The Effects of Reduced Wastewater Load in the Marine Area off Turku in the Archipelago Sea During the Period 1965–2025

In Finland, municipal wastewater treatment has significantly improved in recent decades, leading to a substantial reduction in nutrient loads from wastewater discharged into water bodies. For example, in the marine area off Turku in the Archipelago Sea, located in the northern Baltic Sea, the total phosphorus load from wastewater has decreased to approximately one-eighth of its level in the early 1990s. Simultaneously, the total nitrogen load has been reduced to one-fifth, and the ammonium nitrogen load is now less than 5% of its peak in 1994. This study examines in detail how water quality parameters and phytoplankton indicators in wastewater-affected areas have changed during the same period in which wastewater loads have significantly decreased. This reduction has contributed to positive developments in the marine area off Turku, although the goal of achieving good ecological status remains unmet. In Raisio Bay, chlorophyll a concentrations decreased by 68% following the relocation of wastewater discharge. In Rauvola Bay, the reduction was estimated to be 36%. Over the past 15 years, the biomass of nitrogen-fixing cyanobacteria has increased in northern Airisto. This trend appears to be driven by a decrease in external nitrogen loading in combination with increased internal phosphorus loading. Water bodies in the inner archipelago continue to receive excessive nutrient inputs from the surrounding catchment area, while internal loading significantly delays the restoration process.