Recovery of the aquatic environment following the termination of fish farms [Andersö and Järsö

A study concerning the recovery of the water environment at two former fish farm sites, Andersö and Järsö, in the NW Aland archipelago, was performed in the summer 2003. The environmental effects of the two fish farms have been monitored during their active years (Andersö 1986-2002, Järsö 1981-2003),and now the recovery potential of the affected recipient areas were investigated. Diamond Fish Ab, Andersö is situated in the middle archipelago, in a narrow sound with a dominating north-south current. Alands Forell Kb, Järsö is located in a bay, where a threshold limits water exchange with the outer archipelago. The measured parameters in this study were hydrography, periphyton, zoobenthos and sediment. The warm weather in the summer 2003 resulted in high water temperatures. A stratification of the water column was observed, and it resulted in lower water temperatures in the bottom water especially at deeper stations. The safinity and secchi depth was higher in the Järsö- than in the Andersö-area and the oxygen content in the bottom water was genera/ly high throughout the summer. Measurements of nutrients in the water column showed medium high to high values of total phosphorus and nitrogen values in the two recipient areas. One deep station in the inner archipelago in the Andersö area, and two stations directly affected by the former fish farm in the Järsö area diverged from this pattern throughout the summer. At these stations very high nutrient values were obtained, and in the Järsö-area the content of oxygen in the bottom water was low. The content of chl a in the fotic zone was higher in the Andersö-area than in Järsö. Diverging values were obtained in the end of July, when an algal bloom in the Järsö area caused very high chl a -va/ues. The resu/ts from the periphyton incubation indicated a higher growth rate in the Järsö area. Effects of the fish farms could clearly bee seen in the organic, carbon and nitrogen contents of the sediment, which were higher at stations close to the recipients. The trend in the zoobenthos community is similar in the two recipient areas. At most stations, the abundance and biomass of the bottom fauna has diminished, although the diversity remained at the same level as last year. New opportunistic species have been found at the most affected sites, and the age distribution of Macoma balthica indicates that the recruitment has been successful in later years