Hansen, J.L.S. & Bendtsen, J. 2006. NERI Technical Report No. 598. 50 pp.

 

Summary

The inner Danish marine waters are sensitive to the development of oxygen deficiency due to a permanent stratification which acts as a barrier for the mixing of atmospheric oxygen to the bottom water and the bottom. The bottom water originates from Skagerrak and its content of oxygen is an important factor controlling the total oxygen supply to the bottom water below the mixed layer in the inner Danish marine waters. The analysis in this report assumes an increase in temperature in Denmark of about 3 degrees Celsius within the coming 100 years. Based on this increase in temperature, results from a hydrodynamical 3-D model of the inner Danish Marine waters show a corresponding year round increase in sea temperature of about 3 degrees Celsius.

 

This increase in temperature has the effect that the bottom water is supplied with less oxygen and will furthermore lead to an increase in biological oxygen demand in the bottom water and in the bottom. Both processes will enhance the risk of oxygen deficiency events in the future. The size of the resulting effect is related to the future supply of organic material to the bottom water and the bottom. The size of this future supply is both related to the amounts of nutrients and the structure of the marine ecosystem. Though these changes are not readily predictable it is likely that temperature changes in this order will lead to structural changes of the marine ecosystem. In this analysis it is assumed that there are no qualitative changes of the ecosystem. Results from the model show that oxygen concentrations in the summer/fall will be reduced by 1-2 mg/l, i.e. 30% less than the present level. Predictions of the effect on the marine flora and fauna are associated with an uncertainty. Calculations show that areas with severe oxygen deficiency and "damages" on the benthic fauna will increase by approximately a factor of 3 compared to typical present day year.

 

In case of future extreme climatic events, as in 2002, the relative difference will increase and damages can be expected to be found in the major parts of the Belt Sea and southern Kattegat at depths below 12-15 m. It is expected that the damages on the benthic fauna will have negative consequences on the higher trophic levels in the food chain. Model calculations show that the present nutrient concentrations have to be reduced by 30% to compensate for the effects of the expected increase in temperature.

 

Full report in pdf-format (3,891 kB).