Filter feeding by the freshwater mussel, Diplodon chilensis, as a biocontrol of salmon farming eutrophication

The freshwater mussel Diplodon chilensis (Hyriidae) is abundant in bays where salmon farming takes place in southern Chilean lakes. In order to evaluate the possibility of mitigating salmon farming impacts through the management of benthic communities, we measured the ability of D. chilensis to filter algae and clear the water column of particulates and dissolved nutrients associated with salmon farms. A 3 month experiment growing juvenile salmon with and without mussels was conducted in outdoor tanks where dissolved nutrients and chlorophyll a were measured before, during and after addition of mussels. In addition, the filtering ability of mussels was measured in laboratory aquaria under different algal concentrations ranging from oligotrophic to hypereutrophic. Within 18 days, D. chilensis reduced chlorophyll a concentrations in tanks with fish by two orders of magnitude (from similar to 300 to 3 mu g l(-1)) compared to tanks without it. Concentrations of total phosphorous, PO4 and NH4 were also reduced by about one order of magnitude after 18 days, through to day 39 from the beginning of the experiment. Thus, mussels were able to change a hypereutrophic situation resulting from salmon culture re an oligotrophic one. Since the tanks were closed systems, the effect of mussels declined by day 61, probably due to the excessive accumulation of organic matter. In the aquarium experiments, D. chilensis showed a maximum cell retention rate (60 x 10(6) cells ind(-1) h(-1)) at chlorophyll concentrations between 20 and 30 mu g l(-1). Considering their high filtering rate (ca. 1.31 h(-1) ind(-1)) and high density in Chilean lakes, particularly in coastal areas and bays (50 to 200 ind m(-2)), mussels may exert a considerable filtering effect on lakes. In addition, mussels may play an important role in reducing nutrient loadings. (C) 1999 Elsevier Science B.V. All rights reserved.