Long-term variability of the trophic status of the Curonian and Vistula Lagoons of the Baltic Sea

Based on an analysis of long-term (1991–2007) hydrological and hydrochemical data, as well as data on phytoplankton primary production, the ecological state of the Curonian and Vistula Lagoons of the Baltic Sea is assessed. The main abiotic factors influencing the trophic status of the lagoons are indicated. Water temperature is the key factor determining phytoplankton production and abundance in the Curonian Lagoon. An intensive warming-up of the lagoon water in the 1990s and 2000s (a possible result of climate warming) combined with freshwater and a slow-flow velocity created the conditions for the mass development of bluegreen algae. Despite a significant reduction in the external nutrients load, the occurrences of water hyperblooms” become more frequent, which points to a continuation of the eutrophication processes in the lagoon. The biological productivity of the Vistula Lagoon is below the potentially possible level because hydrodynamic activity and brackish water prevent the intensive development of blue-green algae. The increase in water temperature in the 1990s and 2000s had no significant effect on the ecological state of the lagoon.