COMMUNITY STRUCTURES OF SOFT-BOTTOM MACROFAUNA IN DIFFERENT PARTS OF THE BALTIC)

ABSTRACT The bottom fauna communities in the open Baltic are mainly influenced by the decrease in salinity from about 20% (above the bottom) in the south to about 4% at the heads of the northern gulfs, and by the poor oxygen conditions below the permanent halocline. In 1961 the Institute of Marine Research in Helsinki started a long-term study on the occurrence of benthic macrofauna in the seas around Finland. Some years later the study was widened to comprise the whole of the Baltic Sea inside the transect Trelleborg-Arkona. The study is unique in so far as it is the first attempt to cover the whole Baltic with the same methods and gear. In the present paper some of the results of this investigation, chiefly data on abundance, biomass and species composition, have been used to describe various types of benthic communities in the Baltic Sea in summer 1967. In the deepest part of the southern Baltic, the former mollusc-dominated community has been replaced by a polychaete community. In the Central Basin and the Gulf of Finland, an area totally devoid of macrofauna has developed since the early part of this century. Comparison of the results from summer 1967 with data from the beginning of this century does not reveal any drastic changes in the Gulf of Bothnia.