Phytoplankton response to winter warming modified by large-bodied zooplankton: an experimental microcosm study

While several field investigations have demonstrated significant effects of cool season (winter or spring) warming on phytoplankton development, the role played by large-bodied zooplankton grazers for the responses of phytoplankton to winter warming is ambiguous. We conducted an outdoor experiment to compare the effect of winter warming (heating by 3°C) in combination with presence and absence of <em>Daphnia</em> grazing (<em>D. similis</em>) on phytoplankton standing crops and community structure under eutrophic conditions. When <em>Daphnia</em> were absent, warming was associated with significant increases in phytoplankton biomass and cyanobacterial dominance. In contrast, when <em>Daphnia</em> were present, warming effects on phytoplankton dynamics were offset by warming-enhanced grazing, resulting in no significant change in biomass or taxonomic dominance. These results emphasize that large-bodied zooplankton like <em>Daphnia</em> spp. may play an important role in modulating the interactions between climate warming and phytoplankton dynamics in nutrient rich lake ecosystems.