ADVECTION-INDUCED OXYGEN VARIABILITY IN THE NORTH SEA-BALTIC SEA TRANSITION

Instances of strong oxygen variations are described for two shallow wa ter stations in the Kattegat, situated at the fluctuating frontal zone between outflowing surface water from the Baltic and inflowing bottom water from the Skagerrak/North Sea. The events consist of both a rapi d emergence and a rapid disappearance of oxygen-depletion. Changes in oxygen concentration amounted to more than 20 g m-2 d-1 for the total water columns. Such high rates of change can not be explained by net l ocal bottom oxygen consumption (0.6 g M-2 d-1) or net local water oxyg en consumption (1.6 g M-2 d-1). The oxygen variations were influenced by the local and regional meteorological conditions. The observed inst ance of shallow water oxygen-depletion was connected to upward movemen t of the pycnocline and associated advective transport of oxygen-deple ted Kattegat bottom waters to a shallow water area. Similarly, rapid d isappearance of the bottom water oxygen deficit in a shallow water are a was found to depend more on pycnocline lowering in connection with a dvective transport, than on the effect of local wind driven mixing.