THE FATE OF NITROGEN AND PHOSPHORUS AT THE LAND SEA MARGIN OF THE NORTH-ATLANTIC OCEAN

Five large rivers that discharge on the western North Atlantic contine ntal shelf carry about 45% of the nitrogen (N) and 70% of the phosphor us (P) that others estimate to be the total flux of these elements fro m the entire North Atlantic watershed, including North, Central and So uth America, Europe, and Northwest Africa. We estimate that 61 . 10(9) moles y(-1) of N and 20 . 10(9) moles y(-1) of P from the large river s are buried with sediments in their deltas, and that an equal amount of N and P from the large rivers is lost to the shelf through burial o f river sediments that are deposited directly on the continental slope . The effective transport of active N and P from land to the shelf thr ough the very large rivers is thus reduced to 292 . 10(9) moles y(-1) of N and 13 . 10(9) moles y(-1) of P. The remaining riverine fluxes fr om land must pass through estuaries. An analysis of annual total N and total P budgets for various estuaries around the North Atlantic revea led that the net fractional transport of these nutrients through estua ries to the continental shelf is inversely correlated with the log mea n residence time of water in the system. This is consistent with numer ous observations of nutrient retention and loss in temperate lakes. De nitrification is the major process responsible for removing N in most estuaries, and the fraction of total N input that is denitrified appea rs to be directly proportional to the log mean water residence time. I n general, we estimate that estuarine processes retain and remove 30-6 5% of the total N and 10-55% of the total P that would otherwise pass into the coastal ocean. The resulting transport through estuaries to t he shelf amounts to 172-335 . 10(9) moles y(-1) of N and 11-19 . 10(9) moles y(-1) of P. These values are similar to the effective contribut ion from the large rivers that discharge directly on the shelf. For th e North Atlantic shelf as a whole, N fluxes from major rivers and estu aries exceed atmospheric deposition by a factor of 3.5-4.7, but this v aries widely among regions of the shelf. For example, on the U.S. Atla ntic shelf and on the northwest European shelf, atmospheric deposition of N may exceed estuarine exports. Denitrification in shelf sediments exceeds the combined N input from land and atmosphere by a factor of 1.4-2.2. This deficit must be met by a flux of N from the deeper ocean . Burial of organic matter fixed on the shelf removes only a small fra ction of the total N and P input (2-12% of N from land and atmosphere; 1-17% of P), but it may be a significant loss for P in the North Sea and some other regions. The removal of N and P in fisheries landings i s very small. The gross exchange of N and P between the shelf and the open ocean is much lager than inputs from land and, for the North Atla ntic shelf as a whole, it may be much larger than the N and P removed through denitrification, burial, and fisheries. Overall, the North Atl antic continental shelf appears to remove some 700-950 . 10(9) moles o f N each year from the deep ocean and to transport somewhere between 1 8 and 30 . 10(9) moles of P to the open sea. If the N and P associated with riverine sediments deposited on the continental slope are includ ed in the total balance, the net flux of N to the shelf is reduced by 60 . 10(9) moles y(-1) and the P flux to the ocean is increased by 20 . 10(9) moles y(-1). These conclusions are quite tentative. however, b ecause of large uncertainties in our estimates of some important terms in the shelf mass balance.