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.

Macaronesia, with the exception of the Azores, is one of the few Palearctic provinces where basic taxonomic information on the freshwater copepods is still lacking. We redescribed Eucyclops azorensis, a cyclopid crustacean so far known only in the Azores, and report the occurrence of this species in Algeria and Madeira Island. Eucyclops azorensis was formerly considered to be a subspecies of E. agiloides (East Africa); therefore, the latter species is redescribed here as well based on type and non-type material. Morphological comparisons between E. azorensis, E. agiloides and other taxa (E. serrulatus and E. roseus), assumed to be closely related to our target species, support a closer relationship between E. azorensis and E. serrulatus (Palearctic) than between E. azorensis and E. agiloides (Afrotropical). The slight differences between E. azorensis and E. serrulatus in the surface ornamentation of the antennal coxobasis and intercoxal sclerites of legs 1 and 4 suggest a relatively young separation of these lineages. Eucyclops agiloides is morphologically close to E. roseus (temperate and subtropical Asia, southeastern Europe, East Africa). The numerous and clear-cut differences indicate a species rather than subspecies-level differentiation between E. agiloides and E. roseus, unlike what was formerly proposed in the taxonomic literature.

Thermal tolerance is a critical factor influencing the survival of living organisms. This study focuses on the thermal resistance of copepod species, Thermocyclops crassus (Fischer, 1853) and T. oithonoides (Sars G.O., 1863), with overlapping distribution ranges in Europe. Short-term heat shock experiments were conducted to assess the thermal resistance of these copepods, considering various temperature increments and exposure durations. Additionally, the study explored the influence of heat shock on egg sac shedding, a vital indicator of population dynamics. Results indicate that widely distributed T. crassus exhibits higher thermal tolerance compared to narrowly distributed T. oithonoides, with survival rates varying under different heat shock conditions. Furthermore, T. crassus demonstrated a quicker response in dropping egg sacs in response to thermal stress, suggesting a potential adaptive mechanism for the survival of adults. However, rapid egg sac droppings pose high risks for eggs facing unfavorable conditions. T. crassus, inhabiting environments with greater temperature fluctuations such as the littoral and pelagial zones, exhibited better survival mechanisms compared to T. oithonoides, which predominantly resides in the pelagic zone. The findings have implications for understanding copepod responses to global warming and thermal pollution. This research contributes insights into the adaptive strategies of thermophilic copepod species and their ecological consequences.

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.