NUTRIENT LOADING AND REDUCTION FROM SOURCE-LAKE TO SEA / Närsaltbelastning och -reduktion fran källsjö till hav
So far, the processes behind the eutrophication of coastal waters are poorly understood. As a consequence of this, efforts to counteract this phenomenon have failed. In this study, ecological valuations are used, in comparison with nutrient utilization, in different types of lakes and watercourses in parts of the drainage basin of the River Göta älv, from a source-lake to the river estuary and the sea. The abundance of nitrogen and phosphorus and especially their reciprocal proportions, the N/P-ratio that is necessary for primary production, constitute the important parameters utilized. A high N/P-ratio is initiated at higher altitude by atmospheric input of nitrogen. However, this nitrogen concentration is low when it is compared to that in the inflow from wastewater treatment plants. A high N/P-ratio is then reinforced in lakes that become more oligotrophic. Reduced primary production impoverishes the food chains, including fish production. Abundant nutrients from agriculture, close to an optimal N/P-ratio, are rapidly transformed to biomass, and cause oxygen deficiency. A similar process obviously happens when nitrogen-rich river water is mixed with phosphorus-rich coastal water. New ecological methods are required to transform nitrogen to biomass before it reaches the sea. Such methods have to allow re-use of the finite phosphorus resources.