Effect of salmon cage aquaculture on the pelagic environment of temperate coastal waters: seasonal changes in nutrients and microbial community

The effects of salmon farm inputs on pelagic nutrient concentrations and planktonic microbial abundance and biomass were investigated in Loch Fyne, a temperate fjordic environment off the west coast of Scotland. The concentration of photosynthetic pigments and inorganic and organic nutrients, and the abundance and biomass of the autotrophic and heterotrophic microorganisms, were determined over a complete annual cycle from 3 depths (5, 15 and 25-30 m) at 4 stations located at differing proximities to the fish farm. Ammonium and dissolved organic nitrogen concentrations and heterotrophic microbial abundance and biomass were significantly higher at the stations nearest to the fish farm, suggesting that these and other nutrients derived from the fish farm may be directly or indirectly enhancing heterotrophic microbial activity. This in turn suggests that the heterotrophic microbial food web was responsible, at least in part, for processing matter and energy released into the pelagic environment from the salmon farm. By contrast, pigment concentrations, including chlorophyll a, tended to be similar at all stations, supporting the conclusions of previous studies that failed to establish a clear relationship between fish farm inputs and phytoplankton biomass. As such, the response of the heterotrophic microbial community is probably a more appro-priate indicator than chlorophyll concentration of local ecological effects of fish farms in temperate coastal waters.