Long-Term Changes in the Primary Production of Phytoplankton in the Ecosystem of the Vistula Lagoon of the Baltic Sea

The long-term (2001–2020) data of the primary production and abundance of phytoplankton (according to the concentration of chlorophyll a) in the Vistula Lagoon under significant ecosystem changes have been analyzed. The long-term dynamics is characterized by an increase in primary production to a hypertrophic level (>450 g C/(m² year) in 2006–2010. After the intensive development of the bivalve filter-feeding mollusk Rangia cuneata, the primary production in 2011–2020 remained at the hypertrophic level, despite significant changes in the structure and abundance of phytoplankton. Average annual primary production in 2001–2020 was 60% higher than in 1974–1976. Primary production exceeds the mineralization of organic matter in water by 60%, which leads to the secondary eutrophication of the Vistula Lagoon and pollution of the Baltic Sea.