Herbivorous zooplankton affect phytoplankton community structure by size-selective grazing and by nutrient remineralization, which increments cellular nutrient quotas and enhances growth rates of those algae not eaten. Remineralization effects should be most pronounced when it involves growth-limiting nutrients and when herbivores are a principle source of those nutrients. The objectives of this study were to: 1) use herbivore gradient experiments to test for herbivore size-selective grazing in oligotrophic Lake Michigan during the summer, 2) test for herbivore density-dependent shifts in the nutrient stoichiometry of those algae not grazed, 3) correlate potential growth rates of surviving algae to the herbivore-mediated changes in their stoichiometry. Ultimately, these results have application for assessing the impact of invasive invertebrate predators (Bythotrephes, Cercopagis) that indirectly affect algal biomass and productivity in Lake Michigan. Experimental herbivore gradients (0X-6X ambient) were created with ambient phytoplankton and net-collected zooplankton, then incubated in a water-jacketed plankton wheel at ambient light and temperature in summer 2008. Growth rates were derived from changes in size-fractionated chlorophyll a concentrations. We are currently using HPLC pigment analyses to also explore responses of individual algal taxonomic groups.
Nutrient bioassay experiments indicated that only phosphorus elevated growth rates of algae passing through a 10 µm sieve. Algae in size ranges of 10-50 µm and >50 µm failed to respond to nutrient enrichment. All 8 experiments exhibited size-selective grazing, as evidenced by significant first order regressions of algal size-class growth rates against herbivore abundance. The size classes affected changed seasonally in response to a shift in herbivore dominance from copepods to cladocerans. The net community growth rate response of phytoplankton was correlated with the relative importance of the smallest algae. In some gradient experiments, growth rates of small algae declined while those of larger algae increased; this resulted in large changes in algal size frequency distributions along those gradients. In 7 of 8 experiments, there was a positive correlation between herbivore abundance and the cellular phosphorus content (measured as molar C:P) of the surviving algae. A secondary incubation of these phosphorus-enhanced survivor cells demonstrated a positive correlation between algal growth rates and the “first phase” herbivore abundance. A summary correlation of second incubation growth rates to C:P stoichiometry demonstrates pattern but contains very large variance because the stoichiometry measurement homogenized both “winners” and “losers” from the first incubation.