LONG-TERM CHANGES OF MACROZOOBENTHOS IN THE EASTERN GOTLAND BASIN ANDTHE GULF-OF-FINLAND (BALTIC SEA) IN RELATION TO THE HYDROGRAPHICAL REGIME

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 stud y 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 macrofauna. The major i nflows of North Sea water in 1975-1976 led to rapid benthic recolonisa tion down to approximately 150 m depth in the Eastern Gotland Basin, w here oxygen conditions had improved, but had no effect on the macrozoo benthos communities in the Gulf of Finland. The subsequent, prolonged, stagnation period in 1977-1993 caused a deterioration of the macrozoo benthos as a consequence of anoxia and formation of hydrogen sulphide in sub-halocline (70-250 m) areas. Later, a recovery was observed in t he 70-100 m depth zone when vertical stratification weakened and inten sified vertical mixing resulted in improved oxygen conditions at these intermediate depths. The effect of this process was most pronounced i n the Gulf of Finland. The 1993-1994 inflows led to the highest oxygen levels in the Gotland Deep since the 1930s, Consequently, in 1994 pol ychaetes 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 significa nce of the hydrographical regime, potential impact of eutrophication o n the oxygen balance of the deep waters, and the consistency of long-t erm data sets are discussed.