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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. [ABSTRACT FROM AUTHOR]

Lake Mustijärv was reconstructed by sediment removal following an almost complete siltation. Here we evaluate challenges and opportunities for the management of the lake. We focused on the stream discharging to the lake (i.e., external loading), sediment retention in accumulation basins (i.e., internal processes) as well as the ecosystem-level response to stressors based on biological variables (phyto- and zooplankton, macrophytes, fishes and zoobenthos). Urban and agricultural inputs elevated ammonium and total phosphorus concentrations in the lake, challenging lake reconstruction efforts. Sediment transport highly increased the risk of faster filling of the lake, associated with upstream streambed excavation. Sediments trapped at the accumulation basins release nutrients that enhance eutrophication. We, however, observed a rapid recovery in fish, macrophytes, and zoobenthos, despite the significant disturbances. Lake Mustijärv is in eutrophic condition, reflected by phytoplankton (Pseudanabaena, Closteriopsis dominance) and zooplankton (Keratella, Polyarthra dominance) composition. To improve lake water quality will require controlling external nutrient inputs, underlining the importance of better coordinated activities between the local (lake restoration) and regional (catchment use) scales. [ABSTRACT FROM AUTHOR]

In recent decades, a marked decrease in planktivorous fish (Osmerus eperlanus eperlanus m. spirinchus Pallas and Coregonus albula (L.)) in Lake Peipsi has stimulated research into the potential effect of fish predation on zooplankton. The abundance of planktivorous fish and the abundance and biomass of zooplankton were studied from 1986 to 2014, and the diet of plankton-eating fish was analysed from 2007 to 2013. A linear regression model was used to establish possible trends in the zooplankton assemblages (biomass, abundance, mean weight of individuals) and in the abundance of zooplankton-eating fish. The ANOVA test was used to evaluate differences in the zooplankton assemblages and in fish consumption in the years with large and small fish cohorts. Despite the collapse of the smelt and vendace populations, the abundance of plankton-eating fish remained high due to an increase in the abundance of juvenile fish. Fish juveniles consumed as much as or even more zooplankton than adult planktivores. Since 1986, a significant decrease was observed in the cladoceran and copepod biomass and in the mean body weight of cladocerans. Large cladocerans such as Leptodora kindtii (Focke) and Bythotrephes longimanus Leydig were rare in zooplankton samples and the biomass of Bosmina spp. decreased. Changes in the structure of the zooplankton community were most likely caused by the feeding of juvenile fish as the calculated consumption by fish was high, especially in years with particularly large fish cohorts. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The material of the present study was collected from the pelagial of L. Peipsi (Estonia/Russia) mainly during the vegetation periods of 1997.2008. The zooplankton : phytoplankton biomass ratio (BZp/BPhyt) was followed in different lake parts: the northern eutrophic L. Peipsi s.s. and the southern hypertrophic L. Pihkva (together with L. Lämmijärv connecting the other parts). During 1997.2008 the average values of BZp/BPhyt for the growing season were lower than 0.5 (0.27 and 0.16 for L. Peipsi s.s. and L. Lämmijärv, respectively). The average monthly values of BZp/BPhyt fluctuated in a wide range: from 0.06 to 0.62 for L. Peipsi s.s. and from 0.06 to 0.29 for L. Lämmijärv in May-October. For L. Pihkva, this value was 0.06 in August. Maximum values (up to 2) occurred for L. Peipsi s.s. in June and quite a high value (up to 1) was also found in May. In parallel with the increasing trophic state of the lake, the mean values of BZp/BPhyt for the growing season decreased from 0.34 in 1997 to 0.18 in 2008 in L. Peipsi s.s., and from 0.24 to 0.10 in L. Lämmijärv. The widest range of fluctuations in BZp/BPhyt was found in May and August. For May this is explicable with differences in the melting time of the ice cover and therefore, in the timing of the spring peak of phytoplankton in different years. In August the absolute values of phytoplankton (water bloom caused by cyanobacteria) varied by years and depended mainly on weather conditions. The ratio demonstrated lowest variability in autumn months. This ratio revealed significant differences in the content of nutrients and characteristic features in plankton between the lake parts with different levels of trophy (P-level < 0.0001). We consider mean values of the ratio BZp/BPhyt in a growing season to be quite a reliable indicator for assessing the quality of the ecosystem and water of a large lake. [ABSTRACT FROM AUTHOR]