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Abstract

The northern Benguela upwelling system is a nutrient-replete region with high plankton biomass production and a seasonally changing oxygen minimum zone. Nitrate: phosphate ratios in fresh upwelling water are low due to denitrification in the near-seafloor oxygen minimum zone and phosphate efflux from sediments. This makes the region a candidate for substantial dinitrogen fixation, for which evidence is scarce. Nutrient and oxygen data, N isotope data of nitrate, nitrogen isotope ratios of particulate matter, particulate organic carbon content, and suspended matter concentrations on a transect across the shelf and upper slope at 23 degrees S illustrate N-cycling processes and are the basis for estimating the contribution of N-sources and N-sinks to the reactive nitrogen pool. It appears that N-removal due to denitrification exceeds N gain by N-2 fixation and physical mixing processes by a factor of >6, although inorganic N: P ratios again increase as surface water is advected offshore. Nitrate and ammonium regeneration, nutrient assimilation with N:P < 16, shelf break mixing, atmospheric input, and N-2 fixation all contribute to the restoration of inorganic N: P ratios back to Redfield conditions, but in seasonally changing proportions. The Benguela upwelling system thus is a nutrient source for the oceanic-mixed layer where N-sources and N-sinks are not in balance and Redfield conditions can only re-adjust by advection and mixing processes integrated over time.