Hydrochemical conditions and trophic status of the freshwater Curonian Lagoon

The Curonian Lagoon is the largest lagoon in Europe with freshwater conditions. For a long period, a large amount of nutrients was supplied with the flow of the Neman River, which led to significant eutrophication of the lagoon. The consequence of this was a summer peak increase in phosphorus phosphates, associated with the intensive development of cyanobacteria and water “hyperblooming”. Comparison of data obtained in the southern part of the lagoon in 2019-2024 with an earlier period of water “blooming” (2007-2016) showed a 2-3-fold decrease in summer concentrations of phosphate phosphorus and a change in the N:P ratio (from <16 to ~30-50). As a result, the intensive development of cyanobacteria (water “hyperblooming”) ceased. During the period of water “hyperblooming” (2007-2016), chlorophyll a concentrations during the growing season and in summer exceeded 100 µg/l and characterized the Curonian Lagoon as a hypertrophic water body. In the modern period (2019-2024), chlorophyll a concentrations have decreased by 3-4 times (~40 µg/l). The decrease in chlorophyll and phosphorus concentrations in recent years can be characterized as significant de-eutrophication of the Curonian Lagoon.