Effects of base addition and brook trout (Salvelinus fontinalis) introduction on the plankton community of an acidic Adirondack lake

Abstract

Calcite was added to Woods Lake in 1985 and 1987, and to its watershed in 1990, to restore and maintain water quality conditions suitable for reestablishing brook trout (Salvelinus fontinalis). Juvenile and adult brook trout were introduced following the first chemical treatment and were stocked annually thereafter from 1985 to 1989. In response to the combined chemical and biological manipulation, phytoplankton abundance increased whereas production per unit of biomass decreased. We attribute the increase in phytoplankton abundance and the reduction in biomass turnover rates to a decrease in zooplankton grazing. Zooplankton densities and estimated community grazing rates were lower during the posttreatment period despite a shift from small-bodied taxa (Diaptomus minutus, Bosmina longirostris) to a more diverse assemblage that included four species of daphnids. We attribute reductions in zooplankton abundance to planktivory of introduced brook trout. The changes we observed in phytoplankton and zooplankton species composition were consistent with trends observed during whole-lake acidification experiments, but functional responses in our experiment (decreased zooplankton abundance, increased phytoplankton abundance) were not. Our functional responses were consistent, however, with those observed previously in food webs manipulated to increase planktivore abundance.