Effects of fish farming on growth, epiphytes and nutrient content of Focus vesiculosus L. in theÅland archipelago, northern Baltic Sea

Abstract

The growth (increase in length), species composition and biomass of macroalgal epiphytes, and the contents of total nitrogen and phosphorus of transplanted and native specimens of Fucus vesiculosus L. (Phaeophyceae), were studied during different seasons outside a fish farm and at an unaffected reference locality in the archipelago ofÅland, southwest Finland. The most rapid growth (28 mm month −1) and the highest epiphyte biomass (643 mg g −1 Fucus dry weight) were recorded at a locality with native Focus plants close to the fish farm. Towards the farm, the species composition of epiphytes shifted in dominance from brown and red to green algae. The contents of nitrogen and phosphorus showed significant seasonal variations with low values in summer and high ones in winter. Significantly higher contents were generally measured in Fucus specimens transplanted close to the farm. In summer, the contents of nitrogen showed a slight increase with decreasing distance to the farm. The phosphorus content showed a corresponding increase in autumn and winter.