The dynamics of food chains upon changes in water salinity has been analyzed using the example of six poly- and hyperhaline lakes in the Crimea. The results show that ecosystems of saline lakes are...

Estuaries provide valuable ecosystem services but are threatened by eutrophication caused by increasing nutrient concentrations and climatic factors. We tested the hypothesis that cool and rainy weather in midsummer has a stronger stimulatory effect on indicators of eutrophication in the Neva Estuary than warm weather. The analysis showed that the concentration of total phosphorus in the estuary was positively correlated with the amount of precipitation and the number of rainy days in the region. Significant positive relationships were found between precipitation in midsummer and chlorophyll concentrations and plankton primary production in the upper part of the estuary. Chlorophyll concentrations and plankton primary production were higher in years with a rainy and cool July than in years with warm and dry July. Such conditions could negate existing measures to improve water quality in the Neva Estuary, meaning that greater effort will be needed.

Knowledge of carbon and nitrogen isotopic ratios in organic matter and their changes is important when studying nutrient cycles in aquatic ecosystems. Relationships between δ¹³C and δ¹⁵N values of suspended particulate organic matter (POM), water temperature, salinity, pH, redox potential, chlorophyll a concentration, primary production, and biomasses of different taxonomic groups of phytoplankton in the Neva Estuary were statistically analyzed. We tested the hypothesis that the studied physicochemical and biogeochemical characteristics, as well as the species composition of phytoplankton and its productivity, can be significant predictors of changes in the isotopic ratios of suspended particulate organic matter in estuaries. In the Neva Estuary, δ¹³C_POM (−16.8–−27.6‰) and δ¹⁵N_POM (2.3–7.3‰) changed synchronously. Statistical analysis showed that for both isotopes, the photosynthetic activity and taxonomic composition of phytoplankton are important. For ¹³C_POM, the second most important factor was water salinity, which was apparently associated with the transition of algae from CO₂ to HCO₃ consumption during photosynthesis in estuarine waters. For ¹⁵N_POM changes, the most important abiotic factor was pH. The study showed that the dependences of POM isotopic ratios on environmental variables obtained for continental and oceanic waters are also valid in transitional zones such as the Neva Estuary.