The imprint of anthropogenic activity versus natural variability in the fjords of the Kiel Bight : evidence from sediments

The fjords of Kiel Bight have been inhabited for centuries and thus exposed to long-term anthropogenic influence. The fjords have restricted water exchange as reflected in periodical hypoxia. Overall, the fjords have a high buffer capacity accumulating organic matter and metals in sediments. The sediments contribute to the supply of recycled nutrients for phytoplankton, but they also may be a source of toxic compounds for benthic organisms. The deoxygenation of sediments leads to a diminution of the benthic fauna and a breakdown of the food chain. Therefore, the sediments reflect the state of the local marine system and present a valuable tool for the assessment of the environment, at present time and for the past. The objective of this study was to determine whether anthropogenic activity over the last century has left the traces in the sediments of Kiel and Flensburg Fjords where sewage discharge, shipyards, and harbours are of major importance. Further questions addressed were how the anthropogenic impact superimposes over the natural variations in Kiel and Flensburg Fjords, and what the main factors controlling the accumulation of pollutants are. To investigate these questions, surface and core sediments were analysed to recover the spatial and temporal distribution and accumulation of organic matter and trace metals in the fjords. This study revealed that the type of sediments and depositional regime are the main controls of organic matter and trace metals distribution in the fjords. Secondary factors are the gradient of primary productivity, from the inner to the outer fjord, as well as oxygenation of near-bottom water and sediments. Anthropogenic influences appear to be less significant than these factors. The accumulation of organic matter and trace metals occurs in the inner areas of the fjords. The reasons for that is the depositional regime in the inner fjords where the most intense anthropogenic discharge occurs. Anthropogenic activity does not affect the outer fjords. The only source of exception is the influence of coastal protection measures on cliff erosion and the sediment supply in depositional areas. In general, the conditions in the fjords are mesothrophic to slightly eutrophic, except for the highly eutrophied innermost Flensburg Fjord. The remediation of the fjords after the cessation of enhanced nutrient input takes more than several decades. Anoxic conditions promote denitrification, which favours the loss of nitrogen, but the input of nutrients remains high because of redeposition and resuspension of sediments under the shallow water depth. High concentrations of metals, such as copper and zinc are associated with zones of sediment accumulation in the inner fjords. Only tin, derived from antifouling paints, exhibits extremely high concentrations and only in harbours. Concentrations of lead in the sediments had decreased during the last decades due to the ban on gasoline lead additives. The enrichment of metals and organic matter for the last 70 years in a core from the outer Kiel Fjord is not clearly apparent due to changes in sedimentation rate and the observed upward coarsening of the sediments. Elevated fluxes of trace metals fell during the 1930s, when the harbours and shipyards flourished in Kiel Fjord. An increase of sewage discharges in the 1940-1970s is recorded in the core as the accumulation of organic carbon and nitrogen. The downcore distribution of redox-sensitive metals showed that no drastic changes in the oxygenation of near bottom water and sediments have occurred in the outer Kiel Fjord, in contrast to the inner fjord. In spite of expanding anthropogenic activity, its influence is not so pronounced as originally expected and what influence is seen is veiled by natural variability. However, anthropogenic effects are significant for the input of nutrients and acceleration of the natural eutrophication which finally lead to seasonal near-bottom anoxia.