Primary production and nitrogen regeneration processes in surface waters of the Peruvian upwelling system

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 NH4 assimilated in surface waters (through ammonium regeneration) as well as a variable fraction (2–16%) of nitrate through nitrification.