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The effects of lake acidification on zooplankton community structure and phytoplankton-zooplankton interactions : an experimental approach

University of British Columbia

The effects of lake acidification on zooplankton communities and phytoplankton-zooplankton interactions were investigated by means of two field experiments in Eunice Lake, an oligotrophic, low alkalinity lake in the Coastal Range Mountains of British Columbia. Both experiments were carried out in situ using eight polyolefin enclosures, each holding 28,000 liters of lakewater and plankton. From July to October 1979, acidification with H₂SO₄ and enrichment with NH₄NO₃ and H₃PO₄, were applied to enclosures both as separate treatments and in combination. Acidification alone lowered the epilimnetic pH to 5.6, but did not affect zooplankton, phytoplankton, or transparency. Enrichment alone increased chlorophyll a concentrations, the biomass of edible algal cells and zooplankton biomass. When acidification and enrichment were combined, biotic processsing of NH₄NO₃ lowered the pH to 5.4, causing high mortality to the zooplankton community dominant, Daphnia rosea. The decline of Daphnia allowed chlorophyll a concentrations to increase 6-9 fold. It also led to major changes in the species composition, size structure and amplitude of biomass fluctuations of the zooplankton community. Phytoplankton appeared much more affected by acid-induced changes to zooplankton grazing than by direct abiotic effects of acidification. The 1979 experiment suggested that if lakes of pH 5.0 to 5.5 are enriched, the probability of nuisance algal blooms may be increased, particularly if the herbivorous community dominants are both large and acid-sensitive. In May 1980, unenriched enclosures were acidified over a ten day period to pH 5.5, 5.0 or 4.5, and then maintained at constant pH for seven weeks. Though the acid tolerance of D. rosea was very similar in both years' experiments, the copepod Diaptomus tyrrelli was more sensitive to acidification in 1980 than in 1979, due to differences in either life history stage, temperature or food. High rates of acidification increased toxicity near incipient lethal levels in both D. rosea and Bosmina lonqirostris. Acidification to pH 5.5, 5.0 and 4.5 decreased mean zooplankton biomass by 20%, 63% and 74%. However, both chlorophyll a concentrations and rotifer biomass increased with the level of acidification, due to apparent releases from grazing and competition (respectively). Both my 1979 enclosure experiment and whole-lake manipulations performed elsewhere suggested that acidic lakes might show increased fluctuation in zooplankton biomass over circumneutral lakes. Analyses of unpublished zooplankton data from Ontario acidic lakes support this suggestion. In general, the direction of zooplankton community change is determined by the intersection of acidification episodes with the spatial and temporal distributions of acid-sensitive species, and the competitive relationships within the community at the time of acidification. Both species distributions and competitive relationships are sensitive to seasonal changes in temperature and nutrients, to which zooplankton life histories have been finely tuned. Long-term acidification may create "holes" in the temporal organization of zooplankton communities, with no acid-tolerant species available with the appropriate life history and temperature response physiology to fill them.

Text

2010-04-21

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http://hdl.handle.net/2429/23976

Zoology

University of British Columbia

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