Prevention of growth of potentially dense phytoplankton populations by zooplankton grazing, in the presence of zooplanktivorous fish, in a shallow wetland ecosystem

In two linked, shallow, freshwater basins, phytoplankton densities in summer were very different. Hudsons Bay supported a large stand of water lilies, and the adjacent open water was clear with chlorophyll a concentrations generally <10 µ g·liter ‒1 . In contrast, Hoveton Great Broad, which received inflow water from the same very fertile source, the River Bure, had chlorophyll a concentrations > 100 µ g·liter ‒1 . Hudsons Bay water, in bioassays, could support great phytoplankton growth in summer and did so in spring and fall. The period of clear water coincided with the presence of the lily stand. Zooplankton populations were of rotifers and small‐bodied Cladocera in Hoveton Great Broad, but mostly of Cladocera, including large‐bodied individuals of plant‐associated species, in Hudsons Bay. Zooplanktivorous fish were present and there was evidence of feeding by them in both basins. Coexistence with fish of the large, apparently efficiently grazing Cladocera in Hudsons Bay depended on provision of daytime refuges for the Cladocera among the lilies, and grazing was greatest in the adjacent open water at night. Grazing control was helped by a hydrological regime which favored small and rapidly growing phytoplankters, rather than inedible colonial forms with slow growth. Weed‐associated grazers may be important in maintaining the dominance of aquatic plants in shallow lakes which would otherwise more rapidly become dominated by phytoplankton as nutrient loading increased.