Abstract

Tracer experiments ( 15N and 13C) were carried out off the Peruvian upwelling system in order to evaluate the potential role of regenerated nitrogen in sustaining biological productivity in surface waters (0-15 m depth range). Rates of daily carbon uptake in surface waters showed values ranging between 6 and 200 mg C m -3 d -1. Maximum integrated rates were obtained in northern Peru (up to 5.5 g m -2 d -1). Parallel measurements of nitrogen uptake showed a stronger utilization of ammonium compared to nitrate at all stations, with surface ammonium uptake values reaching up to 0.15 µmol N L -1 d -1. Ammonium was also actively regenerated in surface waters, at rates as high as 0.5 µmol N L -1 d -1, whereas ammonium oxidation to nitrate (nitrification) occurred at lower rates (<0.15 µmol N L -1 d -1). Results obtained in this study showed active regeneration of N within the euphotic layer. Regenerated nitrate and ammonium are, therefore, available for primary producers within the euphotic zone and sustain a fraction of the observed photoautotrophic carbon fixation. The contribution of nitrogen regenerating processes to primary production in terms of DIN could represent as much as 50% of NH 4 assimilated in surface waters (through ammonium regeneration) as well as a variable fraction (2-16%) of nitrate through nitrification. © 2009 Elsevier Ltd. All rights reserved.