The effects of fish farm effluents on the water quality in the Åland archipelago, Baltic Sea

The main aim of this study was to increase our knowledge on how fish farm effluents influences the surface water quality in the Åland archipelago (Baltic Sea). A second aim was to conduct critical model tests of three models that predicts the impact of farm emitted phosphorus on total phosphorus (TP), chlorophyll and Secchi depth. Extensive field studies were conducted between 1997 and 1999 in nine defined coastal areas with, and four reference areas without, fish farms. According to our results the most significant effects on TP concentrations, periphytic growth and phytoplankton standing crop was observed in two coastal bays with small to moderate fish farm production (≈70 tonnes per year). The significant effects in each bay was mainly due to the small size of the bay (0.48 and 0.73 km²) and the shallow mean water depth (3 and 3.8 m), rather than long water retention times (2 and 6 days). Due to a very large fish farm production (800 tonnes per year), we also observed significant effects in a coastal area of moderate size (8 km²), which had a theoretical water retention time of about 8 days. No significant effects of fish farm effluents on the surface water quality were observed in the other six areas. Furthermore, our data shows that, besides the fish farm effluents, the import of nutrients from the surrounding sea and the sediment resuspension may have a decisive effect on the water quality in the studied areas. Finally, we can conclude that at least two of the tested models are useful in context of environmental management.