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The effect of herbivorous zooplankton on summer phytoplankton standing crops in Placid Lake, British Columbia Krause, Edith


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.

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