Recent changes in climate and eutrophication have caused increases in oxygen depletion in both freshwater and marine ecosystems. However, the impact of oxygen stress on zooplankton, which is the major trophic link between primary producers and fish, remains largely unknown in lakes. Therefore, we studied 41 lakes with different trophic and oxygen conditions to assess the role of oxygen stress on zooplankton communities and carbon transfer between phytoplankton and zooplankton. Samples were collected from each lake at the peak of summer stratification from three depth layers (the epilimnion, metalimnion, and hypolimnion). Our results revealed that freshwater zooplankton were relatively tolerant to anoxic conditions and the greatest changes in community structure were found in lakes with the highest oxygen deficits. This caused a switch in dominance from large to small species and reduced the zooplankton biomass in lower, anoxic layers of water, but not in the upper layers of water where the oxygen deficits began. This upper anoxic layer could thus be a very important refuge for zooplankton to avoid predation during the day. However, the reduction of zooplankton in the lower water layers was the main factor that reduced the effectiveness of carbon transfer between the phytoplankton and zooplankton.

This study presents the diversity and structure of pelagic zooplankton in north-eastern Poland. The research was conducted in 47 lakes with different trophic conditions in the middle of summer. Samples were collected close to the deepest part of the lakes to avoid the diverse benthic and littoral zones. We found 119 zooplankton species of which 32 were Cladocera, 16 were Cyclopoida, 4 were Calanoida, and 67 were Rotifera. We determined which species occurred most frequently in the region, as well as the species that were characteristic of different trophic conditions. We also recorded the presence of eight cold-adapted species which some of them are considered as glacial relicts (e.g., Eurytemora lacustris, Heterocope appendiculata, Cyclops lacustris). Our research revealed potential glacial refugia for planktonic species in 14 lakes of NE Poland. Our study suggests that the presence of stenotherm species may be an excellent indicator of the ecological status of deep lakes and could be considered in lake monitoring programs. Furthermore, we did not find Bythotrephes longimanus which has been reported from Poland. Instead, we found that B. brevimanus was the most common representative of the genus in the study area.

Shallow brackish water habitats are characterized by highly unstable environmental conditions, which result in low species diversity. We performed our research in the Vistula Lagoon in three seasons in the years 2019–2021. This lagoon is characterized by extremely variable environmental conditions, and our research covered the period of hydro-technical works before opening the canal through the Vistula Spit, which could be an additional stress factor. The aim of our study was to present the particularities of zooplankton communities (Rotifera; Crustacea) in the shallow coastal ecosystem. Our results pointed out very low diversity of zooplankton which could be caused by multi-stress conditions related to increased turbidity and low salinity. Nevertheless, under these conditions, some species achieved an advantage and strongly dominated the zooplankton communities. Domination of Eurytemora affinis in the Vistula Lagoon as well as in other brackish habitats is a well-known phenomenon. Moreover, the Vistula Lagoon offered favorable conditions for the intense development of non-indigenous, or alien, species like Diaphanosoma mongolianum, D. orghidani, and Acanthocyclops americanus. Therefore, brackish waters seem to be favorable transitional areas for non-native fauna that may subsequently spread into adjacent freshwater and marine systems.