ABSTRACT
The deep basins have, during the last 100 yr, had irregular periods of stagnation causing hypoxia and therefore, temporarily, have been devoid of benthic macrofauna. However, since the early 1950s, the oxygen concentrations in the bottom waters have declined. A prolonged period of stagnation followed the great water inflow in 1951-52. Significant inflows has been observed later, but none of them has been large enough to raise the oxygen concentration to the same level as the inflow in 1951-52 (Falandysz et al. 2000). In addition, the increased eutrophication since the 1950s has, as a secondary effect, led to increased oxygen consumption above the sea bed. Laine et al. (1997) concluded that the oxygen consumption in the deep waters is higher today than at the beginning of the century. They also indicated that the oxygen conditions suitable for macrobenthic communities even after re-oxygenation of the deep waters will be less optimal during periods of stagnation. Consequently, areas with hypoxia and anoxia and deterioration of fauna have extended at depths below the pycnocline (semi-permanently at 70-80 m) since the middle of this century (Fig. 5). Above the pycnocline, on the other hand, biomass and production of benthic fauna have increased as an effect of the eutrophication (Cederwall & Elmgren 1980, 1990).
