Strength of Zooplankton-Phytoplankton Coupling in Relation to Lake Trophic State

Abstract

We examine the interactions of zooplankton and phytoplankton in lakes representing a broad range of environments and trophic states. Comparisons focus on the grazing impact and the regeneration rate of the most limiting nutrient during summer stratification in several large lakes: Lake Michigan (USA), Lake Titicaca (Peru), and Lake Tahoe (USA). These lakes range over an order of magnitude in plankton biomass and productivity, and in all of them copepods are the dominant crustaceans. In Lake Michigan (2.2 mg m−3 chl a biomass and 2.86 mg C-m−3· h−1 primary productivity), zooplankton have a very significant grazing impact on phytoplankton, and they can supply the majority of phytoplankton phosphate demand through regeneration. Daily phosphate demand is much greater than the available ambient concentration, so turnover of the available phosphate pool is quite rapid. Lake Titicaca has similar phytoplankton biomass (0.95 mg m−3 chl. α) and productivity (3.5 mg C-m−3·h−1), yet zooplankton concentrations are lower. Thus, zooplankton have less impact, though size-selective grazing may be significant, and nutrient regeneration may reach 20 percent of phytoplankton N demand. Plankton biomass (0.23 mg m−3 chl. a) and productivity (0.25 mg C-m−3· h−1) are much lower in Lake Tahoe than the other two lakes, and the zooplankton-phytoplankton coupling is much weaker. Grazing by zooplankton is two orders of magnitude lower than phytoplankton growth, and zooplankton regeneration appears to be negligible in relation to other fluxes and to the available concentrations of limiting nitrogen.These examples indicate that there is substantial variability in strength of zooplankton-phytoplankton interactions. For these three large lakes the strength of coupling increases with increasing trophic state. Data from these lakes in combination with smaller and more eutrophic lakes demonstrate that nutrient regeneration and grazing by crustaceans peak at intermediate trophic levels. Thus, zooplankton-phytoplankton coupling is strongest in meso-oligotrophic to mesoeutrophic lakes, and less important in both oligotrophic and very eutrophic lakes.KeywordsTrophic StateEutrophic LakeZooplankton CommunityLarge LakeNutrient RegenerationThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.