The use of an optical plankton counters in zooplankton ecology requires sampling strategies and hypothesis testing that take into account its ability to collect highresolution size-structured data, as well as its inability to distinguish zooplankton from detritus. Studies in Saanich Inlet and the Strait of Georgia, B.C. were performed to 1) compare the temporal resolution of OPC and net samples and 2) compare seasonal variation in the zooplankton community size structure against the predictions of biomass size distribution theory. The first part of this work found that OPC samples have finer resolution and require fewer replicates to approximate the mean abundance of zooplankton than net samples at time scales between 20 minutes and 48 hours. However, the OPC is subject to measurement error in high productivity waters. The second section of this work shows that the size-spectrum dynamics of zooplankton in the Strait of Georgia follow the predictions of biomass size spectrum theory. The interpretation of thse data using size-distribution theory suggests that variation in the trophic dynamics of the zooplankton community may be at the root of the observed seasonality.

Feeding habits of two coexisting species of Chaoborus, C.trivittatus and C.americanus, were investigated in lake and in laboratory experiments, and by an extensive survey of larval crop contents. The field study showed that C.trivittatus has a two year life cycle, migrates dielly during the summer season down to 20 m, and is the more abundant of the two Chaoborus species. C.americanus has a one year life cycle and migrates at most over a distance of only 5 m. Seasonal abundance and distribution of most plankton types found in Eunice Lake were monitored during 1971 and 1972. Zooplankton had a low standing crop and were mostly found above 6 m. Zooplankton size and swimming velocity and size of larval head parts were measured in the laboratory and, together with the above data on vertical distribution, were used to evaluate the availability of zooplankton as prey for Chaoborus. The lake and laboratory experiments showed that rates of larval feeding and digestion increase significantly with temperature. Temperature, however, does not seem to affect feeding rate of the 4th instar C.trivittatus larvae. Feeding rates increase with larval age and vary with prey type and density. From analysis of chaoborid crop contents, diet differences were found among all instars of the two species, and these were related to prey size, abundance, and distribution. The calculated percent of prey standing crop that the Chaoborus in Eunice Lake could potentially remove ranged from a minimum of 3% for nauplii to a maximum of 20% for Diaptomus kenai. Seasonal changes in Choaborus instar composition and in zooplankton species composition result in reduced predation on any single zooplankton group. The observed diet of Chaoborus larvae in Eunice Lake was shown to be closely related to the morphology of Chaoborus and their prey and to the relative distribution of predator and prey. / Science, Faculty of / Zoology, Department of / Graduate

Annual and seasonal changes in distribution and abundance of six species of macro-zooplankton (Cyclops bicuspidatus thomasi, Diaptomus ashlandi, Daphnia galeata mendotae, Diaph- anosoma leuchtenbergianum, Bosmina coregoni, and Mysxs relicta.) twgre studied in Kootenay Lake, a body of water extending over 105 km in southeastern British Columbia. Differences between two years, 1949 and 1964, were examined as well as changes in midsummer distribution and abundance between three consecutive years (1964 to 1966). Seasonal changes during one year and within a summer were also noted as well as diel changes in distribution and abundance. A large increase in zooplankton abundance occurred during the fifteen year period from 1949 to 1964, and this was attributed to an increased inflow of primary nutrients from a major tributary at the south end of the lake. A change in species composition, formerly dominated by Daphnia but now by Diaphanosoma, was related with eutrophication. Changes in food availability due to eutoophication, competition for food with Mysis relicta, and heavy predation by kokanee (Oncorhynchus nerka), were examined as factors possibly responsible for changes in the spatial distribution of Daphnia since 1949. Differences in the midsummer zooplankton distribution and abundance between three consecutive years (1964 to 1966) were minor, suggesting that the changes noted between 1949 and 1964 were attributable to large changes in limnological conditions and not merely the result of annual fluctuations. Examination of seasonal trends in zooplankton distribution and abundance indicated that zooplankton, with the exception of Mysis relicta, were most abundant during the summer. Mysids were most abundant in May when the young mysids were released from the brood pouch. Spatial differences in summer zooplankton abundance was examined in relation to wind-driven currents, and food availability. Spatial differences during the rest of the year were minor, possibly a result of similar limnological conditions at all stations then. Abundance of Mysis relicta was greatest in the north arm of the lake, whereas all other species were most abundant in the central area of the lake. Changes that occurred within the summer were largely due to high turbidity in the south arm during June and July. Diel changes in the vertical distribution of zooplankton, with the exception of Mysis relicta, were not detected. / Science, Faculty of / Zoology, Department of / Graduate

A series of cruises was made over 14 months to determine the effect of physical transport on the zooplankton of Indian Arm, whose deep water is separated from the marine influence of the Strait of Georgia by a series of shallow sills. The dominant transport process during the study period was tidal exchange. The topographic features of the inlet, coupled with density stratification found over the year, restricted the exchange of water and plankton to a surface phenomenon. Four copepod species were sampled using horizontally towed Clarke-Bumpus nets and vertically towed SCOR nets and one meter conical nets. These were analysed to determine the relationship between different life history patterns and tidal exchange, and the effect of this relationship on the populations of these species inside Indian Arm. Corycaeus anglicus is a surface water to mid depth organism found in highest numbers in Vancouver Harbour. The population density reaches a peak in the fall and remains high throughout the winter with transport concomittant with this peak. Euchaeta japonica exhibits ontogenetic depth preferences, nauplii and Stage I copepodites are found mainly in deep water below 200 meters, Stages II-IV are found mainly in shallow water, and Stages V and VI (adults) are found scattered over most of the water column. This species reproduces throughout the year in both Indian Arm and the Strait of Georgia. Euchaeta japonica was transported mainly as the Stage III copepodite and primarily during the winter months despite the fact that the species is found in surface waters in large numbers at other times of the year, thus producing an isolated population in Indian Arm during the summer. Metridia pacifica is a strong diel migrator found extensively in the upper 50 meters at night, and from 250 meters to 50 meters during the day. Transport of the species across the sill occurs at all times of the year but is significantly greater at night, regardless of the phase of tide. Eucalanus bungi is an ontogenetic migrator, overwintering in deep water (greater than 150 meters), and coming to the surface in the spring to spawn; juvenile stages are found in surface waters during the summer. The data show that this species is transported only during the summer while it is in surface water. Consequently nauplii and younger copepodites are the dispersal stages. The data suggest that the species does not reproduce in Indian Arm and that transport of the species during the summer months can account for the entire overwintering population found in the inlet. An analysis of the correlation of changes in the zooplankton community with physical parameters varying over the tidal cycle was made. Species known to migrate dielly show significant differences in numbers between day and night samples. Relatively few species show differences which can be correlated to the direction of tidal movement. The most significant changes seen in the zooplankton community occur in association with changes in hydrographic properties. Moreover, these changes are manifested not at the species level, but at the community level as changes in such parameters as diversity and dominance. The effect of transport on the zooplankton community in Indian Arm varies from species to species. Qualitatively, the effect depends upon the organisms' life cycle including a surface dwelling stage, and/or its ability to survive the surface water transport conditions. Quantitatively, the effect depends on the amount of time spent in surface water, which varies with the depth distribution of the organism, determined by its behavioral characteristics. The overall effect of tidal exchange will be to drive towards equilibrium the population of zooplankton found in Indian Arm and the Strait of Georgia. It is the biology of the individual species which determines the extent of interaction with this transport process and hence the amount of exchange which takes place. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

No description available.

Despite high densities in an upstream lake, the herbivorous calanoid copepod, Diaptomus leptopus, is extremely rare in oligotrophia Eunice Lake in the Coastal Range Mountains near Vancouver, British Columbia. In situ experiments conducted in 1979 and 1980 tested the hypothesis that competition from zooplankton species resident in Eunice Lake prevents immigrant D. leptopus from colonizing the lake. Polyethylene enclosures, each holding 29,000 1 of lake water, contained all experimental treatments. Experiments in 1979 exposed a standard density of D. leptopus to all Eunice Lake zooplankton species at lake densities (control), to all Eunice Lake species at reduced densities (low density), and to all Eunice Lake species except one of Daphnia rosea (Daphnia-removal), Diaptomus kenai (kenai-removal), or Diaptomus tyrelli (tyrelli-removal). Improved performances of D. leptopus in non-control treatments was evidence for competition in controls. Measures of performance included density, number of eggs per female, and adult size. Similarity between D. leptopus performances in control, Daphnia-removal, and kenai-removal treatments plus relatively high concentrations of D. leptopus nauplii in the tyrelli-removal treatment suggested that D. leptopus did not compete with species other than D. tyrelli. However, similarity between tyrelli-removal and low density treatments in concentrations of D. leptopus nauplii confounded competition between D. leptopus and D. tyrelli with diffuse competition from several Eunice Lake species combined. D. leptopus overlapped in vertical distribution and seasonal cycle more with D. tyrelli than with other species. Differences' among experimental treatments in algal size compositions did not conclusively show whether zooplankton partitioned food resources. Experiments in 1980, designed to separate D. tyrelli competition from diffuse competition, did not provide any evidence of interspecific interactions. By increasing zooplankton metabolic efficiencies, cool temperatures in 1980 may have virtually eliminated, competition for algal foods. / Science, Faculty of / Zoology, Department of / Graduate

Understanding the impact which grazers have on their prey is of vital importance in understanding how aquatic ecosystems function. In an attempt to contribute to this understanding, this study examined, at three levels, the effects of zooplankton on phytoplankton biomass in Placid Lake in summer. Examination of selective feeding by the major herbivorous zooplankton species in in situ enclosures revealed that single phytoplankton cells in the range of 6-20 μm long were the preferred food of these organisms. Colonial algae, when dominated by the cyanophyte Merismopedia, did not appear to be grazed. The effect of zooplankton biomass on phytoplankton biomass was examined in in situ enclosures. Generally, phytoplankton biomass decreased only in enclosures where initial zooplankton biomass was very low or very high. A simple model based on the classical logistic model of predator-prey interactions was developed to explain events in the enclosures. I concluded that in summer, Placid Lake phytoplankton depend on nutrients remineralized by zooplankton for growth. Grazing appears to be an important regulating mechanism of the phytoplankton standing crop in the spring but not summer. A third level of study involved examination of the responses of phytoplankton to lake perturbation, namely removal of zooplankton, compared to plankton patterns in previous and subsequent years. In years lacking zooplankton manipulation, major increases in zooplankton biomass in mid spring were followed by phytoplankton biomass increases in late spring. During the first harvesting season, July and August 1979, a 50% reduction in zooplankton biomass was obtained. An enormous bloom of the inedible Merismopedia developed. I hypothesized that removal of zooplankton caused a shortage of available biologically reactive nitrogen which became limiting to eukaryotic phytoplankton, allowing Merismopedia, a blue-green alga which may be able to fix nitrogen, to thrive. In summer, the positive effect of zooplankton on phytoplankton via nutrient remineralization appeared to be more significant than the negative effect of grazing. During the second harvesting season, May, June, and July 1980, no decrease in zooplankton biomass was apparent. Instead of the usual pattern of zooplankton biomass increase preceeding the phytoplankton biomass increase, both increases occurred simultaneously. I concluded that harvesting delayed the rise in zooplankton biomass and decreased the grazing pressure on phytoplankton, allowing it to peak earlier. Grazing may thus be significant in spring in slowing phytoplankton growth. Seasonal variations were introduced to the model for the enclosure experiments to help understand the normal plankton patterns in Placid Lake. The time lag between maximum solar radiation and lake temperature, and the effects of these two physical parameters on phytoplankton and zooplankton growth appear to be instrumental in establishing the pattern of plankton biomass dynamics observed in Placid Lake. / Science, Faculty of / Zoology, Department of / Graduate

A two year study was initiated in 1973 to examine effects of substantial (3.8 fold; from a 1962-66 mean of 39 million to about 150 million in 1973 and 1974) increases in sockeye (Oncorhynchus nerka) Walbaum) fry numbers on zooplankton abundance in Babine Lake. Several lake areas and stationsware sampled for zooplankton bimonthly from May to October during 1973 and 1974 and compared to data gathered between 1958 and 1962 prior to a large scale enhancement program for sockeye stocks. Zooplankton biomass had decreased up to 70% in some areas of the lake during 1973, but only 40% in 1974. Decreases in numbers were also evident. Although seasonal changes in fry diet followed changes in zooplankton species abundance, feeding was selective. The less abundant but larger forms, Daphnia and Heterocope together comprised 70% of the diet during summer, while Cyclops and Diaptomus formed the bulk (87%) of the diet in late fall. Significant decreases in Daphnia and Diaptomus abundance and increases in nauplii-early copepodite abundance had occurred by 1973. The increased 1974 zooplankton abundance relative to 1973 was attributed to decreased mid-summer fry numbers in the lake. Field data suggested low Diaptomus numbers contributed to much higher fry mortality (about double in 1974) compared to 1973. An experimental study of species selectivity by sockeye fry indicated that they selected Cyclops and Diaptomus adults. The larger copepods, Heterocope and Epischura, were rejected by fry encountering zooplankton for the first time. Copepodites and nauplii were rejected, but less so when preferred prey were scarce. Prey activity, in my experiments, could not be used to predict predation vulnerability and hence the species selectivity displayed by the fry. Light and temperature had little effect on Cyclops, Dlaptomus and Heterocope activity. / Science, Faculty of / Zoology, Department of / Graduate