INPUTS, LOSSES AND TRANSFORMATIONS OF NITROGEN AND PHOSPHORUS IN THE PELAGIC NORTH-ATLANTIC OCEAN

The North Atlantic Ocean receives the largest allochthonous supplies o f nitrogen of any ocean basin because of the close proximity of indust rialized nations. In this paper, we describe the major standing stocks , fluxes and transformations of nitrogen (N) and phosphorus (P) in the pelagic regions of the North Atlantic, as one part of a larger effort to understand the entire N and P budgets in the North Atlantic Ocean, its watersheds and overlying atmosphere. The primary focus is on nitr ogen, however, we consider both nitrogen and phosphorus because of the close inter-relationship between the N and P cycles in the ocean. The oceanic standing stocks of N and P are orders of magnitude larger tha n the annual amount transported off continents or deposited from the a tmosphere. Atmospheric deposition can have an impact on oceanic nitrog en cycling at locations near the coasts where atmospheric sources are large, or in the centers of the highly stratified gyres where little n itrate is supplied to the surface by vertical mixing of the ocean. All of the reactive nitrogen transported to the coasts in rivers is denit rified or buried in the estuaries or on the continental shelves and an oceanic source of nitrate of 0.7-0.95 x 10(12) moles NO3- y(-1) is re quired to supply the remainder of the shelf denitrification (Nixon et al., this volume). The horizontal fluxes of nitrate caused by the ocea n circulation are both large and uncertain. Even the sign of the trans port across the equator is uncertain and this precludes a conclusion o n whether the North Atlantic Ocean as a whole is a net source or sink of nitrate. We identify a source of nitrate of 3.7-6.4 x 10(12) moles NO3- y(-1) within the main thermocline of the Sargasso Sea that we inf er is caused by nitrogen fixation. This nitrate source may explain the nitrate divergence observed by Rintoul & Wunsch (1991) in the mid-lat itude gyre. The magnitude of nitrogen fixation inferred from this nitr ate source would exceed previous estimates of global nitrogen fixation . Nitrogen fixation requires substantial quantities of iron as a micro -nutrient and the calculated iron requirement is comparable to the rat es supplied by the deposition of iron associated with Saharan dust. In terannual variability in dust inputs is large and could cause comparab le signals in the nitrogen fixation rate. The balance of the fluxes ac ross the basin boundaries suggest that the total stocks of nitrate and phosphate in the North Atlantic may be increasing on time-scales of c enturies. Some of the imbalance is related to the inferred nitrogen fi xation in the gyre and the atmospheric deposition of nitrogen, both of which may be influenced by human activities. However, the fluxes of d issolved organic nutrients are almost completely unknown and they have the potential to alter our perception of the overall mass balance of the North Atlantic Ocean.