BOTTOM-UP AND TOP-DOWN IMPACTS ON FRESHWATER PELAGIC COMMUNITY STRUCTURE'

For freshwater pelagic ecosystems, the biodiversity and cascading trophic interaction theories both predict that decreased piscivore populations will result in direct, short—term (a few years) increases in planktivore biomass, reductions in crustacean herbivore biomass, and increases in chlorophyll a concentration and phytoplankton biomass. An Alternate view is offered by the bottom—up:top—down theory, which predicts that in eutrophic lakes changes in piscivore biomasses will have strong impacts on planktivore numbers, weaker but observable impacts on zooplankton biomass, and little or no long—term effects on phytoplankton biomass. A partial winterkill at Lake St. George, Ontario, Canada allowed us to test these predictions. The data set comprised measures of: (1) piscivore and planktivore numbers, (2) zooplankton species composition, size structure, and biomass, (3) chlorophyll a concentration and Secchi depth, and (4) water chemistry from 1980 through 1986. Prior to the winterkill of 1981—1982, the piscivore population was high (1000—2000 piscivores/ha), the planktivore population was intermediate (8000—10 000 planktivores/ha), zooplankton biomass was intermediate (2400 µg/L), and chlorophyll a concentration was high (5—12 µg/L). In the year following the winterkill (1982), piscivore and planktivore numbers were low, and zooplankton biomass and chlorophyll a concentration were high. During the next 2 yr (1983—1984) the planktivore population increased rapidly to densities >20 000 individuals/ha, zooplankton biomass density decreased to <1600 µg/L and chlorophyll a concentration decreased. During the final 2 yr of the study, piscivores recruited to near prewinterkill levels, planktivores were reduced to <8000 individuals/ha, zooplankton biomass increased, and chlorophyll a concentration decreased. Over the 7 yr data set, we found a strong negative correlation between numbers of piscivores and planktivores, a weaker correlation between numbers of planktivores and zooplankton biomass, and no between—year correlation between zooplankton biomass and chlorophyll a concentration. There was, however, a positive correlation between total epilimnetic phosphorus and chlorophyll a concentration. These data are consistent with predictions made by the bottom—up:top—down model, and the implication is that at Lake St. George, the trophic cascade uncouples at the zooplankton → phytoplankton link. We speculate that this may be due to the combined effects of lake trophy and Daphnia species composition and size.

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