Seasonal dynamics of phytoplankton and zooplankton communities in the estuarine Elblag Bay (Vistula Lagoon, southern Baltic) dominated by floating-leaved plants

Lagoons and estuaries, as transitional ecosystems between freshwater and marine environments, are characterized by exceptionally strong fluctuations in abiotic and biotic factors. This study aimed to present the seasonal succession of phytoplankton and zooplankton and their interactions in highly variable, shallow estuarine Elblag Bay dominated by floating-leaved macrophytes. The ecosystem experiences temporary shifts between freshwater and brackish water due to the seasonal influx of freshwater from the river during winter and spring, and the substantial inflow of brackish water during summer and autumn. The seasonal changes of plankton in the bay were driven by several factors, among which the most important were water temperature, periodic brackish water inflow, and development of aquatic vegetation. After winter, phytoplankton began to develop first (March), followed by rotifers (April) and crustaceans (May). At the beginning of summer, zooplankton biomass decreased, which could favored the development of phytoplankton, however, the intense growth of algae was limited by shading by plant canopies and nutrient competition between macrophytes and phytoplankton. The development of vegetation was accompanied by an increase in the species richness and diversity of crustacean zooplankton with many plant-associated species. The results of our research indicate that the periodically changing habitat complexity (macrophytes, salinity) in the estuary bay limits the intense development of plankton but increases its diversity.