at this time to support a phytoplankton bloom indicating that it is grazing and not nutrient depletion that is inhibiting an increase in phytoplankton biomass in A p r i l and into mid-May. Parsons et al. (1969 a; 1970), Shim (1977), Stockner et al. (1979), and Y i n et al. (1996; 1997) all observed a similar pattern o f zooplankton grazing during spring in the Strait o f Georgia. 46 Herbivorous calanoid copepods have also been linked to grazing of the spring phytoplankton bloom i n other areas such as the Bering Sea, coastal Washington (Dabob Bay), and the Norwegian Sea. Calanoid copepods feed preferentially on large diatoms, the dominant phytoplankton group in these waters in spring (Frost 1977). A pronounced spring bloom has been documented in all o f these coastal regions, similar in magnitude and composition to the Strait o f Georgia. The timing o f this spring bloom ranges from March, in the more southern locations such as Dabob Bay, to M a y in the more northern locations such as the Bering and Norwegian Seas. In the Bering Sea, V ida l and Smith (1986) reported that N. plumchrus dominated the zooplankton population in the southeastern Bering Sea in spring. They found that N. plumchrus migrated into the surface waters immediately prior to the spring bloom, and the growth rate o f the N. plumchrus community achieved a maximum during the height o f the spring bloom. Cooney and Coyle (1982) found that the outer shelf of the Bering Sea hosts a number o f large calanoid copepods during spring, and that these copepods can routinely graze 20-30% of the primary productivity in this region. In Dabob Bay, W A , numerous studies have been conducted to examine the impact o f grazing on the spring bloom. The dominant calanoid copepod present in Dabob Bay is Calanus pacificus, a diel vertical migrator that typically migrates from below 75 m during the day into the upper 25 m at night (Frost 1988). C. pacificus molts from C5 into the adult stage in late February-early March and produces the first generation in early spring. The population then produces two subsequent generations, one in late spring and one in fall (Osgood and Frost 1994). The three generations coincide with large phytoplankton blooms, indicating that the adults are spawning so that their young are in surface waters when there are large concentrations of 47 available food. A study by Welschmeyer and Lorenzen (1985) found that 61-77% of the chlorophyll production in Dabob Bay was grazed by herbivorous zooplankton, indicating that in this coastal fjord, zooplankton heavily graze the phytoplankton standing stock. In the Norwegian Sea, the ontogenetically migrating copepod Calanus finmarchicus is the dominant zooplankton species during spring. It produces young that arrive in the surface waters at the time o f the spring bloom, like TV. plumchrus in the Strait o f Georgia. During their growth from G1-C5, C. finmarchicus also grazes heavily on the spring bloom (Bathmann et al. 1990; Falkenhaug et al. 1997; Meyer-Harms et al. 1999). In the Strait o f Georgia, once TV. plumchrus begins its downward migration to its overwintering depth, the conditions are ideal for either another phytoplankton bloom or the continuation of the spring bloom, which occurred in 1996 and 1998. This second bloom was previously reported for the Strait by Shim (1977) and Y i n et al. (1997), and therefore its presence was not surprising. In 1997 however, a second bloom was not observed, Y i n et al. (1997) suggest that heavy winds and/or high Fraser River runoff can significantly delay the onset of the spring bloom. This may in turn influence the abundance of TV. plumchrus, since it feeds heavily on the spring bloom. Following initiation, the progression o f the spring bloom in the Strait o f Georgia, like many coastal areas, is primarily controlled by top-down factors such as grazing. Other bottom-up factors such as nutrient availability, and environmental factors such as wind and irradiance do play a role in controlling the progression of the spring bloom, however they play a greater role in the initiation and development of the blooms. 48 Chapter 2 Interannual variation in the coupling of the spring bloom and zooplankton biomass in the Strait of Georgia, BC 2.1 Introduction The Strait o f Georgia serves as a nursery ground and a rearing are salmon stocks. While the Strait traditionally supported one of the world's largest commercial and recreational salmon fisheries, since the early 1970s scientists and fishermen have observed an overall decrease in such salmon stocks (Levy et al. 1996). Although the levels o f production of juvenile salmon in freshwater environments have decreased during this period due to,the degradation o f their habitat by human activity. (Beamish and Boui l lon 1993; Beamish etal. 1995), it is believed that this degradation does not account for all o f the decrease observed in these stocks. Therefore it is assumed that their abundance is also regulated to some degree by the marine environment (Beamish et al. 1996). These observations and assumptions have led to increased interest in the biological oceanography o f the region. It has been proposed that changes in available zooplankton biomass may directly or indirectly influence changes in food availability for juvenile salmon, which may in turn affect salmon returns (Beamish and Bouil lon 1993; Hinch et al. 1995; Levy et al. 1996; and Harrison pers. comm.). Thus some years may be "good food years" and some years "poor food years" for juvenile salmon. To examine this idea, two hypotheses were proposed: 1) In years designated as a "good food year", zooplankton survivability w i l l be greater and therefore there w i l l be more zooplankton available to juvenile salmon or their prey. In these years there w i l l be a tight 49 coupling between the spring bloom and the peak in zooplankton biomass (a good match). A n d , 2) in years designated as "poor food years", zooplankton productivity w i l l be lower, and therefore there w i l l be less food available to juvenile salmon or their prey. In these years there w i l l be weak coupling between the spring bloom and the peak in zooplankton biomass (a mismatch). Tight coupling occurs when phytoplankton are present in the water column just prior to the arrival o f the migrating zooplankton, providing the zooplankton with a source o f food. Weak coupling occurs when either the zooplankton arrive in the surface water prior to the initiation o f the spring bloom, or after the spring bloom has waned (Fig. 2.1). While examining the above relationship, this study also attempted to determine i f greater zooplankton survivability occurs the same year that phytoplankton biomass is high, or in the following year. . The basis o f these hypotheses is the match-mismatch hypothesis developed by Cushing (1972). Cushing's (1972) match-mismatch hypothesis was originally developed for the relationship between the production of larval fish and their prey. Since its inception however, it has been used to describe a variety o f predator-prey relationships in marine environments (Cushing 1990). This hypothesis has been used to determine i f there is a coupling or a decoupling between a species that generally spawns at a fixed time and its prey which is much more variable. For example, large numbers o f sockeye salmon smolts enter the Strait o f Georgia from the Fraser River in late spring. The start o f timing for their residence in the Strait is set by the timing o f the downstream migration, and is therefore controlled by physiological rhythms and by events in freshwater rather than by conditions in the marine environment of the Strait. Most juvenile sockeye remain in the Strait for only 20-30 days before migrating from the Strait to the open ocean (Healey 1980). Yet on average their residence in the Strait is remarkably close in time to the period of peak zooplankton biomass. In the Strait o f Georgia, phytoplankton biomass 50 Mismatch Match Time Figure 2.1. Schematic representation o f the match-mismatch hypothesis, as it applies to the Strait o f Georgia, B C Striped area indicates a match (high survivability) and shaded area indicates a mis-match (low survivability) (modified from Cushing 1990). 51 also reaches its annual maxima in late spring and again in early summer (Harrison et al. 1983). The hypothesis is based on the premise that both the salmon and the zooplankton spawn at a time when their young w i l l , on average, arrive in surface waters o f the Strait in order to take maximum advantage o f available food stocks. 2.1.1. Controls on timing and abundance of phytoplankton and zooplankton biomass in the Strait of Georgia A s was stated in Chapter 1, the timing of the spring bloom is dependent on numerous, highly variable factors and thus it varies from year to year. The controls on the timing of N. plumchrus development and abundance are poorly understood. It is not known what triggers the spawning time, nor what controls the subsequent progression through the developmental stages, although temperature has been shown to play an important role (McLaren 1963; Gardner 1977; McLaren 1978; Huntley and Lopez 1992; Mackas et al. 1998). The effects o f temperature w i l l be discussed in detail in Chapter 3. It is not known i f N. plumchrus is food limited, however, i f it is, then the amount o f food readily available for these copepods once they reach the surface as early stage copepodites may influence how many of these young individuals grow into C5 copepodites, and how large each individual grows, which in turn influences biomass estimates for that year. In addition, i f food is abundant while young copepodites are feeding, they are able to lay down l ipid reserves (Falk-Petersen et al. 1987; Evanson et al. 1999). Therefore, when they descend to the overwintering depth they have a greater chance of surviving to the adult stage and spawning, which may influence biomass estimates for the following year. However, i f this species is not food limited, then the degree o f coupling is unimportant. 52 This chapter examines the coupling between phytoplankton and zooplankton in the Strait o f Georgia during the spring season and therefore is based on the hypothesis that N. plumchrus is food limited. 2.2 Materials and Methods Data were compiled for chlorophyll-a and total zooplankton biomass, spanning the years 1968-1998. Data were available from nine years throughout this period. Sources o f information included data reports, scientific papers, and M S c theses from various sampling programs in the Strait since the 1960s. Sampling dates, locations, methods, depths sampled and sources ;of data are summarised in Appendix B . Unfortunately, the depths sampled and the dates of sampling varied considerably over the years. Most notably the chlorophyll-a values for 1966 and 1967 were derived from single surface samples of chlorophyll-a (Stephens 1968), whereas the mean water column chlorophyll-<2 for the other years was obtained from a series o f discrete depths. The average chlorophyll-^ of all depths was calculated for each sampling date, and reported as . . . . . . . . . « mean water column chlorophyll-a. These mean water column chlorophyll-a values (mg m" ) were used as a proxy for phytoplankton biomass, since these data were readily available for all three decades studied. Surface zooplankton biomass, reported in dry weight (mg m") , was derived from vertical net hauls through the upper water column (usually 20 or 50 m). In the 1960s and the 1970s samples were collected using nets with a mesh size o f 330 pm or 350 pm, however in the 1990s samples were collected using nets with a mesh size o f 202 pm. In the 1960s, the zooplankton weights were reported as wet weights. Therefore, in order to compare these wet weight data with 53 the dry weight data from other years, the wet weights were converted to dry weight using the conversion reported by Stephens et al. (1969) for this dataset. WW(mgm 3)= 8.4 DW (mg-m3) - 2.7 (Eqn. 2.1) A l l samples from the 1990s were collected following the methodology outlined in Chapter 1. Although sampling was regularly conducted at three stations, in this study only data from D F O l were used, as this was the station sampled during the previous decades. The data for the other two stations ( D F 0 2 and D F 0 3 ) is presented in Chapter 1. 2.3 Results and Discussion 2.3.1 Interannual variation in biological coupling of phytoplankton and zooplankton The historical results describe the upper water column (0-20, or 0-50 m) zooplankton and phytoplankton biomass in the central Strait o f Georgia for the years studied. The data are presented in Figure 2.2, and in Table 2.1. The variation in magnitude and timing o f the spring bloom and peak zooplankton biomass is shown in Figure 2.2. Each point represents either a single point or the average o f two samples per sampling date; for the raw data see Appendix C. The onset o f the spring bloom was the most variable, ranging from mid-March to mid-A p r i l in years when it was discernible. This is to be expected since the timing o f the spring bloom is dependent on weather and other physical oceanographic factors. One might expect the timing o f the peak in zooplankton biomass to be constant, since it is dependent on the spawning date. Although it is not known exactly what controls spawning date, the timing of peak 54 zooplankton biomass was less variable, and it occurred on M a y 6 ± 10 d, for the nine years between 1966 and 1998. In 1966 (Fig. 2.2a), two large phytoplankton peaks were observed, one in late March and another in June. The phytoplankton biomass had increased slightly by the time the zooplankton reached surface waters. This suggests that there was food available for the early stage copepodites migrating into surface waters. A s N. plumchrus grazed and grew, a decrease in phytoplankton biomass was observed and it reached a minimum when the zooplankton biomass : reached a maximum. Since N. plumchrus only remains in surface waters as late stage copepodites for up to six weeks, the decrease in zooplankton biomass in early June reflects their downward migration to their overwintering depth. Associated with this downward migration was a substantial increase in phytoplankton standing stock that was likely a result o f several factors, including decreased grazing pressure, increased nutrients in the water column from earlier grazing activity, increased water temperatures, light, and water column stability. 1966 is considered a "good food year" since the development o f the phytoplankton bloom occurred slightly prior to the migration o f zooplankton into surface waters, and phytoplankton remained available during the period o f peak grazing. In 1967 (Fig. 2.2b), the situation was quite different. The data suggest that there was not a large phytoplankton standing stock when N. plumchrus migrated into surface waters. The zooplankton biomass this year was more than twice that o f 1966, however the data indicate that there was a low phytoplankton standing stock and therefore less available food. Although unlikely, the lack of evidence for a spring bloom may however be attributed to the overlap in timing of the onset o f the spring bloom and initiation o f grazing by the zooplankton. I f the spring bloom began once the copepodites had already reached the surface layer, grazing would mask its 55 presence. 1967 is therefore classified as a "poor food year" with respect to coupling, however it is classified as a "good food year" with respect to zooplankton biomass. In 1968 (Fig. 2.2c), the pattern was similar to 1967, although the zooplankton biomass was considerably lower. There were too few sampling dates prior to M a y to resolve a spring bloom, however it does appear that the phytoplankton biomass was increasing as N. plumchrus arrived in the surface waters. Zooplankton arrived earlier this year than had previously been observed, and the window of maximum zooplankton biomass was quite large; spanning almost 70 days. This year is considered a "poor food year", since the coupling between phytoplankton and zooplankton appears does not appear to be tight and the zooplankton biomass is quite low. During the 1970s, sampling did not occur frequently enough to resolve the timing o f the spring bloom, nor the timing of peak zooplankton biomass with any confidence. Sampling, occurred approximately once every six weeks (Fig. 2.2d,e,f), and in most years there was only one sampling date in A p r i l and May , the most critical period. There was a greater sampling frequency (bi-mothly) in the 1990s, and therefore a better estimate of the timing o f the spring bloom and the timing o f peak zooplankton biomass were obtained. In 1996 (Fig. 2.2g), the spring bloom in March and the following bloom in M a y bracket the large zooplankton peak that occurred in A p r i l . There was a relatively high standing stock o f phytoplankton when N. plumchrus migrated into the surface waters, indicating that the onset o f the spring bloom coincided with the development o f the early stage copepodites. A s the zooplankton fed and grew producing increased amounts of ammonium and urea, they exerted more grazing pressure on the phytoplankton, reducing it to a minimum. A s soon as the copepodites began their migration to their overwintering depth, there was an increase in 56 phytoplankton, similar to that observed in 1966. Due to the tight coupling between phytoplankton and zooplankton during 1996, this year is classified a "good food year". In 1997 (Fig. 2.2h), the situation was quite different from all o f the previous years examined. There was a very large phytoplankton standing stock in early A p r i l , and by mid-May the phytoplankton biomass had decreased. It was not until several weeks after this decrease that the zooplankton biomass peaked. Therefore it appears that the phytoplankton bloom may have waned before N. plumchrus arrived at the surface. It is also interesting to note that the zooplankton peak was very late in 1997. The nutrient data support these observations as nitrate concentrations decreased in early A p r i l and remained low into June.- The ammonium concentrations were low throughout the month o f A p r i l and it was not until mid-May before an increase in concentration was observed. This year is classified a "poor food year", since the coupling between phytoplankton and zooplankton appears weak. In 1998 (Fig. 2.2i), conditions were very similar to those in 1966 and 1996. There were two discernible phytoplankton peaks, in March and in May, together bracketing the peak in zooplankton biomass that occurred in late A p r i l . The development o f the phytoplankton bloom, occurred before the arrival o f zooplankton in surface waters, and therefore there was food available for zooplankton when they arrived in late March. A s the zooplankton biomass increased, the phytoplankton biomass decreased (from 13 mg m" to 2 mg m" in 30 days), suggesting heavy grazing. Once again as the zooplankton began to migrate out of surface waters, the phytoplankton standing stock increased substantially most likely as a result of increased ammonium and urea concentrations in the water column from zooplankton grazing, although data are not available for 1998. This year is classified as a "good food year". It is important to note that in 1998 there was a very large zooplankton biomass, almost three times that observed in 57 1996 and 1997, and therefore food requirements of the copepods would have been almost three times as great. The main results for the nine years are summarized in Table 2.1. A large increase in phytoplankton biomass was observed in both March and in June for the majority o f years with available data. The first bloom occurred on March 24 ±12 d. The second bloom, most l ikely occurred in response to a decrease in grazing pressure by N. plumchrus. A s N. plumchrus feeds it lays down l ipid reserves, and releases nutrients into the water. A s it approaches the time when it begins its migration to its overwintering depth it does not feed as heavily on the phytoplankton. A s grazing pressure diminishes, nutrient concentrations are still high enough to support a phytoplankton bloom, sea surface temperatures and light are increasing, and Fraser River discharge.reaching a maximum, all o f which lead to increased water column stability. A l l o f these factors result in ideaL conditions for a large phytoplankton bloom. This second phytoplankton bloom occurred on June 9 ±10 d, and its average magnitude was 7.6 ±1.5 mg C h i nv 3 . In years such as 1966 and 1996, designated as 'good food years', the zooplankton biomass was not substantially higher than in other years. However in 1998, also a "good food year", zooplankton biomass was almost three times greater. In 1997, a "poor food year", the zooplankton biomass was lower than the other years. However in 1967 and 1968, also "poor food years" the zooplankton biomass was not substantially lower than in other years. From these results it cannot be stated that in years when there is a tight coupling between phytoplankton and zooplankton that there is greater zooplankton biomass. The years following a "good food year" (i.e. 1967, 1997) were examined and again there was no relationship suggesting that in years following a "good food year" zooplankton were more abundant. Figure 2.2. Timing o f the spring phytoplankton biomass (--•--) and spring zooplankton biomass ( - • - ) in the Strait o f Georgia, B C , 1966-68, 1975-77,1996-98. Chlorophyll-a data represent the average of 0-20 m values. Zooplankton dry weights represent the average o f 0-20 or 0-50 m vertical tows. Sources of data are given in Appendix B . 59 Table 2.1. Summary o f match-mismatch data, and date, magnitude and duration o f peak phytoplankton and zooplankton biomass. For sources of data and sampling methodology see Appendix B . Year Match/ Date of Dry weight Duration of mismatch peak (mg-m3) peak zooplankto (mean of 0-20 zooplankton n biomass m o r ° - 5 0 m biomass vertical tows) (#daysDW (early Dates of peak Phytoplankton biomass late >150mgm"3) spring spring) Peak Chlorophyll-a (mg-m3) (average of 0-20 m values) (early late spring spring) 1966 match Apr 30 262 -41 Mar 26 June 15 6.0 8.0 1967 Mismatch or match May 20 666 -51 none June 15 none 6.7 1968 mismatch Mar 16-May 2 250-320 -47 none June 11 none 6.5 1975 N/A N/A N/A N/A N/A N/A 1976 N/A May 7 148 N/A May 7 3.25 1977 ,N/A Apr 28 111 N/A none June 22 none 7.2 1996 match Apr 25 271 - 18 Mar 26 May 8 5.5 9.6 1997 mismatch May 22 142 ~ 18 Apr 8 Jun 10 17.0 8.4 1998 match May 8 824 -79 Mar 25 May 21 12.4 10.5 60 2.3.2 Interdecadal variation in phytoplankton and zooplankton timing The average decadal timing of the spring bloom and maximum zooplankton biomass are presented in Figure 2.3 in order to determine interdecadal variations in timing o f the spring phytoplankton bloom and peak zooplankton biomass. In the 1960s, there was a decline in phytoplankton standing stock observed as the zooplankton biomass increased (Fig. 2.3a). Prior to and following the peak in zooplankton biomass, greater phytoplankton biomass was recorded. A s one might expect, the zooplankton exert maximum grazing pressure on the phytoplankton when the zooplankton are largest and most abundant. It is also interesting to note that the zooplankton biomass essentially doubles from Apr i l to May. M i l l e r and Nielsen (1988) determined that, in the subarctic Pacific, the dry weight o f N. plumchrus increased by a factor o f 3.3 from C3 to C4 . In 1993 M i l l e r (1993) determined that the mortality o f C4 and C5 N. plumchrus was approximately 7.6%, which agrees with Fulton's estimate o f 6% for the Strait o f Georgia (Fulton 1973). Therefore it is possible that the observed doubling in zooplankton biomass is a reflection o f the development from C3 to C4 and C5 . In the 1970s a spring bloom was never observed (Fig. 2.3b), however this is more than likely a result of low sampling frequency. There was a bloom in June, comparable in magnitude to those of the 1960s and 1990s. The overall zooplankton biomass was substantially lower and this is most likely a factor of sampling technique. It is important to note that there were many fewer sampling dates and fewer samples were collected in the 1970s than in the other two decades. Therefore, the trends observed may not be representative of actual conditions at this time. 61 In the 1990s, the phytoplankton trend was similar to that observed in the 1960s. The zooplankton biomass, however, reached a maximum approximately one month earlier than in the previous decades (Fig. 2.3c). This observation w i l l be developed in Chapter 3 where the interdecadal variation in timing o f Neocalanus plumchrus development is examined in detail. It should be noted that in the 1990s the mesh size used in the nets was smaller (202 pm) than that used in the 1960s and 1970s (330 and 350 pm respectively). This change should produce higher biomass estimates for the 1990s since a greater percentage o f the smaller zooplankton would be captured. This was not apparent in the zooplankton estimates since values from the 1960s were very similar to those in the 1990s. There are two possible explanations for this observation. The first is that dry weight estimates from the 1960s were calculated (as seen in Eqn. 2.1) from wet weight estimates which may overestimate the biomass. Or secondly, it is possible that such a large fraction of the zooplankton biomass is composed o f the larger animals that changes in the biomass o f the smaller animals is masked. If size fractionated zooplankton estimates had been obtained in the 1960s it would have been possible to determine i f this was the case, however only total wet weight was measured. It is not known why the values for the 1970s were so much lower than the other two decades. 800 C 600 co E. J 400 o Q. O fl 200 0 1960s n=4 .n=5 n=4 i t — 1 — + 6 10 E E 4 1 .c 0 800 600 400 + E. Q Q. | 200 0 1970s n=1 • — 10 8 E, 4 ? .c o + 2 0 800 sr 600 "E o> E J 400 Q Q. O 200 + 0 1990s (c) 10 8 6 E E 4 1 o 0 Figure 2.3. Mean monthly chlorophyll-a (--•--) and zooplankton biomass (H ) estimates during 1966-68, 1975-77, 1996-98 for the Strait of Georgia, BC. Chlorophyll-a data represent the average of 0-20 m values. Zooplankton dry weights represent the average of 0-20 or 0-50 m vertical tows. Error bars represent ± 1 SD from the mean. For sources of data and sampling methodology see Appendix B. 63 2.4 Conclusions Leggett and Dublois (1994), reviewed some of the larger studies which have examined the match-mismatch hypothesis. Although most of the studies they cited compared the timing o f the zooplankton production with the timing o f peak larval fish abundances, Leggett and Dublois (1994) concluded that the relationships described by other authors were weak. In addition, they concluded that the importance of the strength of coupling between a zooplankton species and a fish species is less important in determining marine fish recruitment than was previously hypothesized. In this study it is extremely difficult to predict from the coupling between the phytoplankton standing stock and the zooplankton biomass, which years w i l l be better food years for small fish such as juvenile salmon, herring and hake. The available data are useful for comparing phytoplankton - zooplankton timing relationships. Without the corresponding fish life history characteristics and food preference data it is difficult to make any conclusions about whether a specific year is a "good food year" or a "bad food year" with respect to food availabilty. More information on other biotic and abiotic factors is also required in order to determine i f the relationships documented were in fact representative o f the conditions for that year. It should be noted that very little data for primary production, nutrients, wind, or irradiance exist for 1960s and 1970s. These data do show that in years when phytoplankton biomass was high immediately prior to the arrival of young copepodites in surface waters that the corresponding zooplankton biomass was not always greater for that year, nor for the following year. Therefore it should be noted that estimations of food availability for juvenile salmon could not be made from only phytoplankton and/or zooplankton data, which agrees with Leggett and Dublois (1994). 64 Therefore a better definition of "good food year" specifically with reference to fish might be "a year in which there is a large zooplankton standing stock" regardless o f the degree of coupling o f between phytoplankton and zooplankton. The data do show that there has been a shift in the date o f peak zooplankton biomass from M a y in the 1960s and 1970s to A p r i l in the 1990s. This interesting observation is explored in the next chapter and leads to several key questions: If there is indeed earlier development o f N: plumchrus in the Strait, how does this affect the five different salmon species? Do they adapt and utilize an alternate food source? Do they modify their timing to some degree as well? This is beyond the scope o f this study, however with salmon gut contents and timing data, this could be answered. This study does clearly show the importance of long-term, high frequency: sampling in field studies. Wi th only three years o f sampling each decade it is virtually impossible to establish a pattern. A n d , due to the low sampling frequencies the earlier data, particularly from the 1970s, are; o f marginal value. This should be kept in mind when new monitoring programs are developed whose goal is to determine the timing o f the spring phytoplankton bloom and the date of peak zooplankton biomass. 65 Chapter 3 Interdecadal variation in the timing and development of the copepod Neocalanus plumchrus in the Strait of Georgia, BC 3.1 Introduction Recently there has been an increase in the examination of large datasets for interannual and interdecadal trends due to the increased interest in climatic variations and their effects on the marine environment (Brodeur and Ware 1992; 1996; Mackas et al. 1998). There is heightening evidence that persistent physical and biological changes have occurred in the northeast Pacific Ocean in recent decades. Some indices that have been used to detect the biological changes are primary productivity, secondary productivity, and changes in higher trophic levels, such as salmon. Changes in the marine environment have been linked with significant declines in coho, chinook and sockeye salmon stocks (Beamish and Boui l lon 1993; 1995; 1996; 1999), whereas other authors have attributed changes in the marine environment to decadal-scale fluctuations in primary production (Venrick et al. 1987), and in secondary production (Brodeur and Ware 1992; Roemmich and M c G o w a n 1995; Brodeur et al. 1996; Mackas et al. 1998). Numerous other authors have detected changes in the physical oceanography using atmospheric and oceanic indices (Freeland 1990; Tabata 1991; Freeland et al. 1997). In a study by Mackas et al. (1998) 40 years of zooplankton data from Ocean Station P (OSP) were examined and they found that there has been a significant change in developmental timing of the dominant zooplankter in this region, the calanoid copepod Neocalanus plumchrus. Their study examined the response of a single species to changes in the oceanographic regime, 66 however the response observed by this single species may be used to indicate changes in abundance or the developmental timing of other species. This discovery is recent, and it has not been determined i f this is a widespread phenomenon amongst other zooplankton, or i f it is an isolated response to changes in the local oceanographic regime. There are very few northeast < Pacific regions that have been studied as extensively as OSP, making it difficult to determine the geographical extent o f this change. However the Strait o f Georgia has been studied periodically throughout the last three decades and therefore it might be possible to determine i f a shift in < developmental timing has occurred here also. The objectives o f this chapter were: 1) to investigate the interdecadal variations in the developmental timing o f the copepod N. plumchrus in the Strait o f Georgia, and 2) to determine i f reproduction and development were occurring earlier in the Strait o f Georgia, as was reported at OSP, and i f so to 3) propose some possible hypotheses to explain this change in timing. The data from Chapter 2 indicate that there has been a change in the developmental timing of N. plumchrus since it was determined that the time of the peak in spring zooplankton biomass in the Strait shifted from M a y to A p r i l over a 30 year period. In order to determine i f developmental timing o f TV. plumchrus has changed over the 30 year period, it is important to review the life history cycle of this species. N. plumchrus undergoes a very strong seasonal migration that is closely linked to its developmental and reproductive cycle (Parsons et al. 1970; Fulton 1973; M i l l e r et al. 1984; Mi l l e r and Clemons 1988; Mackas et al. 1998). Adults of N. plumchrus spawn at depth in mid-winter, and several weeks later the females release eggs at depth. The eggs hatch into the first naupliar stage ( N l ) and ascend through the water column towards the surface. During their ascent, development from N l to N 6 occurs, and they reach the surface in late winter as either an N 6 or as a first stage copepodite ( C l ) . A t the surface, active 67 feeding begins, and development from C l to C5 occurs. In late spring or early summer, the C5s migrate back down to their overwintering depth, where they enter diapause, and molt into the adult stage. Each stage duration from C1-C4 is approximately equal (Table 3.1), and it is assumed that each stage responds uniformly to changes in environmental parameters. In the Strait the N. plumchrus population enters and exits diapause synchronously, and is present in surface waters for a relatively short period during spring, about 70-100 days (Fulton 1973). The timing of their presence in surface waters is coincident with the spring phytoplankton bloom (Parsons et al. 1970; Stockner et al. 1979; Harrison et al. 1983), and with the time that juvenile salmon are entering the Strait and beginning to feed (Healey 1980). ; Throughout this chapter there w i l l be a comparison between O S P and the Strait o f Georgia, however, the two N. plumchrus populations are in fact very different. The life history strategy of this copepod is very different in the open ocean than in coastal areas (Fig. 3.1). A t O S P , N. plumchrus overwinters at depths between 400 and 700 m. M i l l e r et al. (1984) in an extensive life history study o f this species, determined that at OSP maturation (C5 to C6) takes place immediately after N. plumchrus reaches their diapausing depth. Reproduction commences soon after maturation, and can continue throughout fall and into mid-winter. Naupli i hatch between October and January and reach the surface as C l s between November and February. Development at the surface is much longer at OSP occurring over a seven month period. N. plumchrus copepodites are present in surface waters at O S P from November through until July. The majority o f the population begins the descent to their overwintering depth in late June and July. Although it was thought for many years that at OSP ./V. plumchrus were mainly herbivorous, this was disproved in the early 1990s by Gifford and Dagg (1991; 1993) and it has since been shown that N. plumchrus feeds primarily on microzooplankton at OSP. In the Strait o f Georgia, 68 evidence suggests that N. plumchrus is mainly an herbivore and grazes heavily on the spring phytoplankton bloom (Parsons et al. 1969b). Therefore while comparing these populations it is important to keep in mind the differences in life history strategy and food preferences. / 69 O N D J F M A M J J A S Month Figure 3.1 Comparison o f the life history timing o f N. plumchrus between the coastal regions (Strait o f Georgia) (A) and open ocean (OSP) (B). (reprinted from Wen 1995) Table 3.1. Stage durations o f naupliar and copepodite stages o f TV. plumchrus at OSP. (Mil ler and Clemons 1988) Stage Durat ion Non-feeding nauplii (N1-N4) Short Feeding nauplii (N5-N6) Longer C l 12.6 d C2 13.4 d C3 13.4 d C4 16.6 d C5 > 7 mo C6 (adult) 1-2 mo 71 3.2 Materials and Methods 3.2.1 Zooplankton data: sources and sampling methods Data were compiled on late winter to early summer N. plumchrus stage composition from the Strait o f Georgia sampling programs during 1967-1998 from various sources, published and unpublished. Appendix D lists geographic location, sampling dates, source publications, sampling methods and depth ranges, and copepodite stages resolved and enumerated. Seasonal coverage, depth ranges, and earliest-stage-identified varied considerably among datasets. This variability made it difficult to compare among all years the full C1-C5 developmental sequence, however the C3-C5 sequence was available for all years. It is important to note that throughout the 30 year sampling period, copepodite developmental stage identification and enumeration was completed by microscope. Resolution of age structure and species composition has improved over this period for two reasons. Since about 1980 there has been a shift in the mesh size used on sampling nets, from approximately 330 urn to approximately 200 jam. Smaller mesh sizes mean that there is a greater chance o f capturing the earlier stage copepodites. In addition, improved taxonomic descriptions have allowed the distinction between N. plumchrus and N. flemingeri in other oceanic regions (Bradford and Jillet 1974; Mi l l e r 1988), but to date there have been no individuals of N. flemingeri observed in the Strait. Conover (1988) reported that Fulton re-examined al l o f his samples from the Strait and concluded that the Neocalanus sp. population in the Strait was composed entirely o f N. plumchrus. 72 3.2.2 Estimation of developmental t iming The seasonal timing o f N. plumchrus development can be estimated several ways: 1) by direct measurement o f stage composition, or 2) by timing o f peak zooplankton biomass. Because N. plumchrus makes up a large fraction (>75%) o f the surface (0-50 m) zooplankton biomass in spring, biomass can be used as a proxy for developmental timing following the method - o f M i l l e r (1993) explained below. Stage composition information is more useful as it is less susceptible to spatial patchiness, a shortcoming o f biomass measurements (Wiebe and Holland 1970). However, biomass measurements are much faster and cheaper and therefore more readily available. . . A.conversion method between the two estimates o f developmental timing was required to . compare years when stage composition was analyzed and years when biomass estimates were computed;, since both estimates were rarely available for the same year. The values computed by Mackas et al. (1998), based on the model by M i l l e r (1993), show that maximum zooplankton biomass occurs when the N. plumchrus population is composed o f between 35 % C5s (for declining mortality rate) to 65% C5s for uniform mortality rate (Fig. 3.2). Uniform mortality rate assumed a constant mortality o f 6% per day throughout all o f the life history stages. Declining mortality rate assumed a mortality rate of 13% per day for early C l copepodites and declined to 3% per day for mid-C5 copepodites. Therefore the average o f these two values, 50% C5s, was used to indicate the timing of peak biomass. Stage composition data were plotted as a fraction o f the total copepodite abundance, and linear interpolation between dates was required to produce plots of percent stage composition by date. The timing o f the occurrence of 50% C5s was also plotted and compared for all years. 73 0) +-* ro OL | T a o E ' c 3 eouepunqe SO- IO J° aBejs Aq % re a: >» +—• 75 t o cn c c 13 o Q inr-gi. inp-01 mr-s unr-OE unp-93 unr-gL unn-01. unr-g ; / 1, j ABW-93 ABIN- « Bil lR ABW-H. AB|/\|-9 AB|/\|-L jdv-93 Jdv-I.3 jdw-91 ra a: ro o 5 E | c 3 ( Z . LU 6) ssewojq QO-ZO ro ra at c o a inr-si. inr-g unr-o€ unr-sz unp-02 unr-si. unr-Ol-unf-g ABW-LE ABW-92 ABW-LZ AEW-9", AeiAm AevM-9 ABW-L Jdv-9Z , jdv-VZ W^WWrfWTO Jdv-9l. o C/3 u o S 3 X) cn > CO Cfl 6 g T 3 o ON II tu Ej •f3 -*-» o o ( 3 . LU 6) sseujoiq 30-20 OH — 3 ex aouBpunqB go-to jo a6B»s Aq % 74 3.2.3 Envi ronmenta l data The environmental data include temperature and salinity measurements from station D F O l , for the period 1968-1998. The data from 1968-1991 are reported in Fissel et al. (1991); data from 1968-1998 are included in the Institute of Ocean Sciences (IOS) database o f oceanographic data. Robin Brown, the database manager at IOS, supplied the raw data. Sampling in this region was primarily conducted by the U S Navy, and in months when samples were collected sampling frequency was relatively high, ranging from 4-16 samples per month. Average monthly temperatures and standard deviations were calculated at three depths for each year and are reported in Appendix E . Three depths were reported, surface, intermediate, and deep water. The 5 m temperatures are the average o f 0-5 m temperatures, the 100 m temperatures are the average o f 90-110 m temperatures, and the 350 m temperatures are the average of all temperatures deeper than 350 m. In addition, the average monthly temperature for all years was calculated using all available monthly data for the three given depth ranges. Since anomalies often give a better indication o f changes, temperature anomalies were calculated for three depth ranges by subtracting the average monthly temperature for all years, from the average monthly temperature for a given year. In order to determine the developmental timing o f N. plumchrus versus SST anomalies for March to May, degree days were calculated using the following equation: Degree days = ^(Temperature - 6°C) j u i i a n days 60-isi E q n . 3.1 Degree day values were calculated following the same methodology used by Mackas et al. (1998). A n arbitrary value of 6°C was subtracted from every daily surface temperature value 75 from March 1 through M a y 31 (Julian days 60-151). To obtain a degree day value these differences were summed to produce a cumulative anomaly total. For days when there was no SST estimate, an exponential interpolation was conducted between available dates for that year. It is important to note that SST data in the Strait o f Georgia are patchy and therefore the degree day estimates are based to some extent on estimates of SST. 3.3 Results 3.3.1 Interdecadal shifts in TV. plumchrus stage composition The stage composition data clearly show that there has been a shift towards earlier developmental timing o f TV. plumchrus copepodites in the Strait o f Georgia (Fig. 3.3). Max imum zooplankton biomass estimated by 50% C5s, occurred on M a y 18 in 1967, and by 1996 the date o f maximum biomass was A p r i l 22. This change in timing appears to have been gradual since in 1971, 50% C5s occurred on M a y 9, and in 1981, on A p r i l 25. When the date o f developmental timing was plotted by year, using both biomass estimates and stage composition estimates, there appeared tO:be little correlation between the time of development and year ( r ^ 0.34, p=0.02) (Fig. 3.4). Biomass data estimates are not as accurate as stage composition analyses, since biomass estimates are more subject to spatial patchiness and to fluctuations in abundance o f other species present in the sample (Wiebe and Holland 1970). Overall, there has been a gradual shift toward earlier developmental timing o f the copepod N. plumchrus in the Strait o f Georgia, and the magnitude of this change has been on the order of 25 days. 76 Figure 3.3 Interannual comparison of N. plumchrus stage composition of C l to C5 versus date for the Strait of Georgia, 1967-1997. The date of maximum zooplankton biomass is approximately 25 days earlier in 1997 than in 1967. Dashed lines connect earliest and latest dates of 50% C5s. 77 145 100 -i 1 1 1 1 1 J — - 1 I 1960 1965 1970 1975 1980 1985 1990 1995 • 2000 Year Figure 3.4. Regression o f date of maximum biomass and year estimated from both biomass data and from stage composition data. The negative relationship indicates the copepodites of N. plumchrus occur about 10-15 days earlier in the 1990s than in the 1960s. (r^=0.34, p=0.02) 78 \\ 'IMS 1968 1970 1972 1974 976 -< CD 0) • 2-2.5 • 1.5-2 • 1-1.5 • 0.5-1 • 0-0.5 •-0.5-0 • -1—0.5 •-1.5-1 • -2--1.5 • -2.5-2 Figure 3.5 Temperature anomalies for the Strait o f Georgia, 1968-1998. Anomalies calculated from average of 0-5 m temperatures. A warming o f surface waters has occurred over the 30 year time period. 1968 79 < CD 0) • 1-1.5 • 0.5-1 • 0-0.5 •-0.5-0 • -1--0.5 •-1.5-1 Month Figure 3.6 Temperature anomalies for the Strait o f Georgia, 1968-1998. Anomalies calculated from average o f 90-110 m. A warming o f mid-waters has occurred over the 30 year time period. 80 fi) • 1.5-2.0 • 1.0-1.5 • 0.5-1.0 • 0.0-0.5 •-0.5-0.0 •-1.0—0.5 •-1.5-1.0 •-2.0-1.5 5 7 9 11 Month Figure 3.7 Temperature anomalies for the Strait o f Georgia, 1968-1998. Anomalies calculated from average of all depths greater than 350 m. A warming o f bottom waters has occurred over the 30 year time period. 81 Figure 3.8 N. plumchrus developmental timing versus anomalies o f March to M a y SST in the Strait of Georgia (degree days relative to a 6°C baseline; see methods for details). Squares ( • ) are biomass estimates, and the circles ( • ) are stage composition estimates. The broken line regression (Julian day = 149.57-0.08(degree-days) ; 1^=0.34, p=0.002) is fitted only to the biomass data. The solid line regression (Julian day = 159.29-0.13(degree-days); r2=0.54, p=0.005) is fitted only to the stage composition estimates. Error bars (n=5) represent standard error associated with each type o f estimate which is largely dependent on sampling frequency. 82 3.3.2 Changes in environmental parameters The temperature anomalies from this area show that there has been a gradual warming o f both surface, intermediate and deep water in the Strait o f Georgia. In the 1960s the surface water temperatures were approximately 1-2 °C colder than they were in the 1990s (Fig. 3.5). Although warming was also seen at 100 m and >350 m, the changes are not as great as in the surface waters (Figs. 3.6 and 3.7). When the date of maximum zooplankton biomass was compared with anomalies o f SST (degree days using a 6°C baseline) it was found that in years when March to M a y temperatures were colder, developmental timing was later (Fig. 3.8). Data for stage composition produce a stronger relationship with temperature than biomass data, however both datasets indicate a negative relationship between date of maximum biomass and SST (Fig. 3.8). 3.4 Discussion 3.4.1 Possible environmental controls for developmental timing Developmental timing o f N. plumchrus is known to differ by a month or more between oceanographic regions. Table 3.2 summarises seasonal cycles for the Strait o f Georgia (Fulton 1973; this thesis), the Sea of Japan (Mil ler and Terazaki 1989), the oceanic Alaska Gyre (Mil ler et al. 1984; Mi l l e r 1993), and the Bering Sea continental shelf and slope (Vidal and Smith 1986). The timing o f N. plumchrus development varies with latitude and with the oceanic environment. N. plumchrus spawns later in the calendar year both further north and further offshore. In addition to the oceanographic region, environmental parameters, especially temperature, have been found to play a role in the development ofN. plumchrus. Gardner (1977) found that the size of the TV. plumchrus overwintering population was positively correlated (C. marshallae negatively correlated) with deep water temperatures in the Strait over the range 8.75-9.4°C. He concluded that a cooling o f the Strait o f Georgia deep water between the 1960s and 1971 had favoured C. marshallae. It would be interesting to examine whether or not C. marshallae is less abundant now since deep waters are warmer. A t the outer edge o f the continental shelf o f the Bering Sea, Smith and V i d a l (1986) observed 1-3 week later development of TV. plumchrus in 1981 than in 1980. They also noted a second cohort o f early copepodites in 1981, which they attributed to summer; reproduction. Smith and V i d a l (1986) characterised 1981 as both warmer and calmer than 1980. A t OSP, M i l l e r (1988) noted large differences in the timing o f TV. plumchrus development between 1984 and 1988. The development in 1984 was about 2 weeks earlier than in 1988; the body size eventually reached by the C5 diapause stage was also larger in 1984. M i l l e r (1988) attributes the size difference to low growth rate in 1988, possibly caused by disruption o f the upper ocean food web by a very intense storm event in mid-Apr i l . A l so at OSP, using data collected between 1956 and 1998 Mackas et al. (1998) observed that the date o f peak zooplankton biomass was increasingly later from 1956 to the mid-1970s, whereas from the mid-1970s to present they observed the opposite trend (Fig. 3.9). They examined TV. plumchrus stage composition data from 1971 to 1998, and observed that there was both substantial interannual and interdecadal variation in the developmental timing, and that overall TV. plumchrus in this region is developing approximately 60 days earlier in the 1990s than in the early 1970s (Fig. 3.10). They attributed the observed long term variability in TV. plumchrus developmental timing to changes in ocean temperatures in this region. The magnitude o f this change may appear much greater at O S P since the window o f surface development is much longer at O S P compared to the Strait and other coastal regions. -a os a IT) £ U T3 a o 93 U OJD s I I-I 1—1 CD -*-» cfl •4—» 3 ,2 < 00 CD cfl CD o a CS T J ' « c O " (D cd CD -4—* ,2 (D >H CD H O CD 3 & a C3 a I CO CD PH I O CD Q •s I C u CD 00 >H CD o cu S "« -8 I O CD Q > o O CD Cu CD CO '•3 -S 9 J3 8 -2 Q a. ° 3 C A cs I * E • « ° UJ cfl ) H ST c u CD co U ex r-in oi o O N i o in in in cn u, « oo O N Os i f -V O ON w O CX) O N oo O N O N T - H I V O in O N oo O N oo O oo O N cu u a CU l -C O r--O N PH PH cfl . „ 1=1 O O N cfl V O O N CD CU *2 O N g oo 03 O N N CD Cd ^ H T3 § »H CD O O 0 0 O N ' -o cfl C3 C cfl O oo O O O N C3 = H C f l CD 4^ O Cfl Cf l ^5 cfl C O O N O N C CD w ^ co co C ^ H r - H o O N O N " 3 0) - J J cfl O N O N Q CD cfl Cfl N O O N cfl O N cfl > oo S 2 CO w a © "ox .2 'ex i . , o C O S O > 2 is O N CO •<* £ j a c o 03 C O Cu '—1 3 ^ C O o cu CO oo C O C3 - _ ^ PL, C 1 ^ J S -CS B o CO C O in CD cu Cu O ^ C3 o CU O O CO vo ex ^ b ° W ^ 85 E 3 E 'x re on re E o m c _re o. o o fsl i*— o at "re O • D a t e s es t imated f r o m % C 5 o Da tes f r o m b i o m a s s t ime s e r i e s Y e a r Figure 3.9 Date of zooplankton biomass maximum at OSP from 1954-1998 derived from both percent C5 and biomass estimates, (from Mackas et al. 1998) 86 Figure 3.10 Interannual comparison of N. plumchrus stage composition of C l to C5 versus date at Ocean Station P, 1967-1997. The date of maximum zooplankton biomass is approximately 60 days earlier in 1997 than in 1967. (Mackas et al. 1998) 87 3.4.2 Proposed hypotheses Three hypotheses are proposed to explain the shift to earlier developmental timing o f N. plumchrus in the Strait o f Georgia. The three hypotheses are: H j : Earlier spawning date due to the breaking o f diapause state, controlled primarily by environmental conditions at depth. H2: Shortening of each copepodite stage, controlled primarily by environmental conditions at the surface. H 3 : Increased survival of earlier spawned individuals within an annual cohort, controlled primarily by food conditions at the surface. In the Strait o f Georgia, any one, or combination of, the above hypotheses may explain the changes observed. When examining the first hypothesis it was found that Fulton (1973) observed that peak egg production by N. plumchrus in the Strait occurred on March 5, however a similar study done by Beaith (1999) determined that peak egg production in 1998-99 occurred on February 15. Therefore TV. plumchrus does appear to be spawning approximately 18 days earlier in the 1990s. A s temperature has been found to increase the metabolic rate o f calanoid copepods (McLaren 1978), it is possible that the increase in bottom water temperature (+ 1-2°C) may have caused N. plumchrus to expend their l ipid reserves faster. If this occurred, females would put their energy into producing and laying eggs sooner. This would lead to the observation that the developmental cycle is "speeding up". However, since very little is known about what causes the breaking o f diapause and the subsequent spawning in this species, it is difficult to determine for certain i f the increase in temperature is the primary influencing factor. 88 Other authors have suggested that in addition to temperature, other factors such as salinity, light, water movements, l ipid depletion in females, and settling phytoplankton particles may trigger spawning in marine zooplankton (Giese and Kanatani 1987; Starr et al. 1990; Starr et al. 1994). The physical parameters in this region are strongly seasonal making it difficult to isolate the causative factor. It is unlikely that sedimentation o f the spring phytoplankton bloom is a trigger for TV. plumchrus spawning, since spawning occurs approximately one to two months prior to the onset o f the spring phytoplankton bloom. In addition, a study by Evanson et al. (1999) in the Strait o f Georgia determined that TV. plumchrus females were not l ipid depleted at the time o f spawning. Warming o f the surface waters may also play a role in the earlier developmental timing o f TV. plumchrus. In the Strait o f Georgia, Fulton (1973) determined that the progression from C l to, C5 in surface waters takes approximately 70-100 days. Assuming that TV. plumchrus arrived in the surface layer at the same time in 1996 as in 1967, in order for development to be 25 days earlier in 1996, each individual stage duration must have been 25-35% shorter. Copepod stage durations have been reported to shorten with increasing environmental temperatures (McLaren 1978; Huntley and Lopez 1992; Mauchline 1998), and Huntley and Lopez (1992) suggest that a 2-3°C warming o f water can lead to a shortening of each stage duration by about 25%. The warming observed in the surface waters of the Strait is on the order of 1-2°C and therefore it is certainly possible that the warming of the upper layer may be responsible for a shortened individual stage duration in TV. plumchrus and for the overall shift in developmental timing. The third hypothesis assumes that the increased survival of earlier spawned individuals is more related to food availability than temperature. In this case, i f there is a large standing stock o f phytoplankton when early stage copepodites reach surface waters and begin feeding, 89 survivability should be greater. If these copepodites are from an earlier portion o f the annual cohort and they feed heavily enough to reduce the phytoplankton bloom to a minimum, there w i l l be less food available for later spawned individuals. The result would be increased survivability o f the earlier individuals within the annual cohort, which would have led to the observed earlier developmental timing. It is most l ikely a combination o f the first and second hypotheses that describe the : changes in N. plumchrus observed in the Strait o f Georgia. There is evidence that spawning is occurring earlier in the year, and there is evidence o f a warming o f surface water temperatures that may be shortening the progression through the copepodite developmental stages. These two together may be reducing the amount of time that copepodites spend in diapause and in the progression through the early copepodite stages. Mackas et al. (1998) suggest that at OSP the earlier developmental timing is most l ikely a result of interannual differences in survival among early versus late portions o f the annual ; copepodite cohort. They do not attribute this increase in survivability directly to food : availability. Instead, they attribute it to warmer spring temperatures that lead to increased water ;• column stability that in turn influenced food availability. These hypotheses are based on environmental factors thought to control the developmental timing of this species, however until there is further evidence to support that temperature and/or food availability play an active role in development of the calanoid copepod N. plumchrus, these hypotheses w i l l remain hypotheses. 90 3.4.3 Potential impact on predator populations The timing o f TY. plumchrus is important as it affects the very narrow seasonal window o f grazing pressure on phytoplankton, and it influences the availability o f these large copepodites to upper ocean predators such as salmon. Juvenile sockeye, pink, chum, and coho salmon from various major nursery lakes migrate down the Fraser River in A p r i l or May. Most spend about 30-60 days (Apri l to June) in the Strait before migrating to the outer coast, although some may spend longer. A s was emphasized in Chapter 2, salmon most likely spawn at a.time when their > young w i l l arrive in the Strait when food is most abundant. I f there is a change in the timing o f . this food availability it may lead to lower survivability of juvenile salmon and other predator .species. • 3.5 Conclusions Earlier developmental timing o f the calanoid copepod TY plumchrus was observed in the • Strait o f Georgia. Although there is little known about the controls on timing o f spawning and on the development of this copepod, several hypotheses were proposed which may help to explain this change. A n examination o f the hypotheses with the data available suggests that a combination o f the first and the second hypotheses, earlier spawning of TV plumchrus and a shortening of each copepodite stage due to a warming of surface waters, might explain the greatest change. \ 91 General Conclusions and Future Research Increasing the understanding of the biological oceanography o f the Strait o f Georgia is vitally important to "managing" the ecosystem well . Currently many management decisions involving lower trophic levels are based on data collected in the late 1960s and 1970s. There have been changes to certain aspects o f the biological oceanography since this time and they were investigated and presented in this thesis. The nutrient (NO3, PO4, N H 4 , and urea) concentrations were reported and at was determined that they do not differ greatly from those reported previously by Stephens et al. (1960), Parsons et al. (1970) and Shim (1977). Chlorophyll concentrations were also measured and it was found that there is a very large spring bloom in late March and another large phytoplankton bloom in June. These two blooms occasionally bracket the large peak in zooplankton biomass that was seen in late A p r i l . The occurrence o f a spring bloom has been wel l documented in this region (Parsons et al. 1969 a; Parsons et al. 1970; Shim: 1977; Stockner et al. 1979; Harrison et al. 1983), although only one other researcher, Shim (1977), has reported the second large bloom in June. It has been well established that the presence o f a large spring bloom is a phenomenon that occurs regularly in coastal regions o f temperate; latitudes. The spring bloom develops in response to several factors including the stabilization of the water column, an increase in nutrients at the surface following winter mixing, and an increase in light availability or irradiance associated with an increase in day length. The sharp decline in phytoplankton biomass can be attributed to a decline in nutrient availability, wind mixing, advection and grazing. A l l these factors are possible in explaining the decrease in phytoplankton biomass in A p r i l in the Strait o f Georgia, although grazing probably accounts for the greatest proportion of the decline since nutrient concentrations are high when the decline commences. 92 The development of a second bloom in the Strait appears to be a response to the relief in grazing pressure associated with the migration of zooplankton, particularly N. plumchrus, out o f surface waters. The remaining zooplankton are also herbivorous, however, they are more likely to undergo diel migration, remaining at depth during the day, allowing the phytoplankton population to regenerate, and migrating up to the surface at night to graze. The presence o f N. plumchrus has been wel l documented in the Strait and its life history cycle was studied extensively by Fulton (1973) and Gardner (1972). These researchers reported that a large peak in zooplankton biomass, composed o f more than 75% N. plumchrus, generally occurred in May. This study observed a peak in zooplankton biomass in late A p r i l . The existence o f several large datasets was vital to the detection of this change and therefore the biological oceanographic properties o f this ecologically and economically important region should continue to be monitored in the future. Copepodite stage composition data were collected for all available years in order to determine i f this observed change in the timing o f the zooplankton peak is a result o f a change in the developmental timing o f N. plumchrus. It was established that there has in fact been a change in the developmental timing o f this copepod in the Strait. This change was on the order o f 25 days, and is similar in magnitude to that observed by Mackas et al. (1998) at OSP. The earlier arrival of this large zooplankton biomass must have an impact on the food webs in this region, and w i l l therefore directly or indirectly impact food availability for juvenile salmon residing in the Strait at this time and who are feeding high up on the food chain. Changes to the availability of prey for juvenile salmon in the Strait have not yet been reported and therefore the extent of this change in N. plumchrus timing is not known. The cause o f this shift in timing is also not known, although preliminary data suggest that a warming o f the surface waters over the past 30 93 years could have precipitated this change. In contrast it, is unlikely that a change in the water temperature at OSP is directly forcing the change in N. plumchrus development. Regardless, further studies are required in order to determine the primary cause of this change in both regions. N. plumchrus developmental timing in the Bering and the Japan Seas has also been studied routinely over the past several decades (Cooney and Coyle 1982; Mi l l e r and Terazaki 1989). These and other areas in the North Pacific where N. plumchrus is dominant, should also be examined in a effort to determine i f the change in developmental timing is a widespread phenomenon. The evidence from both OSP and the Strait suggests this species is responding to a change in oceanic temperature over the past several decades. In addition, this examination should be extended to other species o f copepods, or even other zooplankton in order to determine i f other species are responding to changes in oceanic climate. Future research should also examine the impact of temperature on the duration o f the different copepodite stages of N. plumchrus from both the Strait and from OSP. Future research should include the testing o f the three proposed hypotheses, and a more detailed study concerning the life history development o f JV. plumchrus. These studies should examine feeding characteristics and food preferences, the retention and use o f lipids, and the nature o f migration o f the nauplii to the surface (active vs. passive migration). Changes in copepod timing should be continuously monitored, at both Station P and in the Strait o f Georgia, and efforts should be made to determine the consequences of this timing change on local food webs. For example, gut content analyses o f juvenile salmon and herring should be conducted to determine i f there has been a dietary shift for salmon over the 30 year period, i f samples are available. 94 Literature Cited Bathmann U . V . , T.T. Noj i , and B.von Bodungen. 1990. Copepod grazing potential in late winter in the Norwegian Sea - a factor in the control o f spring phytoplankton growth? Mar. Ecol. Prog. 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Dudley P .L . 1986. Aspects of general body shape and development in Copepoda. In: Schriever G , H . K . Schminke, and C.-T. Shih (ed.). Proceedings of the 2nd International Conference on Copepoda. Ottawa, Canada, 13-17 August 1984. National Museums o f Canada, Ottawa, O N , Canada, p 7-25. Evanson M . E . , E . A . Bornhold, R . H . Goldblatt, P.J. Harrison, and A . G . Lewis. 1999. Temporal variation in body composition and l ipid storage of the overwintering, subarctic copepod, Neocalanus plumchrus (Marukawa) in the Strait o f Georgia. Mar. Ecol. Prog. Ser. in press Falkenhaug T., K . S . Tande, and T. Semenova. 1997. Die l , seasonal and ontogenetic variations in the vertical distributions of four marine copepods. Mar. Ecol. Prog. Ser. 149:105-119. Falk-Petersen S., J.R. Sargent, and K . S . Tande. 1987. L i p i d composition o f zooplankton in relation to the subarctic food web. Polar Biol. 8:115-120. Fissel D . B . , J.R. Birch, and R . A . J . Chave. 1991. 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Spring bloom in the central Strait o f Georgia: interactions of river discharge, winds and grazing. Mar. Ecol. Prog. Ser. 138:255-263. Y i n K . , P.J. Harrison, R . H . Goldblatt, M . A . St. John, and R . J . Beamish. 1997. Factors controlling the spring bloom in the Strait o f Georgia estuary, British Columbia, Canada. Can. J. Fish. Aquat. Sci. 54:1985-1995. 101 Append ix A . Inventory of stations, ships used, samples collected and analyzed. notes Station Date Ship Btl. depth Chi Bongo depth DW lDFOl 12-Feb-96 R. B. Young 1 Y 400 Y 5 Y 400 Y 10 Y 50 Y 15 Y 50 Y 20 Y 50 Y 1DF02 12-Feb-96 Clupea ; I Y 50 Y 5 Y 50 Y 10 Y 15 Y 20 Y 50 Y 1DF03 12-Feb-96 Clupea 1 Y 50 Y 5 Y 50 Y 10 Y 15 Y 20 Y 50 Y 2DF01 Clupea 1 Y 400 Y 5 N 50 Y 10 N 50 Y 15 N 20 N 50 N 3DF01 26-Mar-96 Clupea 1 Y 375 . Y 5 Y 50 Y 10 Y 50 Y . 15 Y 20 Y 50 Y 3DF02 26-Mar-96 Clupea 1 Y 50 Y 5 Y 50 Y 10 Y 15 Y 20 Y 50 Y 3DFG3 26-Mar-96 Clupea 1 Y 50 Y 5 Y 50 Y 10 Y 15 Y 20 Y 50 Y 4DFQ1 08-Apr-96 Caligus 1 Y 400 Y 5 Y 400 Y 102 Station Date Ship Bt l . depth Ch i Bongo D W notes 4DFQ2 08-Apr-96 Caligus 4DFQ3 08-Apr-96 Caligus 5 D F O ! 26-Apr-96 Caligus 5DF02 26-Apr-96 Caligus 5DFQ3 26-Apr-96 Caligus 6DFQ1 08-May-96 Clupea 6DF02 08-May-96 Clupea i depth  10 Y 50 Y 15 Y 50 Y 20 Y 50 Y 1 Y 50 Y 5 Y 50 Y 10 Y 15 Y 20 Y 50 Y 1 Y 50 Y 5 Y 50 Y 10 Y 15 Y 20 Y 50 Y 1 Y 50 Y 5 Y 50 Y 10 Y 15 Y 20 Y 50 Y 1 Y 50 Y 5 Y 50 Y 10 Y 15 Y 20 Y 50 Y 1 Y 50 Y 5 Y 50 Y 10 Y 15 Y 20 Y 50 Y 1 Y 50 Y 5 Y 50 Y 10 Y 15 Y 20 Y 50 Y 1 Y 50 Y 5 Y 50 Y 10 Y 15 Y 20 Y 50 Y Station Date Ship Btl. depth Chi Bongo DW notes depth 6DF03 08-May-96 Clupea 7DFO! 23-May-96 Clupea 7DFQ2 23-May-96 Clupea 7DFQ3 23-May-96 Clupea 8DF01 ll-Jun-96 Clupea 8DFQ2 ll-Jun-96 Clupea 8DFQ3 ll-Jun-96 Clupea 9DFO! 03-Jul-96 Clupea 1 Y 50 Y 5 Y 50 Y 10 Y 15 Y 20 Y 50 Y 1 Y 50 Y 5 Y 50 Y 10 Y 15 Y 20 Y 50 Y 1 Y 50 Y 5 Y 50 Y 10 Y 15 Y 20 Y 50 Y 1 Y 50 Y 5 Y 50 Y 10 Y 15 Y 20 Y 50 Y 1 Y 50 Y 5 Y 50 Y 10 Y 15 Y 20 Y 50 Y 1 Y 50 Y 5 Y 50 Y 10 Y 15 Y 20 Y 50 Y 1 Y 50 Y 5 Y 50 Y 10 Y 15 Y 20 Y 50 Y 1 Y 50 Y 5 Y 50 Y 10 Y 104 Station Date Ship Bt l . depth Chi Bongo D W notes depth 9DF02 03-M-96 Clupea 1 Y 50 Y Y 9DF03 03-Jul-96 Clupea 1 Y 50 Y Y 10DFO1 21-Jan-97 Clupea 10DFO2 21-Jan-97 Clupea 10DFO3 21-Jan-97 Clupea H D F O l 10-Feb-97 R. B . Young 11DF02 10-Feb-97 R . B . Y o u n g 15 Y 20 Y 50 Y  5 Y 50 10 Y 15 Y 20 Y 50 Y  5 Y 50 10 Y 15 Y 20 Y 50 Y 1 Y . 50 5 Y 50 10 Y 200 15 Y 400 20 Y 50 Y 1 Y 50 5 Y 50 10 Y 15 Y 20 Y 50 Y 1 Y 50 5 Y 50 10 Y 15 Y 20 Y 50 Y 1 Y 50 5 Y 50 10 Y 200 15 Y 400 20 Y 400 50 Y 1 Y 50 5 Y 50 10 Y 15 Y 20 Y 50 Y 105 Station Date Ship Btl. depth Chi Bongo DW notes depth 11DF03 10-Feb-97 R.B.Young 12DFO! 04-Mar-97 Caligus 12DFQ2 04-Mar-97 Caligus 12DFQ3 04-Mar-97 Caligus 13DFO! 25-Mar-97 Caligus 13DFQ2 25-Mar-97 Caligus 13DF03 25-Mar-97 Caligus 14DFO! 08-Apr-97 UBC 1 Y 50 5 Y 50 10 Y 15 Y 20 Y 50 Y 1 Y 50 5 Y 50 10 Y 200 15 Y 390 20 Y 400 50 Y 1 Y 50 5 Y 50 10 Y 15 Y 20 Y 50 Y 1 Y 50 5 Y 50 10 Y 15 Y 20 Y 50 Y 1 Y 50 5 Y 50 Y 10 Y 200 Y 15 Y 400 Y 20 Y 50 Y 1 Y 50 Y 5 Y 50 10 Y 15 Y 20 Y 50 Y 1 Y 50 Y 5 Y 50 10 Y 15 Y 20 Y 50 Y 1 Y 400 Y 5 Y 50 10 Y 106 Station Date Ship Btl. depth Chi Bongo DW notes depth 15DFO! 15-Apr-97 Clupea 15DF03 15-Apr-97 Clupea 16DF01 02-May-97 Caligus 1 Y 200 Y Y 16DF02 02-May-97 Caligus 16DF03 02-May-97 Caligus 17DFO! 22-May-97 Clupea 17DF02 22-May-97 Clupea 15 Y 20 Y 50 Y 1 Y 50 5 Y 50 10 Y 15 Y 20 Y 50 Y 1 Y 50 5 Y 50 10 Y 15 Y 20 Y 50 Y   5 Y 50 10 Y 50 15 Y 20 Y 50 Y 1 Y 50 5 Y 50 10 Y 15 Y 20 Y 50 Y 1 Y 50 5 Y 50 10 Y 15 Y 20 Y 50 Y 1 Y 50 5 Y 50 10 Y 200 15 Y 400 20 Y 50 Y 1 Y 50 5 Y 50 10 Y 15 Y 20 Y 50 Y 107 Station Date Ship Btl. depth Chi Bongo DW notes depth 17DFQ3 22-May-97 Clupea 18DFO! 10-Jun-97 Caligus 18DFQ2 10-Jun-97 Caligus 18DF03 10-Jun-97 Caligus 19DFO! 09-Jul-97 Clupea 20DFO1 06-Aug-97 Clupea 21DFO! 03-Sep-97 Caligus 22DFO! 27-Oct-97 J.P. Tully 1 Y 50 5 Y 50 10 Y 15 Y 20 Y 50 Y 1 Y 50 5 Y 50 10 Y 200 15 Y 400 20 Y 50 Y 1 Y 50 5 Y 50 10 Y 15 Y 20 Y 50 Y 1 Y 50 5 Y 50 10 Y 15 Y 20 Y 50 Y 1 Y 400(v) 5 Y 400(v) 10 Y 400(o) 15 •Y 200(o) 20 Y 50(o) 50 Y 1 Y 5 Y 10 Y 15 Y 20 Y 50 Y 1 Y 5 Y 10 Y 15 Y 20 Y 50 Y 0 Y 400(blk) 5 Y 400(wht) 10 Y 200(blk) Y 108 Station Date Ship B t l . depth Ch i Bongo depth D W notes 23DFO! 25-Nov-97 Clupea 24DFO! 18-Dec-97 Clupea 25DFO! 20-Jan-98 Clupea 26DFO! 03-Feb-98 Clupea 27DFO! 26-Feb-98 Vector 27DF02 26-Feb-98 Vector 27DF03 26-Feb-98 Vector 15 Y 200(whte) Y 20 Y 50(blk) Y 30 Y 50(whte) 50 Y 571-285 (o) 250 N 400 N 1 Y 400 Y 5 Y 200 Y 10 Y 50 Y 15 Y 50 Y 30 Y 50 Y 1 Y 400 Y 5 Y 200 Y 10 Y 50 Y 15 Y 50 Y 20 Y 50 Y 1 Y 400 N 5 Y 200 Y 10 Y 50 Y 15 Y 20 Y 50 Y 1 Y 400 Y 5 Y 200 Y 10 Y 50 Y 15 Y 20 Y 50 Y 1 Y 400 5 Y 400 10 Y 200 Y 15 Y 50 Y 20 Y 50 50 Y 1 Y 50 Y 5 Y 50 Y 10 Y 15 Y 20 Y 50 Y 1 Y 50 Y 5 Y 50 Y 10 Y 109 Station Date Ship Btl. depth Chi Bongo depth D W 15 Y 20 Y 50 Y 28DF01 17-Mar-98 Clupea 1 Y 400 Y 5 Y 200 N 10 Y 50 Y 15 Y 20 Y 50 Y 29DF01 25-Mar-98 Clupea 1 Y 400 N 5 Y 200 N 10 Y 50 N 15 Y 20 Y 50 Y 30DFO1 31-Mar-98 Vector 1 Y 400 N 5 Y 200 N 10 Y 50 N 15 Y 20 Y 50 Y 30DFO2 31-Mar-98 Vector 1 Y 50 N 5 Y 10 Y 15 Y 20 Y 50 Y 30DFO3 31-Mar-98 Vector 1 Y 50 N 5 Y 10 Y 15 Y 20 Y 50 Y 32DF01 22-Apr-98 Clupea 1 Y 400 Y 5 Y 200 Y 10 Y 50 Y 15 Y 20 Y 50 Y 33DFO! 08-May-98 Clupea 1 Y 400 Y 5 Y 200 Y 10 Y 50 Y notes ** huge amount of phyto in sample would not sieve *IOS is missing theirs too 110 Station Date Ship Bt l . depth Chi Bongo D W notes depth 34DFO! 20-May-98 Clupea 35DFO! 04-Jun-98 Vector 35DF02 04-Jun-98 Vector 35DF03 04-Jun-98 Vector 15 Y 20 Y 50 Y 1 Y 400 Y 5 Y 200 Y 10 Y 50 Y 15 Y 20 Y 50 Y 1 Y 400 Y 5 Y 200 Y 10 Y 50 Y 15 Y 20 Y 50 Y 1 Y 50 Y 5 Y 10 Y 15 Y 20 Y 50 Y 1 Y 50 Y 5 Y 10 Y 15 Y 20 Y 50 Y Note: (v) = vertical tow (o) = oblique tow (blk) = black nets from IOS (whte) = white nets from U B C I l l Appendix B. Sources o f data, dates sampled, method o f sampling, and depths sampled for data used in Chapter 2. Source Dates of sampling Chl-a sampling method Depth used for surface chl-a estimates Net type Zoop sampling depth Units of reported biomass Stephens (1968) Mar-Sept 1966; Jan-Dec 1967 (monthly) Pump 3 m n/a n/a n/a . Stephens etal. (1969) Approx monthly 1966-68 Bottles 1,5, 10, and 20 m (avg) Hensen net 330 pm 20 m mg-m"3, wet weight Stockner et al. (1979) Approx every 6 weeks 1975-77 Bottles 1 and 3 m (avg) SCOR 350 pm; 50 m mg m"3, dry weight this study Feb-June 1996 Jan-Dec 1997 Jan-June 1998 Bottles 0 and 5 m (avg) Bongo 202 pm 50 m mg m"3, dry weight 112 Appendix C. Raw chorophyll and zooplankton data, 1966-1968,1975-1977, and 1996-1998. Year Julian Day Zoop (mg m 3) Chi (mg m"3) 1966 10 2.06 0.5 20 2:06 0.6 (Stephens et al. 1969) 46 1.11 2.0 59 2.06 64 11.58 69 3.0 71 4.0 74 12.77 5.0 79 94.92 6.0 89 118.73 6.0 95 118.73 5.0 105 130.63 3.0 120 261.58 4.0 135 249.68 4.0 140 237.77 3.0 161 202.06 ' 6.5 166 53.25 8.0 171 6.5 181 35.39 4.5 196 112.77 2.0 211 118.73 1.5 227 4.0 242 9.20 6.5 258 7.42 1.5 288 15.15 334 1.46 349 0.87 1967 20 1.46 0.5 (Stephens et al. 32 1.46 0.7 1969) 46 2.06 1.4 60 4.44 1.5 69 13.96 1.1 79 100.87 1.2 91 118.73 2.3 105 202.06 1.8 130 511.58 3.0 140 666.35 152 166.35 1.0 156 160.39 166 47.30 6.7 181 23.49 6.1 196 106.82 0.8 201 118.73 1.3 244 41.35 1-5 258 13.96 2.1 278 17.54 288 1.82 1.1 Year Julian Day Zoop (mg m"3) Chi (mg m"3) 305 1.11 0.5 335 1.11 0.6 1968 5 1.46 0.5 (Stephens et al. 46 5.63 1.5 1969) 56 6.82 65 23:49 70 29.44 1.5 75 249.68 80 190.15 91 213.96 1.0 122 318.73 136 11.58 3.0 162 29.44 6.5 187 23.49 1.5 218 5.04 254 1.11 294 1.23 339 1.11 1975 84 5.60 0.7 (Stockner et al. 120 17.00 1.0 1979) 168 37.00 296 16.80 0.9 1976 29 2.30 0.5 (Stockner et al. 78 2.70 0.9 1979) 127 148.20 3.3 197 14.70 2.5 251 64.10 ' 2.1 316 3.30 4.7 1977 36 2.90 0.5 (Stockner et al. 76 2.90 1.8 1979) 118 111.50 3.1 173 13.60 7.2 244 6.10 2.2 1996 43 9.25 0.8 (this study) 86 33.50 5.5 98 227.15 0.8 116 271.47 0.9 128 99.60 9.7 143 36.24 9.2 162 74.20 6.3 184 65.34 1.9 1997 21 22.32 0.7 (this study) 41 46.63 0.7 63 1.6 84 30.17 1.3 98 59.27 17.2 105 6.67 6.9 122 12.13 3.6 142 167.06 5.1 161 130.87 8.3 184 5.6 Year Julian Day Zoop (mg m"3) Chi (mg m"3) 1998 20 54.65 1.3 (this study) 34 1.1 57 62.22 1.7 76 299.80 7.7 84 - - 12.4 90 9.6 112 436.21 1.1 128 823.84 0.8 141 148.37 10.5 155 209.24 1.2 115 Appendix D. Sources o f data, dates sampled, method o f sampling, depths sampled, and copepodite stages identified for data used in Chapter 3. Year Reference Date Span Sampling Methods Depth Ranges Copepodite Stages Identified 1967 1971 1981 1996-97 Parsons et al. (1969a,b) Gardner (1972) Black (1984) this study biweekly, Feb 13-May 23 April , May, June biweekly, Mar 30 - Jun 14 biweekly, Jan -July Vertical hauls, 0.25 m 2 ring net, 0.330 mm mesh; Van Dorn bottles; horizontal Miller net tows Horizontal tows, Clarke -Bumpus nets, 0.300 mm mesh Oblique hauls, 0.5 m 2 bongo net, 0.351 mm mesh Vertical hauls, 0.27 m 2 bongo net, 0.202 mm mesh 0 - 2 0 m C 1 , C 3 , C 5 Spaced C1-C5 from 0 -near bottom 0 - near C3-C5 bottom 0 - 5 0 m , C1-C5 0 - 200 m, 0 - 400 m Appendix E . Average nutrient concentrations per depth for Stations 1-3, 1996-1 998. Date Stn Depth (m) N 0 3 NH4 Urea P 0 4 Si 12-Feb-96 1DF01 1 25.07 0.20 0.08 2.07 1DF01 5 23.31 0.26 0.00 1.94 1DF01 10 27.86 0.00 0.06 2.35 1DF01 15 29.87 0.00 0.04 3.03 1DF01 20 29.88 0.09 0.09 3.32 1DF01' 50 26.43 0.03 0.04 2.26 1DF02 1 0.00 0.62 0.75 0.24 1DF02 5 0.14 1.09 0.93 0.22 1DF02 10 14.75 0.97 0.16 1.40 1DF02 15 16.60 0.45 0.00 1.51 1DF02 20 21.07 0.47 0.15 1.86 1DF02 50 22.14 0.41 0.03 1.96 1DF03 1 0.05 0.31 0.12 0.16 1DF03 5 0.04 0.30 0.08 0.19 1DF03 10 0.00 0.47 0.00 0.32 1DF03 15 13.80 0.85 0.40 1.39 1DF03 20 24.62 0.58 0.22 1.99 1DF03 50 27.69 0.36 0.12 2.06 26-Mar-96 3DF01 1 25.04 2.53 2.08 1.42 3DF01 5 24.98 0.24 0.15 1.59 3DF01 10 22.60 0.00 0.00 1.72 3DF01 15 25.77 0.18 0.75 1.77 3DF01 20 28.81 0.35 0.40 2.13 3DF01 50 26.89 0.25 0.33 1.92 3DF02 1 26.88 0.14 0.01 2.05 3DF02 5 27.57 0.06 0.09 2.25 3DF02 10 28.20 0.00 0.02 2.29 3DF02 15 27.42 0.22 0.11 2.25 3DF02 20 27.25 0.02 0.06 2.26 3DF02 50 29.83 0.06 0.00 2.66 3DF03 1 20.13 0.00 0.01 1.72 3DF03 5 26.56 0.31 0.13 2.14 3DF03 10 20.98 0.00 0.05 1.72 3DF03 15 22.67 0.03 0.00 1.97 3DF03 20 26.05 0.13 0.10 2.25 3DF03 50 26.98 0.17 0.07 2.38 8-Apr-96 4DF01 1 8.70 1.19 0.04 1.06 4DF01 5 13.01 2.30 0.17 1.45 4DF01 10 18.71 1.47 0.05 1.89 4DF01 15 23.79 2.19 0.48 2.41 4DF01 20 25.73 1.22 0.04 2.28 4DF01 50 27.12 0.16 0.00 2.27 4DF02 1 17.35 2.37 0.13 1.81 4DF02 5 18.03 4.16 1.47 1.79 4DF02 10 25.91 0.53 0.10 2.40 4DF02 15 25.45 0.25 0.01 2.42 4DF02 20 27.96 1.01 0.19 2.58 4DF02 50 23.09 2.19 0.35 2.21 4DF03 1 " 8.32 2.39 0.13 0.99 117 26-Apr-96 8-May-96 23-May-96 4DF03 5 8.33 3.15 0.38 1.05 4DF03 10 8.65 2.25 0.20 1.17 4DF03 15 7.01 2.06 0.17 1.00 4DF03 20 10.11 3.38 0.30 1.18 4DF03 50 25.89 0.42 0.00 2.39 5DF01 1 6.10 0.31 0.06 0.73 5DF01 5 14.04 0.25 . 0.04 1.50 5DF01 10 12.61 2.39 2.54 1.26 5DF01 15 11.65 0.28 0.00 1.37 5DF01 20 14.72 1.14 0.11 1.46 5DF01 50 29.36 0.12 0.03 2.74 5DF02 1 13.46 1.14 0.54 1.36 5DF02 5 17.42 0.29 0.09 1.80 5DF02 10 11.85 0.67 0.00 1.26 5DF02 15 10.20 0.37 0.03 1.74 5DF02 20 21.47 0.32 0.17 2.10 5DF02 50 16.89 0.81 0.08 1.54 5DF03 1 10.47 1.35 0.57 0.96 5DF03 5 11.49 1.93 0.64 1.13 5DF03 10 12.32 1.36 0.65 1.27 5DF03 15 12.79 0.30 0.00 0.20 5DF03 20 19.20 0.07 0.12 0.20 5DF03 50 22.70 0.81 0.27 0.23 6DF01 1 11.14 0.33 0.04 1.31 6DF01 5 7.85 0.68 0.05 0.27 6DF01 10 20.29 0.33 0.05 6DF01 15 20.66 0.39 0.16 6DF01 20 20.97 0.28 0.00 6DF01 50 23.96 0.16 0.36 6DF02 1 11.40 0.31 0.26 6DF02 5 0.50 0.00 0.26 6DF02 10 21.47 0.65 0.28 6DF02 15 14.02 0.87 0.28 6DF02 20 22.70 0.72 0.24 6DF02 50 24:00 0.25 0.10 6DF03 1 3.73 0.04 0.13 6DF03 5 8.85 0.27 0.15 6DF03 10 18.00 0.57 0.23 6DF03 15 17.00 0.64 0.23 6DF03 20 20.00 0.79 0.23 6DF03 50 22.00 0.54 0.25 7DF01 1 0.00 0.03 0.00 0.00 7DF01 5 0.97 0.04 0.09 0.12 7DF01 10 6.09 0.18 0.05 0.62 7DF01 15 16.68 0.25 0.15 1.76 7DF01 20 21.73 0.14 0.07 2.19 7DF01 50 21.82 : 0.00 0.06 2.27 7DF02 1 0.00 0.00 0.08 0.15 7DF02 5 0.73 0.13 0.07 0.26 7DF02 10 12.75 0.18 0.00 1.33 7DF02 15 16.12 0.33 0.13 1.72 7DF02 20 15.58 0.53 0.06 1.46 7DF02 50 17.93 0.22 0.05 1.71 11-Jun-96 3-Jul-96 21-Jan-97 7DF03 1 0.04 0.79 0.00 0.16 7DF03 5 0.00 0.04 0.06 0.12 7DF03 10 13.94 1.03 0.07 1.37 7DF03 15 18.61 0.59 0.09 1.89 7DF03 20 15.71 0.11 0.01 1.74 7DF03 50 23.73 0.05 0.04 2.59 8DF01 1 0.00 0.35 0.21 0.16 8DF01 5 0.99 0.00 0.05 0.44 8DF01 10 13.07 0.48 0.08 1.78 8DF01 15 10.29 0.59 0.09 1.39 8DF01 20 15.43 0.96 0.30 1.85 8DF01 50 25.91 0.11 0.06 2.84 8DF02 1 0.09 0.14 0.03 0.18 8DF02 5 0.00 0.00 0.03 0.20 8DF02 10 13.09 0.71 0.15 1.61 8DF02 15 20.96 0.32 0.13 2.38 8DF02 20 23.23 0.15 0.16 2.64 8DF02 50 25.48 0.19 0.06 2.68 8DF03 1 0.11 0.00 0.07 0.46 8DF03 5 0.00 0.00 0.10 0.24 8DF03 10 14.33 0.24 0.15 1.65 8DF03 15 21.23 0.05 0.08 2.38 8DF03 20 19.32 0.38 0.13 2.05 8DF03 50 19.91 0.17 0.19 2.08 9DF01 1 0.19 0.37 0.30 0.23 9DF01 5 0.00 0.93 0.27 0.42 9DF01 10 16.48 0.25 0.22 2.05 9DF01 15 16.48 0.12 0.21 2.01 9DF01 20 21.27 0.00 0.15 2.48 9DF01 50 22.31 0.33 0.34 2.36 9DF02 1 0.52 0.50 0.23 0.33 9DF02 5 0.63 0.94 0.26 0.48 9DF02 10 16.33 0.06 0.20 2.14 9DF02 15 22.49 0.00 0.27 2.62 9DF02 20 24.50 0.01 0.19 2.71 9DF02 50 25.81 0.22 0.37 2.77 9DF03 1 2.68 4.03 1.09 0.61 9DF03 5 5.08 0.62 0.26 0.83 9DF03 10 7.67 0.35 0.29 1.31 9DF03 15 5.65 0.30 0.26 1.09 9DF03 20 7.71 0.13 0.32 1.41 9DF03 50 20.44 0.07 0.34 2.25 10DFO1 1 27.28 0.87 10DFO1 5 29.16 1.56 10DFO1 10 24.59 1.20 10DFO1 15 30.63 1.63 10DFO1 20 32.87 1.43 10DFO1 50 30.87 1.41 10DFO2 1 16.83 0.42 10DFO2 5 33.27 1.49 10DFO2 10 26.49 0.95 10DFO2 15 14.81 0.92 10DFO2 20 7.29 0.56 10-Feb-97 4-Mar-97 25-Mar-97 10DFO2 50 9.93 0.77 25.87 10DFO3 1 14.99 0.52 33.12 10DFO3 5 11.23 0.50 29.91 10DFO3 10 15.12 0.86 30.32 10DFO3 15 20.86 1.11 35.12 10DFO3 20 17.75 1.02 23.78 10DFO3 50 24.18 1.51 33.44 11DF01 1 30.73 1.16 15.15 11DF01 5 24.37 1.13 17.76 11DF01 10 27.55 1.00 19.27 11DF01 15 29.74 1.41 25.73 11DF01 20 27.21 1.43 44.75 11DF01 50 35.21 0.91 26.06 11DF02 1 31.13 1.25 27.77 11DF02 5 34.12 1.25 34.43 11DF02 10 41.64 1.59 35.04 11DF02 15 27.53 1.59 30.60 11DF02 20 28.75 1.33 23.38 11DF02 50 47.83 0.78 24.72 11DF03 t 29.15 1.33 30.89 11DF03 5 28.33 1.47 34.85 11DF03 10 29.97 1.46 58.29 11DF03 15 25.37 0.88 22.50 11DF03 20 34.52 1.63 31.01 11DF03 50 37.87 1.00 31.16 12DF02 1 30.18 1.23 40.44 12DF02 5 25.69 0.28 32.08 12DF02 10 30.13 0.74 58.31 12DF02 15 35.57 1.16 81.67 12DF02 20 27.14 0.87 35.83 12DF02 50 40.77 2.11 74.56 12DF03 1 8.63 0.14 20.80 12DF03 5 10.45 0.14 27.43 12DF03 10 5.59 0.15 21.10 12DF03 15 19.37 1.26 40.29 12DF03 20 23.13 1.68 45.34 12DF03 50 20.85 1.50 37.11 13DF01 1 22.70 1.06 1.05 2.98 13DF01 5 17.64 0.55 0.32 1.60 13DF01 10 24.72 0.75 0.32 2.19 13DF01 15 22.74 0.57 0.14 2.37 13DF01 20 23.48 0.40 0.50 2.32 13DF01 50 28.78 0.16 0.14 2.67 13DF02 1 16.86 0.74 0.00 1.77 13DF02 5 19.22 1.18 0.36 2.33 13DF02 10 17.44 0.72 0.55 1.70 13DF02 15 25.35 0.79 0.36 2.48 13DF02 20 25.06 0.90 0.18 2.54 13DF02 50 26.80 0.62 0.00 2.77 13DF03 1 21.10 0.86 0.91 1.51 13DF03 5 17.49 0.63 0.36 1.43 13DF03 10 16.53 1.05 0.36 1.92 13DF03 15 26.90 0.46 0.36 2.57 120 8-Apr-97 15-Apr-97 2-May-97 22-May-97 13DF03 20 29.92 0.86 0.18 2.67 13DF03 50 28.06 1.01 0.55 2.76 14DF01 1 9.99 0.59 0.36 0.49 14DF01 5 22.80 0.84 0.18 1.55 14DF01 10 23.59 0.79 0.36 1.83 14DF01 15 29.27 0.70 0.36 2.27 14DF01 20 29.35 0.88 0.36 2.43 14DF01 50 30.93 0.41 0.18 2.60 14DF03 1 8.46 0.95 0.73 0.50 14DF03 5 3.27 1.10 0.55 0.25 14DF03 10 4.27 1.01 0.55 0.32 14DF03 15 2.86 1.41 0.36 0.53 14DF03 20 23.76 2.47 0.18 2.02 14DF03 50 32.68 0.74 0.18 2.50 15DF01 1 18.59 0.95 0.00 1.55 15DF01 - 5 16.68 0.66 0.00 1.43 15DF01 10 21.37 1.21 0.00 1.91 15DF01 15 13.94 0.82 0.18 1.15 15DF01 20 31.12 1.06 0.36 2.52 15DF01 50 31.24 1.01 0.36 1.88 16DF01 1 12.74 2.27 0.66 16DF01 5 10.63 2.25 1.01 16DF01 10 14.69 2.95 0.99 16DF01 15 26.59 2.42 2.02 16DF01 20 25.83 3.04 2.07 16DF01 50 29.03 0.45 2.17 16DF02 1 10.74 1.85 0.73 16DF02 5 10.07 1.79 0.78 16DF02 10 13.18 1.79 0.65 16DF02 15 11.38 2.24 1.05 16DF02 20 20.90 3.10 1.61 16DF02 50 36.48 0.54 2.64 16DF03 1 3.95 0.38 0.15 16DF03 5 7.29 0.81 0.26 16DF03 10 10.51 2.75 1.12 16DF03 15 28.76 1.39 2.17 16DF03 20 35.41 0.65 1.89 16DF03 50 36.19 0.55 2.31 17DF01 1 9.30 1.22 0.18 0.29 17DF01 5 1.49 0.31 0.18 0.26 17DF01 10 9.21 1.77 0.55 2.85 17DF01 15 20.16 2.50 0.91 3.30 17DF01 20 20.66 2.49 0.18 2.16 17DF01 50 32.40 0.46 0.36 2.86 17DF02 1 0.00 0.07 0.18 0.04 17DF02 5 7.38 0.35 0.55 0.13 17DF02 10 0.91 0.34 0.36 1.89 17DF02 15 5.76 0.42 0.00 0.67 17DF02 20 24.18 0.92 0.18 1.32 17DF02 50 25.88 0.17 0.18 2.35 17DF03 1 6.22 0.59 0.73 1.87 17DF03 5 3.69 0.56 0.36 2.29 10-Jun-97 6-Aug-97 3-Sep-97 27-Oct-97 26-Feb-98 17DF03 10 0.78 0.19 0.00 0.14 17DF03 15 6.88 0.55 0.55 1.95 17DF03 20 22.09 1.67 0.55 1.78 17DF03 50 30.01 1.25 0.18 2.89 18DF01 1 2.98 0.26 0.55 0.06 18DF01 5 3.69 0.44 0.73 0.41 18DF01 10 25.16 0.56 0.18 0.85 18DF01 15 13.18 1.70 0.18 1.55 18DF01 20 19.72 2.44 0.55 2.35 18DF01 50 23.12 0.32 0.36 2.34 18DF02 1 0.66 0.42 0.73 2.41 18DF02 5 2.28 0.71 0.73 1.12 18DF02 10 15.74 2.01 0.18 1.79 18DF02 15 17.23 1.17 0.55 2.00 18DF02 20 32.81 0.68 0.00 3.42 18DF02 50 33.01 0.35 0.36 2.78 18DF03 1 0.03 0.10 0.00 0.15 18DF03 5 13.29 1.02 0.36 1.67 18DF03 10 18.21 1.14 0.55 1.40 18DF03 15 23.88 0.35 0.36 2.43 18DF03 20 26.45 0.45 0.00 2.62 18DF03 50 24.46 0.31 0.36 2.20 20DFO1 1 0.87 0.15 0.36 0.00 20DFO1 5 1.20 0.32 0.36 0.09 20DFO1 10 6.54 0.24 0.00 0.69 20DFO1 15 18.62 0.97 1.09 1.34 20DFO1 20 29.91 0.92 0.73 1.43 20DFO1 50 18.74 0.63 0.55 1.69 21DF01 1 1.05 18.12 21DF01 5 3.29 17.17 21DF01 10 11.14 0.41 23.88 21DF01 15 25.17 0.61 22.24 21DF01 20 14.33 0.87 18.39 21DF01 50 29.68 1.44 26.25 22DF01 0 15.68 1.01 0.55 1.55 22DF01 4 22.21 2.06 0.00 22DF01 5 22.34 0.11 0.55 22DF01 10 24.90 0.12 0.18 22DF01 15 14.18 0.84 0.00 22DF01 20 15.34 0.06 0.36 22DF01 30 26.59 0.13 0.00 22DF01 50 26.13 0.15 0.55 22DF01 250 20.63 0.15 0.18 22DF01 400 19.59 1.65 0.55 27DF01 1 26.24 1.42 25.35 27DF01 5 31.13 1.27 21.09 27DF01 10 35.40 1.91 25.79 27DF01 15 37.53 2.06 37.19 27DF01 20 34.98 2.11 30.05 27DF01 50 31.39 2.01 24.50 27DF02 1- 17.70 0.84 18.81 f 122 27DF02 5 21.21 0.89 15.02 27DF02 10 11.10 0.66 19.03 27DF02 15 27.14 1.19 10.62 27DF02 20 12.30 0.67 13.76 27DF02 50 41.32 2.24 35.24 27DF03 1 36.70 1.71 28.32 27DF03 5 22.51 0.95 21.28 27DF03 10 36.42 2.02 36.56 27DF03 15 37.53 2.19 32.82 27DF03 20 29.97 1.64 25.13 27DF03 50 28.21 1.22 23.43 25-Mar-98 29DF01 1 14.48 0.53 8.68 29DF01 5 8.70 0.60 10.40 29DF01 10 17.17 0.63 10.06 29DF01 15 26.47 0.86 7.72 29DF01 20 51.67 1.49 14.02 29DF01 50 12.96 0.70 10.16 31-Mar-98 30DFO1 1 15.91 0.48 17.50 30DFO1 5 9.03 0.76 21.45 30DFO1 10 18.36 0.18 13.94 30DFO1 15 35.37 2.85 32.44 30DFO1 20 38.50 0.74 10.72 30DFO1 50 48.32 2.53 24.23 22-Apr-98 32DF01 1 16.49 • 6.31 32DF01 5 4.92 0.40 9.14 32DF01 10 10.66 0.75 17.56 32DF01 15 3.70 6.71 32DF01 20 23.91 1.67 18.88 32DF01 50 14.14 1.03 13.16 8-May-98 33DF01 1 3.79 0.15 6.14 33DF01 5 3.58 0.34 9.19 33DF01 10 5.08 0.10 5.66 33DF01 15 25.07 1.40 11.49 33DF01 20 31.66 2.07 33.07 33DF01 50 27.00 1.36 23.21 

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