Environmental conditions in the coastal zone of the Curonian Lagoon in the absence of water hyperbloom

The Curonian Lagoon of the Baltic Sea is one of the most highly eutrophic water bodies (hypertrophic type) and is of great fishery and recreational importance. The coastal zone has the most important function in the ecosystem of the lagoon. Monthly studies in January–October 2022 showed the absence of algal “hyperblooming”. Based on the results of a detailed spatial study in August 2022 along the Curonian Spit in 4 sections, hydrobiological and hydrochemical characteristics of the “background” state of the coastal zone were obtained without the adverse effects of Cyanobacteria blooming. The oxygen content (80–160 % saturation) was favorable for hydrobionts, while the values of BOD5 and ammonium nitrogen indicated a slight accumulation and decomposition of organic matter (including algae biomass). The concentration of chlorophyll "a" was 15–43 μg/l, and abundance of saprophytic bacteria was 9–35 thousand CFU/ml. Increased values of phosphates and chlorophyll in the area of the set. Lesnoy, possibly indicate local anthropogenic pollution. In the coastal zone in 2022, the indicators of the ecological state corresponded mainly to eutrophic, α-mesosaprobity, low or moderately polluted waters in terms of the concentration of chlorophyll “a” and the abundance of saprophytic bacteria. The water quality was significantly higher than was observed in some years during the “hyperblooming” of the lagoon, when these indicators corresponded to the maximum hypertrophic and hypersaprobic levels that adversely affect the ecosystem (disease and death of aquatic organisms). Consequently, in the years of absence of summer algal “hyperblooming”, the coastal zone is characterized by favorable conditions for fishery and recreational use in the area of the Curonian Spit National Park.

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