Deterioration of soft-bottom benthos along the Swedish Skagerrak coast

Oxygen deficiency in the near-bottom water associated with eutrophication has been shown to have negative impacts on marine benthos in many Scandinavian waters. In the present study, the benthic fauna and the sedimentary habitat have been studied between 7 and 34 m depth; at 14 stations in three Swedish fjords in four different years, and at 12 stations along the Swedish coast in three different years. Ignition loss of the top sediment indicated generally enriched conditions. The benthic fauna showed temporal declines in number of species, numerical abundance and biomass, particularly since the first samplings in 1976 at the fjord stations, and in 1987 at the coastal stations. Benthic macrofauna was lacking at some stations and very low numbers were recorded in several samples. Highest quantities were recorded at the deepest fjord and coastal stations. Multivariate analysis showed that the faunal composition changed with time. Analysis of sediment profile images classified the benthic habitat as disturbed with a thin vertical depth distribution of the apparent redox potential discontinuity. It is the first time reductions in benthic fauna have been shown to occur in relatively shallow waters over large coastal areas at the Swedish Skagerrak coast. The cause of the deterioration of the fauna is suggested to be low oxygen concentrations at the bottom in association with detached vegetation, leading to organic enrichment and locally even to anoxic sediment conditions.