One reason why the world is green may be that many plants are chemically defended from herbivory. In green lakes, toxin-producing cyanobacteria have frequently been hypothesized to limit the ability of herbivorous zooplankton (such as Daphnia) to suppress total phytoplankton biomass. Toxic cyanobacteria may inhibit Daphnia feeding and population growth, and in extreme cases, cause outright Daphnia mortality. We tested the hypothesis that high levels of toxic cyanobacteria prevent Daphnia from grazing down the phytoplankton with an enclosure experiment in a shallow, nutrient-rich lake. The lake has extremely high densities of zooplanktivorous fish, no Daphnia, and high levels of toxic cyanobacteria. In contrast to most previous experiments of this kind, the phytoplankton in the enclosures was totally comprised of toxic cyanobacteria at the start of the experiment, with concentrations of the toxin, microcystin, above 3 µg L^-1. A large species of Daphnia (D. pulicaria) was collected from a nearby fishless lake, cultured in the laboratory for 3 weeks, and then stocked at low density (~1 L^-1) into 3 of the enclosures.
Results/Conclusions

Over the first two weeks of the experiment, toxin levels increased to ~5 µg L^-1 in the enclosures, yet the enclosed Daphnia populations were increasing. By the third week, Daphnia had suppressed phytoplankton biomass by ~66% relative to Daphnia-free controls.  Reduced phytoplankton biomass in the Daphnia treatment persisted until the end of the experiment (7 weeks). The ability of Daphnia to increase and suppress phytoplankton biomass, despite initially high levels of cyanobacteria and associated toxins, suggests that nutrient-rich lakes are green largely because they lack Daphnia (as a result of high fish densities), rather than because the phytoplankton are well defended against Daphnia grazing.