Trends in nutrient and oxygen concentrations in the Skagerrak-Kattegat

The Skagerrak and Kattegat form a transition zone between the Baltic and the North Sea. Both areas are subject to increased nutrient loads. In this paper a non-parametric method is used to calculate the changes in nutrient and oxygen concentrations in the area. The period chosen was 1971 to 1990 and the parameters were dissolved inorganic nitrogen, phosphorus, silicate, total nitrogen and total phosphorus together with oxygen and oxygen saturation. Data have been sorted after salinity and the analyses have been carried out for different water masses and subareas. The results demonstrate that both surface and deep water in the Kattegat show increasing trends during winter for all nutrients except silicate. During summer there is an increase in total nitrogen and total phosphorus while silicate shows a decrease. In the Skagerrak the picture is more variable; in the eastern part, however, there is a clear increase for all inorganic nutrients during winter in the coastal water. Oxygen shows a declining trend in most areas.