Long-term changes of macrozoobenthos in the Eastern Gotland Basin and the Gulf of Finland (Baltic Sea) in relation to the hydrographical regime

Abstract Changes in the macrofauna of the sub-halocline bottoms of the Eastern Gotland Basin and the Gulf of Finland during the 1965–1994 period have been studied in relation to the salinity and oxygen regimes. The study period covers several significant major inflows of North Sea water, as well as the longest stagnation period during the last hundred years, with semi-permanent anoxia affecting the distribution and community structure of the macrozoobenthos. At the beginning of the study period vast areas below the halocline were devoid of benthic macrofauna. The major inflows of North Sea water in 1975–1976 led to rapid benthic recolonisation down to approximately 150 m depth in the Eastern Gotland Basin, where oxygen conditions had improved, but had no effect on the macrozoobenthos communities in the Gulf of Finland. The subsequent, prolonged, stagnation period in 1977–1993 caused a deterioration of the macrozoobenthos as a consequence of anoxia and formation of hydrogen sulphide in sub-halocline (70–250 m) areas. Later, a recovery was observed in the 70–100 m depth zone when vertical stratification weakened and intensified vertical mixing resulted in improved oxygen conditions at these intermediate depths. The effect of this process was most pronounced in the Gulf of Finland. The 1993–1994 inflows led to the highest oxygen levels in the Gotland Deep since the 1930s. Consequently, in 1994 polychaetes were found at 243 m depth indicating a recolonisation of even the deepest part of the basin. The fluctuations in salinity have also affected the distribution of marine species. The biological significance of the hydrographical regime, potential impact of eutrophication on the oxygen balance of the deep waters, and the consistency of long-term data sets are discussed.