Grazing and growth rate of a cyclopoid copepod fed with a phytoplankton diet constituted by a filamentous cyanobacterium

Many tropical freshwaters experience persistent cyanobacterial blooms throughout the year. However, to date, most of the cyanobacteria–zooplankton interaction studies focused on temperate regions using large generalist grazers, which rarely coexist with cyanobacteria. Here a short-term (3 h) laboratory experiment was conducted to examine the grazing rate of Thermocyclops decipiens exposed to different combinations of phytoplankton (T1 = 100% Nitzschia-0% Cylindrospermopsis, T2 = 75% Nitzschia-25% Cylindrospermopsis, T3 = 50% Nitzschia-50% Cylindrospermopsis, T4 = 25% Nitzschia-75% Cylindrospermopsis and T5 = 0% Nitzschia-100% Cylindrospermopsis). In addition, an 8-day experiment was performed to determine the effect of the diet treatments on Thermocyclops survivorship and growth. The total biomass of phytoplankton in treatments with higher proportion of Nitzschia (i.e., T1 and T2) showed abrupt decline (i.e., down to 61% and 71%, respectively) over time when compared to other treatments. While Thermocyclops largely excluded Cylindrospermopsis when Nitzschia is abundant, they were capable of shortening the longer filaments of Cylindrospermopsis into significantly smaller fragments and ingesting them when provided as a sole food. This, however, negatively influenced their survivorship and growth. Our results generally suggest that dominantly occurring selective grazers in tropical and warmer-temperate lakes may shift the phytoplankton community composition to favor cyanobacteria by grazing on the alternative food source, at least before the cyanobacteria becomes dominant.