A mathematical model driven by meteorological forcing and sea level differences has been established in order to describe the water exchange and vertical mixing in the Kattegat and Samsø Belt.

The model has been run with a time-step of 1 day for the period April - October from 1961 to 1981. The mixing has been represented by equations from the theory of turbulence with the normally used coefficients, i.e. no calibration has been necessary. Results from earlier investigations have been used to describe the flows in the Belts and the Sound.

The vertical exchange between the upper, brackish and the lower, saline water is strongly dependent on the meteorology. A small meteorological activity results in small exchange flows which implies that the oxygen supply to the lower layer becomes smaller and unfavourable biological conditions may occur. A simple calculation has been carried out on the assumption that the chemical and biological oxygen demand is a first order process with a time scale for the oxygen decay, which is halfed for each 10° increase of temperature. Thus the model describes the effect of the meteorological activity on the oxygen concentration, all other parameters kept constant.