Lake Peipsi sensu lato consists of limnologically and hydrochemically different parts, Lake Peipsi sensu stricto, Lake Lämmijärv, and Lake Pihkva. The eutrophic L. Peipsi s.s. and the hypertrophic Lake Pihkva were studied. The aim was to find out if the ecological state of these lake parts has improved, stabilized or deteriorated during a ten-year period, 2003–2012. For this purpose, data on loadings, in-lake nutrient concentrations, water transparency, water level, chlorophyll a concen­tration, as well as phyto- and zooplankton were compared for two five-year sub-periods (2003–2007 and 2008–2012). Comparison demonstrated a decline in the loading of total phosphorus (TP) from rivers on the Estonian and Russian sides as well as in its mean concentration in both lake parts. Both phytoplankton biomass and cyanobacterial biomass decreased in response to the reduced nutrient content in the lake water. The responses of zooplankton were contradictory. Changes in the occurrence of indicator species and declining mean zooplankter weight reflected a continuous eutrophication process while changes in the abundance of rotifers and the genus Daphnia indicated a subtle shift towards recovery. Our results show a modest improvement in the ecological condition of both lake parts.

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 Microcystis-specific mcyE 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 Microcystis in Lake Peipsi, a large eutrophic lake in Estonia, Northern Europe. Microcystis cells with mcyE genes were found in all crustaceans examined. However, some species, such as the cyclopoid copepod Mesocyclops leuckarti, were more efficient in ingesting potentially toxic Microcystis than other co-occurring cladocerans (Daphnia spp., Bosmina spp., Chydorus sphaericus) and copepods (Eudiaptomus gracilis). The amount of toxigenic Microcystis cells grazed by crustacean population changed temporarily, and copepods were the predominant consumers of toxigenic Microcystis during several months of the 5-month study period. Crustacean ingestion of toxigenic Microcystis was not related to Microcystis biomass or mcyE 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 Microcystis, indicating that some species may play a more significant role in linking toxic cells within the food web than others.