Effects of changing nutrient loads to the North Sea

The environmental effects of river nutrient loads to the North Sea have been investigated using a numerical biophysical model, NORWECOM, to perform different reduction scenarios. The simulations demonstrate that the river nutrients have a significant contribution on the annual primary production, both in the southern North Sea, in Skagerrak and along the Norwegian west coast. A 50% reduction in the loads of N and P reduces the primary production with 10–30% in the southern North Sea, and 5–10% in Skagerrak and along the Norwegian west coast. Scandinavian rivers only contribute to the 1–2% level in these reductions, thus, continental rivers have the major effect on the environment in all downstream areas. However, it should be noted that this reduction, even in the southern North Sea, is less than the natural variability of the production of phytoplankton. A reduction only in the P values shows that the production regime in the southern North Sea is phosphorous limited, while nitrogen is the limiting nutrient in the northern North Sea. Focusing on the N/P ratio as a possible proxy for eutrophication, a reduction in the N and P loads reduces this ratio by a similar factor, while a reduction in the P loads only, increases it. Based on this, it is proposed to use the N/P ratio for eutrophication assessment, and this is demonstrated.