Climate, hydrographic variations and marine benthic hypoxia in Koljِ Fjord, Sweden

Since the late 1970s, Scandinavian waters have been extensively investigated for human-induced marine pollution, especially marine eutrophication, oxygen deficiency in bottom waters and subsequent benthic mortality. The most serious oxygen deficiencies are noted in the sill fjords along the Swedish west coast, in southern Norway and in large areas of the southern Kattegat and Baltic Sea. One of these sill fjords, Koljِ Fjord, is located on the Swedish west coast. This fjord is characterised by frequently occurring episodes of hypoxia/anoxia which last for months or even years. Sediments are laminated and the fjord is generally regarded as seriously affected by human-induced eutrophication. We detail the environmental development of a well-documented fjord by combining high resolution sediment records with long hydrographical and meteorological instrumental data, and we present ultra high-resolution sediment information together with long-term instrumental records of air-temperatures, NAO indices and hydrography from Koljِ Fjord. These data show, in contrast to the current opinion focusing on anthropogenic eutrophication, that natural causes are the most important factors controlling the marine environment in this sparsely populated area. Natural variables concerned are fjord physiography, weather and hydrography (including the macro-nutrients DIN and PO4-P), sediment laminations and organic carbon. Interactions between fjord physiography, weather and hydrography regulate the possibility for water exchange and deep-water renewals. The present study points to the importance of natural causes for the environmental status of sill basins and semi-enclosed areas along the west coast of Sweden.