On the importance of quantifying bioavailable nitrogen instead of total nitrogen

Monitored and modeled data provided the basis for the establishment of two nitrogen (N) budgets covering the Kattegat-Belt Sea area in the period 2000–2009: one for total nitrogen (TN) and one for bioavailable nitrogen (N_bio). Our results show a significant difference between the two budgets, and we argue that N_bio is more important than TN for our understanding of the sources causing marine eutrophication. Consequently, an optimal strategy for abatement of eutrophication aims at minimizing N_bio rather than TN. The TN budget shows that advection from the adjacent seas is the dominant source of N to the Kattegat-Belt Sea area. The loadings from land and atmosphere only account for 14 and 9 % of the TN loadings, respectively. However, when the bioavailability of the different N sources is taken into account, the supply from land and atmosphere becomes relatively more important, now accounting for 21 and 16 %, respectively (37 % in total). The ecological relevance of land and atmosphere loadings is most likely even larger since a fraction of the input from the Skagerrak is exported again before it can support primary production. Water action plans have reduced the direct loadings of TN from land and atmosphere by about 35 % since the 1980s. The contributions from land and atmosphere accounted for 47 % of the N_bio loadings in the 1980s. Hence, loadings from land and atmosphere have only decreased by 10 % points since the 1980s: from 47 to 37 %. The largest sink of TN in the study areas is advection to the adjacent seas (71 %) whereas denitrification and burial only accounts for 17 and 11 %, respectively.