Temporal and Spatial Large-Scale Effects of Eutrophication and Oxygen Deficiency on Benthic Fauna in Scandinavian and Baltic Waters – a Review

Abstract

Eutrophication has been an increasing ecological threat during the past 50 yr in many Scandinavian and Baltic marine waters. Large sedimentary areas are seasonally, or more or less permanently, affected by hypoxia and/or anoxia with devastating effects on the benthic macrofauna in, for example, the Baltic Sea, the Belt Seas and Oresund between Denmark and Sweden, the Kattegat and the Skagerrak coast towards the North Sea. In this review figures for the input of nitrogen and phosphorus to different sea areas are presented, and in several cases also changes of nitrogen and phosphorus concentrations in the water. The nutrient input is related to production levels, and related to macrobenthic infauna. Changes of dominant benthic species, abundance and biomass are presented in relation to both changes in organic enrichment and hypoxia and/or anoxia in time and space. Since the 1950s-60s, the benthic faunal biomass has increased in the Gulf of Bothnia as a result of increased organic enrichment. In the Aland Archipelago, the number of benthic species decreased since the 1970s but abundance and biomass increased. Drifting algae at the sediment surface has also been an increasing problem. The changes were caused by increasing eutrophication. In the Finnish Archipelago Sea, large-scale eutrophication has resulted in periodic bottom water hypoxia and drifting algal mats with negative effects on benthic fauna. In the Gulf of Finland, the benthic fauna has been negatively affected by hypoxic bottom water below 70 in depth since the 1960s, but with a period of improved oxygen conditions during 1987-94. In the Baltic Proper, large sea-bed areas of 70 000-100 000 km(2) below 70-80 in water depth have been more or less hypoxic and/or anoxic since the 1960s with no or reduced sediment-dwelling fauna. This process was a result of increased eutrophication and lack of larger inflows of oxygenated water from the Kattegat. Several coastal areas and larger basins in the southern Baltic (e.g. the Bornholm Basin, the Arkona Basin and the Kiel Bay), have, on occasions, been similarly negatively affected by hypoxic bottom water. Many sedimentary areas below similar to 17 in in the Danish Belt Seas have been affected by seasonal hypoxia since the 1970s with negative consequences for the bottom fauna. On the Danish Kattegat coast, the benthic fauna in the Limfjord, the Mariager fjord and the Roskilde fjord have been particularly negatively affected. In the southeast, open Kattegat, increased input of nutrients in combination with stratification have resulted in seasonal hypoxia since 1980 with negative effects on benthic animals and commercial fish species in most years. Several fjords on the Swedish and Norwegian Skagerrak coast have shown negative temporal trends in bottom water oxygen concentrations, and some of them lack benthic fauna in the deeper parts for several months or more. In this review the temporal development of bottom water hypoxia and/or anoxia is discussed and consequent possible losses of sediment-dwelling faunal biomass are roughly calculated. In total for the areas investigated, the worst years of hypoxia and/or anoxia combined may have reduced the benthic macrofaunal biomass by 3 million t. This loss is partly compensated by the biomass increase that has occurred in well-flushed organically enriched coastal areas. Tolerance of some Baltic species to hypoxia and/or anoxia is discussed and also their different strategies to cope with hypoxia and/or anoxia and H2S.