PhD Thesis Defence by Lars Jonasson – Niels Bohr Institutet - Københavns Universitet

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PhD Thesis Defence by Lars Jonasson

Variability and Dynamics of Dissolved Oxygen in the Transition Zone of the North Sea and the Baltic Sea

The bottom water in the Transition Zone of the North Sea and the Baltic Sea suffers from seasonal hypoxia, usually during late summer and autumn. Though hypoxia occurs naturally in this region, the concentration of dissolved oxygen has decreased considerably during the second half of the last century. On top of this long-term trend, the interannual variations in hypoxic conditions are large. This thesis seeks to provide new knowledge about the major drivers for short- and long-term oxygen variations in the bottom water of the Kattegat and the Belt Seas.

A simplified oxygen consumption model in combination with a 3D circulation model was proven to be a very useful tool in these investigations. A thorough assessment proved that the model is able to reproduce the seasonal and interannual oxygen variations in this region. Studies with this couple model showed that the year-to-year variations in hypoxia can to a large extent be explained by the autumn wind conditions and inflows of oxygen-rich water from the Skagerrak to the Kattegat.

On decadal timescales, both physical and biogeochemical processes significantly impact the oxygen conditions. The Kattegat and the Belt Seas have experienced both large climatic and environmental changes during the past 4 decades. Nutrient concentrations and river loads have varied as well as bottom water ventilation and water temperatures. A method which combines observations and model results was developed to assess the relative importance of these changes in relation to the oxygen conditions.

The results suggest that increased eutrophication have lowered the oxygen concentration approximately 2 µmol l-1 per year between 1975 and 1990. After 1990, the biological oxygen consumption was reduced, concurrent with decreasing river loads. Furthermore, it was found that the estimated changes in oxygen consumption were significantly correlated to the concentration of inorganic nitrogen. However, large climatic changes have occurred concurrent with the reduced oxygen consumption. The bottom water temperature has increased almost 2 oC between mid 1990s and late 2000s. The warmer water might have lowered the oxygen concentration as much as 15-30 µmol l-1. Thus, the positive effects of reduced nutrient concentration on the hypoxic events have been overshadowed by climatic changes, especially the increased water temperatures.