Does predation by planktonic organisms influence the size structure of phytoplanktonic algae in a black water lake in the Amazon?
Phytoplanktonic organisms may be categorized as pico, nano and microplankton, and zooplanktonic organisms as micro, meso and macroplankton. Because they are different sizes, zooplanktonic organisms can feed on varying sizes of phytoplankton. The study objective was to test whether microcrustaceans and planktonic rotifers consumed pico-, nano- and microphyoplankton non-selectively during low- and high-water periods in Lake Tupé, Amazonian Brazil. An experiment was carried out across 24 hours in the low- and high-water periods, with zooplankton and phytoplankton samples collected from the lake with a PVC tube 4 m in length. Zooplankton were counted and measured, while the phytoplankton sample was divided into pico-, nano- and microphytoplankton and the biomass of each fraction measured. During low water, total initial biomass was 1.92 μg/L and, by fraction, contained pico 0.82, nano 0.55 and microphytoplankton 0.55 μg/L. Total biomass was1.09 μg/L, corresponding to pico- 0.55, nano- 0.27 and microphytoplankton 0.27 μg/L. During high water, total initial biomass was 2,91µg/L and by fraction, contained pico- equal to zero, nano- 0.54µg/L and micro- 2.37 µg/L. Total biomass was 0.81µg/L corresponding only to picophytoplankton. The highest density of organisms occurred in the low-water sample. We conclude that predation pressure from zooplankton does not influence phytoplankton size structure in the studied environment, since it impacts the different size classes equally.