IMPROVING WATER-QUALITY IN A EUTROPHIED FJORD SYSTEM WITH MUSSEL FARMING

The flow of dissolved inorganic nitrogen (DIN) in an eutrophied fjord system on the Swedish west coast has been studied. The aim was to iden tify possibilities for improving water quality through mussel cultivat ion. It is suggested that off-bottom mussel farms should be introduced in the fjords as extra filters, to convert passing particulate-bound nutrients into mussel meat and into mussel faeces that will form sedim ent under the farms. Nutrients will be removed from the fjords when th e mussels are harvested, and the enriched sediments under the farms ca n be dredged up. Mussels are produced commercially in Sweden and the n utrient removing process could become more effective and economically viable than the methods used today. Using an empirical model it was sh own that mussel farms covering 1%-2.4% of the fjord surface water DIN concentrations in the water flowing out of the fjords will be lowered by 20%, i.e. almost to the same level as DIN levels in the Skagerrak. The biological oxygen demand (BOD) in the deep basin waters, below sil l depth, could be lowered by 26%. Decreased BOD could imply improved s urvival opportunities for higher life in the deep basins if oxygen con centrations are increased. It is also shown that if mussel farms are l ocated so that faeces from the farms accumulate in the deep basins for more than a decade, the net effect of the farms will be increased BOD in the basin water. In addition to this type of extreme situation, th e impact on the ecosystem due to mussel cultivation appears to be posi tive even when mussel sediments are not dredged up.