The aim of this study was to answer the following questions: (1) which environmental variables, biotic (fish predation) or abiotic factors, have a greater influence on the rate of zooplankton changes in lake outlet sections and (2) which plankters suffer the greatest reduction in the outlet section. Samples were collected in two locations at each of 18 lakes. The first site was right at the outflow; the second site was 0.2 km downstream from the outflow. At downstream sites the percentage contribution of Rotifera in zooplankton species number and abundance was higher than in the outflow, whilst the percentage contribution of Cladocera and Copepoda (except nauplii) in the zooplankton species number and abundance was lower at downstream sites than in outlets. The most important variables affecting the rate of zooplankton reduction in the lake outlets are the biomass of cyprinids and hydrological parameters along the outlet, such as discharge, current velocity, and depth. The most dramatic reductions involved daphnids, adult copepods, small cladocerans, and copepodites, while the reduction in the abundance of rotifers and nauplii was statistically insignificant.

Aquatic ecosystems are under pressure due to human activity. In the summer of 2022, the Odra River (Central Europe) suffered a massive death of fish and mollusks. This paper aims to show selected water quality parameters before and during the ecological disaster and find which parameters may have been crucial to the development of this disaster. We used the Kruskal–Wallis test and Spearman’s correlation to check the water parameters’ spatial and temporal diversification. In addition, non-metric multidimensional scaling was performed. The water quality parameters of the Odra system were analyzed in sections: middle Odra, lower Odra, transitional waters, and Szczecin Lagoon. Human activity has led to the formation of a system with unusual characteristics, disturbing the river’s natural continuum and related processes. The year 2022 in the middle Odra differed from the previous years in having a high water temperature (>24 °C), high ammonium concentrations (>4 mg/L), and relatively low nitrate concentrations (<6.5 mg/L). At the same time, salt pollution in the river was very high (maximum 1.4 g/L). In June and July 2022, we observed low chlorophyll a concentrations in the middle Odra (on average 2.3 µg/L and 4.4 µg/L, respectively), presumably due to salinity stress and high temperatures, suppressing freshwater phytoplankton taxa. This circumstance has created the appropriate conditions for developing euryhaline and thermophilic Prymnesium parvum. Because of decaying organisms, hypoxia occurred in the lower Odra (0.26 mg/L of dissolved oxygen in surface waters), and oxygen conditions worsened in the transitional waters (<4 mg/L). The zonal deaths of fish and mollusks result from multiple stressors induced by human activity. This disaster has proven that it is necessary to improve selected water quality parameters to reduce the risk of such disasters. The most urgent recommendations are to reduce the salt pollution of the upper section of the Odra, reduce nutrient inputs, and improve the condition of smaller rivers feeding the main course of the Odra.

The abundance and the biodiversity of summer zooplankton in the waters of the Drawa drainage (NW Poland) were studied, as was their relation to selected environmental conditions. The conditions upstream, especially in the outlets of lakes, did not affect the zooplankton communities downstream. This was also true of tributaries which had no influence on the shape of the zooplankton communities in the main river. The number of zooplankton in the outlets of eutrophic lakes was greater than in those of mesotrophic lakes. Increased vegetation cover significantly affected mainly the crustacean communities of zooplankton. Larger amounts of zooplankton were observed in rivers where the riparian zone was not covered with vegetation, but this difference was not significant. The hydrological conditions of the rivers and the Secchi depth visibility strongly impacted the composition of the zooplankton. The influence of abiotic factors was most pronounced on the abundance of cladocerans, and least pronounced on the abundance of rotifers.

Background: Lake depth and the consequent mixing regime and thermal structure have profound effects on ecosystem functioning, because depth strongly affects the availability of nutrients, light, and oxygen. All these conditions influence patterns of zooplankton diversity. Zooplankton are a key component of the aquatic environment and are essential to maintaining natural processes in freshwater ecosystems. However, zooplankton biodiversity can be different regard to depth, mixing type and trophic state. Therefore, the aim of this study was to examine how depth and mixing regime affect zooplankton diversity in lakes. We also investigated the vertical distribution of diversity across a trophic gradient of lakes.Methods: A total of 329 zooplankton samples from 79 temperate lakes (36 polymictic and 43 dimictic) were collected. The biodiversity of zooplankton was calculated using species richness (SR) and the Shannon index (SI). An index based on Secchi disc visibility was used to determine the trophic state index (TSI_SD) of lakes. The one-way ANOVA with Duncan’s post hoc test were used to determine differences in zooplankton biodiversity between mictic lake types and thermal layers. To find the best predictors for zooplankton biodiversity a multiple stepwise regression was used. The rarefaction method was used to evaluate the impact of mixing types, thermal layers, and the TSI_SDon zooplankton biodiversity indices. A Sørensen similarity analysis and nonmetric multidimensional scaling (NMDS) were performed to describe the similarity patterns in species composition among lakes.Results: We identified a total of 151 taxa from 36 polymictic and 43 dimictic lakes. Lake depth and the TSI_SDwere significantly correlated with the biodiversity of lake zooplankton. The results of ANOVA and Duncan tests show that mictic type and thermal zones had a significant effect on zooplankton biodiversity. The rarefaction curve showed significant differences in zooplankton biodiversity, which was greater in lakes with lower trophic state. Ordination by NMDS showed clustering of different mictic types, thermal layers, and composition changes throughout the TSI_SDprofile. Moreover, we determined that polymictic lakes are more heterogeneous than dimictic lakes in regard to zooplankton similarities.Discussion: Both mictic lake types were characterized by varying levels of zooplankton biodiversity, which is shaped by the communities’ response to lake depth, thermal layers and TSI_SDvalues. The zooplankton SR and SI (during daylight hours) depends greatly on the mixing type. Lake type also indicates the importance of the metalimnion in shaping zooplankton biodiversity in dimictic lakes. In addition, data from NW Polish lakes indicated that the increase of the TSI_SDleads to taxonomic shifts and has a negative effect on the diversity of all groups of zooplankton.