Crustacean Zooplankton Ingestion of Potentially Toxic Microcystis: In Situ Estimation Using mcyE Gene Gut Content Detection in a Large Temperate Eutrophic Lake

Grazing by zooplankton can regulate bloom-forming cyanobacteria but can also transfer toxin-producing cells, as well as toxic metabolites, to the food web. While laboratory investigations have provided extensive knowledge on zooplankton and toxic cyanobacteria interactions, information on zooplankton feeding on toxin-producing cyanobacteria in natural water bodies remains scarce. In this study, we quantified <i>Microcystis</i>-specific <i>mcyE</i> synthase genes from the gut contents of various cladoceran and copepod taxa to assess the in situ crustacean community and taxon-specific ingestion of potentially toxic <i>Microcystis</i> in Lake Peipsi, a large eutrophic lake in Estonia, Northern Europe. <i>Microcystis</i> cells with <i>mcyE</i> genes were found in all crustaceans examined. However, some species, such as the cyclopoid copepod <i>Mesocyclops leuckarti</i>, were more efficient in ingesting potentially toxic <i>Microcystis</i> than other co-occurring cladocerans (<i>Daphnia</i> spp., <i>Bosmina</i> spp., <i>Chydorus sphaericus</i>) and copepods (<i>Eudiaptomus gracilis</i>). The amount of toxigenic <i>Microcystis</i> cells grazed by crustacean population changed temporarily, and copepods were the predominant consumers of toxigenic <i>Microcystis</i> during several months of the 5-month study period. Crustacean ingestion of toxigenic <i>Microcystis</i> was not related to <i>Microcystis</i> biomass or <i>mcyE</i> gene copy numbers in the environment but was instead related to the abundance of major crustacean grazers. Our findings emphasize the close interaction between crustacean zooplankton and toxigenic <i>Microcystis</i>, indicating that some species may play a more significant role in linking toxic cells within the food web than others.