REGIONAL NITROGEN BUDGETS AND RIVERINE N-AND-P FLUXES FOR THE DRAINAGES TO THE NORTH-ATLANTIC OCEAN - NATURAL AND HUMAN INFLUENCES

We present estimates of total nitrogen and total phosphorus fluxes in rivers to the North Atlantic Ocean from 14 regions in North America, S outh America, Europe, and Africa which collectively comprise the drain age basins to the North Atlantic. The Amazon basin dominates the overa ll phosphorus flux and has the highest phosphorus flux per area. The t otal nitrogen flux from the Amazon is also large, contributing 3.3 Tg yr(-1) out of a total for the entire North Atlantic region of 13.1 Tg yr(-1). On a per area basis, however, the largest nitrogen fluxes are found in the highly disturbed watersheds around the North Sea, in nort hwestern Europe, and in the northeastern U.S., all of which have river ine nitrogen fluxes greater than 1,000 kg N km(-2) yr(-1). Non-point s ources of nitrogen dominate riverine fluxes to the coast in all region s. River fluxes of total nitrogen from the temperate regions of the No rth Atlantic basin are correlated with population density, as has been observed previously for fluxes of nitrate in the world's major rivers . However, more striking is a strong linear correlation between river fluxes of total nitrogen and the sum of anthropogenically-derived nitr ogen inputs to the temperate regions (fertilizer application, human-in duced increases in atmospheric deposition of oxidized forms of nitroge n, fixation by leguminous crops, and the import/export of nitrogen in agricultural products). On average, regional nitrogen fluxes in rivers are only 25% of these anthropogenically derived nitrogen inputs. Deni trification in wetlands and aquatic ecosystems is probably the dominan t sink, with storage in forests perhaps also of importance. Storage of nitrogen in groundwater, although of importance in some localities, i s a very small sink for nitrogen inputs in all regions. Agricultural s ources of nitrogen dominate inputs in many regions, particularly the M ississippi basin and the North Sea drainages. Deposition of oxidized n itrogen, primarily of industrial origin, is the major control over riv er nitrogen export in some regions such as the northeastern U.S. Using data from relatively pristine areas as an index of change, we estimat e that riverine nitrogen fluxes in many of the temperate regions have increased from pre-industrial times by 2 to 20 fold, although some reg ions such as northern Canada are relatively unchanged. Fluxes from the most disturbed region, the North Sea drainages, have increased by 6 t o 20 fold. Fluxes from the Amazon basin are also at least 2 to 5 fold greater than estimated fluxes from undisturbed temperate-zone regions, despite low population density and low inputs of anthropogenic nitrog en to the region. This suggests that natural riverine nitrogen fluxes in the tropics may be significantly greater than in the temperate zone . However, deforestation may be contributing to the tropical fluxes. I n either case, projected increases in fertilizer use and atmospheric d eposition in the coming decades are likely to cause dramatic increases in nitrogen loading to many tropical river systems.