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Ecology of the amphipods Parathemisto pacifica (Stebbing) and Cyphocaris challengeri (Stebbing) in the… Haro-Garay, Martha Jeannette 2002

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ECOLOGY OF THE AMPHJPODS Parathemisto pacifica (STEBBING) AND Cyphocaris challengeri (STEBBING) IN THE STRAIT OF GEORGIA: MANDIBLE MORPHOLOGY, FEEDING HABITS, AND FOOD DISTRIBUTION by MARTHA JEANNETTE HARO-GARAY B.Sc, Instituto Politecnico Nacional, 1983 M.Sc, Instituto Politecnico Nacional, 1991 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (Department of Earth and Ocean Sciences) We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA October 2001 © Martha Jeannette Haro-Garay, 2001 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 C ^ / " 7 X •g-v\-<?C C?Ce ^S^te. *\ ct^ The University of British Columbia Vancouver, Canada Date DE-6 (2/88) 11 Abstract In amphipods, morphological adaptations for feeding provide taxonomic evidence that helps to differentiate them into species. Intuitively, this indicates that the morphology of feeding structures may be related to the type of food the amphipods consume, and that their distribution would depend on the distribution of their food. These hypotheses were examined using zooplankton collections from three locations from the Strait of Georgia, British Columbia: the Fraser River plume, the estuarine plume and the Strait. The study investigated the mandible morphology, the stomach contents and the potential food for the amphipod species Parathemisto pacifica and Cyphocaris chalkngeri. Mandible morphology obtained with SEM photography and morphometric analyses were used to infer feeding habits. The mandible of P. pacifica has a wide molar indicative of microphagous habits and a cutting incisor related to carnivorous habits, while in C. chalkngeri, strong molar and sharp incisor in the mandible indicate carnivorous habits. The population dynamics indicated that the mean recruitment event for both species occurs in late spring. Recruits born then benefit from the available food during spring and summer, the most productive seasons in the Strait of Georgia. Temporal changes of zooplankton abundance and composition coincided, in general, with the seasonal increase of the river discharge volume; spatial variation coincided with the interaction of wind and tides with the Fraser River. The zooplankton species most commonly found in stomachs were the copepods Pseudocalanus minutus, Paracalanus parvus, Oithona sirrdUs, Oithona spinirostris, Oncaea borealis, Metridia pacifica, young stages of the euphausiid Euphausia pacifica, ostracods Conchoecia spinirostris and C. alata minor, and the pteropod Limacina helicina. These species had an overlapping distribution with the amphipod species studied. The presence of these microzooplankton species in the diet of amphipods suggests amphipods are an important link between nannoplankton (e.g. Dinophysis sp., Peridinium sp., Mesodinium sp.) and fish that consume amphipods (e.g. salmonids and clupeoids). This can be particularly influential in the trophic ecology of the Strait during the years of low abundance of the copepod Neocalanus plumchrus and the euphausiid Euphausia pacifica, the most abundant hervibores which salmonids feed on. The suggested trophic pathway is nannoplankton (20-30 u,m, e.g. flagellates, ciliates: Dinophysis sp., Peridinium sp. Mesodinium sp.) -> microzooplankton (20-200 urn, e.g. protozoans, tintinnids) -> mesozooplankton (2-20 mm, e.g. copepods, cladocerans, ostracods) macrozooplankton (2-20 cm, e.g. amphipods, euphausiids) -> fish (e.g. herring, salmon). This type of food chain is considered typical of oceanic areas such as the Pacific subarctic. Its occurrence simultaneous to the phytoplankton (chain forming diatoms) -> zooplankton (copepods, euphausiids) -> fish (e.g. herring, salmon) suggests that the model most appropriate to describe the trophic dynamics of the Strait of Georgia would be the multivorous food web. Ul Table of Content Abstract ii List of Tables v List of Figures vii Acknowledgments xii Chapter 1. Introduction 1 General introduction 2 Study area 3 Study subject 6 Study objective 7 Chapter 2. The mandible of Parathemisto pacified and Cyphocaris challengeri: Shape and function. 9 Introduction 10 Materials and methods 11 Results 15 Parathemisto pacifica 15 Description of mouthparts 15 Function of mouthparts 20 Cyphocaris chxdkngeri 20 Description of mouthparts 20 Function of mouthparts 24 Morphometries 24 Parathemisto pacifica 24 Cyphocaris cliailengeri 27 Discussion 30 Parathemisto pacifica 30 Cyphocaris challengeri 32 Chapter 3. Feeding habits and food distribution of amphipods 35 Introduction 36 3A: Stomach contents reflecting diet and zooplankton as potential food 36 Introduction 36 Materials and methods 37 Stomach contents 37 The relation between stomach content and zooplankton species abundance 38 Results 41 Stomach contents 41 Type of prey consumed 44 Parathemisto pacifica 44 Cyphocaris challengeri 45 Zooplankton as potential food for amphipods 45 Similarity analysis on zooplankton species composition 49 Discussion 56 iv 3B: Zooplankton composition and salinity structure as environmental cues of the habitat of amphipods 61 Introduction 61 Materials and methods 62 Samples source 62 Subsample preparation and species identification 64 Standardization of zooplankton abundance 65 Analysis of zooplankton composition and amphipod size-group distribution 67 Results 69 Zooplankton general abundance 69 Zooplankton composition, amphipod size-group distribution and salinity structure 70 I. Spring and summer changes (1990, 1991) 71 May 29-June 6 1990 71 April 7-16 1991 74 June (11-14) 1991. 77 II. Annual changes 80 January - December 1997 80 Discussion ; ; 86 I. Changes during spring and summer (1990, 1991) 87 II. Annual changes 88 Chapter 4.. Population dynamics of amphipods: Growth and mean size changes associated with plankton production 92 Introduction 93 Materials and methods 94 Escape and retention in a plankton net 95 Estimation of growth and mortality from size data 96 Results 101 Simulated size distributions and estimation of K and Z 101 Parathemisto pacifica 105 Parathemisto pacifica mean size changes and plankton variation 109 Cyphocaris challengeri - I l l Cyphocaris challengeri mean size changes and plankton variation 117 Discussion 118 Parathemisto pacifica 119 Cyphocaris challengeri 121 Chapter 5.. General discussion and conclusions 124 Discussion 125 Conclusions 132 Future work 134 Bibliography 136 Appendix A 149 List of Tables Table 3.1. Number of prey items (numerator) identified per number of stomachs analyzed (denominator) for samples collected during three years from the Strait of Georgia 41 Table 3.2. Zooplankton identified in the stomach contents of P. pacifica and C. challengeri. collected in the Strait of Georgia during 1990, 1991, and 1997 (see Table 3.1 for number of stomachs sampled). 42 Table 3.3. Zooplankton species related to P. pacifica with a Bray-Curtis similarity value > 25%. Symbols indicate: n = number of samples with the species, VV = double square root of individuals per 1000 m3, / = species present in stomach contents, • item identified only to genus level, f = female, m = male, CI-C5 = Copepodite 1 - Copepodite 5 46 Table 3.4- Zooplankton species related to C. challengeri with a Bray-Curtis similarity value >25%. Symbols indicate: n = number of samples with the species, VV = double square root of individuals per 1000 m3, / = species present in stomach contents, • item identified only to the genus level, f = female, m = male, CI-C5 = Copepodite 1 -Copepodite 5 48 Table 3.5. Kruskal - Wallis test by ranks to test Ho = Abundance is the same across the Fraser River plume, estuarine plume and the Strait of Georgia, for a = 0.05 (for June 1991, a = 0.01). Data used for the analyses are in Table A7. Nomenclature follows Zar (1996) 51 Table 3.6. Label code for the serni-diurnal sampling locations 63 Table 3.7. List of authors whose criteria were used to identify the zooplankton groups 65 Table 3.8. Abundance of zooplankton groups (in individuals per 1000 m3) present in the samples collected during three years from the Strait of Georgia. Data from 1997 correspond to a complete year 69 Table 3.9. Loadings of the most important species on the principal components for zooplankton. Samples collected from the riverine plume, estuarine plume and the Strait during May 29 - June 6, 1990. Stations where the highest variation occurred are in parentheses. Abbreviations indicate: f = female, frc = furcilia, C-3 = Copepodite 3 73 Table 3.10. Loadings of the most important species on the principal components for zooplankton. Samples collected from the riverine plume, estuarine plume and the Strait during April 7-16 1991. Stations where the highest variation occurred are in parentheses. Abbreviations indicate: f = female, est = estuarine, riv = riverine, plm= plume i 75 vi Table 3.11. Loadings of the most important species on the principal components for zooplankton. Samples collected from the riverine plume, estuarine plume and the Strait during June 11-14, 1991. Stations with highest variation are in parentheses. Abbreviations indicate: riv = riverine, est = estuarine, pi = plume 79 Table 3.12. Correlation coefficients between the proportion of individuals per group size and average salinity per depth layer from January to May, 1997 81 Table 3.13. Loadings of the most important species on the principal components for zooplankton. Samples collected from station DFOl during 1997. Seasons with the highest variation are indicated in parentheses. Abbreviations indicate: f = female, m= male, C-2 = Copepodite 2, C-3 = Copepodite 3 82 Table 4.1. Monthly mean length of Parathemisto pacifica collected at station DFOl during 1997 105 Table 4.2. Monthly mean length of Cyphocaris challengeri collected at station DFOl during 1997 113 Vll List of Figures Figure 1.1. Amphipod species studied. A) Cyphocaris chalkngeri (adult, approx. 12 mm), B) Parathemisto pacifica (adult, approx. 10 mm) 2 Figure 1.2. Area of study. Central Strait of Georgia. R, E, S, and DFOl are the sampling stations in this study. R = riverine plume, E = estuarine plume, and S = Strait 4 Figure 2.1. Schematic representation of an amphipod. A) Body parts*, B) Head showing mouthparts: MX1= maxillae 1, MX2= maxillae 2, MXP= maxilliped. C) Mandible**. *Redrawn from Bowman and Grunner (1970), ** Redrawn from Schmitz (1992) 13 Figure 2.2. Scanning electron microscope image of Parathemisto pacifica mandibles 16 Figure 2.3. Schematic representation of Parathemisto pacifica left mandible showing the incisor, molar, seta row, and a detail of a seta 16 Figure 2.4- Scanning electron microscope image of Parathemisto pacifica mandible. Notice the lamelliform rows with blunt cusps on the molar 17 Figure 2.5. Scanning electron microscope image of Parathemisto pacifica mouthparts showing the mandible and the mandible palp 18 Figure 2.6. Mouthparts of Parathemisto pacifica (left column) and Cyphocaris challengeri (after Bowman, 1960, and Barnard, 1958) 19 Figure 2.7. The labrum of Cyphocaris challengeri 21 Figure 2.8. Schematic representation of Cyphocaris challengeri left mandible showing its position with respect to mouthparts 22 Figure 2.9. Parts of the left mandible of Cyphocaris chalkngeri 22 Figure 2.10. Scanning electron microscope image of the right mandible of Cyphocaris challengeri showing the incisor, the seta row, and the molar 23 Figure 2.11. Morphometric relation between body size and mandible characteristics of Parathemisto pacifica. Molar length and body size (rilled circles, continuous line) follow the relation y = 45.68 LN(x) - 4.088 (r2 = 0.84, n = 38). Incisor length and body size (open circles, dotted line) follow the relation y = 10.26 e a 2 1 4 x (r2 =0.71, n = 38) 25 Figure 2.12. Morphometric relation between body size and molar height of Parathemisto pacifica. The trend line follows the relation y = 70.46 - 18.29 x + 1.87 x2 (r2 = 0.81, n = 38) 26 vm Figure 2.13. Morphometric relation between body size and number of teeth in the incisor of Parathemisto. pacifica. Trendline follows the relation y =3.637 LN(x) + 5.573 (r2 = 0.92, n = 38) 26 Figure 2.14. Morphometric relation between body size and incisor height of Parathemisto pacifica. Trendline follows the relation y = -0.093 x 2 + 2.047 x + 0.066 (r2 = 0.90, n = 38) 27 Figure 2.15. Morphometric relation between body size and molar dimensions of Cyphocaris challengeri. Molar length and body size (filled circles, continuous line) follow the relation y = 7.852 x + 10.068 (r2 = 0.95, n = 32). Molar height and body size (open circles, dotted line) follow the relation y = 9.943 x - 20.45 (r2 =0.96, n = 32) 28 Figure 2.16. Morphometric relation between body size and incisor dimensions of Cyphocaris challengeri. Incisor length and body size (filled circles, continuous line) follow the relation y = 15.83 x - 7.89 (r2 = 0.98, n = 32). Incisor base width and body size (open circles, dotted line) follow the relation y = 7.03 x - 7.11 (r2 =0.93, n = 32) 28 Figure 2.17. Morphometric relation between body size and incisor height of Cyphocaris challengeri. Trend line follows the relation y = 4.188 x - 0.152 (r2 = 0.95, n = 32) 29 Figure 2.18. Mandibular position during extreme abduction and adduction for Malacostracan crustaceans. Scheme of Anaspides tasmaniae. Redrawn fromManton (1977) 31 Figure 3.1. Prey items identified at species or group level in the 63 stomachs examined for Parathemisto pacifica. Parentheses in legend indicate the number of times the prey was found in the stomach contents 44 Figure 3.2. Prey items identified at species or group level in the 57 stomachs examined for Cyphocaris challengeri. Parentheses in legend indicate the number of times the prey was found in the stomach contents 45 Figure 3.3. Dendrogram showing zooplankton similarity between sampling areas during May-June 1990. B) Whisker plot of mean abundance of P. pacifica and C . challengeri per area. * indicates a double square root transformation. C . challenged in the Fraser plume was present in only one sample 50 Figure 3.4. Dendrogram showing zooplankton similarity between sampling areas during April 1991. B) Whisker plot of mean abundance of P. pacifica and C . challengeri per area. * indicates a double square root transformation 52 Figure 3.5. Dendrogram showing zooplankton similarity between sampling areas during June 1991. B) Whisker plot of mean abundance of P. pacifica and C . challengeri per area. * indicates a double square root transformation 53 ix Figure 3.6. Dendrogram showing zooplankton similarity between sampling areas during 1997. B) Abundance of P. pacifica and C. challengeri per area. * indicates a double square transformation 55 Figure 3.7. A) Principal components for zooplankton from May 29 -June 6, 1990. Components are described in Table 3.9. B) Salinity contour plot. R = riverine plume,..72 Figure 3.8. Size distribution of P. pacifica (A) and C. challengeri (B) during May 29 - June 6, 1990 •. 73 Figure 3.9. Principal components for zooplankton samples from April 7-16, 1991.Components are described in Table 3.10. B) Salinity contour plot. R = riverine plume, E =estuarine plume, and S =Strait of Georgia (Nomenclature of stations as in Fig. 1.2, Table 3.6). ...76 Figure 3.10. Size distributions of P. pacifica (A) and C. challengeri (B) during April (7-16), 1991 77 Figure 3.11. A) Principal components for zooplankton samples from June 11-14, 1991. Components are described in Table 3.11. B) Salinity contour plot. R = riverine plume, E=estuarine plume, and S =Strait of Georgia (Nomenclature of stations as in Fig. 1.2, Table 3.6) 78 Figure 3.12. Size distributions of P. pacifica (A) andC. chalkngeri (B) during June 11-14, 1991. 79 Figure 3.13. A) Principal components for zooplankton collected at station DFOl during 1997. Components are described in Table 3.13. Symbols indicate: J=January, F= February, Mr4=March 4, Mr25 = March 25, Ap8= April 8, Apl5= April 15, My 2= May 2, My 22= May 22, Jn=June, Jy=July, Au=August, Sp= September, Oc= October, Nv= November, Dc= December. B) Salinity contour plot. February and November data were interpolated, data from March to July include only to 100 m 83 Figure 3.14. Parathemisto pacifica collected at station DFOl. Percentage distribution of individuals per size interval and depth during January (A) and May (B) of 1997 related to the vertical salinity structure 84 Figure 3.15. Cyphocaris challengeri collected at station DFOl. Percentage distribution of individuals per size interval and depth during January (A) and during May (B) of 1997 related to the vertical salinity structure 85 Figure 4.1. Simulated (circles) monthly mean size values of individuals in a population with one recruitment pulse during June. Predicted (line) values were estimated with the ML method. Values used in the simulation were Z = 1.8 and K = 1.8. Estimated values were Z = 1.94 and K = 1.82 101 Figure 4.2. Simulated monthly mean size of individuals (A) and recruitment pattern (B) in a population with one recruitment pulse during June (circles). Predicted values for mean size and recruitment pattern (line) estimated with the RP-ML method. Values in the simulation were Z = 1.8 and K = 1.8. Estimated values were Z = 1.89 and K = 2.11.102 Figure 4.3. Simulated (circles) and predicted (line) monthly mean size values of individuals in a population with one recruitment event during May (20%), June (60%) and July (20%). Parameter values (Z = 1.94 and K = 1.74) were estimated with the ML method. Values used in the simulation were Z = 1.8 and K = 1.8. The May data point was not used in the fitting 103 Figure 4.4. Simulated monthly mean size values (circles) of individuals in a population (A) and recruitment pattern distributed during May (20%), June (60%) and July (20%) (B). Predicted values (line) were estimated with the RP - ML method. Values used in the simulation were Z = 1.8 and K = 1.8. Estimated values were Z = 1.99 and K = 1.71.103 Figure 4.5. Simulated (circles) and predicted (line) monthly mean size values of individuals in a population with two recruitment events during May (20%), June (60%) and July (20%), and October (20%) - November (20%). Parameter values (Z = 0.47 and K = 4.19) were estimated with the ML method. Values used in the simulation were Z = 1.8 andK= 1.8 104 Figure 4-6. Simulated monthly mean size values (circles) of individuals in a population (A) and recruitment pattern distributed during May (20%), June (60%) and July (20%), and October (20%)-November (20%) (B). Predicted values (line) were estimated with the RP-ML 104 Figure 4.7. Monthly size distribution of Parathemisto pacifica collected at station DFOl during 1997 106 Figure 4.8. Predicted (line) and observed (circles) values of mean size and recruitment pattern for P. pacifica during 1997 107 Figure 4.9. Sum of squares surface plot of K and Z for P. pacifica 107 Figure 4.10. Predicted (line) and observed (circles) values of mean size and recruitment pattern for P. pacifica during 1997 assuming size-dependent mortality 109 Figure 4.11. Sum of squares surface plot of K and Z for P. pacifica under the assumption of size dependent mortality 109 Figure 4.12. Mean length size of Parathemisto pacifica (circles) with a one month lag, and chlorophyll a_(squares) during 1997 at DFOl station. Late spring to summer data relate logarithmically: y= 2.16 Ln (x) + 0.52 (r2 = 0.91), and linearly during fall-winter y =0.18 (x) +4.99 (r2=0.6) '. 110 Figure 4.13. Mean length size of Parathemisto pacifica (circles) with a one month lag, and zooplankton biomass (squares) during 1997 at DFOl station. Spring to early summer xi data exhibited a logarithmic relation: y= 0.86 Ln(x) + 1.33 (r2 = 0.68). During winter the data relate exponentially: y= 6.12 e a 0 0 4 6 l c (r2 = 0.58) 110 Figure 4.14. Monthly mean size distribution of Cyphocaris challengeri collected at station DFOl during 1997 112 Figure 4.15. Predicted (line) and observed (circles) values of mean size and recruitment pattern for C. challengeri during 1997 assuming constant mortality 113 Figure 4.16. Sum of squares surface plot of K and Z for C. chalkngeri under the assumption of constant mortality 114 Figure 4.17. Predicted (line) and observed (circles) values of mean size and recruitment pattern for C. challengeri during 1997 assuming size dependent mortality 115 Figure 4.18. Predicted (line) and observed (rilled circles) values of mean size and recruitment pattern for C. challengeri during 1997 assuming size dependent mortality. April data (open circles) were not considered in the fit 116 Figure 4-19. Sum of squares surface plot of K and Z for C. chalkngeri under the assumption of size dependent mortality. A) Surface plot including all data points. B) Surface plot without values for April 116 Figure 4-20. Mean size of Cyphocaris challengeri (filled circles) with a one month lag, and zooplankton biomass (open circles) during 1997 at DFOl station showing the logarithmic relationship y = 1.49 Ln (x) + 3.35 (r2 = 0.88) 117 Xll Acknowledgments I am greatly indebted to several people for their contribution to this project. My supervisor Dr. Alan Lewis provided me with advice, guidance and countless hours of editorial work. His opportune financial and moral support is sincerely appreciated. To Dr. Susan Allen, Dr. Paul Harrison, Dr. Geoffrey Scudder, and Dr. Max Taylor for their timely help and insightful advice throughout the review of the manuscripts. Dr. John Dower shared his broad perspective on numerical analysis of biological oceanography, his library and helpful suggestions. Dr. Carl Walters suggested a method used to estimate growth/mortality. An anonymous reviewer made valuable suggestions to the overall improvement of the manuscript. This work was partially funded by scholarships from CONACyT and the Instituto Politecnico Nacional (Mexico). Dr. Elaine Humphrey (Electron Microscope Facility), with her vast experience and extensive support, made possible my work on SEM imaging. Dr. Paul Harrison allowed me to use the zooplankton samples and data obtained for his project "Plankton Production and Nutrients Dynamics in the Fraser River Plume". Robert Goldblatt and the people involved in the project collected data and zooplankton samples for the years 1990 and 1991. Beth Bornhold kindly provided me with recorded data, and together with Melissa Evanson, obtained the field data and zooplankton samples for the year 1997. Robin Williams (Department of Fisheries and Oceans) provided hydrographic data. People in Dr. Harrison's, Dr. Dower's and Dr. Lewis's labs were always helpful: Sharon DeWreede, Christianne Wilhelmson, Mike Henry, Rob Campbell, and Christine Ortlepp were generous, sharing their time, skills, and equipment. Friends and family provided me with soul-vitamins when most needed, and for that I am deeply thankful to Octavio, Francisca, Luz, Amada, Elva, Fausto, Javier, Dora, Octavio, Lupita, Kugako, and Edgar. Especially, I am grateful to Leonardo: because life could be easier, but he instead made it better. Chapter 1. Introduction 2 General introduction Amphipods rank 3 r d i n abundance i n cold environments behind copepods and euphausiids (Bowman and Grunner, 1973). They are abundant at highly productive areas (Nair et al., 1973) as the Strait of Georgia (St John et al., 1992; Parker and Kask, 1972a, 1972b), and can be an important part of the diet of whales (Nemoto, 1970) and fish (St John et al., 1992, Birtwell et al., 1984), including salmon (Brodeur, 1990). A s a crustacean group, amphipods are highly morphologically diversified and occupy various habitats (Abele, 1982). They are present i n marine, freshwater, semi-terrestrial, and terrestrial habitats (Barnes, 1974). Their adaptations to the different environments are as varied as the morphology of their oral appendages, to the extent that the morphology of mouthparts is a distinct element i n the taxonomy (Barnard, 1958). This is indicative of the determinant role the morphology of mouthparts may play i n feeding where they live. The purpose of this study was to investigate the relationship between the morphology and the ecology of the planktonic amphipods Cyphocaris challengeri and Parathemisto pacifica (Fig. 1.1). In particular, how the adaptations for feeding relate to the type of food they consume, and how this may determine their distribution. 3 Study area The Strait of Georgia is a very productive estuary including a variety of habitats (Parsons et al., 1970; -Harrison et al., 1983). Its biological oceanography (Harrison et al., 1983), and physical oceanography (Stommel and Farmer, 1952; Tully and Dodimead, 1957; Waldichuk, 1957; Thomson, 1981 and LeBlond, 1983) have been extensively reviewed. Hence, the following is a reduced summary that only includes information considered directly relevant to the present study. The Strait of Georgia is located on the Pacific coast of Canada between mainland British Columbia and Vancouver Island. It extends about 222 km from a latitude of 48° 50' to 50° 00' N, has an average width of 33.3 km, covers an area of 6800 km^, and has a mean volume of 298 km^. Its mean depth is 156 m, with a maximum of 420 m. It connects and exchanges waters with the Pacific ocean at the north through Johnstone Strait and at the south through Juan de Fuca Strait (Thomson, 1981). The main source of freshwater in the Strait is from the Fraser River, which makes up to 80% of the total freshwater discharged in the Central Strait (Thomson, 1981). Semi-diurnal tides, Fraser river runoff, and winds, are the most important factors related to estuarine circulation and salinity structure (LeBlond, 1983). The general flow of surface water is clockwise to the north (Tabata, 1972). Two periods of high runoff occur during the year, one in early summer associated with snow melt, and another in winter associated with the peak in coastal precipitation. The salinity structure in the area results from dilution of the seawater with fresh water from runoff, precipitation, and from the mixing action of tides (Thomson, 1981). The Strait of Georgia (Fig. 1.2) is divided into three areas. The Northern, the Central, and the Southern Strait (Thomson, 1981). The biological material for this study was collected in the Central Strait, from now on referred to as the Strait. Phytoplankton production in the Strait has two seasonal blooms, occurring during spring and summer (Parsons and LeBrasseur, 1969; Stockner et al., 1979). Light limits phytoplankton productivity during winter, and nutrients 4 and grazing during summer. Zooplankton biomass also peaks twice a year, during late spring and late summer, following the peaks in phytoplankton production (Harrison et al., 1983). 1 2 4 ° 0 0 ' W 1 2 3 ° 0 0 ' W Figure 1.2. Area of study. Central Strait of Georgia. R, E, S, and DFOl are the sampling stations in this study. R = riverine plume, E = estuarine plume, and S = Strait. The freshwater from the Fraser River flows into the Strait of Georgia, and because of its lower density, flows out over the estuarine water. This process creates a distinct surface layer, the riverine plume (Pond and Pickard, 1983). This layer is characterized by its high turbidity, low salinity (< 15), and by a sharp halocline occurring in the upper 2-5 m. During the flood tide the riverine plume remains in the Strait of Georgia where it is subject to wind and tidal mixing. At the next flood tide, the riverine plume mixes further with water from the Strait and becomes the estuarine plume. These two plumes are recognizable from horizontal mapping and vertical profiles of salinity and nutrients (Yin, 1994). 5 Thus, the interaction between the Fraser River outflow and tides forms the riverine plume, the estuarine plume, and the transition between them or fronts (Parsons et al., 1981; LeBlond, 1983). Furthermore, the outflow of river water over the seawater entrains salt water (Pond and Pickard, 1983) and new nutrients from beneath (Harrison et al., 1994; Yin, 1994). This fresh supply of nutrients results in different plankton species composition and abundance compared to that of the neighboring areas, where plankton production depends on regenerated nutrients (Eppley and Peterson, 1979). Furthermore, the spatial heterogeneity of the area also generates a wide range of plankton densities (Hayward, 1986). This implies that the plankton community from riverine and estuarine plumes would expectedly differ from that of areas distant to the plumes, and would result in a more heterogeneous series of environments in the Strait. Zooplankton biomass concentrates in frontal areas (Mackas et al., 1980) as a result of the convergence occurring at the frontal areas (Le Fevre, 1986). Although there is good correlation between phytoplankton and physical processes, the causes of zooplankton variability are less clear, and presumably due to ecological rather than physical causes (Steele and Henderson, 1992). Thus biological interactions occurring at frontal areas can lead to different outcomes, some of them to increased stocks of fish and their commercial exploitation (Le Fevre, 1986). In the Strait of Georgia, studies showed that estuarine and riverine plume areas had higher densities of fish than other areas. St. John et al. (1992) recorded the highest catches of juvenile salmon in the front between the estuarine plume and the Strait of Georgia; they were feeding on juvenile fish, including herring, while juvenile herring were feeding on several zooplankters including amphipods. Different salinity structure, contrasting enriched environments, and potentially distinct plankton communities between waters of the Strait and those neighboring the Fraser River were determinants in the selection of the three sampling locations for this study: the Fraser river plume, the estuarine plume, and the Strait (Fig. 1.2). The complex and dynamic environment present in the Strait was considered capable of providing the wide variety of 6 feeding opportuni t ies necessary to investigate the re la t ion between feeding morphology and the ecology o f amphipods. Study subject Amph ipods are ecologically diverse and widely d is t r ibuted crustaceans. There are about 7000 described species o f amphipods (Bousfield and Shih, 1994) i n more than 125 families d iv ided among the suborders Gammaridea, Lemopidodea (Caprel l idea), Hyper i idea, and Ingolf iel l idea (Abele, 1982; Brusca, 1979; Brusca and Brusca, 1990). A m p h i p o d s can be aquatic, semi-terrestrial and terrestr ial. I n freshwater habitats they are present i n lakes, ponds, springs, spring runs, rivers and creeks. I n the marine env i ronment they are present f r o m the dr i f t l ine to the abyss. A m p h i p o d s are h ighly morphological ly diversif ied, their bodies adapted for bur rowing, hopping, crawl ing, c l inging, grasping, and swimming. T h e i r body shape can vary f r o m globular to elongated, their appearance varies f r o m transparent to opaque, and their size may be smaller t han 1 m m and as big as 28 c m (Barnes, 1974; Brusca a n d Brusca, 1990). T h e amphipods studied here are the hyper i idean Parathemisto pacifica a n d the garnmaridean Cyphocaris challengeri, bo th c o m m o n i n zooplankton col lect ions f r o m the Strai t o f Georgia (Fu l ton, 1968; Gardner, 1977) These two species were f o u n d i n suff ic ient ly abundant for an analysis o f their diet and potent ia l food present i n the env i ronment . V inog radov et al. (1996) indicates that the Hyper i idea suborder is composed o f 233 described species. A l l o f t h e m are ent i rely pelagic mar ine species mostly oceanic, a l though some species occur i n coastal water. Hyperi ids have short compact bodies w i t h wel l -developed musculature l ike ly used for s w m m i n g . I n species o f the genus Parathemistd, their eyes occupy most o f the big, spherical head (Bowman and Gruner, 1973). T h e Gammaridea comprises the highest percentage o f amphipods, w i t h about at least 5700 k n o w n species (Holsinger, 1994; W a t l i n g and Thomas, 1995), that can be f o u n d i n freshwater, the marine env i ronment , and even semi-terrestrial envi ronments (Schmitz, 1992). Gammarids have sturdy bodies w i t h strong musculature, their 7 head is narrow and their eyes are small (Barnes, 1974). The pelagic Cyphocaris challengeri in particular is a fast swimmer, as observed in zooplankton collections before preservation. Study objective The main objective of this thesis was to investigate the relationship between the morphology and the ecology of the planktonic amphipods Parathemisto pacifica and Cyphocaris challengeri in the Strait of Georgia. It particularly focuses on how their adaptations for feeding relate to the type of food they consume, and how this relationship determines their distribution. To achieve this goal the study was subdivided into three sub-objectives, covered in chapters 2-4. Each chapter includes a materials and methods section. Chapter five presents the general discussion and conclusions of this study. Given the remarkable morphological diversity of amphipods (Abele, 1962), and in particular that of their oral appendages (Barnard, 1958), a main premise of this research was that morphological adaptations for feeding allow organisms to better use particular food resources. Since the mandible is a mouthpart observed to change shape in amphipods in relation to feeding habits (Manton, 1977; Wading, 1993), the mandible was the main target of study for the morphology analysis. The first objective was to investigate the relationship between the amphipod's mandible and their food. Chapter two examines the morphology of the mandible of Parathemisto pacifica and Cyphocaris challengeri. Bowman (1960) and Barnard (1958), respectively described and provided drawings of the mandibles of these species. However, their research does not provide information on the functional morphology or the morphometric changes the mandible undergoes during growth. This information was considered fundamental to show what type of food the mandible would enable these animals to eat. The goal of this chapter is to provide a morphological description of the mandible, to propose an explanation of the way it functions and to gain insight into the feeding habits of the two species. 8 The second objective (chapter three) was to investigate whether the distribution of amphipods was determined by their association with their food or resulted from differences in environmental conditions, indicated by salinity structure. Salinity was selected as the cue to detect environmental change because it is the dominant factor controlling the vertical structure of the water column during most of the year and over most of the Strait of Georgia, as a result of strong freshwater runoff (LeBlond, 1983). Chapter three is divided in two parts. The goal of the first part was to determine the diet of Parathemisto pacifica and Cyphocaris challengeri, and to identify potential prey from the zooplankton collected simultaneously with the amphipods. It was thought that stomach contents analysis would help to distinguish potential food sources from actual food ingested in the field (Mauchline, 1966; Dalley and McClatchie, 1989). Therefore, stomach contents were examined and identified, and complete prey items were measured to establish the prey size range. The similarity between P. pacifica and C. challengeri and zooplankton (from collections) within prey-size range was analyzed to determine potential prey. The goal of the second part of the chapter was to investigate the relationship between amphipods distribution, zooplankton distribution and salinity structure of the water column. Principal components driving the spatial and temporal variation of the zooplankton cornmunity, amphipods included, were estimated and analyzed with respect to salinity structure of the locations where collections were obtained. The third objective (chapter four) was to examine amphipod growth and mortality relative to the plankton productive season. Different size groups and corresponding stomach contents had been analyzed in Parathemisto pacifica (Bowman, 1960), but only a small fraction of the size structure of the population was included. Similar information does not exist for Cyphocaris challengeri, nor does previous estimation of amphipod growth. I estimated individual growth and mortality rates were using a minimum square fitting technique (Hilborn and Walters, 1992) using the recruitment pattern - mean length series of observed data. Also, I examined how the average individual size changed as the plankton season progressed. 9 C h a p t e r 2 . The mandib le of Parathemisto pacifica a n d Cyphocaris chaiiengerh Shape and funct ion 10 Introduction Amphipods are an ecologically diverse, morphologically diversified, and widely distributed group of crustaceans (Lutz, 1986). While most of them are marine, some occupy freshwater habitats (Abele, 1982). There are also semi-terrestrial and some completely terrestrial amphipods (Barnes, 1974), and even some can lead a commensal or parasitic life (Schmitz, 1992; Abele, 1982). The multiple adaptations of amphipods to their environments parallel the varied morphology of their mouthparts. Mouth morphology has been widely used for taxonomic classification (Barnard, 1958) and its variation across species has also proved quite useful in phylogenetic studies (Wading, 1993; Bousfield, 1995). Yet only a handful of studies has focused on the connection between mouth morphology and ecology (e.g. Saint Marie, 1984; Coleman 1991a, 1991b; Moore and Rainbow, 1989; Steele and Steele, 1993) and these studies have been mostly on gammarideans. The mouthparts of amphipods are located ventrally in the head or cephalon and form the buccal mass (Fig. 2.1A). In total there are six pairs of appendages in the cephalon. Two pairs of antennae, one pair of mandibles, two pairs of biramous maxillae, and one pair of basally fused maxillipeds (Fig. 2.IB). The mandibles, the maxillules, and the maxillipeds each bear a palp (Bousfield, 1973), except in the hyperiids, which lack the palp in the maxillipeds (Bowman and Gruner, 1973). Among the mouthparts, the amphipod mandible has the greatest variation in shape across species which is related to the feeding habits of each species (Watling, 1993). The basic mandible (Fig. 2.1 C) consists of a compact coxa or mandible body bearing a toothed incisor, a lacinia mobilis, a row of setae leading dorsally to a molar designed for crushing, and a 3-article palp (Manton, 1977; Watling, 1993). Parts most susceptible to change are the incisor and the molar, and the changes can include a reduction of the incisor, loss of the seta row, reduction 11 or loss of the molar, or all of these (Watling, 1993). Most of these modifications are related to the feeding strategy of the species, being either predation and/or scavenging. Thus, the idea that morphological differences among species are related to differences in their ecology (Wainwright, 1994) is the main hypothesis of the present chapter. The objective of this chapter is to investigate the mandible morphology of planktonic amphipods and to make inferences on the prey that they may be able to consume. Materials and methods I identified all specimens, sorted them by size, and produced a size distribution profile for each zooplankton sample. Whenever possible, I selected individuals from the entire size range for mandible dissection. The description of cuticular extensions of mouthparts and mandible, such as spines and setae, follows the terminology proposed by Watling (1989). The mandible of those selected individuals was then dissected under a stereoscopic microscope as follows: I dissected the exoskeleton slicing between the head and the first segment to remove the head, then dissected the mandible by cutting around the joint between the mandible and the head. The dissected mandible was immersed in a drop of corn syrup mixed with formalin (Pennak, 1978). This mixture provided a medium to preserve and to embed the mandible, and the high viscosity of the medium also secured the mandible in the desired position for measurement and drawing. For drawing the mandible, as well as for measuring its components I used an inverted microscope and a camera lucida. The mandible dimensions measured to examine their morphometry were the following: For Parathemisto pacifica, the length and height of the molar, and the height, length of the cutting area of the incisor, and number of teeth of the incisor. For Cyphocaris challengeri, the length and height of the molar, and the height, length of the cutting area of the incisor, and length of the incisor base. All dimensions were regressed 12 against the body size of each species in order to identify changes in the mandible structure during growth. 13 Figure 2.1. Schematic representation of an amphipod. A) Body parts*, B) Head showing mouthparts: MX1 = maxillae 1, MX2 = maxillae 2, MXP= maxilliped. C) Mandible**. *Redrawn from Bowman and Grunner (1970), ** Redrawn from Schmitz (1992). 14 Selected images of the mandibles were obtained by scanning electron microscopy, and laser scanning microscopy. Preparation for scanning microscopy consisted of a series of alcohol dehydration, critical point drying with C02, and gold coating. Laser scanning samples were immersed in distilled water as a retention medium. Electron microscopy was performed in the UBC Botany-Zoology Electron Microscopy laboratory. 15 Results Parathemisto pacifica Description of mouthparts The mandibles are elongated and have a thin integument; they insert laterally on the head and meet frontally under the labrum. The labrum has a medioapical notch that covers the dorsal areas of the mandibles and partially overlaps them. The labium is bilobate; each lobe is oval and wide (Fig. 2.2). The mandibles fit between the labrum and the labium (Fig. 2.2), and surround the mouth. The incisors are wide and toothed, laterally flanked on the external edge with a brush-looking set of long setae (Fig. 2.3). The teeth are sclerotized. Only the left mandible has a lacinia mobilis, which is about 30% smaller than the incisor, although similarly toothed. The left mandible embraces the right one between the incisor and the lacinia mobilis when closed. The distal tooth in the left mandible fits between the two distal teeth of the right one. The molar arises from the frontal part of the mandible. It is broad and round in small specimens and elongated and lanceolated in the large specimens. Its surface is covered with rows of lamellae with blunted, sclerotized ridges (Fig. 2.4). The molar has a fringe structure, with longer setae in the internal margin than those in the external margin. Between the molar and the incisor processes sharp setae protrude forming the seta row (Fig. 2.3). 200 |jm Figure 2.2. Scanning electron microscope image of Parathemisto pacifica mandibles. Figure 2.3. Schematic representation of Parathemisto pacifica left mandible showing the incisor, molar, seta row, and a detail of a seta. 17 100 L j m Figure 2.4. Scanning electron microscope image of Parathemisto pacifica mandible. Notice the lamelliform rows with blunt cusps on the molar. The mandibular palps are directed frontally on both sides of the labrum. The distal segments rest mediobasally to the second antennae and are bent laterally (Fig. 2.5). The surface of the distal segment is covered with comb spines used to clean the basal region of the antennae and the region between them. The first maxilla has some robust apical spines and the inner marginal setae directed medially and towards the incisors; the palp inserts laterally, it is flat and apically denticulated. The second maxilla is flat, with an almond-shape endite. Both have margins covered with slender, medium-sized setae from the medial area to the top. The palp also has 3-5 long apical spines. I S Figure 2.5. Scanning electron microscope image of Parathemisto pacifica mouthparts showing the mandible and the mandible palp. The maxillipeds are simplified and lack palps; the inner lobe is fused, covered with setae and points towards the mouth area. The posterior part is covered with long sharp spines directed to the mouth (Fig. 2.6; Maxilliped). 19 Figure 2.6. Mouthparts of Parathemisto pacifica (left column) and Cyphocaris challengeri (after Bowman, 1960, and Barnard, 1958). 20 Function of mouthparts The frontal disposition of the incisors and their toothed cutting edges indicate they work as a pair of trimming scissors. Food may cling on the setae flanking the external edges of the incisors during the cutting action. The medioapical notch of the labrum allows the use of the anterior incisor regions to produce a wide bite, in which the right mandible crosses between the incisor and the lacinia mobilis of the left mandible, in a cutting fashion. The insertion of the distal tooth of the left incisor between the pair of the distal teeth of the right one suggests that when the mandibles close, the incisors come together in a clasping action, for trimming and cutting. The row of setae adjacent to the incisor is related to grasping and directing food towards the molars. While the incisors are cutting, the molars meet and grind the food. The surface area of the molar process is slightly sclerotized. During feeding, the long sharp spines of the maxillipeds and the palps of the first maxillae point to the mouth area (Fig. 2.5), which helps in handling and holding the food and facilitates the work of the mandibles. Cyphocaris challengeri Description of mouthparts The mandibles are slender and elongated, with a strong integument; they insert laterally on the head and meet frontally under the narrow apex of the labrum. The labrum is small, flattened, and drop-shaped with a frontal apex (Fig. 2.7). It only covers the proximal parts of the incisors. The labium is bilobate, with triangular, pointed lobes. The mandibles fit between the labrum and the labium, with the mouth located above the mandibles (Fig. 2.8). The incisors are toothless, axe-like, and sclerotized. The left incisor has hook-shaped endings (Fig. 2.9). The right incisor is shaped like a wedge, with the posterior end slightly swollen and a small vertical indentation (Fig. 2.10). The lacinia mobilis is present only on the left mandible; its appearance is columnar, narrow and toothed. When the mandibles close, the left mandible 21 embraces the right one between the right incisor and the lacinia mobilis. The setae row between the incisor and the molar is composed of 6 long, strong setae that bear small serrations. The molar is cuboid, short, columnar and highly sclerotized. It has a triturative surface formed by a regular pattern of ridges across the molar. The molar is fringed with bundles of short, fine setae longer at the internal margin. The mandibular palps are strong, the interior margin of the last segment is covered with slender setae, arranged in a brush-like manner. The setae continue over a small area of the next segment. The palps are oriented frontally at both sides of the labrum and operate both frontally and on the basal area of the antennae. Figure 2.7. The labrum of Cyphocaris chalkngeri. 22 Figure 2.8. Schematic representation of Cyphocaris challengeri left mandible showing its position with respect to mouthparts 40 nm Figure 2.9. Parts of the left mandible of Cyphocaris challengeri. 23 100 Mm Figure 2.10. Scanning electron microscope image of the right mandible of Cyphocaris challengeri showing the incisor, the seta row, and the molar. The proximal endite of the first maxilla is slender with inner marginal setae and long, blunt apical spines. The distal endite is crowned with blunt spines. The palp is inserted laterally and follows the shape of the distal endite (Fig. 2.6, right column). It has two long slender spines and some short ones at the tip. The second maxilla is triangular and flattened, with short plumose setae on the inner margin. The maxillipeds are well developed. The palp is strong, with long spines on the external margin and short slender spines close to the last two segments, with a slender almost straight dactyl. The distal endite is flattened, and almond-shaped with apical spines and short blunt, denticles on the internal margin. The proximal endite is flattened, thumb-shaped with marginal setae from the tip to the internal margin. 24 Function of mouthparts The incisors are frontally oriented and their toothless, axe-like cutting edges indicate they work as a pair of sharp scalpels. The mandibles fit between the labrum and the labium and meet in a narrow region under the apex of the shield-shaped labrum (Fig. 2.6, 2.7, and 2.8). This arrangement suggests a limited opening since the labrum shape and size determine the bite gap (Manton, 1977). During hiring, the right incisor may glide between the left incisor and the lacinia mobilis, following a curved path that their wedge shape dictates. The distal indentation on the left incisor may be clasped onto the hook ending of the right incisor. The labrum has a mid-line incision and projects towards the mouth; the labium is bilobate, covered with hair-like processes facing the mouth (Fig. 2.7). The setae row adjacent to the incisor is composed of long and strong setae that seem capable of holding big pieces of food and directing them towards the molar. The space between the incisor and the molar is large and suggests a capability for handling relatively big pieces of food. When the mandibles close, the molars come together, presumably crushing the food between them. While feeding, the palps from the mandible and the maxillipeds would meet in the mid-line, probably helping to hold the food for biting, or to avoid it escaping. The long spines and the tooth-like spines of the proximal endite, all oriented medially, may help holding the food to support and guide it towards the mandibles. Morphometries Parathemisto pacifica The changes in dimension for the incisor and molar during growth are in Figures 2.11 to 2.14. The molar length, molar height, incisor base length, incisor height, and number of teeth on the incisor were regressed against body size and the results are shown in these figures. The change in molar dimensions related to the body growth of Parathemisto pacifica is allometric in 25 nature. The plot of molar length shows a logarithmic relationship with body size. The molar length rapidly increases with body size from 2 to 7 mm and then remains almost constant for the remaining sizes. In comparison, molar height exhibits a slight increase in the 2-7 mm size interval, and beyond this size its height increases. In fact, the molar process changes shape throughout growth: in small specimens it is round and short with a wide surface area that changes into a tall, columnar molar process, with a slender and lanceolate surface area in large specimens. The number of teeth on the incisor increases from 7 to 12 in the 2-5 mm size range then, in the >5 - 13 mm size range, the number of teeth changes from 12 to 14. From 13 mm up to a maximum size of 16 mm, the incisor only acquires one more tooth, while the length of the incisor increases exponentially. Finally, the height of the incisor and the body size follow a quadratic relationship suggesting allometry in the morphological change of the incisor during growth. Figure 2.11. Morphometric relation between body size and mandible characteristics of Parathemisto pacifica. Molar length and body size (filled circles, continuous line) follow the relation y = 45.68 LN(x) - 4.088 (r2 = 0.84, n = 38). Incisor length and body size (open circles, dotted line) follow the relation y = 10.26 e a 2 H x (r2 =0.71, n = 38). Figure 2.12. Morphometric relation between body size and molar height of Parathemisto pacifica. The trend line follows the relation y = 70.46 - 18.29 x + 1.87 x2 (r2 = 0.81, n = 38). Figure 2.13. Morphometric relation between body size and number of teeth in the incisor of Parathemisto. pacifica. Trend line follows the relation y =3.637 LN(x) + 5.573 (r2 = 0.92, n = 38). 2 7 16 n 0 -I 1 1 1 1 0 5 10 15 20 Body size (mm) Figure 2.14. Morphometric relation between body size and incisor height of Parathemisto pacifica. Trendline follows the relation y = -0.093 x 2 + 2.047 x + 0.066 (r2 = 0.90, n = 38). Cyphocaris challengeri Changes in incisor and molar dimensions with body size were measured. Molar height, incisor length, incisor height, and the length of the incisor base were regressed against body size (Figs. 2.15-2.17). The dimensions of the molar and incisor processes increase linearly with body size. The only difference among them depends on how rapidly those dimensions change as the animal grows. The height and the base of the incisor grow slower than the molar height and the incisor length. This indicates that during growth, the incisor widens while the molar process becomes shorter, narrower and presumably sturdier. These relationships indicate that there is no allometry in the growth of C. challengeri. 28 Figure 2.15. Morphometric relation between body size and molar dimensions of Cyphocaris challengeri. Molar length and body size (filled circles, continuous line) follow the relation y = 7.852 x + 10.068 (r2 = 0.95, n = 32). Molar height and body size (open circles, dotted line) follow the relation y = 9.943 x - 20.45 (r2 =0.96, n = 32). c o CO o c 300 240 180 H 120 60 OQO'OOPO Q.O-D" oaSO 200 h 150 f h 100 o CO X ) k_ o 50 w o c — I 1 1 1 1 1 1 n 1 1 1 1 1 1 1 — 5 10 15 20 Body size (mm) Figure 2.16. Morphometric relation between body size and incisor dimensions of Cyphocaris challengeri. Incisor length and body size (filled circles, continuous line) follow the relation y = 15.83 x - 7.89 (r2 = 0.98, n = 32). Incisor base width and body size (open circles, dotted line) follow the relation y = 7.03 x - 7.11 (r2 =0.93, n = 32). 29 Body size (mm) Figure 2.17. Morphometric relation between body size and incisor height of Cyphocaris challengeri. Trend line follows the relation y = 4.188 x - 0.152 (r2 = 0.95, n = 32).. 30 Discussion Parathemisto pacifica The mandible in amphipods is attached to the outside of the head by a membrane that defines the hinge line (Watling, 1993). As in other Malacostracan crustaceans, adductor and abductor muscles attached to the mandible body move it about the hinge in the transverse plane of the body (Snodgrass, 1952; Manton, 1977). The muscles causing the mandibular movement are a pair of abductor and adductor muscles connected to the mandible (Fig. 2.18). The abductor muscles arise from the pre-axial, dorsal part of the mandible (Snodgrass, 1952). In the amphipod mandible, the adductor muscle originates from an apodeme at the posterior edge of the mandible, and from it extends laterally deep into the head (Watling, 1993; Steele and Steele, 1993). Amphipod mandibles with wide molars and grinding surfaces are present in species with microphagous or herbivorous habits (Watling, 1993). In Parathemisto pacifica, the mandible is elongate with the toothed incisor at its front. These features have been associated with the ability of cutting large pieces of food, and are found in carnivorous species (Watling, 1993). However, the molar is relatively large compared to the body of the mandible and to the incisor, suggesting microphagous feeding habits. The rows of blunt cusps in the molar surface seem designed to smash the food mass. Toothed incisors as those found in P. pacifica are present in hyperiids that feed on soft food, such as Lestrigonus schizogeneios and Hyperietta stebbingii (Bowman, 1973). L. schizogeneios is known to feed on marine snow, and has also been found to burrow on the mesoglea of the medusa Aequorea. sp. (Harbison et al., 1977), which may indicate that it also feeds on gelatinous plankton. L. schizogeneios has a wide molar with blunt cusps (Bowman, 1973) similar to those of P. pacifica. The molar of H. stebbingii is reduced compared to that of P. pacifica, and has sharp cusps on the molar surface (Bowman, 1973). This species feeds on egg masses and radiolarians (Harbison et al., 1977). Thus, the lamelliform blunt cusps found in 31 the molar of P. pacifica may serve for crushing masses of soft food such as the organic matter found in marine snow, and perhaps for breaking the exoskeleton of microzooplankton that may be present on marine snow or on particulate matter. This action would facilitate the chemical digestion of the material in the gut of Parathemisto. Figure 2.18. Mandibular position during extreme abduction and adduction for Malacostracan crustaceans. Scheme of Anaspides tasmaniae. Redrawn from Manton (1977). The masticatory action in hyperiids occurs entirely in the mouthparts and the small masticatory ridges of the foregut (Sheader and Evans; 1975). In Parathemisto gaudichaudii the foregut is reduced but has two membranous sheets that are suggested to protect it from the abrasive action of ingested material (Sheader and Evans; 1975). A reduced foregut, an extensive digestive chamber, and a single pair of digestive caeca is a gut configuration that appears to be common in hyperiids, since it has also been found in P. Ubettula, P. abyssorum, P. australis, Hyperia galba and Hyperoche medusarum (Sheader and Evans, 1975). These authors also indicate that, with the exception of genus Parathemisto, hyperiid species feed to varying degrees on soft-bodied animals such as gelatinous zooplankton, and relate the presence of an 32 ample digestive chamber with the ability of ingesting large quantities of food overt short periods of time. The ability of the mandibles for cutting large food pieces, the ample digestive chamber, and the affinity of hyperiids for feeding on soft tissue as that found in gelatinous plankton suggests this too may be a source food for P. pacifica. However, the presence of a molar, the protected nature of the foregut, and evidence of raptorial habits in the related species P. gaudichaudii also suggests that P. pacifica may be feeding on skeletal covered organisms as well. The morphometric relationships between elements of the mandible of P. pacifica are non-linear, indicating that the change in mandible shape during growth is allometric. This in turn suggests that the mandible capability of P. pacifica may change during growth, and likely its feeding habits as well. Small individuals have a wider molar than adults, indicating perhaps a more acute microphagy early in life, while the incisor of adults is wider and taller than that of small individuals, which is indicative of carnivorous habits. Cyphocaris challengeri The main characteristics of C. challengeri mandible are a reduced seta row, a sharp, wide, and toothless incisor and a short, cuboid, and compact molar. These features, with the exception of the reduced seta row, are very similar to the basic mandible shape of amphipods described by Watling (1993). The shape of the molar and the ridges on its surface indicate a strong crushing effect on the food during feeding. The shape of the incisor is commonly found in species belonging to the Lysianassidae family, which includes C. challengeri, and is considered an adaptation for shearing-off large pieces of food tissue (Watling, 1993; Steele and Steele, 1993). Mandibles with sharp incisors and reduced or absent molars are present in amphipods that feed on soft food. The mandibles of the stegocephalid Parandaia boecki have long and smooth incisor resembling cutting blades, with a molar reduced to a short blunt process (Moore and 33 Rainbow, 1989). The laboratory experiments conducted by Moore and Rainbow (1989) showed that the concave and elongate shape of the mandibles of P. boecki, feeding on the medusa Aequorea parva and on fragments of crustaceans, allows them to take large pieces of tissue in each bite. While the sharp incisor and the absence of a molar in the mandible of Stegocephxdoid.es christianensis, limited their feeding to the sea - pen Pennatuh phosphorea (Moore and Rainbow, 1984). An extreme case is the complete absence of a molar process in some parasitic amphipods (e.g. Lafystius morhuanus), which have been found in mouths and gills of fish (Bousfield, 1987). Furthermore, Lyssianasids with incisor-bearing mandibles but reduced or lacking molar processes have predominantly necrophagous habits (Saint-Marie, 1984; Broyer and Thurston, 1987). In AUcella gigantea, an abyssal necrophagous species, the molar process is vestigial, and its stomach contents revealed large pieces of soft food (Broyer and Thurston, 1987). This indicates that feeding on soft food may not require the triturative action of a molar, which is present in scavenger and predatory species in a varying range of sizes (Saint-Marie, 1984). Some scavengers that posses a vestigial molar, as Anonyx sarsi, also exhibit some predatory activity (Saint Marie and Lamarche, 1985). However, predation was apparently limited to weak prey and only observed in large A. sarsi individuals with high swiirnriing capability. The mandibles of Cyphocaris challengeri seem more structured for carnivorous habits when compared with necrophagous lyssianasiids. Along with their cuboid molar and sharp incisor, the mandible of C. challengeri bears a small row of long and strong setae to hold the food cut by the incisor for further processing by the molar. In contrast, necrophagous species present mandibles with a large setae row that often extends from near the incisors to the vestigial molar, or overrides the space where the molar would have been (Saint-Marie, 1984; Broyer and Thurston, 1987), or is comprised of short bushy setae (Olerod, 1975). The presence of sharp incisors in the mandibles of amphipods, as those found in C. challengeri, enables them to cut large pieces of soft food, which allow for necrophagous and/or carnivorous habits (Watling, 1993). However, the presence of a strong molar in the mandible of C. challengeri also indicates the ability for crushing food. This could be construed as an adaptation 34 that effectively widens the trophic spectrum of C. challengeri by allowing them to access food from exoskeletal-protected, small planktonic invertebrates. If this is indeed the case, it may suggest a predacious nature in C. challengeri. The morphometric relationships between elements of the mandible of C. challengeri are linear, indicating that the change in the mandible shape during growth is isometric. This suggests that as C. chalkngeri grows, there is an increase in mandible capabilities for crushing and cutting. This would allow them to feed on bigger food particles. It also suggests that C. challengeri is a carnivore all of its life. 35 Chapte r 3. Feed ing habi ts and food d is t r ibut ion of amphipods 3A: Stomach Contents Reflecting Diet and Zooplankton as Potential Food. 3B: Zooplankton Composition and Salinity Structure as Environmental Cues of the Habitat of Amphipods. 36 Introduction Chapter 3 is divided in two parts. The first part focuses on investigating the diet of amphipods and their potential food. Diets were determined by examining the stomach contents and by identifying the zooplankton species caught from the same net tow sample that contained the amphipods studied. The second part of this chapter presents the analysis of amphipods and zooplankton together with the salinity structure in the study area. 3A: Stomach contents reflecting diet and zooplankton as potential food. Introduction The food that an organism captures and consumes can be considered as a subsample selected from the environment that the organism inhabits. The proportional abundance of the prey species in the diet, compared to the proportion of the prey in the environment, indicates how selective is the diet of a species. This information can be partly derived from the analysis of stomach contents of the individual of interest (Mauchline, 1996; Dalley and McClatchie, 1989). A drawback of stomach contents analysis is that soft tissue organisms in the diet may not be identified. Thus the presence of a food item in the stomach contents can be used as evidence of a trophic relationship, but the absence of prey from the gut cannot be used as evidence of no trophic relationship. Previous studies on the diet of planktonic amphipods are scarce. Sheader and Evans (1975) reviewed many of them in their work on Parathemisto gaudichaudii on the Atlantic coast. They found that the diet of P. gaudichaudii is composed of copepods (1.5 - 3 mm), decapod larvae (3 - 4 mm), chaetognaths (7 - 15 mm), and euphausiids (10 - 12 mm), with larger individuals feeding on progressively larger prey. In the North Pacific Ocean and adjacent Arctic Ocean, 37 Bowman (1960) found diatoms and crustacean parts in the stomach contents of oceanic Parathemisto pacifica, although most of the content was unrecognizable. In this section I investigate the diet of the amphipods Parathemisto pacifica and Cyphocaris challengeri in the Strait of Georgia, British Columbia. The specific objectives are 1) To determine the nature and size of prey consumed as could be inferred from stomach contents analysis, and 2) To determine how selective the diet is by comparing it with the composition and abundance of zooplankton from the same samples as those from which in which amphipods were collected. It is possible that prey items may be destroyed beyond identification during feeding (Coleman, 1991b), as Bowman (1960) found. However, it is also reasonable to expect that samples from productive areas/seasons, like the Strait of Georgia (Parsons et al., 1969; Harrison et al., 1983), increase the probability of finding identifiable food items, primarily hard tissue, in the stomach contents. For that purpose, around 500 stomach contents of amphipods collected in the Strait of Georgia were examined, and the zooplankton simultaneously collected were also identified. The periods studied were May-June 1990, April 1991, June 1991 and the entire year of 1997. Samples taken during 1990 and 1991 were over a 24-h period (see table 3.6), while those from 1997 followed a monthly and/or semi-monthly schedule. Materials and methods Stomach contents The samples used in this study were taken semi-diurnally in the estuarine and riverine plumes and at a station away from the river influence, near Texada Island. Description of the study site is provided in chapter 1. Samples were taken during May - June 1990, April 1991 and June 1991 in oblique hauls from 0 - 15 m, from 0 -15 m (1990) and from 0 - 25 m (1991) depth with a Bongo net of 296 or 300 /xrn mesh size. Semi-monthly and monthly samples were also collected during 1997 at station DFOl near Nanaimo (Fig. 1.2) with a 38 bongo net with a mouth diameter of 0.52 m, and 202 [im mesh netting (Bornhold, 1999). Vertical net hauls were collected monthly for most of the year and semi-monthly during March, April and May, at 0-50, 0-100, 0-200 and 0-400 m depths. During some months only the 0-400 m depth was sampled. All samples were preserved and stored with a solution of borax-buffered formalin (4%) and seawater. Stomachs were dissected using entomology pins. An adhesive ring was placed in the middle of a glass slide labeled with the amphipod's species name and size, as well as the date, depth and the station where it was collected. The amphipod was placed in the middle of the ring in a drop of distilled water and the exoskeleton sliced at the dorsal joint between the head and the first segment with two dissecting pins. Foregut content was then emptied in the glass slide, covered with a drop of a solution of corn syrup and formalin (Pennak, 1978) and sealed with a cover glass. Stomach contents were examined with an inverted microscope, and each item was categorized and recorded. After the stomach content was separated in pieces, the appearance of each particle was recorded with as much detail as possible (e.g. roundish particle, spine, cuticle pieces, brownish material, etc.). Food items found complete were measured and identified to the nearest taxonomic level, with the same criteria used for the identification zooplankton (this chapter, section 3B). The size of complete prey items was plotted against the size of the animal in which they were found. The range and mean size values of prey were also estimated. Diatoms were identified following the criteria of Tomas (1997) and F. J. R. Taylor (pers. comm.). The relation between stomach content and zooplankton species abundance The co-occurrence between the amphipods and the zooplankton species present in the samples was determined with the Bray-Curtis or Czechanowaki similarity index (Legendre and Legendre, 1983; Clarke and Warwick, 1994). I chose this index because estimations of 39 similarity values between species are not affected by joint absences, which is a highly desirable property in a similarity index (Clarke and Warwick, 1994).' The formulation of the index is as follows: 5M=100| i,\yu-yi* I 1=1 Where S j k = Percent similarity between species j andk Vy = VVAbundance of the j t h species in the i c h sample y jk = VVCount for the k th species in the i t h sample p = Number of samples For this analysis the zooplankton species that were present only once or twice were eliminated before calculating the similarity index. Abundance was calculated by standardizing the number of individuals to 1000 m3 of water according to Postel et al. (2000). This procedure is described in section "Standardization of zooplankton abundance" in the second part of this chapter. These values were then transformed using a double square root (VVabundance). This transformation allows both common and rare species to play a role when analyzing the zooplankton community (Clarke and Warwick, 1994). Finally, the similarity in zooplankton species composition among the riverine plume and the estuarine plume of the Fraser River and the Strait of Georgia was calculated with the percent disagreement index = Number of (xt * yt)/i- Similarity between areas was estimated using the complete linkage method of cluster analysis, and the results are presented as dendrograms calculated using the "Statistica" software. This index emphasizes the absolute difference of species present between compared areas. Resulting dendrograms were analyzed with respect to the abundance of amphipods and the most abundant zooplankton species in each zone. The non-parametric Kruskal -Wallis test by ranks was performed on the abundance of C. challengeri and P. pacifica collected in the three locations sampled to test for differences in 40 abundance between sampling sites (Zar, 1996). The reason for using this method was that zooplankton distribution is highly variable (Longhurst, 1981), which makes it difficult to estimate differences in distribution with parametric statistical methods (Solow et al., 2000). 41 Results Stomach contents Five hundred and sixty five amphipods (307 Parathemisto pacifica and 258 Cyphocaris challengeri) were dissected and their stomach contents were analyzed. The number of stomachs analyzed per species per year is given in Table 3.1. Ingested prey was identified in about 50% of the stomachs dissected, 140 items were documented for Parathemisto pacifica and 116 for Cyphocaris challengeri. The number of items identified (species or groups) for each amphipod species is also given in Table 3.1. Table 3.1. Number of prey items (numerator) identified per number of stomachs analyzed (denominator) for samples collected during three years from the Strait of Georgia. Year 1990 1991 1997 Total Parathemisto pacifica 48/116 47/114 45/77 140/307 Cyphocaris challengeri 40/99 38/79 38/80 116/258 Total 88/215 85 /193 83/157 256/565 The identified prey items listed in Table 3.2 reveal that the most common groups identified in stomach contents were copepods, cladocerans and ostracods. Amphipod species were frequently predating on each other and even oh their own species. Table 3.2 summarizes the numbers and times when this occurred. Larvae of barnacles, bryozoans, crabs and euphausiids were also present in the diet of both species although less frequently (other items with low numerical importance are also provided in Table 3.2). 42 Table 3.2. Zooplankton identified in the stomach contents of P. pacifica and C. challengeri. collected in the Strait of Georgia during 1990, 1991, and 1997 (see Table 3.1 for number of stomachs sampled). 1990 1991 1997 Group Species in stomach contents Cyphocaris chaUengeri Parathemisto pacifica 1990 Total Cyphocaris chaUengeri Parathemisto pacifica 1991 Total Cyphocaris chaUengeri O o J 1997 Total Grand Total Amphipods N . I. amphipod 1 1 1 Cyphocaris challengeri 11 3 14 9 2 11 4 3 7 32 Parathemisto pacifica 11 3 14 2 1 3 8 11 19 36 Primno macropa 1 1 1 Scina borealis 1 1 1 Scina sp. 1 1 1 Bryozoan Cyphonautes larvae 3 3 4 1 5 8 Cirripeds Barnacle larvae 1 1 2 2 4 5 Cladoceran Cladoceran 1 3 4 6 3 9 2 3 5 18 Evadne nordmanni 2 2 2 Podon leuckartii 3 3 1 1 4 Copepods Copepod 7 14 21 13 21 34 1 12 13 68 Corycaeus anglicus, m 1 1 2 2 hletridia pacifica, f 2 2 2 hietridia pacifica, m 1 1 1 Ivietridia sp. 1 1 2 2 MonstrUla sp. 2 2 2 Oithona similis, f 1 1 2 3 3 5 Oithona sp. 1 1 1 1 2 Oithona spinirostris, f 2 2 2 Oithona spinirostris, m 1 1 1 Oncaea boreaUs, i 1 1 1 1 2 Oncaea sp. 1 1 1 Paracalanus parvus, f 1 1 2 2 Pseudocalanus minutus, CI 2 2 2 Pseudocalanus minutus, CII 1 1 1 1 2 Pseudocalanus minutus, f 1 1 1 1 2 3 2 5 8 ScolecithriceUa minor, f 1 Crabs Zoea larvae 1 1 1 1 2 Diatoms Chaetoceros sp. 1 1 1 N.I. diatom 1 2 3 4 4 1 2 3 10 Rhizosolenia alata 1 1 1 Stephanophyxis palmeriana (c.f.) 2 2 2 Euphausiids Euphausiid furcilia 3 3 3 Medusae Nematocyst (unshoot) 1 1 1 1990 1991 1997 N.I. Egg 1 1 1 1 4 nauplii 1 1 1 1 2 Nematodes N.I. nematode 1 1 1 Ostracods Conchoecia alata minor 1 1 3 3 4 Ostracod 1 1 2 5 7 2 2 10 Pteropods Umacina helicina 1 1 1 1 2 Thaliacean N.I. thaliacean 1 1 1 Grand Total 40 48 87 38 47 85 38 45 83 256 44 Type of prey consumed Parathemisto pacifica The results of the size, frequency and nature of the prey found in stomach contents indicate Parathemisto pacifica with body size from 2-12 mm (Fig. 3.1) primarily consume copepods. The size range in which they ingest ostracods and the amphipod Cyphocaris challengeri was from 3 to 9 mm. Cyphonautes larvae were found in P. pacifica specimens measuring 2 to 6 mm, while diatoms were found in specimens measuring 3 to 7 mm. The size range of the prey found in the stomach contents of Parathemisto pacifica was 0.3 to 3 mm (Fig. 3.1), with a mean prey size of 0.96 mm and a standard deviation of 0.58 mm. The size range of P. pacifica studied was from about 2 to 12 mm. Throughout its size range, P. pacifica ingested prey within the minimum prey size and progressively incorporated bigger prey as its body size increased. Large and complete prey items were more frequently found in the stomach contents of individuals measuring 6 to 9 mm. 0.5 i 0.0 • • ^p> D #> nrj cgj m • A A £ tP A * - i 1 1 r- A i A i 1 — — i 1 1 1 3 6 9 Body size (mm) 12 o invertebrate lar\ae(1) • Limacina helidna (1) o Diatoms (10) A Cyphonautes (4) A Barnacle larvae (2) • Cladocerans(11) • Copepods (67) © Ostracod(16) - 3 K— Parathemisto pacifica (16) 0 Cyphocaris challengeri (8) X Nematods(1) 15 A Eggs (3) Figure 3.1. Prey items identified at species or group level in the 63 stomachs examined for Parathemisto pacifica. Parentheses in legend indicate the number of times the prey was found in the stomach contents. 45 Cyphocaris challengeri Cyphocaris challengeri ingested copepods, cladocerans, ostracods, and members of its own species and also preyed upon P. pacifica (Fig. 3.2). Cyphonautes and barnacle larvae were found in the stomachs of C. challengeri ranging from 8 to 14 mm, and diatoms were found in the stomachs of individuals ranging from 9 to 14 mm. Figure 3.2 shows the size of the prey found in dissected stomachs of Cyphocaris challengeri. The minimum size of prey was of 0.45 mm and the maximum 2.5 mm, with a mean size of 0.94 mm and a standard deviation of 0.39. Data indicate that Cyphocaris challengeri ingested small prey throughout the size range studied of (2 to 16 mm). As expected, the largest prey size present in the stomach contents increased at larger body sizes of C. challengeri. 2.5 2.0 E CD N 'tn >» 1.0 1 <D 0_ 0.5 -\ X 0.0 X • A * A * * • o \ 6 9 12 Body size (mm) 15 18 x Invertebrate larvae (2) • Barnacle larvae (5) A Cladocerans (13) • Copepods (36) o Cyphocaris challengeri (24) x Parathemisto pacifica (21) + Cyphonautes (4) o Ostracods (5) — Scina borealis(1) * Limacinahelicina(l) o Diatoms (8) @ Larvaceans(l) Figure 3.2. Prey items identified at species or group level in the 57 stomachs examined for Cyphocaris challengeri. Parentheses in legend indicate the number of times the prey was found in the stomach contents. Zooplankton as potential food for amphipods Zooplankton species that had percent similarity values of 25 and higher with respect to Parathemisto pacifica and to Cyphocaris challengeri are shown in Tables 3.3 and 3.4- The list 46 contains those species that based on co-occurrence, were considered to qualify as potential food for amphipods. These tables also show those species that were previously found in the stomach contents (Table 3.2). For Parathemisto pacifica, the stomach contents included the amphipod Cyphocaris challengeri, the ostracods Conchoecia alata minor and Conchoecia elegans, euphausiid furcilia larvae, and the copepods Corycaeus anglicus, Metridia pacifica, Oithona sirnilis, Oithona spinirostris, Oncaea borealis, Paracalanus parvus, Pseudocalanus minutus, and Scolecithricelh. minor. Table 3.3. Zooplankton species related to P. pacifica with a Bray-Curtis similarity value > 25%. Symbols indicate: n = number of samples with the species, VV = double square root of individuals per 1000 m 3, / = species present in stomach contents, • item identified only to genus level, f = female, m = male, CI-C5 = Copepodite 1 - Copepodite 5. Zooplankton species In collections 1990 Amphipod Copepod Copepod Copepod Copepod Ostracod Copepod Thaliacean Copepod Euphausiid Euphausiid Euphausiid Copepod 1991 Amphipod Ostracod Copepod Copepod Copepod Copepod Copepod Copepod Polychaete Copepod Ostracod Copepod Copepod Chaetognath u CB S £3 2 c to S c c o „ Cyphocaris challengeri (Stebbing) ^ 20 Oncaea borealis, f (Sars) 18 Oithona spinirostris, f (Claus) 14 Metridia pacifica, f (Brodsky) • 14 Pseudocalanus minutus, f (Kroyer) S 13 Conchoecia elegans (Sars) 10 Oithona sirnilis, f (Claus) S 10 Fritilaria borealis (Lohman) 7 Microcalanus pigmaeus pusillus, f 6 Euphausia pacifica (Hansen) 6 Eggs (presumably E. pacifica) 6 Euphausiid furcilia larvae S 6 Scoleciihricella minor, f (Brady) S 4 Cyphocaris challengeri (Stebbing) S 13 Conchoecia elegans (Sars) • 12 Pseudocalanus minutus S 18 Calanus pacificus (Brodsky) 15 Oithona spinirostris • 13 Metridia pacifica 14 Oithona sirnilis • 18 Oncaea borealis 16 Tomopteris septentrionalis 12 Acartia longiremis (Lilljeborg) 8 Conchoecia alata minor (McHardy) 7 Neocalanus plumchrus (Marukawa) 12 Chiridius polaris (Farran) 9 Sagitta elegans (Verril) 8 S > CO 87 76 64 65 30 31 27 80 66 66 65 61 60 59 53 50 48 cu c 3 cu 00 cd cu > < 0 0 0 0 55 0 45 0 51 0 35 0 35 0 29 0 44 1 42 1 41 1 38 1 46 3.96 48 2.54 69 3.34 45 3.00 56 2.82 61 2.62 37 3.33 88 2.53 31 3.06 55 1.94 86 4.07 48 2.48 50 3.20 09 5.09 19 3.26 93 8.56 90 4.52 11 3.79 04 5.96 .90 3.48 93 3.34 .94 2.66 .54 3.22 .65 2.86 .11 5.34 .02 2.41 .20 2.96 J ca -a cs 8 « co Q 1.81 1.22 2.31 1.35 1.57 1.61 1.23 2.22 0.96 1.06 3.51 1.19 1.61 2.44 2.73 9.19 5.05 3.40 5.73 3.85 3.59 2.83 2.08 1.73 4.79 2.36 2.46 8 cu £ 8 <^  CS 79 46 47 42 37 26 33 18 18 12 24 15 13 58 30 119 49 38 63 47 40 23 21 16 50 15 18 Copepod Paracalanus parvus (Claus) 9 35 1.18 4.12 3.50 28 Copepod Eucalanus califomicus (Johnson) 4 26 2.68 2.48 0.93 8 Euphausiid Euphausia pacifica 8 25 1.50 7.70 5.15 50 1997 Copepod Pseudocalanus minutus, f / 26 69 0.43 3.29 1.42 86 Amphipod Cyphocaris chaUengeri / 24 70 0.57 8.35 4.77 200 Copepod Metridia pacifica, f / 23 73 0.26 3.66 0.94 84 Copepod Pseudocalanus minutus, C2 20 59 0.27 3.16 0.85 63 Copepod Metridia pacifica, m • 16 64 0.29 2.87 0.84 46 Ostracod Conchoecia alata minor • 15 54 0.27 3.11 0.85 40 Pteropod Limacina helicina (Phipps) 14 39 0.40 2.52 1.00 35 Copepod Pseudocalanus minutus, C4 12 56 0.49 3.26 1.59 39 Copepod Acartia longjiremis, f 11 42 0.59 2.81 1.65 31 Copepod Pseudocalanus minutus, m 17 57 0.39 2.61 1.01 44 Copepod Pseudocalanus minutus, C3 10 34 0.23 2.97 0.68 30 Ostracod Conchoecia elegans • 9 35 0.34 2.37 0.81 21 Copepod Corycaeus anglicus, f (Lubbock) / 12 39 0.44 2.08 0.92 25 Copepod Oithona simSis, f 26 64 0.44 4.09 1.81 106 Copepod Oncaea borealis, f / 17 62 0.41 3.38 1.39 57 Copepod Paracalanus parvus, f / 6 34 0.24 2.71 0.65 16 48 Table 3.4- Zooplankton species related to C. chaUengeri with a Bray-Curtis similarity value >25%. Symbols indicate: n = number of samples with the species, VV = double square root of individuals per 1000 m3, / = species present in stomach contents, • item identified only to the genus level, f = female, m = male, CI-C5 = Copepodite 1 - Copepodite 5. 1990 1991 1997 Zooplankton species Stomach (A milarity ff. Var. CD viation indance In collections Stomach •i-c B milarity ff. Var. 00 CO U i CD Q < c t—i c £ ti c 3 CL) > < -B cn Amphipod Parathemisto pacifica / 22 87 2.46 3.56 1.44 78 Copepod Oithona simiUs, f / 10 61 0.37 3.33 1.23 33 Chaetognath Sagitta elegans / 12 60 0.59 2.35 1.39 28 Amphipod Primno macropa (Guerin- 10 47 0.38 1.70 0.64 17 Copepod Calanus pacificus, f 8 45 0.62 2.25 1.39 18 Copepod Metridia okhotensis, f (Brodskv) • 11 44 0.45 2.50 1.13 30 Amphipod Calliopius laeviesculus (Krover) 10 44 0.52 1.54 0.80 15 Euphausiid Eggs 6 37 0.86 4.07 3.51 24 Euphausiid Euphausiid furcilia larvae 6 37 0.48 2.48 1.19 15 Copepod Corycaeus angUcus, f 10 35 0.39 1.65 0.64 16 Copepod Paracalanus parvus, f 7 32 0.42 2.03 0.84 14 Copepod Chiridius Polaris, f 4 31 0.34 2.72 0.93 11 Euphausiid Euphausia pacifica • 6 29 0.55 1.94 1.06 12 Amphipod Scina borealis 4 28 0.62 2.21 1.38 9 Copepod Metridia pacifica, m • 6 27 0.51 2.30 1.17 14 Copepod Acartia longiremis, f 6 25 0.51 2.13 1.08 13 Amphipod Parathemisto pacifica 14 69 0.83 3.26 2.73 53 Copepod Oncaea borealis • 16 50 1.11 3.48 3.85 40 Copepod Calanus pacificus • 15 65 1.07 3.34 3.59 49 Copepod Metridia pacifica • 14 65 0.48 5.09 2.44 63 Copepod Oithona spinirostris • 13 50 0.67 7.70 5.15 38 Copepod Neocalanus plumchrus 12 61 1.07 8.56 9.19 50 Ostracod Conchoecia elegans • 12 45 1.12 4.52 5.05 30 Polvchaete Tomopteris septentrionalis 12 44 0.98 2.41 2.36 23 Copepod Paracalanus parvus • 9 48 0.60 2.86 1.73 28 Copepod Chiridius Polaris • 9 30 0.90 5.34 4.79 15 Copepod Acartia longjiremis • 8 29 1.06 2.66 2.83 21 Amphipod Parathemisto pacifica / 23 70 1.52 6.53 4.31 150 Polvchaete Tomopteris septentrionalis / 13 57 0.28 2.13 0.59 28 Copepod Pseudocalanus minutus, m 17 56 0.39 2.61 1.01 44 Pteropod Limacina helicina / 14 48 0.40 2.52 1.00 35 Copepod Euchaeta japonica, f (Marukawa) 7 39 0.13 1.64 0.21 11 Ostracod Conchoecia elegans • 9 37 0.34 2.37 0.81 21 Chaetognath Eukrohnia hamata (Mobius) 7 37 0.45 2.08 0.93 15 Copepod Acartia longjiremis, f 11 36 0.59 2.81 1.65 31 Copepod Pseudocalanus minutus, C3 • 10 36 0.23 2.97 0.68 30 Copepod Chiridius Polaris, f 7 32 0.31 2.28 0.70 16 Copepod Calanus pacificus, C4 6 29 0.48 2.88 1.37 17 Copepod Pseudocalanus minutus, C5 • 6 29 0.30 3.64 1.08 22 Chaetognath Sagitta decipiens (Fowler) 6 28 0.23 2.00 0.45 12 Copepod Eucalanus calif amicus, f 5 0.28 0.25 1.96 0.49 10 49 The stomach contents of Cyphocaris challengeri included species with high similarity values such as the amphipods Parathemisto pacifica, Prim.no macropa, the euphausiid Euphausia pacifica as well as its eggs and furcilia larvae, the pteropod Limacina helicina, the ostracod Conchoecia elegans, and the copepods Acartia longiremis, Calanus pacificus, Chiridius polaris, Metridia okhotensis, Metridia pacifica, Oithona sirnilis, Oithona spinirostris, Oncaea borealis, Paracalanus parvus, and Pseudocalanus minutus (Table 3.4). The polychaete Tomopteris septentrionalis and chaetognaths Sagitta elegans and Eukrohnia hamata appeared in the similarity results but neither was observed in the stomach contents. Conversely, cyphonautes larvae, barnacle larvae, and cladocerans were not important in the similarity analysis, although they were found in the stomach contents Similarity analysis on zooplankton species composition During May-June 1990, the similarity of zooplankton species composition between the Fraser River plume, the estuarine plume and the Strait is expressed as a dendrogram in Figure 3.3a. The figure shows the riverine plume and the estuarine plume were more similar with each other than with the Strait. However, the Kruskal - Wallis test for °= = 0.05 showed no difference in the abundance between areas for P. pacifica andC. challengeri (Table 3.5). 50 A) STRAIT Of GEORGIA ESTUARINE PLUME FRASER RIVER PLUME i Cyphocaris challengeri =i Parathemisto pacifica Min-Max • 25%-75% — Median value Strait of Georgia Estuarine plume Fraser River plume Figure 3.3. Dendrogram showing zooplankton similarity between sampling areas during May-June 1990. B) Whisker plot of mean abundance of P. pacifica and C. challengeri per area. * indicates a double square root transformation. C. challengeri in the Fraser plume was present in only one sample. 51 Table 3.5. Kruskal - Wallis test by ranks to test Ho = Abundance is the same across the Fraser River plume, estuarine plume and the Strait of Georgia, for a = 0.05 (for June 1991, a = 0.01). Data used for the analyses are in Table A7. Nomenclature follows Zar (1996). May - June Cyphocaris challengeri Paratkemisto pacifica 1990 H = 3.28 H= 1.61 Ho,0.05,4,3,2 = 5.44 Ho,0.05,4,3,2 = 5.44 H < Ho H < Ho Do not Reject Ho P>>0.1 Do not Reject Ho P>>0.1 April 1991 Tied ranks C 0.9 He 3.2 H= 2.46 Ho,0.05,4,2,2 = 5.33 Ho,0.05,4,2,2 = 5.33 He < Ho H < Ho Do not Reject Ho O . K P < 0.25 Do not Reject Ho P>>0.1 June 1991 Tied ranks C 0.97 He 1.54 H 1.14 Ho.0.1,2,2,2 = 4.57 Ho (0.1,2,2,2) 4.57 He < Ho H < Ho Do not Reject Ho P>0.1 Do not Reject Ho P >> 0.1 1997 H 4.52 H = 0.95 Ho (0.05,4,4,3,3) 7.04 Ho,0.05,4,4,3,3 = 7.04 H < Ho H < Ho Do not Reject Ho P>0.1 Do not Reject Ho P>> 0.1 During April 1991 the zooplankton composition was more similar between the Strait of Georgia and the estuarine plume (Fig. 3.4A). However, the Kruskal - Wallis test showed no difference in the abundance between areas for P. pacifica and C. challengeri (Table 3.5). During June 1991 there was greater similarity between the Strait and the estuarine plume, than between it and the riverine plume (Fig.,3.5a). The abundance of C. challengeri and of P. pacifica however, was not statistically different between areas (Table 3.5). 52 A) 0.B8 0.86 0.84 0.82 cu 5 0.80 b ro 5 0.76 0.74 0.72 0.70 I 1 L 1 RIVERINE PLUME STRAIT of GEORGIA ESTUARINE PLUME B) n E S S Cyphocaris challengeri ZT~_ Min-Max c = i Parathemisto pacifica • 25%-75% — Median value ED Fraser River strait of Estuarine plume Georgia plume Figure 3.4. Dendrogram showing zooplankton similarity between sampling areas during April 1991. B) Whisker plot of mean abundance of P. pacifica and C . challengeri per area. * indicates a double square root transformation. 53 A) FRASER RIVER PLUME ESTUARINE PLUME STRAIT of GEORGIA i Cyphocaris challengeri ^ Parathemisto pacifica Min-Max • 25%-75% — Median value Fraser River plume Strait of Georgia Estuarine plume Figure 3.5. Dendrogram showing zooplankton similarity between sampling areas during June 1991. B) Whisker plot of mean abundance of P. pacifica and C. challengeri per area. * indicates a double square root transformation. 54 The similarity analysis of the zooplankton composition during 1997 compared the October-December period with the period of April-June, leaving January-March and July-September as distinct groups (Fig. 3.6a). However, the Kruskal - Wallis test showed no seasonal difference in the abundance of C. challengeri and P. pacifica for the periods selected. There was a high abundance of copepod eggs during January-March, and some copepod species such as Pseudocalanus minutus, Metridia pacifica, Oncaea borealis and Oithona sirnilis were relatively high throughout the year. 55 A) 1.00 0.98 CD <J C CO I 0.96 cu O) CO ^ 0.94 0.92 0.90 JUL-SEP OCT-DEC APR-JUN JAN-MARCH B) 20 16 O § 12 1-H s cn *2 8 •8 > 5 Cyphocaris challengeri Parathemisto pacifica I Min-Max I I 25%-75% • Median value Jul - Sep Oct - Dec Apr - Jun Jan - Mar Figure 3.6. Dendrogram showing zooplankton similarity between sampling areas during 1997. B) Abundance of P. pacifica and C . challengeri per area. * indicates a double square transformation. 56 Discussion Based on gut analyses alone, the organisms that Parathemisto pacifica and Cyphocaris challengeri preyed upon most frequently were crustaceans, particularly copepods, cladocerans and ostracods. The prey was similar in mean size for both amphipod species, with P. pacifica being able to consume prey on a somewhat wider size range. There was no evidence of a change of prey type consumed over the size range analyzed, but a change did occur in size for the same prey type. Small amphipods consumed earlier developmental stages of copepods of the same species that they consumed at larger body sizes. There was also a high coincidence between the amphipods and female copepods, suggesting females as a preferred prey type for amphipods, since females can have a higher nutritious value when bearing eggs. What is missing is prey that may have been eaten but did not show up in the analyses. The absence of a species in the stomachs cannot be used as evidence of lack of trophic relationship. This is a disadvantage inherent to the method of inferring diet from stomach contents. The type of prey most likely to be missing in the stomachs would include jelly-like food (e.g. coelenterates) or food that is rapidly digested. If this is the case for P. pacifica and/or C. challengeri, then the stomach contents analysis is providing a diet composition biased towards organisms with hard to digest or indigestible structures (e.g. crustaceans). Previous studies on stomach contents of hyperiidean amphipods in general, and Parathemisto pacifica in particular are scarce. In their review, Sheader and Evans (1975) reported that gut contents of P. gaudichaudii (from the Atlantic ocean), a close relative of P. pacifica, contained copepods from the genera Calanus and Temora as well as young specimens of Euphausia superba. Hyperiids amphipods have also been associated with gelatinous plankton, both as predators and as symbionts (Harbison et al., 1977). Madin and Harbison (1977) hand-collected Vibilia and Lycea hyperiids from salps. They found that adults of the genus Vibilia position themselves in the esophagus of the salp and feed on the particulate matter collected by the salp, although their larvae initially feed parasitically on the salp tissue. Members of the genus Lycea grazed on the cilia rows of the gill bars of the salp, and also chewed the interior body of the salp and consumed developing embryos. Madin and Harbison (1977) also noticed 5 7 juvenile individuals of Parathemisto gaudichaudii on the outside of the salp Pegea bicaudata. However, they did not find evidence that P. gaudichaudii were feeding on the salp. Furthermore, they indicated that several individuals of P. gaudichaudii swam away when they collected the salp. These authors concluded that although the raptorial P. gaudichaudii is not as free-living as once thought, its relationship with the salp Pegea bicaudata is tenuous. Although the possibility exists that P. pacifica feeds on gelatinous zooplankton, the analysis of zooplankton composition presented here indicates that gelatinous zooplankton (ctenophores, medusas, and siphonophores) were scarce (Table A l - A4). Furthermore, considering that P. pacifica is closely related to P. gaudichaudii, it seems reasonable to expect P. pacifica to hold a similarly loose relationship with gelatinous plankton. Owing to a lack of evidence from gut analysis as well, the possibility of P. pacifica feeding on gelatinous zooplankton can not be considered. Sheader and Evans (1975) found P. gaudichaudi to be an indiscriminate feeder; its gut contents reflected the composition of the local plankton. This was also evident to some extent for Parathemisto pacifica in the Strait of Georgia, except that some zooplankton species within the prey size range, were not identified in the stomach contents. This suggests that these amphipods do not necessarily eat every potential prey they encounter. On the other hand, cladocerans were present in the stomach contents. However, they had such a low score in the Bray-Curtis similarity index that they were not considered to be significant. This may be because of their small size prevents them from being caught effectively in the zooplankton net used in this study. The stomach contents of P. pacifica contained species of small copepods, cladocerans and ostracods that were abundant in the zooplankton, and presented high scores in the Bray -Curtis index of similarity analysis between amphipods and zooplankton species (Tables 3.2 and 3.3). The species present in the diet that the similarity analysis also indicated as potential prey were small individuals of P. pacifica itself, and of the amphipod C. challengeri. Also the copepods Corycaeus anglicus, Metridia pacifica, Oithona similis, Oithona spinirostris, Oncaea 58 borealis, Pseudocalanus minutus, Scolecithricella minor, small euphausiid furcilia presumably of Euphausia pacifica, and the ostracods Conchoecia alata minor and Conchoecia elegans. The literature does not contain information on Cyphocaris challengeri stomach contents. This is an amphipod adapted to a pelagic life that has been previously observed to be abundant in plankton samples in the Strait of Georgia (Legare, 1957; Gardner, 1977). The results presented here indicate that the stomach contents of C. challengeri are similar to those of P. pacifica, although there are some differences in species and proportion of groups (Table 3.2). Cladocerans were more abundant in stomachs of C. challengeri and copepods were more numerous in the diet of P. pacifica. The cladoceran Podon leuckartii appeared in C. challengeri diet, while Evadne nordrnanni appeared in the stomach of P. pacifica. The species present in the diet of C. challengeri, and the similarity analysis, indicated as potential prey were small individuals of C. challengeri, and the amphipod P. pacifica. Also the copepods Calanus pacificus, Corycaeus anglicus, Chiridius polaris, Metridia pacifica, Oithona spinirostris, Oncaea borealis, Paracahnus parvus, Pseudocalanus minutus, the euphausiid Euphausia pacifica, the ostracod Conchoecia elegans, and the pteropod Limacina helicina. A high coincidence of zooplankton potential prey and stomach contents may also be a result of predator/prey interactions owing to confinement during the net towing. Consequently, the analysis of zooplankton diets using field samples can be biased towards the zooplankton species caught in the sample. However, this is difficult to accept considering that a net sample collection often takes less than an hour which is less than the several hours needed for amphipods (P. gaudichaudi) to digest crustacean prey (e.g. Sheader and Evans, 1975). One must also consider the fact that both prey and predator are being tumbled and twisted by the action of both the net and the plankton bucket both during and after a tow, prior to preservation. Although the possibility of feeding during confinement could occur for some zooplankters, Sheader and Evans (1975) observed that Parathemisto gaudichaudi specimens had to be exposed to prey for 3-5 days to accept it as a food source. Their results from both laboratory 59 prey selection experiments and gut content analysis o f field samples also ind ica ted that a prey species was more l ike ly to become their food i f i t h a d previously fo rmed part o f the diet o f Parathemisto. Th is suggests that the stomach contents o f Parathemisto are close to its true diet. T h e simi lar i ty between the zooplankton composi t ion among areas showed a shift between these (Figs. 3.3a and 3.4a). I n May-June 1990 was estuarine p lume - Fraser River plume and changed to estuarine plume - Strai t o f Georgia i n A p r i l and June 1991. T h e zooplankton similar i ty between estuarine plume and Fraser River p lume as f o u n d dur ing M a y - June 1990 was expected, since the mu tua l inf luence o f river discharge and t idal water combined and fo rmed first the riverine and then the estuarine plume (Harr ison et al., 1991; Y i n et al., 1995a). O n the other hand , the zooplankton similar i ty between the estuarine plume and the Strait o f Georgia f o u n d dur ing A p r i l and June 1991, may have resul ted f r o m the estuarine plume being in f luenced by water f r o m the Strai t , w h i c h penetrates and mixes w i t h the estuarine p lume dur ing the flood tide (Y in , 1994). Areas w i t h similar zooplankton composi t ion (Figs. 3 .3A, 3 .4A a n d 3 .5A) were expected to exhib i t similar levels o f abundance o f ei ther P. pacifica or C . challengeri (Figs. 3.3B, 3.4B and 3.5B), but this was n o t observed. Wh isker plots show some differences i n the abundance o f P. pacifica and C. challengeri between areas; however K ruska l -Wa l l i s tests showed no statistical differences for any o f the two species between areas (Table 3.5). There may be some differences, a l though the h igh variance that characterizes zoop lankton samples obscured any possible pat tern o f abundance. T h e Bray-Curt is index o n the other hand , prov ided evidence that abundance o f P. pacifica and C. challengeri were related each to a group o f re-current species. W h e t h e r P. pacifica or C. challengeri, and the group o f associated species, exh ib i t a spatial pa t te rn w o u l d be more readily detected using a numer ica l technique to account for bo th the spatial and commun i t y structure. Th is aspect is analyzed i n the nex t section using pr inc ipa l components analysis. T h e organisms f o u n d i n the stomach contents o f bo th amph ipod species have small bodies (0.7 to 2 m m ) representing small size components o f the mesozooplankton. By foraging o n 60 this fraction of the zooplankton, amphipods may serve as a trophic link to bigger zooplankton consumers such as fish species. In the Strait of Georgia, salmon and herring have been observed to feed in the Fraser River plume and estuarine plume areas (St. John et al., 1992). Feeding experiments revealed that juvenile pink salmon measuring 34 mm (fork length) preferred Neocalanus plurnchrus over the small Pseudocalanus minutus and Microcakmus sp. (LeBrasseur et al., 1969). They possibly preferred N. plurnchrus because of the obtained nourishment value or because trying to feed on small-bodied copepods may be more energy-expensive and rime consuming. At larger juvenile sizes, pink salmon were observed to consume Parathemisto pacifica (Beacham, 1986). Juvenile herring have been observed eating Pseudocalanus sp. (Raymont, 1983), and adult herring were observed feeding on hyperiid amphipods (Raymont, 1983). Sockeye salmon have been observed feeding on Parathemisto pacifica both during juvenile and adult stages (Beacham, 1986). Considering that type and quantity of prey may limit the survival of young fish (Arthur, 1976), amphipods may benefit the fish species that prey on them by concentrating food energy and biomass from small zooplankters. 61 3B: Zooplankton composit ion and salinity structure as environmental cues of the habitat of amphipods Introduction The most important factors limiting primary productivity in the Strait of Georgia have been identified as light during winter months and nutrients and grazing during the late spring and summer (Harrison et al., 1983). Turbidity, not considered in this study, depresses irradiance and therefore photosynthesis and is also a source of suspended nutrients (Pinet, 1992). The two abundant periods for zooplankton are late spring and late summer, both following phytoplankton peak events. Spring to early fall encompass the most productive periods in the Strait and, at these times, many zooplankton species are present. Of these two seasons, spring is the most productive and the period between March and May has been identified as the best for zooplankton (Parsons et al., 1970; Harrison et al., 1983). The Fraser River has a major influence in controlling the phytoplankton production and zooplankton abundance in the Strait of Georgia. This is due to nutrient enrichment produced by entrainment (Yin, 1994) and the subsequent increase in phytoplankton and zooplankton. The effect of the estuarine and riverine plumes on zooplankton diversity and abundance has been studied at higher taxonomic levels (St. John et al., 1992). In the present study, the analysis of zooplankton was conducted at the species level. The research objective of this section was to investigate the zooplankton community with which amphipods coexist in the Strait of Georgia, along with the salinity structure prevailing at the sampling stations. 62 Materials and methods Samples source Samples for this study were obtained for the research project "Nutrient and Phytoplankton Dynamics in the Fraser River Plume, Strait of Georgia, British Columbia" (Clifford et. al. 1990, 1991a, 1991b, 1992; Harrison et al., 1991). The objective of the project was to investigate the dynamics of the phytoplankton production, through the monitoring and analysis of the nutrients and changes in the chlorophyll maxima under tidal interactions between the Fraser River and the Strait of Georgia. Samples were taken over a period of 24 h in the Fraser River Plume, the estuarine plume and the Strait of Georgia. Since high zooplankton biomass has been associated with the high chlorophyll maximum (Anderson et al., 1972; Mullin and Brooks, 1972; Hobson and Lorenzen, 1972; Youngbluth, 1975; Haury et al., 1976; Castro et al., 1991), or at the depth of maximum phytoplankton production (Venrick et al., 1973; Longhurst 1976; Herman et al, 1981), it seemed a good opportunity to investigate some of the zooplankton dynamics as well. Semi-diurnal sampling was done during May - June 1990, April 1991, and June 1991. Also, semi-monthly and monthly sampling was conducted during 1997. The semi-diurnal samples were taken in the estuarine and riverine plumes and at a station away from the river influence, near Texada Island (indicated as R, E, and S respectively in Fig. 1.2). The zooplankton samples included oblique plankton net hauls from 0 -15 m (1990) and from 0 - 25-m (1991) m depth with a Bongo net of 296 or 300 /xm mesh size. Salinity data were extracted from the project reports (see first paragraph of materials and methods section) and included data from the upper 25 m. The locations of sampling stations in this study were: Location Station Latitude Longitude Strait of Georgia S 49° 21'04" N 124° 11' W Riverine plume R 49° 05' 10" N 123° 22' 05" W Estuarine plume E 49° 07'25" N 123° 34'10" W 63 and the sampling periods were: May 29,1990 - June 6,1990 April 7,1991 - April 16, 1991 June 11, 1991 - June 14, 1991 The labelling code used to identify the semi-diurnal sampling locations for the statistical analysis in this section is in Table 3.6. Monthly and semi-monthly samples taken during 1997 were also included in this study to characterize an annual cycle. These samples were taken at a location near Nanaimo (station DFOl, located at 49° 15' 0" N, 123° 44' 9" W. See Fig. 1.2). The samples consisted of plankton and hydrographic data collected monthly for most of the year and semi-monthly during March, April and May, at depths of 0-50, 0-100, 0-200 and 0-400 m. The zooplankton samples consisted of oblique plankton net hauls done with a Bongo net of 202 /zm mesh size. During February, March, October, November, and December only the 0-400 m depth interval was sampled. Table 3.6. Label code for the semi-diurnal sampling locations. Location Label used in plot Date Hour 11:00 23:00 20:00 18:00 14:00 17:00 23:00 13:00 1:00 Estuarine plume Estuarine plume Riverine plume Riverine plume Estuarine plume Estuarine plume Strait Strait Strait RI R2 E l E2 SI S2 S3 E3 E4 29-May-90 29- May-90 30- May-90 31- May-90 l-Jun-90 l-Jun-90 l-Jun-90 5-Jun-90 6-Jun-90 Riverine plume Riverine plume Estuarine plume Estuarine plume Estuarine plume Estuarine plume Strait RI R2 E l E2 E3 E4 SI 7- Apr-91 8- Apr-91 9- Apr-91 9- Apr-91 10- Apr-91 10-Apr-91 16-Apr-91 22:40 10:49 9:30 22:05 10:40 10:50 23:06 64 Strait S2 16-Apr-91 23.15 Riverine plume Estuarine plume Riverine plume Strait RI E l R2 SI S2 ll-Jun-91 11-Jun-91 12-Jun-91 13-Jun-91 14-Jun-91 14:10 14:25 2:10 14:25 1:12 Strait Subsample preparation and species identification Zooplankton samples of one liter volume were concentrated to 250 mL in the laboratory. Each sample was sieved with a 33 fxm mesh nitex cone filter. Then the zooplankton was transferred to 250 mL jars which were filled to the top with a filtered solution of borax-buffered formalin (4%) and seawater from the original sample. The sample was stirred and then fractionated using a 50 mL graduated pipette with a tip diameter of 1 cm to avoid clogging, attached to a 10 mL syringe with a piece of flexible rubber tubing. Three to five subsamples of one mL were prepared for each sample and the zooplankton identified and counted. This is a standard method that insures a random distribution of the organisms in the sample and provides a set of subsamples of controlled volume which has to be considered for calculations of abundance (Postel et al., 2000). Under these conditions the accuracy of a subsample depends on the numbers of specimens counted (Cassie, 1971); more than 100 specimens yields a precision of ±20% at the 95% confidence level, which is considered acceptable (Postel et al., 2000). All of the organisms contained in the zooplankton subsamples were identified and counted under a stereoscopic microscope (Wild M20). Whenever possible, animals were identified by gender and developmental stage. The zooplankton species were identified following the criteria of authors listed in Table 3.7. 65 Table 3.7. List of authors whose criteria were used to identify the zooplankton groups.. Taxonomic group Author (s) Amphipods Bowman and Grunner (1973) Chaetognaths Alvarino (1965) Cladocerans Fulton (1968), Smith (1977), Raymont (1983) Copepods Brodskii (1950), Fulton, (1973), Gardner andSzabo (1982) Crustacean decapods Campbell (1935) Euphausiids Bodenetal. (1955) Ichthyoplankton Moseretal. (1996) Medusae Fulton (1968), Arai and Brinckmann-Voss (1980) Ostracods McHardy (1964), Fulton (1968), Smith (1977), Raymont (1983) Polychaets Fulton (1968) Siphonophores Totton and Bargman (1965), Alvarino (1967) Thaliaceans Fulton (1968) General zooplankton Fulton (1968), Smith (1977) Standardization of zooplankton abundance The zooplankton specimens were identified, counted, and standardized to number of individuals per 1000 m3 of water as this is a conventional practice in the analysis of plankton samples (Postel et al., 2000). First, the number of individuals collected in the sample was estimated from the number of individuals in the subsample as follows (UNESCO, 1968): X T * V S N = n * — vs where: N = Number of organisms collected n = Number of individuals in subsample VS = Volume of sample jar (250 mL) vs = Volume of subsample in mL Then, the number of organisms collected per 1000 m3 was calculated as follows (UNESCO, 1968): 66 N * = — *1000 w where: N* = Number of organisms per 1000 m 3 of water N = Number of organisms collected W = Volume of water filtered during the tow in m 3 and, according to Tranter (1968) : W = F * A * D Where : F = Filtration efficiency A = Mouth area of the sampler D = Distance the plankton net was towed Filtration efficiency of the net used during the 1997 field sampling was determined as follows: The net was hauled vertically to the surface from 200 m with a calibrated flow meter placed half way between the rim and the centre of the opening. Flow meter revolutions were then compared with identical hauls of the flow meter and the net frame without the nets attached. This procedure yielded an estimated filtration efficiency of 95% during spring conditions (Bornhold, 1999). However, for the samples taken in 1990 and 1991, this procedure was not done, and the filtration efficiency was then estimated following a method proposed by Tranter and Smith (1968): (1 + 0.01AT) Where: K = -—Jr-*6Rin = Resistance coefficient B = = Mesh porosity H (d + mf V d Re = - Reynolds number P v 67 m = Mesh width d = Diameter of strands in the meshwork V = Flow velocity v = Kinematic viscosity of the water = 1 x IO-6 m2 s'1 Analysis of zooplankton composition and amphipod size-group distribution The zooplankton community was investigated by examining the abundance of species through principal component analysis (PCA) using the commercial computer software "Statistica". For the samples collected during 1990 and 1991, PCA was performed only on those collected in the first 0-15 m (1990) and 0-25 m (1991) because the corresponding hydrographic data were only collected for those depths (Table 3.6). Species found only once were eliminated prior to PCA analysis because, in terms of abundance, they had a low contribution to the community composition but added computing load during the process of big data matrices. The PCA analyses were performed after abundance values were transformed by applying a double square root (VV). This transformation allows both common and rare species to play a role during analyses of the zooplankton community (Clarke and Warwick, 1994). The PCA results were plotted and presented with the respective salinity data. The salinity data were presented in contour plots and results of both data sets compared; salinity data were selected over temperature for analysis because the Strait of Georgia is a salt-controlled environment (Waldichuck, 1957). Salinity values from 0 to 10 ppt corresponded to the riverine plume, from 10 to 15 to the estuarine plume and from 25 to 30 to the Strait (Harrison et al., 1991; St John et al., 1992) Also, to examine the distribution per size group relative to the locations studied, amphipods were organized in 4 size groups. The first group included from the hatching size to the size of first reproduction, 1.5-5.5 mm for P. pacifica and 2-5.5 mm for C. challengeri. The second 68 group comprised young adults (>5.5-8.5 mm), followed by old adults (>8.5- 15 mm), and an extra group of older adults (> 10.5-16 mm) for C. chaUengeri. Data from May-June 1990, and April and June 1991 provided information on spatial changes in size composition of amphipods between the Strait and areas neighboring the Fraser River. Data from January-December 1997 supplied information on how.size composition changed relative to seasonal variation of the salinity vertical structure. Results from winter and summer were selected to illustrate those contrasting seasons. 69 Results Zooplankton general abundance Zooplankton abundance per species per sampling is in Tables A l to A4 in Appendix A. Copepods, amphipods and euphausiids were the numerically dominant groups over all the periods studied (Table 3.8). Ostracods, cladocerans and thaliaceans were occasionally abundant, but of less numerical importance. Table 3.8. Abundance of zooplankton groups (in individuals per 1000 m3) present in the samples collected during three years from the Strait of Georgia. Data from 1997 correspond to a complete year. Date May29-Jun6 Apr 746 Jun 11-14 Jan-Dec Subtotal 1990 1990 1991 1997 Group Amphipods 5568 4475 13732 1104 24880 Barnacle larvae 147 33 20 90 290 Bryozoan larvae 173 274 101 1823 2371 Chaetognaths 315 5 249 838 1407 Cladocerans 803 0 184 464 1451 Copepods 10556 9150 6230 86092 112027 Decapod Crustaceans 153 158 28 1200 1539 Ctenophores 21 0 0 0 21 Euphausiids 2471 1229 1317 1298 6316 Gastropods 12 0 0 0 12 Ichthyoplankton 14 75 15 336 439 Invertebrate larvae 0 0 563 0 563 Medusae 16 64 . 20 4 • 404 Mysids 0 0 0 8 8 Ostracods 338 286 871 2254 3749 Polychaets 55 107 122 7.10 993 Pteropods 109 208 82 1188 1586 Siphonophores 36 87 99 83 305 Thaliaceans 401 165 936 0 1502 Subtotal 21187 16315 24569 97650 70 Zooplankton composition, amphipod size-group distribution and salinity structure The abundance of zooplankton species collected in the different periods was analyzed using PCA. The first three components explained a cumulative proportion between 60 and 78% of the total variance in the data. These first three components were selected considering that: (1) the cumulative variance of consecutive components was closest to the total variance (100%), and (2) a component would individually contribute at least 15% of the total variance (Daukrey, 1976). The analysis was station (space) oriented in order to use zooplankton species as descriptors of the environment, and to examine possible existing patterns of zooplankton composition among locations. This was convenient because when using species to characterize the environment in a principal component analysis, the total variance associated with a site is equivalent to an index of ecological diversity (Ter Braak, 1983). Since factor loadings represent the particular contribution of each species to the total variance that particular component explains (Dillon and Goldstein, 1984), species selected for each component were those with the highest values. In terms of ecological diversity, these would represent the dominant species (positive loadings) and the species whose abundance decrease along the component (negative loadings) at the site (Jongman et al, 1995). Tables 3.9 to 3.11 and 3.13 show these loadings, as well as the areas (riverine plume, estuarine plume, and Strait) or months (for the annual sampling) that played the most important role in the zooplankton variation. Salinity data results are presented as contour plots and given with the zooplankton PCA results and with the size group distribution per zone in Figures 3.7 to 3.15. Although presenting salinity results as isopleths is artificial because the sampling locations were not contiguous, it provided a convenient way to show the vertical structure of salinity during the semi-diurnal sampling, and during the months during the annual sampling. 71 The relative abundance per size group of P. pacifica and C. challengeri is shown in Figures 3.8, 3.10, and 3.12 for semi-diumal samples and together with the salinity contour plots in figure 3.14 for the monthly samples. Data are in Tables A5 and A6 in Appendix A. Correlation coefficients between proportion per group size and average salinity for the depth layers of 0-10 m, 10-50 m and >50 m are shown in Table 3.12. The results first shown refer to the changes during spring and summer in the Fraser River plume, the estuarine plume and the Strait of Georgia, and correspond to the periods of May 29 - June 6 1990, April (7 - 16) 1991, and June 11- 14 1991. The second set of results corresponded to changes occurring during 1997. Notice that stations are named according to the location in which they were sampled (Fig. 1.2, Table 3.6), and that provided salinity profiles shown reflect the dynamic nature of estuarine areas. I. Spring and summer changes (1990, 1991) M a y 2 9 - J u n e 6 1 9 9 0 During May - June 1990, the estuarine plume stations dominated the variance in the 1st component (Fig. 3.7A), two stations from the riverine plume dominated the variance in the 2nd component, and stations from both estuarine and riverine plumes had high values in the 3rd component. Strait stations were low on both the l 5 t and 3rd component and showed moderate values on the second component. In the upper 5 m the salinity for Strait stations was in the 26 - 29 ppt range (Fig. 3.7B), from 8 to 29 in the estuarine plume stations and from 14 to 29 ppt in stations from the riverine plume. Below 15 meters, salinity values were 29 ppt for all stations sampled during May-June 1990. Station E4 from the estuarine plume area had surface values within the range of the riverine plume water (0-10 ppt) and the stations from the riverine plume area had values within the range of the estuarine plume water (>10 - 20 ppt). 72 20 4 1 I 'I' I I I l l ||| I I l l i l l I I | | 4 5 6 7 8 9 Sampling day I II I I , I E2 S1 S2 S3 E3 E4 Figure 3.7. A) Principal components for zooplankton from May 29 -June 6, 1990. Components are described in Table 3.9. B) Salinity contour plot. R = riverine plume, E=estuarine plume, and S = Strait of Georgia (Nomenclature of stations as in Fig. 1.2, Table 3.6). I I R1 R2 E1 73 A) io 0.8 U 0.6 tu 3 a" 0.4 0.2 0.0 1-5.5 5.5-8 8.5-10.5 Size Intervals (mm) 11-15.0 B)10 0.8 g 0.6 CU 0.2 0.0 • Riverine plume A Estuarine plume ° Strait 1 I . 1.5-5.5 >5.5-8 >8-10.5 Size Intervals (mm) >10.5-15 Figure 3.8, Size distribution of P. pacifica (A) and C. challengeri (B) during May 29 - June 6, 1990. Young amphipods (1-5.5 mm) of Parathemisto pacifica were dominant in the size composition and most abundant in the estuarine plume (Fig. 3.8A), while Cyphocaris chaUengeri adults (>5.5 mm) were the most abundant group and concentrated in the riverine plume (Fig. 3.8 B). Table 3.9. Loadings of the most important species on the principal components for zooplankton. Samples collected from the riverine plume, estuarine plume and the Strait during May 29 - June 6, 1990. Stations where the highest variation occurred are in parentheses. Abbreviations indicate: f = female, frc = furcilia, C-3 = Copepodite 3. 1st Component (34%) (estuarine plume) 2 n d Component (13%) (riverine plume) 3 r d Component (11%) (estuarine and riverine plume) Oithona spinirostris, f 4.08 Metridia okhotensis, f 3.57 Pseudocalanus minutus, f 4.78 Cyphocaris challengeri 3.71 Pseudocalanus minutus, f 3.29 Cyphocaris challengeri 3.88 Parathemisto pacifica 3.21 Oithona similis, f 2.70 Parathemisto pacifica 2.96 Metridia pacifica, f 1.65 Oncaea borealis, f 2.68 Euphausia pacifica, frc 2.27 Oncaea borealis, f 1.32 Metridia pacifica, f 2.59 Paracalanus parvus, C-3 2.03 Euphausia pacifica, frc 1.21 Parathemisto pacifica 2.32 Paracalanus parvus, f 1.40 Pseudocalanus minutus -1.77 Euphausia pacifica eggs -1.33 Metridia pacifica -1.77 Oithona similis —1.65 Calyptopis larvae -0.99 Oncaea borealis —1.35 Corycaeus anglicus —1.53 FritUaria borealis -0.97 Scolecithricella minor —1.29 Paracalanus parvus -1.45 Nauplii larvae -0.95 Table 3.9 shows zooplankton species with the highest factor score contributing to the total variance of the three first components. Oithona spinirostris was the dominant copepod in the 1st 74 component, Metridia okhotensis in the second and Pseudocalanus minutus in the third component. The components had several species in common, Parathemisto pacifica appeared listed under the three components, while Cyphocaris challengeri and furcilia larvae of Euphausia pacifica appeared in the 1st and 3rd component. Pseudocalanus minutus had a negative relation to the 1" component. April 7-16 1991 Figure (3.9A) shows stations from the estuarine plume with the highest values in the 1 s t component. Two stations from the river and one from the strait had high values on the 2 n d component, and station RI from the riverine plume area had a distinctly high value in the third component. Figure (3.9B) shows station RI as the only station sampled in the riverine plume area that actually had salinity within the range 0-10 ppt expected for a riverine plume. Station R2 had salinity within the range for the estuarine plume area, and the remaining stations had salinity values within the range >25 - 30 ppt identified as Strait water. Likely this is a result of the interaction between the river discharge and the tide during sampling (see Harrison et al. (1991) and Yin et al. (1995a) for an explanation about this type of interaction). Table 3.10 shows that the zooplankton species that contributed the most to the variation in the first component were euphausiids (Euphausia pacifica), copepods (Calanus marshallae, Calanus pacificus, Pseudocalanus minutus) and barnacle nauplii. P. minutus was common to stations from both the riverine and estuarine plumes and was the dominant species listed under the 2nd component. Neocalanus plurnchrus was listed under the 2nd and was the main species for the 3rd component, and it showed a negative relation to the 18C component. In this occasion, the amphipods Parathemisto pacifica and Cyphocaris challengeri were only listed in the 2nd component. Young individuals of both amphipods species dominated the size composition in the estuarine plume, while young adults presented different distributions; Parathemisto pacifica had the 75 greatest abundance in the estuarine plume (Fig. 3.10A) while the highest number of Cyphocaris challengeri individuals occurred in the Strait (Fig. 3.10B). Table 3.10. Loadings of the most important species on the principal components for zooplankton. Samples collected from the riverine plume, estuarine plume and the Strait during April 7-16 1991. Stations where the highest variation occurred are in parentheses. Abbreviations indicate: f = female, est = estuarine, riv = riverine, plm= plume. 1st Component (37%) (estuarine plume) 2nd Component (28%) (Strait) 3rd Component (12%) (Strait, est.plm; riv. plm) Euphausia pacifica 8.50 Pseudocalanus minutus 9.04 Neocalanus plumchrus 7.87 Calanus marshallae 3.68 Cyphocaris challengeri 2.17 Metridia pacifica 3.69 Eucalanus bungii 1.40 Oithona spinirostris 0.97 Euphausia pacifica 2.32 Calanus pacificus 0.27 Neocalanus plumchrus 0.90 Apolemnia uvaria 0.53 Pseudocalanus minutus, f 0.17 Metridia pacifica 0.83 Metridia okhotensis 0.46 Barnacle nauplii 0.16 Parathemisto pacifica 0.82 Metridia curticauda 0.46 Neocalanus plumchrus -1.79 Conchoecia alata minor -1.84 Oncaea borealis -1.98 Paracalanus parvus -1.74 Oncaea borealis -1.72 Pseudocalanus minutus -1.69 Ctenocalanus vanus -1.45 Acartia longiremis -1.05 Corycaeus anglicus -1.66 Apolemnia uvaria -1.21 Euphausia pacifica -0.89 Barnacle nauplii -1.47 Clausocalanus arcuicomis -1.11 Chiridius polaris -1.34 76 Figure 3.9. Principal components for zooplankton samples from April 7-16, 1991.Components are described in Table 3.10. B) Salinity contour plot. R = riverine plume, E =estuarine plume, and S = Strait of Georgia (Nomenclature of stations as in Fig. 1.2, Table 3.6). 77 A) 1.0 0.8 >< nc 0.6 0> 3 cr 0.4 u. 0.2 0.0 1.5-5.5 5.5-8 8.5-10.5 Size Intervals (mm) 10.5-15 B) 1.0 0.8 >• nc 0.6 ai 3 SX 0.4 cu u. 0.2 0.0 • Riverine plume A Estuarine plume ° Strait f L 7??7A 2-5.5 5.5-8 8-10.5 Size Intervals (mm) 10.5-16 Figure 3.10. Size distributions of P. pacifica (A) and C. chalkngeri (B) during April (7-16), 1991. June (11 - 14) 1991 Results for June 1991 (Fig. 3.11 A) show stations from the estuarine and Strait areas contributing to the highest values in both the first and third components. Stations from the riverine plume area had the highest values in the second component, and station SI was set apart with low values in the three components. Figure 3.1 IB shows that salinity for stations sampled in the estuarine plume had values within the range 0-10 ppt characteristic of riverine plume water in the upper 5 m, while the stations in the riverine plume had salinity values closer to the expected in the estuarine plume (10 - 20 ppt). The 24 ppt isoline found at the surface in the Strait stations appears about 6 m depth in the riverine and estuarine plume stations. Strait stations showed salinity values lower than 25 at surface, indicating the influence of brackish water. 78 II E1 E2 -i—i—|—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—r 2 Sampling Day 3 4 I I I I R1 R2 S1 S2 Figure 3.11. A) Principal components for zooplankton samples from June 11-14, 1991. Components are described in Table 3.11. B) Salinity contour plot. R = riverine plume, E=estuarine plume, and S =Strait of Georgia (Nomenclature of stations as in Fig. 1.2, Table 3.6). 79 A) 1.0 0.8 >» nc 0.6 at 3 CT 0.4 (!) L_ LL 0.2 0.0 1.5-5.5 >5.5-8 >8.5-10.5 Size Intervals (mm) >10.5-15 • Riverine plume A Estuarine plume o Strait 2-5.5 >5.5-8 >8-10.5 Size Intervals (mm) >10.5-16 Figure 3.12. Size distributions of P. pacifica (A) andC. challengeri (B) during June 11-14, 1991. During this month young individuals of both species dominated the size composition, and were more abundant in the Strait than in the riverine plume and the estuarine plume (Figs.3.12A and 3.12B). Young Cyphocaris chaUengeri were slightly more abundant in the Strait, while adults were more abundant in the estuarine plume. Table 3.11. Loadings of the most important species on the principal components for zooplankton. Samples collected from the riverine plume, estuarine plume and the Strait during June 11-14, 1991. Stations with highest variation are in parentheses. Abbreviations indicate: riv = riverine, est = estuarine, pi = plume. 1st Component (39%) (estuarine plume and Strait) 2nd Component (21%) (riverine plume) 3rd Component (16%) (Strait-riv.pl-est.pl) Pseudocalanus minutus 6.84 Euphausia pacifica 7.11 Eggs (0.39 mm) 4.29 Euphausia pacifica 5.01 Acartia longiremis 1.03 Fritilaria borealis 3.90 Parathemisto pacifica 2.61 Calanus pacificus 1.03 Parathemisto pacifica 3.71 Cyphocaris challengeri 2.10 Paracalanus parvus 1.01 Oithona similis 3.42 Metridia pacifica 1.60 Sagitta planktonis 0.82 Euphausia pacifica 2.71 Paracalanus parvus 1.17 Parathemisto pacifica -5.07 Pseudocalanus minutus -2.38 Acartia longiremis 0.43 Metridia pacifica, f -2.16 Oncaea borealis -2.03 Oithona similis -2.04 Pseudocalanus minutus -2.13 Monstrilla sp. -1.98 Oithona spinirostris -1.43 Metridia pacifica, m -1.70 Chelophyes appendiculata -1.98 Calanus pacificus -1.22 Conchoecia alata minor -0.84 Tomopteris septentrionalis -1.83 Euchaeta japonica -1.03 Paracalanus parvus -1.70 Calanus pacificus -1.25 80 The maximum zooplankton values in the l 5 t component (Fig. 3.11 A) occurred in stations from the estuarine plume, with P. minutus, E. pacifica, P. pacifica, and C. challengeri among the main contributing species (Table. 3.11). E. pacifica, Acartia longiremis, and Paracalanus parvus were important in estuarine plume stations and also present in stations from the riverine plume. Eggs (presumably Euphausia pacifica as indicated by size), Fritilaria borealis, P. pacifica and E. pacifica were important species in waters with mixed characteristics of the Strait, riverine and estuarine plume. P. pacifica and a group of small zooplankters were negatively related to the 2nd component. II. Annual changes January - D e c e m b e r 1997 Figure 3.13A shows results for the zooplankton analysis. The 1st component indicates an overall change occurring from late spring to summer and, except for January, with the highest variance in summer months. Fall and winter months (except early March) contributed with the highest variance to the second component, and spring months to the third component. Figure 3.13B shows the change in vertical salinity structure. The 28 ppt isoline varies from surface in April to more than 15 m depth in August. This period also shows the most variation in salinity for the upper 15 m in the vertical water column, including the lowest salinity values found during the year. Between August and September the 28 ppt isoline starts surfacing again and continues through fall and winter months. From January to March salinity increases from 28 to 29 ppt at the surface and, by early April, the 28 ppt isoline begins its descent again. Figure 3.14 shows that during January the water column exhibited almost homogeneous high salinity, with values from 28 ppt at surface to 31 ppt at 400 m depth indicating winter conditions. At this time the most important size groups found were adults in the size ranges of 5.5-8 mm and 8.5-15 mm. Whereas during May, when the water column changed to lower surface salinity, indicating the freshwater influence of the Fraser River, small Parathemisto pacifica in the 1.5-5.5 mm size interval dominated the first 50 m of the water column. 81 Figure 3.15 shows an important proportion of small C. chaUengeri in near surface water during January, when the vertical column exhibited high salinity values consistent with winter conditions. Animals in the 8.5 - 10.5 and 10.5 - 16 mm interval sizes comprised the highest proportion of the population collected in deep water. Whereas during July, when the water column exhibited lower surface values indicative of the Fraser River influence, combined groups mostly comprising small sizes (2 - 5.5, 5.5 - 8.5) occupied the first 50 m of the water column while the proportion of bigger animals increased with depth. Results from correlating the change in proportion of group size interval data collected from January to May from 0 to 50 m, and the vertical salinity structure (discrerized in layer of 0-10 m, 10 -50 m, and 50 -100 m) indicates that the smallest individuals of P. pacifica and C. chaUengeri are inversely related to salinity in the 0-10 m layer. This correlation decreases at depths >50 m (Table 3.12), which suggests that the increase in the proportion of small amphipods from January to May is related to low salinity at the surface. Conversely, adults exhibited a positive correlation with salinity in the 0-10 m layer. Table 3.12. Correlation coefficients between the proportion of individuals per group size and average salinity per depth layer from January to May, 1997. Parathemisto pacifica r2 per depth layer (m) Size interval (mm) 0 - 1 0 m 10-50m 50 -100 m 1.5-5.5 -0.73 -0.53 -0.12 5.5-8.5 0.52 0.22 0.45 8.5 -15.0 0.67 0.65 -0.24 Cyphocaris chaUengeri Size interval (mm) 0 - 1 0 m 10-50m 50 -100 m 1.5-5.5 -0.92 -0.51 -0.40 5.5-8.5 -0.96 -0.50 -0.49 8.5 -10.5 0.23 -0.24 0.58 10.5 -16.0 0.61 0.10 0.32 The PCA zooplankton results in Table 3.13 show that the overall zooplankton variation occurred in spring and summer, including from late March to August. During these months 82 zooplankton was plentiful, with the amphipods Cyphocaris challengeri and Parathemisto pacifica as the top dominant species, and small copepod species as co-dominant, except for the ostracod Conchoecia alata minor. The second component includes species dominant during fall and winter months. This group of species included P. pacifica, P. minutus and O. sirnilis and remained dominant from the summer. The third component includes species dominant during late winter and early spring. Species in this group are the copepods O. sirnilis, P. minutus, Metridia pacifica, and the pteropod Limacina helicina. P. pacifica and C. challengeri appeared negatively related to this component. Table 3.13. Loadings of the most important species on the principal components for zooplankton. Samples collected from station DFOl during 1997. Seasons with the highest variation are indicated in parentheses. Abbreviations indicate: f = female, m= male, C-2 = Copepodite 2, C-3 = Copepodite 3. 1st Component (54%) (late spring - summer) 2 n d Component (9%) (fall - winter) 3 r d Component (5%) fl^ate winter - spring) Cyphocaris challengeri 6.65 Parathemisto pacifica 6.44 Oithona sirnilis, f 3.88 Parathemisto pacifica 3.45 Oncaea borealis, f 2.04 Limacina helicina 3.22 Oithona sirnilis, f 1.67 Oithona sirnilis, f 1.41 Pseudocalanus minutus, f 2.14 Metridia pacifica, f 1.67 P. minutus, f 1.11 Metridia pacifica, f 2.09 Pseudocalanus minutus, f 1.29 P. minutus, m 0.90 P. minutus, C-3 1.59 P. minutus, C-2 0.96 Conchoecia spinirostris 0.79 Conchoecia spinirostris 1.48 Conchoecia alata minor 0.66 Cyphocaris challengeri -3.96 Parathemisto pacifica -2.66 Calytopis larvae -0.60 Conchoecia alata minor -0.80 Cyphocaris challengeri -1.77 Nauplii larvae -0.56 Cyphonautes larvae -0.79 N . I. eggs -1.26 Euphausia pacifica eggs -0.55 Eukhronia hamata -0.75 Paracalanus parvus, f -1.15 Paracalanus parvus, C2 -0.55 Tomopteris septentrionalis -0.63 Calanus pacificus, C4 -0.94 Zoea larvae.. -0.53 83 Figure 3.13. A) Principal components for zooplankton collected at station DFOl during 1997. Components are described in Table 3.13. Symbols indicate: J=January, F= February, Mr4=March 4, Mr25= March 25, Ap8= April 8, Apl5= April 15, My 2= May 2, My 22 = May 22, Jn=June, Jy=July, Au=August, Sp= September, Oc= October, Nv= November, Dc= December. B) Salinity contour plot. February and November data were interpolated, data from March to July include only to 100 m. S 4 A) B) • Size 8.5-15 • Size 5.5-8 DSize1.5-5 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 Month Figure 3.14. Parathemisto pacifica collected at station DFOl. Percentage distribution of individuals per size interval and depth during January (A) and May (B) of 1997 related to the vertical salinity structure. S 5 A ) B ) •2-.5.5D>5.5-8H>8.5-10.5B>10.5-16 1 2 3 4 5 6 7 8 9 10 11 12 Month Figure 3.15. Cyphocaris chaUengeri collected at station DFOl. Percentage distribution of individuals per size interval and depth during January (A) and during May (B) of 1997 related to the vertical salinity structure. 86 Discussion Spring and summer are the most productive seasons for plankton in the Strait of Georgia. The increase in solar radiation and the formation of a shallow mixed layer precede this period. Between March and April a phytoplankton bloom develops and is the grazed by herbivorous copepods (Parsons et al., 1970; Harrison et al., 1983). The most important species in this group are Calanus marshallae, Calanus pacificus, Metridia pacifica and Neocalanus plumchrus. This last species dominates in size and number until late spring or early summer, when the population migrates to deep waters (Fulton, 1973). A second phytoplankton bloom occurs between June and July (Stockner et al., 1979), and then other zooplankton groups including amphipods, euphausiids, chaetognaths, medusae, cladocerans, ostracods and invertebrate larvae become more numerous (Fulton, 1968; Harrison 1983). From an overall perspective, the total variance accumulated in the three principal components analyzed pre set of data indicated that zooplankton data presented more spatial (semi-diurnal sampling) variation than temporal (annual sampling) variation. Total variance accumulated as 58%, 87% and 76% during May - June 1990, April 1991 and June 1991 respectively and as 68% during the year of 1997. However, from comparisons of data sets, it became apparent that the variation indicated in the first component of the annual data (54%) was much higher than that from the semi-diurnal collections (34%, 37%, and 39% for May -June 1990, April 1991 and June 1991 respectively). The high variation during the annual sampling coincided with the period between April and August, the most productive season in the Strait (Parsons et al, 1970; Harrison et al., 1983). Spatial variation of zooplankton collected in the semi-diurnal sampling was higher in the estuarine plume than in the Strait and riverine plume stations. There was however a considerable degree of similarity between the three locations, most likely resulting from the overlapping distribution of dominant species. 87 I. Changes during spring and summer (1990, 1991) Results from the semi-diurnal sampling showed that zooplankton variance was highest at the stations sampled in the estuarine plume location during May-June 1990 and April 1991, then at stations sampled in the riverine plume area during May-June 1990 and June 1991. In May - June 1990, the salinity contour data indicate that during the first sampling days (days 1 to 2 in Fig. 3.7B) the riverine plume showed salinity values higher than 10 ppt indicating the influence of saline marine water. The estuarine plume (days 3 and 4 in Fig. 3.7B) showed salinity values higher than the usual 15-20 ppt range. Salinity in the Strait (days 4 and 5 in Fig. 3.7B) were within the expected range (25 - 30 ppt). Salinity in the estuarine plume (sample E3, day 6) showed salinity values as high as that within the range for the Strait. Three days later (day 9, sample E4) the estuarine plume showed low surface salinity, indicating freshwater influence. During this period, PCA results showed that the zooplankton variance was highest at the estuarine plume, then at the riverine plume and finally in Strait stations. PCA results indicated that the amphipods Cyphocaris chaUengeri and Parathemisto pacifica, together with the small copepods Metridia pacifica, Oithona similis and Oithona spinirostris were dominant in the estuarine plume stations. Metridia pacifica is a diel migrant (Stone, 1977) while Oithona simiUs and Paracalanus parvus are found typically in upwelled water (Raymont, 1983). In the present study, their presence in samples from the estuarine plume area possibly relates to the entrainment caused by the Fraser River water (Yin, 1994). However, considering the ample variation of salinity in the estuarine plume area, the dominance of these species could also relate to a high tolerance to salinity change. Analysis of size distribution per area indicated that young Parathemisto pacifica concentrated in the estuarine and riverine plumes, while the highest proportion of C. chaUengeri adults were in the riverine plume and younger stages were in the estuarine plume. In the PCA results, C. chaUengeri appeared related to the estuarine plume and to water with mixed characteristics of riverine and estuarine plume origin. 88 During April 1991, the most noticeable result was the apparent absence of surface salinity values corresponding to a riverine plume. Salinity values in the stratified region agreed more with the range indicated for an 'old estuarine plume' (St John et al., 1992). However, this salinity structure most likely resulted from wind mixing the surface water in the neighbouring river area (Yin, 1994). The amphipods were distributed differently with C. challengeri adults in the river, and young P. pacifica in the estuarine plume. Besides the euphausiids and barnacle larvae, the most important species were three copepod species common to the area and particularly abundant during the spring, Calanus marshallae, C. pacificus and Pseudocalanus minutus, (Parsons et al., 1970; Gardner, 1977). Neocalanus plumchrus, the most abundant herbivorous copepod in the Strait of Georgia (Harrison et al., 1983), was found to be important in the semi-diurnal samplings only during April 1991. This finding is compatible with the life history of the species given that by summer, individuals start their ontogenic migration to deep water (Fulton, 1973; Mackas et al., 1998). During June 1991, zooplankton species composition from the estuarine plume stations was similar to that of stations from the Strait of Georgia. Although on this occasion the freshwater appears only as a very superficial lens, the Strait water had low salinity values comparable to those found close to the estuarine plume (St John et al., 1992). This was due to seasonal high river discharge (Thomson, 1981). The species dominating in this situation were P. minutus and E. pacifica, the amphipods P. pacifica and C. challengeri, and the copepods M. pacifica, P. parvus and A. longiremis. These two latter species are typical of surface water in summer and fall (Gardner, 1977). During this time, young amphipods dominated the size composition, and had similar abundance in the three areas. Considering that the three areas also contained small copepod species previously found as part of their diets, it is likely the two species of amphipods were distributed according to their prey distribution. II. Annual changes The zooplankton analysis during 1997 indicated that the highest variation occurred between April and August. A fall-winter zooplankton group provided a secondary source of variation, 89 and a small group of species contributed the minor variation during late winter-spring. At the same time, vertical salinity contours indicated that during spring and summer the Fraser River influence was apparent in surface water in the location studied. This influence was detected as the presence of less saline water that increased as spring progressed into summer. In April, the 28 ppt isoline was found near the surface and was observed deeper in subsequent months. The freshwater influence peaked in August, when the 28 ppt isoline was found deepest during the year. The observed near-surface water dilution coincided with the period of seasonal increase of the Fraser River discharge (Thomson, 1981). Zooplankton species dominant during the late spring - summer of 1997 were the amphipods P. pacifica and C. challengeri, the copepods Metridia pacifica, Pseudocalanus minutus, Oithona sirnilis, and the ostracod Conchoecia alata minor. It was surprising not to find the typically dominant copepods in this group, especially Neocalanus plumchrus which is the most abundant copepod during spring in the Strait (Parsons et al., 1969). The fall-winter group was integrated mostly by small copepods (exceptions are the amphipod P. pacifica and the ostracod Conchoecia spinirostris). These zooplankters are omnivorous and their dominance during a season of low phytoplankton production seems reasonable. Their food source during this time may be nannoplankton, likely flagellates and ciliates1 that are . abundant in late summer, fall and winter (Stockner et al., 1979; Harrison et al., 1983). The species forming the late winter-spring group are also mostly small copepods (except the pteropod Limacina helicina), and shares O. sirnilis and P. minutus with the fall-winter group. These species cannot feed on the big diatoms characteristic of the spring bloom, therefore they likely relate with conditions prior to the spring bloom. 1 Flagellates, e.g. Peridimium sp., Gymnodimium spp., Dinophysis spp. (Stockner et al., 1979) and Micromonas pusilla: (Taylor 1983 in Harrison et al., 1983). Ciliates e.g. Mesodinium rubrum (Harrison et al., 1983). 90 During the most productive season, small amphipods were very abundant and their proportion increased with low salinity values at the surface resulting from the runoff, whereas during winter months (e.g. January) there was mostly a predominance of big animals. In this context, the relation of amphipods with salinity seems indicative of feeding opportunities derived from high plankton production during spring, with the subsequent population growth. In general, the species expected to be important in the zooplankton composition were the copepods Neocalanus plurnchrus, Calanus marshaUae, Calanus pacificus and Pseudocalanus minutus (Parsons et al., 1969) and the euphausiid Euphausia pacifica. These species are typically abundant in the Strait of Georgia (Harrison et al., 1983), and are responsible for the transfer of energy from the primary producers to consumers. Variation in the abundance of these species can have a great influence on the zooplankton biomass of the Strait of Georgia, except P. minutus, which is extremely numerous but very small. During 1997, Neocalanus plurnchrus and Euphausia pacifica showed an unusually low abundance, compared to 1996 and 1998 (Bomhold, 1999; Kane, 1998). For N. plurnchrus, this was explained by its arrival at the surface layer coinciding with the waning of an early spring phytoplankton bloom (Bornhold, 1999), preventing the copepods from taking full advantage of this food source. The early phytoplankton bloom during 1997 has been related to a strong El Nino event (Macdonald, 2000). However, it has not been clearly documented how it affects primary production within the Strait. What has been observed is that in some years, weak winds at the beginning of the freshet allow the early onset of the phytoplankton spring bloom (Yin et al., 1997). It has also been observed that strong wind mixing can disrupt an early bloom. After the mixing event subsides the phytoplankton bloom recovers, giving the impression of a 'double' spring bloom, as it happened during 1997 (Bomhold, 1999). Yin et al. (1997) concluded that the timing in the spring bloom development must be a significant factor in the trophodynamic phasing in food chains. 91 Low abundance of Neocalanus plurnchrus and Euphausia pacifica affect the zooplankton biomass and may influence the trophic ecology in the Strait. Five species of Pacific salmon2 feed in the Strait during their juvenile stage (Healey, 1980). Although it has been argued that they are either opportunistic or selective feeders, salmon species in the Strait feed on macrozooplankton (Brodeur, 1990). This indicates that all salmon species may be affected to some degree by changes in the zooplankton composition. It also indicates that Parathemisto pacifica, the most abundant of the hyperiid amphipods in the estuary, may play an important part in the trophic transfer when Neocalanus plurnchrus and Euphausia pacifica are scarce. The small zooplankters in the diet of amphipods are mostly omnivorous and/or facultative predators that feed on nannoplankton, and in turn amphipods serve as food for big consumers such as commercial fish. This suggests that amphipods are an important link in a food chain of the type: nannoplankton (2-30 urn, e.g. flagellates, ciliates) microzooplankton (20-200 u,m, e.g. protozoans, Tintinnids) mesozooplankton (2 - 20 mm, e.g. copepods, cladocerans ostracods) macrozooplankton (2- 20 cm, e.g. amphipods) fish (e.g. herring, salmon). This is an alternative to the phytoplankton -> zooplankton fish (e.g. herring, salmon) food chain that typically connects diatoms, herbivorous zooplankton, and fish in the Strait of Georgia. 2 Oncorhynchusgorbuscha (pink), O. keta (chum), O. kisutch (coho), O. nerka (sockeye) and O. tshawytscha (chinook). 92 Chapte r 4 . . Popu lat ion d y n a m i c s of amph ipods : Growth and mean s i ze c h a n g e s a s s o c i a t e d w i t h p lankton product ion 93 Introduction The size-frequency distribution contains succinct information on the demography of a population. Since the size-frequency distribution of a population results from its recent history of recruitment and mortality integrated with the growth rates of individuals, the size-frequency distribution contains valuable information on the dynamics that the population is undergoing (Barry and Tegner, 1989). Constraints in the ability to directly quantify vital rates have led researchers to infer demographic parameters from size data (Barry and Tegner, 1989), and very often size data are the only available clues to the growth, mortality and recruitment processes of a population (Ebert et al., 1993). Methods that analyze size-frequency distributions to estimate demographic parameters of a population are basically of three types. Some methods are based on the separation of the size-distributions into components (Macdonald and Pitcher, 1979; Fournier et al., 1990). Other methods use size data to either estimate mortality when growth is known or to estimate both growth and mortality (Green, 1970; Ebert, 1973). Finally, others are based on the simulation of size-distributions to acquire a better understanding on the underlying processes that generate the observed distributions (Fournier and Breen, 1983; Goodyear, 1997; Ebert, 1999). The methods that separate size distributions into component parts assume that if there is an annual pulse of recruits to a population, growth parameters can be extracted by following peaks in size-frequency histograms (Ebert, 1999). An inconvenience of using these methods is that they can be highly subjective (Gayanilo and Pauly, 1997) because the primary approach is usually graphic (Macdonald and Pitcher, 1979), but the most serious problem is that populations, particularly invertebrates, hardly ever have only one recruitment (Ebert et al., 1993). On the other hand, it can be assumed that a population undergoes continuous and constant recruitment if the size-structure does not change through time (Ebert, 1999), a condition not observed in the field data for this study. 94 I decided to estimate individual growth and mortality under several conditions for recruitment, ranging from a pulsed recruitment event to two recruitment events spread over several months. I also decided to use a simulation method to generate a size-distribution in which to test the capability of a method based on mean size to recalculate the mortality rate and the individual growth parameters (Ebert, 1999), and an age-structured dynamic method. The simulation model was a simple size-structured model that would express population dynamics in terms of one or several recruitment events, mortality as an exponential decay process, and individual growth as following a Brody-Bertalanffy function. The model would use these three processes to predict monthly size distributions in the population without any distinction of age. This chapter focuses on aspects of the growth and mortality dynamics of amphipods and plankton production in the Strait of Georgia. Particularly the estimation of growth and mortality using size-frequency distribution data as well as the observed changes in size associated with chlorophyll a and zooplankton biomass from the Strait of Georgia. Materials and methods Amphipods were separated from collections and measured using a stereoscopic microscope with an attached camera lucida and an electronic pad connected to a computer. Because amphipods have curved bodies, the total length of each individual was measured by adding up measurements from sequential segments of the body, starting at the separation between the first antennae and the head and continuing along the dorsal edge to the tip of the uropod. Total lengths were then organized in frequency tables in intervals of 0.5 mm, from 1.5 mm up to 16 mm. This range covered recently hatched small animals up to the biggest ones found. The size-frequency data were corrected for retention and escape of the net (as percentage composition in tables A5 and A6), and also were standardized to the volume filtered while sampling according to the procedure outlined below. 95 Escape and retention in a plankton net Plankton nets selectively retain organisms according to their size and the net porosity (Saville, 1958). Thus the length frequency data are biased because large organisms may avoid the net, and small organisms escape through the mesh (Somerton and Kobayashi, 1989, 1992). Very fast swimmers also may avoid the net, but so far there is no numerical method to correct plankton data for this factor. Selectivity curves estimated for different nets revealed that the size distribution of captured organisms consistently exhibited sigmoid shapes, going from complete escape to complete retention (Saville, 1958). These results showed that retention and escape occur simultaneously during net towing. However, methods to correct plankton catches treated them as independent events and focused either on one or the other (e.g. Barkley, 1972; Lenarz, 1972; Murphy and Clutter, 1972). Recently, Somerton and Kobayashi (1989) provided a method to correct zooplankton samples for size selection that accounts at once for the probability of escape and retention during sampling as follows: PL = Pe(L) Pr (L) Where PL = True proportion of individuals of size L in the sampled path Pr (L) = 1 / (1+ a e'bL) = Probability of retention for length L (Ricker,1975) Pe(L) = 1 - Pr (L) = Probability of entry for length L Once PL is known, the number of organisms (N0) that 'should' be present in the sample is estimated as a proportion from the number captured (Nc): N 0 = PL N c 96 Once amphipods were corrected for escape and retention, the number of individuals obtained in each sample was standardized to a volume of 1000 m.3 (UNESCO, 1968), following the same method outlined in the Materials and Methods section of chapter 3B. Estimation of growth and mortality from size data Growth and mortality parameters were estimated from monthly size distributions using two methods. The first method is based on the monthly mean lengths of a population with pulsed recruitment and was proposed by Ebert (1999). The second method is an age structured model that predicts a monthly recruitment pattern and the monthly mean length of individuals of a population. The description of these two methods follows: A) The monthly mean lengths of a population with pulsed recruitment method (Ebert, 1999). This method (referred as the ML method hereafter) is based on the idea that the mean size of individuals in the population is a function of growth, survival and recruitment time. Growth is represented with the Richards function, and mortality with an exponential decay equation. The Richards equation is a generalized growth equation capable of generating a family of growth curves, like the Brody Bertalanffy, the logistic, and Gompertz models. A detailed description of the method can be found in Ebert (1999; 254-255). The mean size ST of individuals in the population after a recruitment pulse can be estimated with the following equation: S T =S M (l-e- z )^e- z (l-b.e- K ( T + 1 ) r t=0 Where Z = instantaneous mortality rate (on an annual base) K = instantaneous growth rate (on an annual base) 97 SM = asymptotic size So = size at recruitment T = time within a single year starting at time of recruitment t = age in years n = a shape parameter co = maximum age T varies from 0 to 1. The month in which a first minimum size appears is considered the recruitment month and is assigned as T = 0 in the calculations. For example, if recruitment was in March (T = 0) and the sampling was in May, then for May T = 2/12. Size at recruitment can be calculated as the mean of the first mode in the size frequency distribution. B) The recruitment pattern ' mean lengths method (RP-ML). This procedure is based on the idea that growth and mortality can be estimated through the use of a simple population dynamic model that starts from a monthly recruitment pattern. This recruitment pattern can be defined as the relative contribution of the smaller size intervals present in the data at each month. From this starting point, the relative frequency of this group for the following month is given as: fu = S k i Where fi|t = Relative frequency of cohort of age t born during month i S = Monthly survival rate Notice that this forward propagation has to be carried out for the entire life span (m) of the individuals in the cohort, which is assumed known, for each month. This assumption can be easily relaxed by assuming a sufficiently long life span. The mean size of individuals in each cohort is estimated from a simple Brody-Bertalanfry equation: l t = U( l -e - K ( t ) ) Where 98 l t = Length at age t L» = Asymptotic size of individuals in the population K = Instantaneous growth rate t = Age in months In this structure, the mean size of individuals in the population at rime j can be estimated as the sum across all ages of the product between relative frequency of each age group and its mean length as estimated from the Brody-Bertalanffy equation, as follows: _ in t=i With this structure, we can then estimate a set of values for the Brody-Bertalanffy equation and the recruitment pattern that minimize the sum of squares differences (SSQ) between the observed and the expected values for the monthly mean size and the monthly recruitment pattern simultaneously, as follows: ssQ=wlf{i°-ii c}+w2fj(f°l-fi:ly 1=1 1=1 Here the superscript o stands for observed, and c for calculated. Also, wu and w2 are weights that can be set to increase or reduce the effect of one series over the other in the sum of squares (see Hilborn and Walters (1992), p. 325). To test the reliability of these methods to estimate parameters from a size-frequency distribution with multiple age groups, a simple growth-mortality dynamics model was used to simulate monthly size frequency distributions. The structure of this model is composed of a Brody-Bertalanffy equation to simulate individual growth: L ^ a - b e - ^ 0 ' ) where tQ is time at which size would be 0. Each age group was modelled as a normal distribution with mean l t and standard deviation (rj) equal to 0.1 l t . Each normal component 99 was discretized into predefined size intervals and its percentage contribution to the age group was estimated from: limit b of age t. ft, ab was integrated using the method of Abramowitz and Stegun (1972, in Cooke et al., 1990). Cohort survival was simulated as an exponential decay of the form e~z. Recruitment was introduced at selected months as the appearance of an age 0 size distribution. Monthly size distributions were generated by recalculating mean size at age with the Brody - Bertalanffy equation with the appropriate time index value and reintegrating the normal components as indicated. The number of individuals of each age group that survive to next month was updated as follows: N t + i = N t e-z/12 The size distribution of the population at any given month was then created by summating the number of individuals with body sizes between interval limits a and b for all intervals in the size distribution, as follows: The parameter values used in the simulation were as follows: asymptotic size See = 12 mm, growth rate K = 1.8, and t o = -0.02 for the Brody - Bertalanffy growth function, a mortality rate value Z = 1.8. The recruitment pulse was estimated from a Ricker curve with parameter values of a = 8 and b = 0.000009. The values used to parametrize the Brody - Bertalanffy equation were chosen so as to mimic the range of values observed for the two species studied. The simulation generated a series of monthly size distributions from which I calculated mean length and proceeded to estimate the values of K and Z used to generate the data in the first >-ii where ft,ab is the relative frequency of the size interval within lower limit a and upper NNa b=5Xa bN t t=0 100 place. The estimation method was implemented as a sum of squares minimization procedure carried out in Solver, a non-linear optimization method in Excel. The age-structured method uses two data series in the fitting procedure, and the method allows for the possibility of weighting one of the series more than the other by defining weights wt and w2. In all the estimations carried out with this method the weights were w,= l and w2= 100. These values resulted in the best fit to both data series. 101 Results Simulated size distributions and estimation of K and Z Predicted mean size calculated using the ML method were remarkably well adjusted to the simulated population for one short recruitment pulse per year (during June) as Fig.4.1 shows. Furthermore, the estimated values of K and Z are very close to those used to generate the data in the simulation. The mortality rate was a slightly overestimated, however when expressed as survival rate the values are not that different (16.5% simulated vs. 14.3% estimated annual survival rate). Figure 4.1. Simulated (circles) monthly mean size values of individuals in a population with one recruitment pulse during June. Predicted (line) values were estimated with the ML method. Values used in the simulation were Z = 1.8 and K = 1.8. Estimated values were Z = 1.94 and K = 1.82. The RP-ML method also did a very good fitting to the simulated values (Fig. 4.2). The procedure predicted a mortality value of 1.89, which is very close to the 1.8 used in the simulations. However, the method estimated a value of 2.11 for the growth rate K, which is higher than the value of 1.8 used in the simulation. This seems to be a direct consequence of the slow shift in the size distribution of recruits as a result of growth, which the procedure considers as a recruitment event in July (see Fig. 4.2B), the month following the true recruitment. The removal of the value in the recruitment series for July results in values of K = 1.95 and Z =1.73. Both values are very close to those used in the simulation. 102 i 1 5 3 B) ^T-#T^-rO-r4-i—i—r^T<H^Mh-©-i J F M A M J J A S O N D Month J F M A M J J A S O N D Month Figure 4.2. Simulated monthly mean size of individuals (A) and recruitment pattern (B) in a population with one recruitment pulse during June (circles). Predicted values for mean size and recruitment pattern (line) estimated with the RP-ML method. Values in the simulation were Z = 1.8 and K = 1.8. Estimated values were Z = 1.89 and K = 2.11. A short recruitment pulse event lasting only one month.is not very realistic for a zooplankton population. More likely a recruitment event lasts more than one month. For this reason I tested the performance of both methods assuming that the recruitment pulse lasts three months, from May to July, with a peak in June. The ML method did not provide a good fit to the data, and the parameter values were also not very close (K = 1.51 and Z = 1.42). However, I did notice that most of the problem was caused by the mean length value during May, the first month where recruitment occurred. The value was low, but not the lowest in the series, so it was not defined as T=0. The removal of this point from the analysis resulted in a better fit to the data and the method estimated' the values at K=1.94 and Z=1.74, which are very close the values used in the simulation (Fig. 4.3). However, it should be noticed that the natural variability commonly present in field data would likely prevent this kind of manipulation. On the other hand, the RP-ML method did not require removal of any data points and gave a good fit to the data series (Fig. 4.4). The method estimated parameter values of K = 1.99 and Z = 1.71, which are also very close to the values used in the simulation. 103 Figure 4.3. Simulated (circles) and predicted (line) monthly mean size values of individuals in a population with one recruitment event during May (20%), June (60%) and July (20%). Parameter values (Z = 1.94 and K = 1.74) were estimated with the ML method. Values used in the simulation were Z = 1.8 and K = 1.8. The May data point was not used in the fitting. Month Month Figure 4.4. Simulated monthly mean size values (circles) of individuals in a population (A) and recruitment pattern distributed during May (20%), June (60%) and July (20%) (B). Predicted values (line) were estimated with the RP - ML method. Values used in the simulation were Z = 1.8 and K = 1.8. Estimated values were Z = 1.99 and K = 1.71. A final test was conducted to determine the performance of these two methods under the assumption of two recruitments per year, each lasting more than one month. The first recruitment occurs during May (20%), June (60%), and July (20%), and the second during October (20%) and November (20%). The percentage values given here refer to the fraction of available spawners that reproduced in that month. Under these conditions the ML method does not provide a good fit to the data (Fig. 4.5), and the best estimates (K = 4.19 and Z = 0.47) depart considerably from the values used in the simulation. The estimation by the RP-104 ML method, on the other hand provided a very good fit to the recruitment series and the mean size series (Fig. 4.6). The estimated values were also very close, with Z = 1.84 and K = 2.07. The three tests conducted here show that the estimations done with the RP-ML method are quite robust to the underlying dynamics of recruitment in the population over a broad range of recruitment conditions. This method does not require knowledge of the timing of the event as long as the user provides a recruitment pattern series along with an annual series of mean individual size. For this reason I chose the RP-ML method to estimate the Z and K for P. pacifica and C. chalkngeri. Analysis of these results follows. 12 -I 10 -8 -E, 6 -0) N 4 -U 2 -CO 0) 0 -E A S O N D Month M 1 1 A M Figure 4.5. Simulated (circles) and predicted (line) monthly mean size values of individuals in a population with two recruitment events during May (20%), June (60%) and July (20%), and October (20%) - November (20%). Parameter values (Z = 0.47 and K = 4.19) were estimated with the ML method. Values used in the simulation were Z = 1.8 and K = 1.8. 15 <D N W (0 53 0 A) — i — i 1 —i 1 — i 1—i 1 — i 1 — J F M A M J J A S O N D Month 1.0 « 0.8 C g. 0.6 g 0.4 I 0.2 o 0.0 4-©-rO-r© OC B) J F M A M J J A S O N D Month Figure 4.6. Simulated monthly mean size values (circles) of individuals in a population (A) and recruitment pattern distributed during May (20%), June (60%) and July (20%), and October (20%)-November (20%) (B). Predicted values (line) were estimated with the RP-ML method. Values used in the simulation were Z = 1.8 and K = 1.8. Estimated values were Z = 1.84 and K = 2.07. 105 Parathemisto pacifica The monthly size distributions of Parathemisto pacifica are shown in Fig. 4-7. These were used to define the recruitment pattern and the mean body size data series required to estimate K and Z. The recruitment series was created by summating the relative frequency of individuals measuring from 2 to 3.5 mm in length. The mean body size series was estimated as the summation of the product of size interval and its corresponding relative frequency (see Table 4.1). In the estimations presented below, Loo was not treated as a parameter to be estimated and was defined as 90% - 95% of the largest measured size (Ebert, 1990; p. 255). The actual value used was 14 mm. Table 4.1. Monthly mean length of Parathemisto pacifica collected at station DFOl during 1997. Month N Mean (mm) St. Dev. (mm) Min (mm) Max (mm) Ian. 7832 6.85 0.46 5.5 12.0 Feb. 1894 6.41 0.42 3.0 9.5 Mar. 9853 3.60 0.21 1.5 10.0 Apr. 1736 3.76 0.49 3.5 4.0 May. 1049 6.36 0.24 1.5 11.5 Jun. 2004 3.33 0.21 2.0 13.0 Jul. 2280 6.25 0.23 1.5 15.0 Aug. 1763 9.02 0.92 2.5 10.0 Sep. 1531 6.39 0.46 4.5 12.0 Oct. 2537 6.66 0.24 2.5 11.0 Nov. 460 6.65 0.40 3.5 10.5 Dec. 100 8.25 1.08 7.0 9.5 Note: N = Number of individuals net retention/escape corrected, and standardized to 1000 m3. 106 1.0 0.8 0.6 0.4 0.2 0.0 1.0 • 0.8 0.6 0.4 0.2 0.0 MARCH APRIL JULY innnnnnn7nn, i?-AUGUST ,,ivw?nnl|l|l[lllil|,,flT _ nllllnnn nn7n7nllnllririnr,n NOVEMBER I'V'I' i i | i T i | i i i | i DECEMBER 0 2 4 1 1 " " i * " 1 1 11 11 1 1 1 1 i 1 1 1 1 8 10 12 14 16 o 2 4 6 8 10 12 14 16 0 2 4 6 8 10 12 14 16 mm Figure 4-7. Monthly size distribution of Parathemisto pacifica collected at station DFOl during 1997. The RP-ML method provided a good fit to the observed values of both series (Fig. 4.8), the values that minimize the sum of squares are K = 0.26 and Z = 0.65 (on a monthly basis). A surface plot of the sum of squares (SSQ) (Fig. 4.9) shows an elongated region where the lowest SSQ values occur. This region is narrow for parameter K (from around 0.15 to 0.3), and wide for parameter Z (from 0.4 to 1.2), which indicates that Z is not as well determined as parameter K. 107 Figure 4.8. Predicted (line) and observed (circles) values of mean size and recruitment pattern for P. pacifica during 1997. 0.0 -I . 1 . , , , 1 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 z Figure 4.9. Sum of squares surface plot of K and Z for P. pacifica. The values of Z and K obtained here depict a situation in which individuals grow very fast and are subjected to a high mortality. Under these conditions individuals will reach 12 mm in eight months and experience a total mortality rate of 99%, which implies that the life span of P. pacifica in the area is about 8 months. There is also the possibility that P. pacifica individuals may be experiencing size-dependent mortality which is a process thought to occur across a 108 broad spectrum of species (Peterson and Wroblesky, 1984; Miller et al., 1988). In this case the dominance of small sizes in the size distributions (Fig. 4.7) could be construed as an indication o of size-dependent mortality. Thus I decided to explore this possibility by assuming that the allometry of mortality can be represented as follows: St = exp(-Za L o o / L t ) where St = Survival from age t-1 to age t Za = Instantaneous mortality of large adults This structure, which is very similar to that proposed by Lorenzen (2000), can be readily implemented in the RP-ML estimation procedure outlined in the Methods section. The method would then estimate Za rather than Z. The parameter values obtained for this case were K = 0.26 and Za = 0.3, and the fit to the observed data was just as good as for the constant mortality case (Fig. 4.10). Notice that the method predicts the same value for K as the constant mortality case did. The mortality structure predicts that 99% of the individuals in a cohort will be dead by month 8. This value differs in only one month from that estimated under the assumption of constant mortality. Perhaps the main difference resides in the sum of squares surface (Fig. 4.11) that shows a region with a range of values for K from 0.18 to 0.37 and Z from 0.16 to 0.6. Compared to the case of constant mortality, it seems that the assumption of size dependency as stated here reduces the range of possible values for Z that can explain the data equally well. 109 Figure 4.10. Predicted (line) and observed (circles) values of mean size and recruitment pattern for P. pacifica during 1997 assuming size-dependent mortality. 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 z Figure 4.11. Sum of squares surface plot of K and Z for P. pacifica under the assumption of size dependent mortality. Parathemisto pacifica mean size changes and plankton variation Results of analyzing mean size changes in relation to those of chlorophyll a (mg.m'3) and zooplankton (mg.m'3 wet weight) are shown in Figures 4.12 and 4.13. The mean size data of Parathemisto pacifica showed a correlated variation with the chlorophyll a values for two periods. One period included late spring and summer and another included late fall and winter 110 months (Fig. 4.12). The mean size data also correlated well with zooplankton during the two periods. The first included spring and early summer, and the second late fall and winter (Fig. 4.13). Figure 4.12. Mean length size of Parathemisto pacifica (circles) with a one month lag, and chlorophyll a_(squares) during 1997 at DFOl station. Late spring to summer data relate logarithmically: y= 2.16 Ln (x) + 0.52 (r2 = 0.91), and linearly during fall-winter y =0.18 (x) + 4.99 (r2= 0.6). Figure 4.13. Mean length size of Parathernisto pacifica (circles) with a one month lag, and zooplankton biomass (squares) during 1997 at DFOl station. Spring to early summer data exhibited a logarithmic relation: y= 0.86 Ln(x) + 1.33 (r2 = 0.68). During winter the data relate exponentially: y= 6.12 e 0 0 0 4 6 x (r2 = 0.58). 111 Cyphocaris challengeri The monthly size distributions of Cyphocaris challengeri are shown in Fig. 4.14, and were used to estimate the recruitment series and the mean individual size series (Table 4-2) as before. For this species the recruitment series was assembled from the relative frequency of length intervals 2.5 to 4 mm. As with P. pacifica, Loo was not treated as a parameter to be estimated and was set at 15 mm. 112 JANUARY T T^i;Hiiiiiiipi|i,M, FEBRUARY T* | . . ' ? | . ' . . , . T . ; W . | MAY JUNE ^ n J r n w l i i ? T O R • i • * • i • i OCTOBER ,,,tTT^M^w,..^Hi' MARCH , T J l f l | JULY NOVEMBER in.BiBi.,Bl?jB APRIL i 1 " i • • 1 1 " 1 1 1 " i AUGUST • • -T^ • • 1 J„l]nn[lnllgnnnnnn n DECEMBER TT i^lM 0 2 4 6 8 10 12 14 16 18 0 2 4 6 8 10 12 14 16 18 0 2 4 6 8 10 12 14 16 18 mm Figure 4-14. Monthly mean size distribution of Cyphocaris chaUengeri collected at station DFOl during 1997. The fitting obtained to C. chaUengeri data (in Fig. 4.15) under the assumption of constant mortality shows a good fit to the mean size series. However, it underestimates the recruitment values during January, February, and April, and overestimates those of June and July. The parameter values obtained were K = 0.37 and Z = 0.33 (on a monthly basis). Notice that the value of Z is equal to a monthly survival rate of 72%, and implies that 99% of a cohort will be dead after 15 months. This is a life span nearly twice as long as that estimated for P. pacifica. 113 Table 4.2. Monthly mean length of Cyphocaris challengeri collected at station DFOl during 1997. Month N Mean (mm) St. Dev. (mm) Min (mm) Max (mm) Jan 79160 12.05 0.78 3.0 15.0 Feb 92948 12.93 0.80 3.5 15.0 Mar 57949 12.89 0.72 2.5 15.0 Apr 1640 7.06 0.75 3.5 15.0 May 5093 9.23 0.35 3.5 15.0 Jun 13942 6.76 0.28 3 15.0 lul 29300 7.18 0.23 3.5 15.0 Aug 10091 8.27 0.23 2.0 16.0 Sep 16819 10.93 0.43 5.0 15.5 Oct 5495 9.76 0.38 3.5 16.0 Nov 3625 9.35 0.40 2.5 14.5 Dec 11602 12.24 0.59 4.5 15.5 Note: N = Number of individuals net retention/escape corrected, and standardized to 1000 m3. The sum of squared differences (Fig. 4.16) shows an elongated region containing the lowest SSQ values that can equally explain the data used in the fitting. This region is wider along the Z axis, ranging from 0.2 to 0.7, than along K, which ranges from 0.25 to 0.55. It seems that the data contain less information about Z than about K. J F M A M J J A S O N D J F M A M J J A S O N D Month Month Figure 4.15. Predicted (line) and observed (circles) values of mean size and recruitment pattern for C. challengeri during 1997 assuming constant mortality. 114 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Z Figure 4.16. Sum of squares surface plot of K and Z for C. challengeri under the assumption of constant mortality. The possibility of size-dependent mortality present in the population dynamics of C. challengeri was also considered here; under this assumption the values estimated were K = 0.36 and Za = 0.21. The estimated mortality value indicates that 99% of all the individuals in a cohort will be dead by month 18, which is three months longer than that estimated under the assumption of constant mortality. The estimated growth rate is very close to that estimated under the assumption of constant mortality. The value indicates that an average individual will reach 12.5 mm in five months. As before, these parameter values result in a very close prediction of the mean size (Fig. 4.16). However, this is not the case for the recruitment series: January, February and April are underestimated and June, July and August are overestimated, just as was the case under the assumption of constant mortality. 115 14 -1 ~ 12-S 0.15 \ a 0.1 -\ O 0.05 ] © 0) oc o o o J F M A M J J A S O N D Month J F M A M J J A S O N D Month Figure 4-17. Predicted (line) and observed (circles) values of mean size and recruitment pattern for C . challengeri during 1997 assuming size dependent mortality. Looking at the size distributions from which the two series were created I noticed that the number of size intervals present in the Apri l distribution is quite low compared to the other distributions (see Fig. 4-14), and is also the month with the lowest number of individuals (see Table 4.2). Apri l has only four size intervals, three of which correspond to small sizes. The removal of this month from the series improved the fit to both series (Fig. 4.18). However, the recruitment values for January and February are underestimated. The predicted parameter values are K =. 0.51 and Z a = 0.37. Notice that the value K is higher than the previous case, and individuals will now reach 13 mm in four months. Also 99% of the individuals in a cohort will be dead by month 9, six months earlier than for the previous case. Although the removal of the value for April gave a better fit to the data series, it also resulted in a wider area containing the lowest values in the sum of squared residuals (Fig. 4.19). This indicates that the data can be explained equally by a wider range of values for both parameters (i.e. the problem is less well determined). Considering all the data points in the series, the best fit occurs in the range 0.27 < K < 0.58 and 0.17 < Z a < 0.5 (Fig. 4.19A). Removing the value for April results in the ranges 0.27 < K < 0.72 and 0.18 < Z a < 0.72 (Fig. 4.19B). 116 Figure 4.18. Predicted (line) and observed (filled circles) values of mean size and recruitment pattern for C. challengeri during 1997 assuming size dependent mortality. April data (open circles) were not considered in the fit. Z Z Figure 4.19. Sum of squares surface plot of K and Z for C. challengeri under the assumption of size dependent mortality. A) Surface plot including all data points. B) Surface plot without values for April. 117 Cyphocaris challengeri mean size changes and plankton variation Results of analyzing mean size changes in relation to those of zooplankton (mg m'3 wet weight) are shown in Figure 4.15. The mean size data of Cyphocaris chaUengeri showed a good relationship with zooplankton data (mg.m'3 wet weight). Data correlated for almost all year, except for winter and early spring. Notice that both series follow the same trend for most of the year, in contrast to the pattern observed in P. pacifica, where two phases were present. J F M A M J J A S O N D Figure 4.20..Mean size of Cyphocaris challengeri (filled circles) with a one month lag, and zooplankton biomass (open circles) during 1997 at DFOl station showing the logarithmic relationship y = 1.49 Ln (x) + 3.35 (r2 = 0.88). 118 Discussion Unless all individuals can be counted, marked, followed over time, or have their age determined, the alternative to inferring vital rates may be to estimate them from observed changes in population structure and abundance (Caswell and Twombly, 1988). This is precisely the case for zooplankton populations, particularly those that cannot be identified to stage (Asknes et al., 1997). Most methods based on size-frequency distribution assume vital rates are constant, hence many sources that could possible change the population dynamics are not considered (e.g. food availability) (Ohman and Wood, 1996). Being aware of this limitation, I compared the mean size of amphipods, chlorophyll a concentration and zooplankton biomass were compared. Results indicated that departure from the estimated mean size coincided with changes in chlorophyll a and zooplankton, and correlation between them was particularly high during spring-summer, when growth was fast. Mean size of amphipods had a month lag with respect to plankton production, apparently enough time for small invertebrates to develop and attain a prey size (Lasker, 1975; Arthur, 1976). For this study, I examined the population structure and abundance of amphipods using the RP-ML age structured method after testing its reliability to adjust to a size-frequency distribution with multiple modes, as those present in the observed data. The occurrence of several modes in a size-frequency distribution is often related to various recruitments (Ebert et al., 1993). Another possible source of polymodal distribution has been attributed to a combination of an extended spawning period and the advantage that larger organisms (fish) have in obtaining food and consequently growing faster (De Angelis and Coutant, 1982). However, to obtain this information from field data requires an ability to identify and follow a cohort over time, which is difficult to achieve with zooplankton populations. Therefore, in this study, each major decrease in mean size, linked to a high increase in abundance of hatchlings, was considered as recruitment in the calculations. 119 Parathemisto pacifica Growth and mortality rates were estimated under two different assumptions about how the mortality process operates: constant mortality, and size-dependent mortality. Both assumptions were able to closely predict the observed data used in the fitting. They also predict the same value for growth rate (K = 0.26). Although the mortality structure in each assumption is quite different, the life span they predict is very similar, 8 months under constant mortality and 9 months for size-dependent mortality. The similarity of results made it difficult to discriminate which of these mortality processes is more likely to be occurring in the population dynamics of P. pacifica. Perhaps the only difference between the two assumptions is that the range of Z values that can explain the observed data is twice as wide under constant mortality than under size-dependent mortality (see Figs. 4.9 and 4.11) (the range for K is about the same). In my opinion, the P. pacifica data used here cannot distinguish between these two alternatives. In this light and for no other reason, I decided to base my discussion on the simplest alternative, which is constant mortality. P. pacifica had a recruitment pattern with peaks during March, June, and October. These peaks likely correspond to reproductive events occurring the previous month, since hatchling size is 1.75 mm and this size interval was seldom observed in the size distributions (Fig. 4.7). Thus, an individual born in February (with recruitment peak in March) will experience a fast growth phase during the next four months, attaining a size of 9 mm by June. Likely, it starts contributing to recruitment after reaching 4.5mm of body size (Bowman, 1960). This individual may contribute to recruitment events during summer and probably during the fall although, by October, 99% of its cohort would be dead. Those individuals born during April (and recruited during June) likely follow a similar pattern of growth and mortality. However, those individuals that are recruited in October seem to follow a different pattern that is not captured in the estimates of growth and mortality rates (which were not assumed to vary seasonally). This cohort seems to have a slower growth, which shows as a lack of individuals of 10 mm and larger in the size distribution during January and February (Fig. 4.7) (but see below 120 for an alternative explanation). Yet, this is the cohort that sets the stage for the large summer recruitment peak. Individuals born between late spring and summer are most likely to benefit from the food available at this time, since these are the most productive seasons in the Strait of Georgia (Harrison et al., 1983). Here I found that the mean size of individuals was correlated well with both chlorophyll a and zooplankton biomass from May to August, which coincides with the most intense recruitment period during the year. Seston, a conglomerate of particulate material, follows the same seasonal trend as chlorophyll a in the Strait of Georgia (Harrison, et al, 1983). Seston can be rich in carbohydrates, fats and proteins, and can also be embedded with microorganisms (Parsons et al, 1984). It is worth noticing that P. pacifica has a molar with a wide masticatory area, related to microphagous feeding habits, and a toothed incisor related to carnivorous habits. These two attributes suggest that P. pacifica may be able to feed both on seston and zooplankton. This would provide a mechanism to explain the correlation of mean body size with chlorophyll a (as an indirect indicator of seston) and zooplankton biomass. Conversely, P. pacifica may be a carnivorous species only, and the correlation with chlorophyll a may arise from the fact that their prey (zooplankton) also track it. However, as will be shown later, the mean size of the carnivorous C. challengeri was only correlated with zooplankton biomass. During 1997, a large phytoplankton standing stock appeared early in April and had decreased by mid-May. Apparently this event caused the dominant population of copepod Neocalanus plumchrus to miss the spring bloom, and as a result they were present in reduced numbers (Bornhold, 1999). Populations of small copepod species (e.g. Calanus marshallae, C. pacificus, Metridia pacifica, Pseudocalanus minutus) are known to succeed in the absence of N. plumchrus (Harrison et al, 1983). From these species, at least Metridia pacifica and Pseudocalanus minutus were found in the stomach contents of P. pacifica during 1997. It is possible that P. pacifica individuals present during late spring and summer of 1997 may have benefited from food present in these conditions. 121 The size for adults of Parathemisto pacifica found in this study was in the range of 4.5 - 13 mm. These values slightly exceed those reported in the North Pacific ocean and the California Current (4.5- 8.5 mm) (Bowman, 1960). The maximum size found here was in the interval of 15 mm and represented only 3% of the total P. pacifica individuals. Although these individuals were collected on only two occasions, their rare presence in collections from deep water agrees with the reported tendency in P. pacifica to reach larger body size at lower temperatures and deeper waters (Bowman, 1960), and to be solitary individuals (Vinogradov et al., 1996). The scarcity of large individuals may relate to the mentioned tendency of adults of the genus Parathemisto to inhabit deeper waters as their size increases, which may seasonally reflect their vertical distribution. In the Atlantic coast, small P. gaudichaudii are abundant at the surface and adults occur in deeper water (Bowman, 1960). In the Strait of Georgia, Legare (1957) found young amphipods (presumably including P. pacifica) in the upper 50 m in June, and 95% of the adults below 65 m in November. More specifically, Gardner (1977) found 27% of P. pacifica adults at 0-75 m, and 51% at 75-200 m depth during October-December. This information, together with the results obtained in this study, indicates that as P. pacifica individuals grow, those few that survive and attain the biggest sizes migrate toward deeper waters. Thus big animals, after reaching 10 mm, become scarce and virtually disappear from the sampled area. Cyphocaris challengeri As with P. pacifica, growth and mortality were estimated under the assumption of constant mortality and size-dependent mortality. The growth rate values estimated under these two assumptions were very similar (K = 0.37 and K = 0.36). Differences in size at age estimated using those rates were insignificant, in the order of a fraction of a millimeter. Using any of those growth rates I estimated that once an individual is born it experiences a fast growth within the next three months, reaching a size of 10 mm. The life span predicted from the estimated mortality values for both assumptions were also very close. Of the initial numbers in 122 a cohort 99% will be dead within 15 months assuming constant mortality, and 18 months under size dependent mortality (97.9% die within 15 months). Therefore, under any of these two alternative mechanisms for mortality an individual recruited in June (born in May) will experience a fast growth in the following 3 months, reaching the size of 10 mm in August. This individual will likely contribute to recruitment events occurring during summer and fall, and less likely the next spring. Those individuals born during the fall will reach 10 mm during next February and most likely will contribute to spring and summer recruitments. Perhaps, they may also contribute to the fall recruitment but their chances are very low. Model prediction under constant mortality and size-dependent mortality are very close to the observed monthly mean size data. However, neither of these mortality structures was able to properly predict the recruitment series. Looking for a better fit to the recruitment series I removed the data for the month of April. This removal resulted in a closer fit to the recruitment series at the expense of increasing the region of best fit in the SSQ surface. Furthermore, it generated parameter values that predict a faster growth rate (K = 0.51) and a considerably shorter life span for the individuals (99% of individuals die within 9 months). These parameter values shorten the 3-month period to 2 months for the individual to attain the 10 mm size (K = 0.51), and reduce the life span of individuals to 9 months (Za = 0.37). This life span is comparable to that obtained for P. pacifica. In that context, such a life span does not seem unrealistic; what is uncertain is whether C. challengeri organisms can actually attain 10 mm only 2 months after they were born. Using all of the data, I did not find any differences between the alternatives of constant mortality and size-dependent mortality. However, the parameter values for K and Z yielded a very different life pattern when the datum for April was not used in the fitting. Unfortunately, the data do not provide any guidance in regard to which of these cases is the more reasonable. In my opinion, any of the life histories long life span-low growth rate and short life span-high growth is reasonable for this species. C. chxllengeri is an active carnivore whose mean size, accordingly, was correlated well with zooplankton biomass during most of the year. The individuals born during late spring in 1997 123 may have benefited most sensibly from zooplankton then present, to judge by the trend of the mean size and zooplankton biomass observed during April to July. It seems reasonable to assume that during this period there are prey type and/or prey concentrations that favor C. challengeri nourishment and its consequent growth. That is a likely possibility since copepod species also found in the stomach contents of C. challengeri (chapter 2), and others considered potential prey, are abundant during those months. Over all, results indicated that C. challengeri grows bigger and faster than P. pacifica. Also, the first recruitment for C. challengeri within a year occurred one month later than it did for P. pacifica. This difference in timing may relate to the carnivorous feeding habits of C. challengeri, since after phytoplankton blooms, it takes some time for herbivorous crustacean populations to develop (Frost, 1980). Chapte r 5 . . Genera l d i s c u s s i o n and c o n c l u s i o n s 125 Discussion This research was based on the hypothesis that, because of environmental adaptations, there is a strong relationship between morphology and ecological characteristics (Reilly and Wainwrigth, 1994). The study had the premises: (1) that the morphology of the feeding structures, particularly the mandible of amphipods, and their food preferences are closely related, and (2) that the distribution of amphipods would depend on the distribution of their food. Testing these hypotheses involved investigating the mandible morphology of amphipods, examining their stomach contents and contrasting these results with zooplankton as potential food. It involved as well the analysis of the zooplankton community and the salinity structure, since it was assumed that environmental heterogeneity would provide the habitat variability needed to test these ideas. Preceding plankton studies have reported the presence of 17 species of planktonic amphipods for the Strait of Georgia (Fulton, 1968). Five out of those 17 species were found in the material available for this study. Only two of these species, Parathemisto pacifica and Cyphocaris chalkngeri were sufficiently abundant for an analysis of their diet and potential food present in the environment. The species respectively belong to the Hyperiidea and the Gammaridea suborders and, while hyperiideans are regarded as typically planktonic and gammarideans are benthic, the gammaridean species was the most abundant. A previous plankton study reported Cyphocaris challengeri to be dominant in fall-winter collections from 1969 to 1974 (Gardner, 1977); the co-dominant species Primno macropa (reported as Euprimno abbysalis), and Parathemisto pacifica was third in abundance. The general morphology of the mandible of P. pacifica and C. chalkngeri showed similar elements. Each has a mandibular palp, an incisor, a molar, and a row of setae. These respectively serve for holding the prey, cutting, crushing, and introducing morsels into the gut. From a general perspective it could be inferred that the mandibles perform equivalent work in the two species. A closer look at the particular morphology of each species implied functional 126 differences in their feeding, and presumably in their diet. This could affect what they ate, and where and when they could be found. Examination of the mandible of Parathemisto pacifica revealed a relatively wide molar, with a lamelliform surface area and a toothed incisor. Both the molar surface area and the incisor tooth were sclerotized. These mandibular characteristics suggested an ability to feed on food the animals could cut or trim with the incisor and that the molar would then crush. On the other hand, the inspection of the mandible structure in Cyphocaris chaUengeri showed a cuboid, short, columnar molar with a ridged surface area and a toothless, axe-shaped incisor. The molar surface area and the incisor are highly sclerotized, which suggests a capacity for strong crushing action on food the sharp incisor had cut or snipped. Previous work on functional morphology of amphipods related a wide molar, the type present in Parathemisto pacifica, to herbivorous/microphagous habits (Watling, 1993), while the stout crushing molar of the gammaridean Cyphocaris chaUengeri has been related to carnivorous habits (Saint Marie, 1984). Strong mandibular palps and sharp incisors in C. chaUengeri suggested an ability to deal with prey, and comparatively weaker mandibular palps and toothed, comb-like incisors in P. pacifica seemed indicative of a lesser predacious capability. Considering the mandibular elements observed in Parathemisto pacifica, I anticipated that the type of food in its diet might consist of detritus, small invertebrates, soft tissue of coelenterates or perhaps fish larvae. The mandibular structure of Cyphocaris chaUengeri suggested the type of food would include hard-shelled crustaceans and soft fish larvae as well. Surprisingly, stomachs of both species contained a varied assortment of small crustaceans, including their own and each other's young, as well as some diatoms. C. chaUengeri was more abundant and grew faster than P. pacifica. This indicated that C. chaUengeri rapidly got out of the prey size-range of P. pacifica, consequently appearing less frequently in stomachs of P. pacifica. It also help to explain that C. chaUengeri preyed on more heavily on P. pacifica than conversely. 127 Cannibalistic activity on young stages has been considered a result of a dense congregation in spawning areas of individuals that continue feeding, as it happens in clupeiform planktivorous fish (Hunter and Kimbrall, 1980; Hunter, 1981). An equivalent situation could possibly generate cannibalism in planktonic amphipods since it is highly likely they spawn in the same areas in which they concentrate to feed as well. Amphipods have been found to swarm at very productive upwelling and land drainage areas (Parker and Kask, 1972a, 1972b; Nair et al., 1973). They have been reported in the estuarine plume in the Strait of Georgia (St John et al., 1992) and river influenced areas where they were found feeding and releasing their hatchlings in the present study. The most common crustaceans found in stomachs of both species were copepods, and frequently their bodies were almost intact in P. pacifica, facilitating their identification to species. Whereas stomach contents in C. challengeri were commonly found cracked and shattered to pieces finer than in P. pacifica, evidence perhaps of a more powerful, crushing mandibular action, making it harder to identify prey at the species level in the gut contents. The mandible with a wide molar in P. pacifica is indicative of microphagy, while the toothed incisor indicates carnivory. P. pacifica feeds at surface waters where seston (marine snow) and zooplankton occur. Considering the molar and incisor characteristics of P. pacifica, zooplankton and seston seem reasonable food sources. The mandible of C. chalkngeri with a strong crushing molar and sharp incisor is indicative of carnivorous habits. The carnivorous feeding habits of C. chalkngeri are consistent with the good correlation obtained between mean body size and zooplankton biomass. Population dynamics indicated that individuals of both species born during spring benefit from food available during the spring and summer productive seasons. The spring generation reproduces during summer and a surviving fraction of them will produce the recruits of fall which in turn generate the spring recruits. An individual P. pacifica born in spring, with an estimated lifespan of 8 months, will experience its fastest growth within the first four months. It will reproduce during summer and, if it survives, will also reproduce during the fall. An individual C. challengeri born during late spring with a life span of 15 months may reproduce 128 during summer, fall, possibly in late spring of the following year and will experience its fastest growth within the first three months. However, if the life span is of 9 months, as resulted when sparse recruitment data were omitted, estimations predict that it would contribute only to recruitment events occurring during summer and fall. The analyses of stomach contents and size of amphipods suggest that when young they ingest small copepodites, and as they grow, shift to ingest older stages of copepods and /or increase the number of prey. Occasionally, small Parathemisto pacifica individuals were observed to ingest copepodite stages of Pseudocalanus minutus; adults of this same copepod were also found in stomachs of adult P. pacifica. Something similar seemed to occur for C. challengeri with Oithona spp., although this is hard to confirm given the increasing triturative capacity of C. challengeri with growth. These results suggest that the two species of amphipods grow by preying steadily upon a group of small copepods common in the Strait of Georgia. These species are Corycaeus anglicus, Metridia pacifica, P. minutus, Oithona spp., Oncaea spp., Paracalanus parvus. Other probably steady food sources seem to be euphausiid furcilia, cladocerans, and ostracods. Bray-Curtis similarity analyses between amphipod abundance and zooplankton species composition indicate that the number of species estimated as potential food was greater than those identified in the stomach contents. Sheader and Evans (1975) suggested that species of the genus Parathemisto are indiscriminate feeders and their gut contents reflect the species composition of the local plankton. In the present study, the limited assortment of species found as food suggest that, regardless of the rich complex community available to feed on, amphipods either have a restricted selection of food items or that some prey are completely destroyed during chewing and ingestion. Conversely, species that scored high in both the stomach contents and in the potential food list were frequent and abundant. On occasion, some species appeared in the stomachs (e.g. copepods Corycaeus anglicus, Scolecithricella minor, and cladocerans Evadne nordmanni and Podon leuckartii), but were scarce in the zooplankton countings. This may be an indication of feeding selection by amphipods or that the plankton net used in this study was not efficient in sampling such small-bodied organisms. 129 Principal component analyses indicated that a recurrent group of prey-size species coincided where amphipods were most abundant. These species included the copepods Pseudocalanus minutus, Paracalanus parvus, Oithona sirnilis, Oithona spinirostris, Oncaea borealis, Metridia pacifica, young stages of the euphausiid Euphausia pacifica, ostracods Conchoecia spinirostris and Conchoecia alata minor and the pteropod Limacina helicina. Principal component analyses of monthly zooplankton composition also indicated that maximum variation occurred between late May and August. This is the most luminous part of the year in temperate areas (Libes, 1992), and the most productive season in the Strait of Georgia (Parsons et al., 1970; Harrison et al., 1983). This period also coincided with the seasonal discharge of the Fraser River (Thomson, 1981) and it was correspondingly reflected in the near-surface conditions of the vertical salinity structure (e.g. Tully and Dodimead, 1957). The secondary source of variation was found during the months between September and December, when light and plankton production decline (Harrison et al., 1983). Zooplankton composition during the months between February and late April had the lowest contribution to the total variation. These two last periods were characterized by two small groups of omnivorous and facultative species of copepods, which seems reasonable considering both the low phytoplankton production during those months. Similarity analyses indicated that zooplankton composition from the estuarine plume resembled that of the riverine plume during May - June 1990. This was expected since the mutual influence of river discharge and tidal water combine and form first a riverine then an estuarine plume (Harrison et al., 1991; Yin et al., 1995a). During this process, entrained nutrient-rich water containing phytoplankton is advected seaward, as river sediment settles, light conditions improve, and this allows phytoplankton to bloom (Yin et al., 1995b). This presumably causes zooplankton to aggregate at the edge of the riverine plume (Parsons et al., 1969) and at the estuarine plume (St John et al., 1992). During April and June 1991, the zooplankton composition from the Strait and the riverine plume were more similar. In April 1991, the riverine plume was practically absent during collecting time. The salinity structure in the Fraser River area was high and corresponded with 130 an estuarine plume, and the zooplankton composition was more similar to that collected in the Strait. During April 1991, a strong wind prevailed for several days, and mixed the water from the Strait with the low salinity water near the river (Yin et al., 1996). The similarity of the zooplankton composition between the Fraser River and the Strait may have resulted from the dominance of Neocalanus plumchrus in both areas (Yin et al., 1996). In June 1991, zooplankton composition in the estuarine plume was also similar to that in the Strait. Maximal tidal ranges occur during summer (Thomson, 1981) and, during June 1991, samples were taken during the flood phase of the spring tides. It is likely that the observed similarity in zooplankton composition between the Strait and the estuarine plume had resulted from spring tide action. Intense flooding during the spring tide may have mixed waters from the Strait and the estuarine plume area, minimizing the river influence on the estuarine plume water (Yinet al., 1995). Analyses of principal components of zooplankton composition indicated that P. pacifica and C. challengeri were associated with the distribution of the species involved in their diets. Pseudocalanus minutus, prevalent in the collections studied and commonly important to both amphipods, changed hierarchy value with the copepods Oithona sirnilis, Oithona spinirostris, Oncaea borealis, Metridia pacifica, Paracalanus parvus, cladocerans, ostracods and the euphausiid Euphausia pacifica. Main changes were detected between spring (April 1991) and early summer (May-June 1990, June 1991). For instance, during April 1991, closer to the spring bloom (Yin et al., 1997), P. minutus was co-dominant with typical herbivores (barnacle larvae and the copepods Calanus marshallae, C. pacificus, Eucalanus bungii, and Neocalanus plumchrus). During summer (June 1991), the co-dominant species were the euphausiid Euphausia pacifica and the copepods, M. pacifica, Acartia longirernis, P. parvus, and Oithona sirnilis among others. These species are commonly found during summer in the Strait (Harrison et al., 1983). They are facultative predators that feed on nannoplankton typical of summer conditions when diatoms are scarce (Raymont, 1983). The community found during summer (1990, 1991) differed from that in previous years (July 1987, see Harrison et al., 1991) in that Metridia pacifica dominated instead of P. minutus. 131 There were also differences in the zooplankton composition of 1997 compared with 1996 and 1998, in terms of typically dominant herbivores Euphausia pacifica and Neocalanus plurnchrus (Kane, 1998; Bornhold, 1999). This may have some consequences in the trophic ecology of juvenile salmon species feeding on copepods, euphausiids and hyperiids during their residency in the Strait of Georgia. The presence of small copepods in the diets of Parathemisto pacifica and Cyphocaris challengeri seems an important link between nannoplankton and bigger animals that may consume amphipods in the Strait of Georgia. Copepods (Oncaea and Oithona species) are considered to link nannoplankton and large-sized copepods in conditions where diatoms are not sufficient to form a major dietary source (Parsons and Lalli, 1988). A trophic connection between amphipods and microcopepods in a productive area such as the Strait of Georgia, implies that an alternative trophic link occurs, parallel to the classic diatom-copepod-fish pathway. Results from fish stomach contents seem to suggest this. Examination of zooplankton at the pycnocline in the Squamish river (Parker and Kask, 1972a, 1972b), and stomach contents of fish, revealed a high abundance of amphipods upon which chum salmon were feeding, and of herring that were feeding on copepods. Also, juvenile chinook, coho and steelhead at the riverine and estuarine plumes of the Fraser River (St. John et al, 1992) were feeding on juvenile fish, including clupeoids, while juvenile herring were feeding on several zooplankters including amphipods. The implications of these trophic connections, and potential consequences of zooplankton composition sustaining them are beyond the main objective of this study, but certainly no less enticing. 132 Conclusions Overall, this research indicates that planktonic amphipods Parathemisto pacifica and Cyphocaris challengeri have diets consistent with the plankton production sequence in the central Strait of Georgia. Parathemisto pacifica released their hatchlings relative to the phytoplankton bloom, and their subsequent growth was associated with an increase in both chlorophyll a (as an indirect indicator of seston) and zooplankton biomass. This coincided with a change in mandible structure, suggesting microphagy at young stages, changing towards carnivory in adulthood and perhaps to dettitivory in old stages. In Cyphocaris challengeri, the mandible shape showed a wide molar and a big sharp incisor that relates to carnivory, and its stomach contents coincidentally exhibited an almost exclusive animal diet, which indicates carnivorous feeding habits. This coincides with C. challengeri abundance best correlating with zooplankton biomass. The two amphipod species co-existed, exhibited cannibalistic habits, and preyed on each other's young. Cannibalism in epipelagic animals has been related to spawning and feeding in the same areas, coinciding with results of the present study. Most of the time Cyphocaris challengeri out-numbered and heavily preyed on Parathemisto pacifica. C. challengeri grew faster and bigger than P. pacifica, escaping the prey size-range of P. pacifica. This may explain why P. pacifica preyed less on C. chaUengeri as indicated by stomach contents. Regardless that at times the two amphipod species had very similar diets, their particular composition and distribution possessed enough qualitative and temporal differences to relate each amphipod with specific groups which they used as food. For instance, while the two species preyed on Euphausia pacifica, C. challengeri had more euphausiid furcilia in their stomachs and had a similar distribution with adult euphausiids whereas P. pacifica distribution coincided with high concentrations of E. pacifica eggs1. Both species prey on a resident group 1 Eggs found during summer were considered to belong to Euphausia pacifica; E. pacifica spawns a second time in temperate areas (first in spring; Mauchline and Fisher, 1969 in Raymont, 1983) 133 of small copepod species, which suggests an important link between nannoplankton and bigger animals (e.g. salmonids) that may consume amphipods in the Strait of Georgia. The suggested trophic pathway is: nannoplankton (2-30 p,m, e.g. flagellates, ciliates) -> microzooplankton (20-200 p,m, e.g. protozoans, Tintinnids) mesozooplankton (2 - 20 mm, e.g. copepods, cladocerans ostracods) macrozooplankton (2- 20 cm, e.g. amphipods) fish (e.g. herring, salmon). This is an alternative to the phytoplankton zooplankton fish (e.g. herring, salmon). This type of food chain is considered typical of oceanic areas such as the Pacific subarctic (Parsons and LeBrasseur, 1970). Its occurrence simultaneous to the phytoplankton -> zooplankton fish suggests that the model most appropriate to describe trophic dynamics in the Strait of Georgia may be the multivorous web (Legendre and Rassoulzadegan, 1995). Temporal changes in zooplankton composition and abundance coincided, in general, with the seasonal intensification of the freshet. Spatial variation observed in the semi-diurnal sampling coincided with the interaction of wind and tides with river runoff. A specific result of change in the abundance of Neocalanus plurnchrus and Euphausia pacifica seems to have been related to strong winds altering the development of the spring bloom in 1997. Although beyond the scope of this study, the population dynamics of the two amphipod species supplemented some information on their life history. P. pacifica adults migrate towards deep water during the fall-winter season and in doing so may avoid low-food environmental conditions near the surface, where it may be more energetically expensive to find food, and may put them at higher exposure to predators. While at depth, it is possible they enter a low metabolic phase2. The change in mandible shape in big animals ( > 15 mm) probably indicates an adaptation to feeding on softer, decomposing organic material that they can cut with their incisors. Since the few biggest animals found at depth were all pregnant females, it is possible that an added advantage of this behavior is a "seed bank" strategy for the population that 2 3 females kept in cold (7 °C) and dark conditions in which they were caught (200-400 m depth) were quiet until light, food or water movement nearby would stimulate them. 134 skillfully stores surviving individuals for the next spring bloom. For C. challengeri, the key to success may be growing rapidly, judged by their rapid growth and high levels of abundance. These animals are voracious feeders and vigorous swimmers with big eggs, probably indicating a high investment of energy in their offspring. That, in turn depends on accessing energy-rich food that a carnivorous diet supplies for a very actively feeding organism. Future work Data on prey size together with changes in the mandible and the distribution of individuals suggest that this type of study may provide valuable information on morphological adaptations associated with habitat shifts during their ontogeny. That type of research would require a sampling program designed for that purpose. For instance, the results found here indicated that large adults are distributed in deeper waters. A study focusing in the ecology of amphipods would then require sampling in a vertically stratified manner, using opening/closing nets. This sampling would also provide information necessary to better establish the spatial interactions between amphipods and their prey, and between the species of amphipods present in the area. It is desirable to explore the effect turbidity may have on the predatory efficiency of amphipods. Particularly in the riverine and estuarine plumes, where both turbidity and the presence of food are high. At one hand the presence of abundant food there can be highly beneficial for the amphipods, and on the other hand turbidity can reduced the efficiency of finding and collecting food. In this context, seems similarly important the analysis of the gut fullness at different time during the day. Altogether, the study of these aspects may conduct to better understand the role of estuarine environments in modulating the biological production in coastal areas, and conversely provide information on some of the adaptations that make amphipods succeed there. 135 This study focused on amphipods as predators of the zooplankton community of the Strait of Georgia. However, their role as prey to higher trophic levels must be considered if we are to understand the role of these species in the trophic ecology of the Strait. It would be useful to investigate the effect of interannual changes in their population levels, and how in turn these changes reflect on higher trophic levels. 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Group Species n Amphipoda Caltiopius laeviesculus 51 Cyphocaris challengeri 3721 Parathemisto pacifica 2915 Primno macropa 52 Scina borealis 49 Amphipoda Total 6787 Bryozoa Cyphonautes larvae 173 Bryozoa Total 173 Chaetognatha Eukrohnia hamata 1 N.I. Chaetognath 18 Sagitta decipiens 32 Sagitta elegans 256 Sagitta planktonis 2 Sagitta scrippsae 6 Chaetognatha Total 315 Cirripedia Cypris larvae 56 Nauplii 91 Cirripedia Total 147 Cladocera Conchoecia elegans 4 Evadne nordmanni, f 131 Evadne mrdmanni, m 4 Evadne tergestina 32 Podon leuckartii, f 65 Podon leuckartii, m 5 Podon polyphemdides, f 563 Cladocera Total 803 Copepoda Acartia clausii, f 15 Acartia longiremis, f 95 Acartia longiremis, m 13 Aetidopsis rostrata, f 1 Calanidae nauplii 53 Calanus cristatus 1 Calanus marshaUae, f 59 Calanus marshaUae, m 18 Calanus pacificus, C5 6 Calanus pacificus, f 121 Calanus pacificus, m 20 Chiridius gracilis, f 1 Chiridius polaris, f 46 Clausocalanus arcuicornis, f 9 Clausocalanus arcuicornis, m 3 Clausocalanus cf lividus 2 Corycaeus anglicus, f 135 Corycaeus anglicus, m 12 Eucalanus bungii, f 13 Eucalanus califomicus, f 3 Eucalanus sp, larvae 19 Eucalanus sp, nauplii 17 Euchaeta califomica, f 7 Euchaeta ehngata, f 53 Euchaeta japonica, f 20 Gaetanus intermedius 1 Labidocera acutifrons c.f. 5 Metridia curticauda, f 30 Metridia curticauda, m 15 Metridia iucens, f 63 Metridia lucens, m 34 Metridia okhotensis, f 625 Metridia okhotensis, m 1 Metridia pacifica, f 1013 Metridia pact/iica, m 166 Metridia sp 181 Microcalanus pygmaeus pusillus, 208 Neocalanus plumchrus, C2 2 Neocalanus plumchrus, C4 34 Neocalanus plumchrus, C5 101 Oithona sirnilis, f 1532 Oithona sirnilis, m 62 Oithona spinirostris, f 1389 Oithona spinirostris, m 97 Oncaea borealis, f 587 Oncaea borealis, m 5 Paracalanus parvus, C3 65 Paracalanus parvus, f 241 Paracalanus parvus, m 20 Pseudocalanus minutus, m 8 Pseudocalanus minutus, C2 380 Pseudocalanus minutus, C3 6 Pseudocalanus minutus, C4 697 Pseudocalanus minutus, C5 489 Pseudocalanus minutus, f 1487 Pseudocalanus minutus, m 78 ScolecithriceUa minor, f 138 Spinocalanus brevicaudatus 37 Spinocalanus sirnilis, f 8 Tharybis fukoni 5 Copepoda Total 10556 Crustacea Decapoda Megalopa larvae 33 Paguridae larvae 21 Peneidae larvae 11 Prezoea Paguridae 1 Zoea crab larvae 85 Zoea Peneidae 3 Crustacea Decapoda Total 153 Ctenophora Pleurobrachia pileus 21 Ctenophora Total 21 Euphausiacea Calyptopis larvae 91 Euphausia pacifica 61 Euphausia pacifica, furc 91 Euphausia pacifica, juv 89 Euphausiid eggs 1981 Euphausiid nauplii larvae 29 Euphausiid pseudonauplius 4 Furcilia larvae 102 Metanauplius larvae 8 ; Pseudonauplius larvae 4 Thysanoessa hngipes 2 Thysanoessa hngipes, fur 4 Thysanoessa hngipes, juv 6 Euphausiacea Total 2471 Gastropoda Bivalve larvae 2 gastropod larva 11 Gastropoda Total 12 Ichthyoplankton Ammodytes hexapterus 14 Ichthyoplankton Total 14 Thaliacea Fritilaria borealis 400 Thaliacea Total 400 Medusae Aegina citrea 2 Aglantha digitale 14 Medusae Total 16 Ostracoda c.f. Phillomedes sp. 7 Conchoecia alata minor 22 Conchoecia elegans 271 Conchoecia spinirostris 38 Ostracoda Total 338 Polychaeta Tomopteris septentrionalis 34 Trochophore larvae 21 Polychaeta Total 55 Pteropoda Limacina sp. 11 Limacina helicina 98 Pteropoda Total 109 Siphonophora Agalma okeni, nectophore 2 CheUophyes appendiculata 3 Dimophyes arctica, eudoxia 17 Dimophyes arctica, poligastrica 6 Dyphyes dispar, eudoxia 2 Muggieaea athntica, eudoxia 3 Nanomia bijuga 2 Siphonophora Total 33 Grand Total 22406 153 Table A 2. Total abundance (individuals per 1000 m3) of zooplankton species for April 7-17, 1991. Abbreviations indicate: cf = similar to, f = female, m= male, furc = furcilia, CI - C5 = Copepodite 1- Copepodite 5, N.I. = non-identified. Group Species n Amphipoda Cyphocaris challengeri 3360 Parathemisto pacifica 1329 Primno macropa 7 Scina borealis 3 Amphipoda Total 4700 Bryozoa Barnacle larvae 74 Cyphonautes larvae 200 Bryozoa Total 274 Chaetoenatha Sasitta eleeans 5 Chaetognatha Total 5 Cirripedia Barnacle nauplii 33 Cirripedia Total 33 Copepoda Acartia longiremis, f 63 Calanidae nauplii 71 Calanus marshallae, f 497 Calanus pacificus 832 Calanus pacificus, f 9 Calanus pacificus, m 3 Centropages abdorninalis, f 16 Chiridius polaris, f 64 Clausocalanus arcuicornis, f 24 Corycaeus anglicus, f 6 Ctenocalanus vanus, f 80 Eucalanus bungii, f 623 Eucalanus califomicus, f 56 Euchaeta japonica, f 2 Gaidius minutus, f 2 Metridia curticauda, f 23 Metridia lucens, f 248 Metridia okhotensis, f 33 Metridia pacifica, f 657 Microcalanus pigmaeus 122 Neocalanus plumchrus,CA 2350 Oithona sirnilis, f 639 Oithona spinirostris, f 227 Oncaea borealis, f 501 Oncaea englishi, f 32 Paracalanus parvus, f 107 Pseudocalanus minutus, f 1737 Scolecithricella minor, f 104 Spinocalanus brevicaudatus, f 10 Copepoda Total 9150 Crustacea decapoda Crab zoea larvae 80 Galateidae zoea larvae 77 Crustacea decapoda Total 158 Euphausiacea Euphausia pacifica 1205 Euphausiacea nauplii larvae 9 Furcilia larvae 0 Thysanoessa rashii 5 Euphausiacea Total 1229 Ichthyoplankton Clupea paUasi 45 Eggs 5 Gonostomatidae 10 Gonostomatidae larvae 15 Ichthyoplankton Total 75 Larvacea Oikopleura dioica 39 Oikopleura sp 19 Oikopleura vanhofenii 107 Larvacea Total 165 Medusae Aglantha digitale 64 Medusae Total 64 Ostracoda Conchoecia alata minor 129 Conchoecia elegans 157 Ostracoda Total 286 Polychaeta Polychaeta larvae 35 Tomopteris septentrionalis 71 Polychaeta Total 107 Pteropoda Limacina helicina 42 Limacina inflata 68 Limacina sp. 47 N.I. Pteropoda 51 Pteropoda Total 208 Siphonophora Apolemmia uvaria 31 Eudoxoides spiralis, eudoxia 56 Siphonophora Total 87 Grand Total 16540 155 Table A 3. Total abundance (individuals per 1000 m3) of zooplankton species for June 11-14, 1991. Abbreviations indicate: cf = similar to, f = female, m= male, furc = furcilia, C l - C5 = Copepodite 1- Copepodite 5, N.I. = non-identified. Group Amphipoda Amphipoda Total Bryozoa Bryozoa Total Chaetognatha Chaetognatha Total Cladocera Cladocera Total Copepoda Species Cyphocaris chaUengeri ' Parathemisto pacifica Cyphonautes larvae Sagitta c.f. bedoti Sagitta elegans Sagitta planktonis Sagitta scrippsae Conchoecia alata minor Conchoecia elegans Conchoecia sp Evadne nordrnanni Podon leuckartii Acartia longiremis , f Calanidae nauplii Calanus cristatus, f Calanus pacificus, f Calanus pacificus, m Candacia columbiae, f Centropages abdominalis, f Chiridius polaris, f Clausocalanus papergens, f Corycaeus anglicus, f Corycaeus catus, f Epilabidocera longipedata, f Eucalanus bungii, f Eucalanus bungii califomicus, f Eucalanus calif omicus, f Euchaeta japonica, f Euchaeta nauplii Gaidius minutus, f Labidocera c.f. japonica, f Metridia lucens, f Metridia pacifica Metridia pacifica, f Metridia pacifica, m Microcalanus pigmaeus pusillus, f Microcalanus pygmaeus pusillus, m n 3452 5133 8585 117 117 4 198 43 5 249 12 45 4 21 103 184 821 34 4 571 3 30 16 39 8 4 30 20 8 21 38 74 63 3 45 21 1065 126 8 16 4 156 MonstriUa sp 13 Neocalanus plumchrus, C-4 372 Oithona sirnilis, f 401 Oithona spinirostris, f 636 Oncaea borealis, f 277 Onceae borealis 3 Paracalanus parvus, f 396 Paracalanus parvus, m 19 Paracalanus parvus, C3 11 Pseudocalanus minutus, f .2051 Pseudocalanus minutus, C3 13 Scaphocalanus brevicornis, f 100 Scaphocalanus subbrevicomis, f 72 Scolecithricella minor, f 3 Spinocalanus brevicaudatus, f 17 Copepoda Total 7456 Crustacea Cirripedia Barnacle larvae 20 Crustacea Cirripedia Total 20 Crustacea decapoda crab megalopa 3 crab zoea 25 Crustacea decapoda Total 28 Euphausiacea Euphausia pacifica, juv 5 Euphausiid eggs 222 Euphausia pacifica 1149 Furcilia larvae 20 Euphausiacea Total 1396 Ichthyoplankton Clupea pallasi 15 Ichthyoplankton Total 15 Invertebrate eggs, 0.39mm 563 Invertebrate Total 563 Thaliacea Eritilaria borealis 842 Oikopleura dioica 94 Thaliacea Total 936 Medusae Aglantha digitale 20 Medusae Total 20 Ostracoda Conchoecia alata minor 112 Conchoecia elegans 737 Conchoecia spinirostris 60 Phillornedes sp. 40 Ostracoda Total 950 Polychaeta Tomopteris septentrionalis 122 Polychaeta Total 122 Pteropoda Clione limacina 21 Limacina helicina 61 Limacina sp. 47 Pteropoda Total 129 Siphonophora Chehphyes appendiculata, eudoxia 13 Dimophyes arctica, eudoxia 47 Muggiaea atlantica, eudoxia 33 Namrnia bijuga 5 Siphonophora Total 99 Grand Total 20868 158 Table A 4. Total abundance (individuals per 1000 m3) of zooplankton species for January-December 1997. Abbreviations indicate: cf = similar to, f = female, m= male, furc = furcilia, C l - C5 = Copepodite 1- Copepodite 5, N.I. = non-identified. Group Species n Amphipoda Cyphocaris challengeri 353449 Parathemisto pacifica 192458 Scina borealis 8 Amphipoda Total 545915 Barnacle Nauplius larvae 90 Barnacle Total 90 Bryozoa Bryozoa larvae 109 Cyphonautes larvae 1873 Bryozoa Total 1983 Chaetognatha Eukrohnia hamata 334 N.I. chaetognath 29 Sagitta decipiens 122 Sagitta elegans 150 Sagitta planktonis '91 Sagitta scrippsae 111 Chaetognatha Total 838 Cladocera N.I. cladocera 223 Podon polyphemoides 241 Cladocera Total 464 Copepoda Acartia danae. f 153 Acartia longiremis, f 2726 Acartia longiremis, m 145 Calanus marshaUae, C4 67 Calanus marshaUae, f 8 Calanus marshaUae, m 20 Calanus pacificus, C3 159 Calanus pacificus, C4 1053 Calanus pacificus, C5 345 Calanus pacificus, f 505 Calanus pacificus, m 47 Candacia Columbia, m 8 Centropages abdorninalis, f 8 cf Neocalanus plurnchrus nauplii 712 Chiridius gracilis, f 158 Chiridius gracilis, m 24 Chiridius polaris, C5 10 Chiridius polaris, f 301 Chiridius polaris, m 17 Clausocalanus arcuicomis, f 26 Clausocalanus parapergens, f 223 Copepod eggs 22074 Copepod nauplii 1128 Corycaeus angUcus, C5 3 Corycaeus anglicus, f 592 Corycaeus anglicus, m 3 Ctenocalanus vanus, f 278 Ctenocalanus vanus, m 82 Eucalanus bungii, f 7 Eucalanus califomicus, C4 31 Eucalanus caUfornicus, f 100 Eucalanus caUfornicus, m 64 Eucalanus caUfornicus, nauplii 25 Euchaeta ehngata, m 38 Euchaeta japonica, f 55 Euchaeta japonica, m 25 Euchaeta pacifica, f 52 Gaetanus inteTTnedius, f 24 Gaidius minutus, f 68 Gaidius minutus, m 20 Gaidius variabiUs, f 20 Gaidius variabilis, m 3 Macrocheiron sargassi, f 29 Metridia pacifica, { 5732 Metridia pacifica, m 1682 Microcalanus minutus 208 Microcalanus p^ gmaeus pusiilus, C5 56 Microcalanus pygmaeus pusillus, f 528 Microcalanus pygmaeus pusillus, m 8 Microcalanus pygmaeus pusillus, 20 MicroseteUa rosea, m 29 Nauplii 57 Nauplius, N i l , c.f. Neocalanus 29 Neocalanus cristatus, C4 3 Neocalanus plumchrus, C4 326 Neocalanus plumchrus, C5 488 Neocalanus plumchrus, V 105 Oithona sirniUs, C4 1146 Oithona simiUs, f 16646 Oithona sirniUs, m 448 Oithona spinirostris, f 1262 Oncaea boreaUs, f 4504 Oncaea conifera, f 7 Oncaea conifera, m 7 Paracalanus parvus, C2 228 Paracalanus parvus, C5 349 Paracalanus parvus, f 429 Paracalanus parvus, m 3 Pseudocalanus minutus,C-1 29 160 Pseudocalanus minutus, C2 2901 Pseudocalanus minutus, C3 1030 Pseudocalanus minutus, C4 4109 Pseudocalanus minutus, C5 1609 Pseudocalanus minutus, f 8515 Pseudocalanus minutus, m 1733 Spinocalanus brevicaudatus, f 399 Spinocalanus brevicaudatus, m 35 Copepoda Total 86092 Crustacea Decapoda Megalopa Crab larvae 684 N.I. Decapoda 56 Pandalidae 31 Zoea Crab larvae 429 Crustacea Decapoda 1200 Euphausiacea Calyptopis larvae 33 Euphausia pacifica 504 Euphausia pacifica eggs 133 Euphausia pacifica, juv 51 Euphausiid eggs 296 Furcilia larvae 281 Euphausiacea Total 1298 Ichthyoplankton Fish eggs 203 Fish larvae 133 Ichthyoplankton Total 336 Medusae c.f. Aglantha digitale 4 Medusae Total 4 Mysidacea Boreomysis arctica, a 8 Mysidacea Total 8 Ostracoda Conchoecia alata minor 1006 Conchoecia elegans 506 Conchoecia spinirostris 742 Ostracoda Total 2254 Polychaeta Polychaeta larvae 313 Tomopteris septentrionalis 397 Polychaeta Total 710 Pteropoda CUone limacina 3 Limacina helicina 1185 Pteropoda Total 1188 Siphonophora Dymophyes arctica, eudoxia 57 Dymophyes arctica, nectophore 3 Dyphyes bojani, eudoxia 8 Eudoxoides spiralis, nectophore 3 Muggieae atlantica, eudoxia 8 Nanomia bijuga 4 Siphonophora Total 83 Grand Total 642461 Table A 5. Relative frequency per size interval of Parathemisto pacifica during 1997. DATE 21-Jan 21-Jan 21-Jan 10-Feb 10-Feb 4-Mar 25-Mar 25-M Depth 50 200 400 50 400 400 50 400 Size 1.5 0.00 0.00 0.00 0.00 0.00 0.00 0.06 0.00 2 0.00 0.00 0.00 0.00 0.00 0.19 0.19 0.00 2.5 0.00 0.00 0.00 0.00 0.00 0.00 0.46 0.00 3 0.00 0.00 0.00 0.00 0.07 0.00 0.07 ? 0.00 3.5 0.00 0.00 0.00 0.00 0.00 0.00 0.07 0.00 4 0.00 0.00 0.00 0.00 0.00 0.00 0.14 0.00 4.5 0.00 0.00 0.00 0.00 0.08 0.00 0.00 0.00 5 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 5.5 0.15 0.08 0.10 0.11 0.08 0.00 0.00 0.00 6 0.15 0.16 0.10 0.00 0.23 0.00 0.00 0.04 6.5 0.23 0.25 0.40 0.40 0.15 0.20 0.00 0.04 7 0.23 0.25 0.20 0.00 0.23 0.20 0.00 0.08 7.5 0.08 0.00 0.00 0.31 0.08 0.00 0.00 0.12 8 0.00 0.26 0.00 0.00 0.00 0.00 0.00 0.04 8.5 0.08 0.00 0.10 0.18 0.00 0.20 0.00 0.36 9 0.08 0.00 0.00 0.00 0.00 0.00 0.00 0.08 9.5 0.00 0.00 0.00 0.00 0.08 0.00 0.00 0.04 10 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.04 10.5 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.16 11 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 11.5 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 12 0.00 0.00 0.10 0.00 0.00 0.00 0.00 0.00 12.5 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 13 0.00 0.00 0.00 0.00 0.00 0.22 0.00 0.00 13.5 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 14 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 14.5 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 15 0.00 0.00 0.00 0.00 0.00 0.00 0.00 . 0.00 N 5579 1599 653 224 1671 341 7964 1248 Table A5 continuation DATE 8-Apr 2-May 2 2-May 22-May 18-Jun 18-Jun 9-Jul 9-Jul Depth 50 50 50 400 50 200 60 210 Size 1.5 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 2 0.00 0.00 0.08 0.04 0.35 0.08 0.15 0.00 2.5 0.00 0.00 0.08 0.12 0.47 0.11 0.00 0.00 3 0.00 0.00 0.15 0.06 0.19 0.07 0.00 0.00 3.5 0.46 0.00 0.23 0.06 0.00 0.00 0.32 0.00 4 0.54 0.00 0.15 0.07 0.00 0.00 0.17 0.05 4.5 0.00 0.00 0.16 0.03 0.00 0.03 0.17 0.07 5 0.00 0.00 0.08 0.01 0.00 0.03 0.00 0.19 5.5 0.00 0.00 0.08 0.01 0.00 0.06 0.00 0.09 6 0.00 0.00 0.00 0.03 0.00 0.06 0.00 0.05 6.5 0.00 0.00 0.00 0.06 0.00 0.00 0.00 0.10 7 0.00 0.33 0.00 0.05 0.00 0.06 0.19 0.14 7.5 0.00 0.00 0.00 0.03 0.00 0.12 0.00 0.19 8 0.00 0.00 0.00 0.02 0.00 0.03 0.00 0.00 8.5 0.00 0.67 0.00 0.06 0.00 0.03 0.00 0.05 9 0.00 0.00 0.00 0.08 0.00 0.05 0.00 0.02 9.5 0.00 0.00 0.00 0.11 0.00 0.03 0.00 0.03 10 0.00 0.00 0.00 0.05 0.00 0.05 0.00 0.00 10.5 0.00 0.00 0.00 0.03 0.00 0.03 0.00 0.00 11 0.00 0.00 0.00 0.06 0.00 0.03 0.00 0.00 11.5 0.00 0.00 0.00 0.03 0.00 0.05 0.00 0.00 12 0.00 0.00 0.00 0.00 0.00 0.03 0.00 0.00 12.5 0.00 0.00 0.00 0.00 0.00 0.02 0.00 0.03 13 0.00 0.00 0.00 0.00 0.00 0.02 0.00 0.00 13.5 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 14 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 14.5 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 15 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 N 1736 1112 8337 95486 15892 4148 3327 7001 Table A5 continuation DATE 9-Jul 6-Aug 3-Sep 3-Sep 27-Oct 22-Nov 18-Dec Depth C' 400 200 200 400 400 400 400 o i z e 1.5 0.05 0.00 0.00 0.00 0.00 0.00 0.00 2 0.00 0.00 0.00 0.00 0.06 0.00 0.00 2.5 0.05 0.00 0.00 0.00 0.11 0.00 0.00 3 0.05 0.05 0.00 0.00 0.08 0.00 0.00 3.5 0.09 0.00 0.00 0.00 0.06 0.12 0.00 4 0.00 0.05 0.00 0.00 0.05 0.00 0.00 4.5 . 0.05 0.11 0.13 0.00 0.04 0.00 0.00 5 0.05 0.00 0.13 0.49 0.00 0.22 0.00 5.5 0.05. 0.12 0.14 0.00 0.03 0.11 0.00 6 0.15 0.20 0.00 0.00 0.04 0.11 0.00 6.5 0.05 0.14 0.29 0.00 0.02 0.00 0.00 7 0.00 0.07 0.00 0.00 0.03 0.00 0.50 7.5 0.05 0.15 0.15 0.00 0.02 0.11 0.00 8 0.05 0.00 0.00 0.51 0.03 0.11 0.00 8.5 0.10 0.00 0.16 0.00 0.09 0.11 0.00 9 0.10 0.00 0.00 0.00 0.04 0.00 0.00 9.5 0.00 0.00 0.00 0.00 0.09 0.00 0.50 10 0.00 0.10 0.00 0.00 0.04 0.00 0.00 10.5 0.05 0.00 0.00 0.00 0.04 0.00 0.00 11 0.00 0.00 0.00 11.5 0.00 0.00 0.00 12 0.00 0.00 0.00 12.5 0.00 0.00 0.00 13 0.00 • 0.00 0.00 13.5 0.00 . 0.00 0.00 14 0.00 0.00 0.00 14.5 0.00 0.00 0.00 15 0.06 0.00 0.00 N 12479 5879 1420 0.00 0.06 0.12 0.00 0.00 0.02 0.00 0.00 0.00 0.05 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 "0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 111 25372 460 100 Table A 6. Relative frequency per size interval of Cyphocaris chaUengeri during 1997. Date 21-Jan 21-Jan Depth (m) 50 200 Size 2 0.00 0.00 2.5 0.00 0.00 3 0.05 0.00 3.5 0.05 0.00 4 0.39 0.00 4.5 0.15 0.00 5 0.00 0.00 5.5 0.00 0.00 6 0.00 0.00 6.5 0.00 0.00 7 0.00 0.00 7.5 0.00 0.00 8 0.00 0.00 8.5 0.00 0.13 9 0.00 0.07 9.5 0.00 0.07 10 0.00 0.00 10.5 0.00 0.00 11 0.00 0.00 11.5 0.10 0.10 12 0.05 0.11 12.5 0.05 0.24 13 0.05 0.13 13.5 0.00 0.00 14 0.11 0.15 14.5 0.00 0.00 15 0.00 0.00 15.5 0.00 0.00 16 0.00 0.00 N = 8154 3705 21-Jan 10-Feb 10-Feb 400 50 400 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.06 0.00 0.00 0.18 0.00 0.00 0.24 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.01 0.00 0.00 0.02 0.00 0.03 0.06 0.00 0.00 0.05 0.00 0.00 0.08 0.07 0.06 0.03 0.00 0.12 0.12 0.22 0.09 0.05 0.00 0.07 0.06 0.00 0.04 0.33 0.08 0.14 0.08 0.08 0.16 0.00 0.08 0.06 0.11 0.00 0.23 0.00 0.00 0.00 0.00 0.00 0.00 67301 6783 86165 4-Mar 25-Mar 8-Apr 400 400 50 0.00 0.00 0.00 0.04 0.00 0.00 0.06 0.00 0.00 0.04 0.00 0.00 0.02 0.00 0.00 0.02 0.00 0.50 0.00 0.00 0.00 0.00 0.00 0.50 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.01 0.00 0.02 0.01 0.00 0.07 0.05 0.00 0.14 0.09 0.00 0.07 0.07 0.00 0.15 0.16 0.00 0.10 0.15 0.00 0.11 0.13 0.00 0.06 0.16 0.00 0.06 0.09 0.00 0.03 0.06 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2630 55320 846 Table A6 continuation Date 15-Apr 22-May 18-Jun 9-Jul 9-Jul 9-Jul 6-Aug Depth (m) 50 400 200 60 210 400 200 Size 2 0.00. 0.00 0.00 0.00 0.00 0.00 0.02 2.5 0.00 0.00 o.oo 0.00 0.00 0.00 0.02 3 0.00 0.00 0.04 0.00 0.00 0.00 0.00 3.5 ' ' 0.50 0.01 0.02 0.03 0.00 0.01 0.00 4 0.00 0.02 0.02 0.07 0.02 0.01 0.02 4.5 0.00 0.02 0.12 0.10 0.01 0.08 0.06 5 0.00 0.07 0.08 0.07 0.07 0.05 0.02 5.5 0.00 0.14 0.08 0.20 0.10 0.07 0.11 6 0.00 0.03 0.10 0.07 0.10 0.00 0.04 6.5 0.00 0.05 0.06 0.10 0.08 0.02 0.04 7 0.00 0.10 0.09 0.14 0.06 0.03 0.09 7.5 0.00 0.04 0.15 0.11 0.09 0.07 0.07 8 0.00 0.05 0.07 0.00 0.09 0.05 0.09 8.5 0.00 0.01 0.02 0.04 0.09 0.07 0.05 9 0.00 0.02 0.02 0.04 0.04 0.10 0.03 9.5 0.00 0.01 0.02 0.04 0.03 0.04 0.03 10 0.00 0.00 0.05 0.00 0.04 0.02 0.05 10.5 0.00 0.00 0.00 0.00 0.04 0.04 0.05 11 0.00 0.03 0.02 0.00 0.03 0.02 0.05 11.5 0.00 0.01 0.00 0.00 0.01 0.05 0.06 12 0.00 0.11 0.00 0.00 0.00 0.07 0.00 12.5 0.00 0.05 0.00 0.00 0.03 0.04 0.03 13 0.00 0.02 0.00 0.00 0.01 0.03 0.00 13.5 0.00 0.06 0.00 0.00 0.03 0.02 0.00 14 0.00 0.07 0.00 0.00 0.00 0.00 0.00 14.5 0.00 0.04 0.00 0.00 0.00 0.07 0.02 15 0.50 0.04 0.03 0.00 0.00 0.04 0.01 15.5 0.00 0.00 0.00 0.00 0.00 0.00 0.01 16 0.00 0.00 0.00 0.00 0.00 0.00 0.03 794 5092 13942 15336 9002 4962 9900 Table A6 continuation Date 6-Aug 3-Sep 3-Sep 27-Oct 22-Nov 18-Dec Depth (m ) 340 200 400 400 400 400 Size 2 0.00 0.00 0.00 0.00 0.00 0.00 2.5 0.00 0.00 0.00 0.00 0.02 0.00 3 0.00 0.00 0.00 0.00 0.00 0.00 3.5 0.00 0.00 0.00 0.02 0.00 0.00 4 0.32 0.00 0.00 0.02 0.00 0.00 4.5 0.00 . 0.00 0.00 0.01 0.00 0.01 5 0.00 0.02 0.01 0.05 0.03 0.01 5.5 0.00 0.02 0.00 0.05 0.02 0.00 6 0.00 0.02 0.01 0.06 0.07 0.00 6.5 0.33 0.03 0.02 0.07 0.09 0.00 7 0.00 0.06 0.00 0.05 0.01 0.00 7.5 0.00 0.08 0.00 0.12 0.06 0.00 8 0.00 0.05 0.03 0.05 0.04 0.00 8.5 0.00 0.06 0.02 0.01 0.08 0.02 9 0.35 0.08 0.01 0.02 0.06 0.03 9.5 0.00 0.05 0.04 0.01 0.01 0.03 10 0.00 0.09 0.06 0.00 0.04 0.04 10.5 0.00 0.13 0.07 0.00 0.08 0.06 11 0.00 0.11 0.13 0.00 0.15 0.10 11.5 0.00 0.05 0.06 0.01 0.04 0.12 12 0.00 0.05 0.14 0.06 0.07 0.04 12.5 0.00 0.02 0.18 0.05 0.08 0.09 13 0.00 0.00 0.06 0.10 0.01 0.17 13.5 0.00 0.02 0.09 0.04 0.01 0.07 14 0.00 0.05 0.00 0.11 0.00 0.06 14.5 0.00 0.00 0.08 0.06 0.01 0.00 15 0.00 0.00 0.00 0.01 0.00 0.04 15.5 0.00 0.04 0.00 0.02 0.00 0.10 16 0.00 0.00 0.00 0.02 0.00 0.00 N= 191 6597 10222 5495 3625 11602 167 Table A 7. Abundance of Cyphocaris chalkngeri and Parathemisto pacifica from Estuarine plume, Fraser River plume and Strait of Georgia locations, n = number of data per area, R = sum of ranks per area. Data are double square root standardized. Cyphocaris challengeri Parathemisto pacifica May - June 1990 Estuary River Strait Estuary River Strait 4.39 (7) 0(1) 2.24 (2) 4.62 (8) 4.28 (6) 2.41 (2) 3.80 (5) 2.3 (3) 3.97 (6) 3.69 (5) 1.63 (1) 3.4 (4) 3.03 (4) 4.94 (8) 2.82 (3) 4.4 (7) 5.56 (9) 5.07 (9) n 4 2 3 4 2 3 R 25 4 16 25 7 13 April, 0 (2.5) 0 (2.5) 4.77 (7) 2.10 (1) 2.38 (2) 3.77 (6) 3.4 (5) 6.5 (8) 3.91 (6) 2.93 (3) 4.69 (8) 4.16 (7) 0 (2.5) 3.54 (4) 0 (2.5) 3.62 (5) n 4 2 2 4 2 2 R 12.5 10.5 13 13 10 13 June, 1991 2.38 (3) 4.72 (4) 0 (1.5) 4.11 (4) 4.31 (5) 3.09 (3) 0 (1.5) 4.64 (5) 5.84 (6 0(1) 2.82 (2) 7.5 (6) n 2 2 2 2 2 2 R 4.5 9 7.5 5 7 9 1997 Jan -March Apr - Jun Jul - Sep Oct -Dec Jan -March Apr - Jun Jul - Sep Oct -Dec 16.11 (13) 5.39 (3) 8.39 (6) 13.95 5.05 (7) 5.43 (8) 10.57 12.62 (13) 17.13 (14) 5.31 (2) 3.71 (1) 7.76 (5) 6.39 (10) 2.31 (2) 0(1) 4.63 (6) 7.16 (4) 8.44 (7) 10.05 (8) 10.37 (9) 4.29 (5) 17.8 (14) 3.24 (4) 3.16 (3) 15.33 (12) 10.86 (10) 5.94 (9) 8.02 (11) n 4 4 3 3 4 4 3 3 R 43 22 15 25 31 35 17 22 

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