THE SIGNIFICANCE OF NITROGEN RUNOFF FOR EUTROPHICATION OF FRESH-WATERAND MARINE RECIPIENTS

This article explores the effects of nitrogen loadings to freshwater a nd marine recipients in two large watersheds of southern Norway. Chang es in nitrogen retention over larger areas would severely affect nitro gen concentrations in freshwaters, and subsequently increase the N loa d to marine recipients. Increased atmospheric deposition, changed hydr ology, reduced nitrogen uptake by vegetation due to nitrogen ''saturat ion'' and/or root damage caused by acidification, could contribute to this leakage. Climatic fluctuations strongly affect the seasonal runof f patterns, and a more long-term climate change could mobilize the hug e stores of organic N following increased mineralization. The bioavail ability and seasonal patterns combined will be major determinants to r esponses in the recipients, as will the nutrient ratios. For the spars ely populated heathland dominated Bjerkreim watershed, the stoichiomet ry of nutrient elements are strongly skewed towards high N:P ratios. P concentrations are far too low to support freshwater eutrophication, and a further increase in N would only strengthen the prevailing P lim itation. This would also hold for the marine recipient where excess of N relative to P or Si in riverine runoff could drive the nearshore ar eas towards temporal P or Si limitation. In the Auli watershed, intens e agricultural activity yields high outputs of both N and P, but very scattered and unpredictable runoff peaks with variable N:P ratio. The low concentrations of mineral nutrients in the Bjerkreim river imply a diluting effect relative to seawater concentrations of N and P, while the Auli river may support eutrophication both in the freshwaters and the inner fjord recipient. Due to a pronounced seasonality in nutrien t concentrations and fluxes in the coastal current, the relative contr ibution of riverine sources would peak during summer. Data from both w atersheds with their subcatchments underline the overall importance of agricultural activity for nitrogen runoff.