Interannual variability of the North Sea primary production: comparison from two model studies

The North Sea is known to be a very productive area. Several models and in situ measurements have been used to quantify the primary production and its spatial variability, but large uncertainties still exist. Except for general statements about the level of production, very little is known about the interannual variability of it. In this paper two state of the art ecological models have been run with realistic forcing for 10 different years to investigate this interannual variability. The focus has been on differences due to changing wind fields, and differences due to changes in river discharges. Separated into ERSEM boxes, both spatial and temporal variability is discussed. The models suggest that the interannual variability in the North Sea primary production is around 15%, and that the variability locally is higher than this, thus an increase in one area is often compensated by a decrease somewhere else. The impact of the river nutrient inputs is estimated to be less than 10% of the total production. The modelled variation in primary production, can in both models be related to, and explained from, actual variability found in the modeled physics. The large variability in primary production found due to these changes in the underlying physical fields, suggests that a realistic three-dimensional circulation model is essential for this kind of modelling.