Temporal and spatial changes in distribution and abundance of macro-zooplankton in a large British Columbia lake

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 ofZoology, Department ofGraduat