Grazing rates of crustacean zooplankton communities on intact phytoplankton communities in Canadian Subarctic lakes and ponds

Zooplankton grazing can potentially affect the biomass and composition of phytoplankton communities directly and indirectly. Low chlorophyll a concentration for a given TP concentration and simplified fishless food webs lead to the expectation that zooplankton community grazing rates are high in Subarctic regions; however, zooplankton community grazing rates have not been determined for Subarctic lakes/ponds. We estimated zooplankton community grazing rates on phytoplankton in 12 lakes and ponds in Wapusk National Park, Canada using a microcosm grazing experiment. Lakes and ponds differed in zooplankton taxonomic composition, Chl-a concentration, and zooplankton biomass. We found that the grazing rates on the total chlorophyll a (GRTotal) ranged 0–13.7% grazed per day and the grazing rates on the edible (<30 μm, GR<30) chlorophyll a was 0 to 16.7% per day. GRTotal increased with lake Daphnia and cladoceran biomass, as did GR<30, which also had a negative relationship with the total in-lake Chl-a. The calculated zooplankton grazing rates were within the range found for larger, temperate lakes.