Microbial uptake and regeneration of inorganic nitrogen off the coastal Namibian upwelling system

Abstract

We used 15N-labeled substrates to measure microbial nitrate (NO3−) and ammonium (NH4+) uptake, regeneration and associated dissolved organic nitrogen (DON) release in a coastal upwelling system off Namibia (Benguela Current) in the austral winter of 2011 with the aim of quantifying rates of new production (Pnew) and regenerated production (Preg). These measurements were made during four consecutive coastal-offshore transects. The water parcels sampled at the different stations over the transect were classified into three groups according to the time passed from the first contact of the water with the surface during coastal upwelling (‘pseudo-age’). The average Pnew was high in freshly upwelled waters with a pseudo-age < 13 d (17.8 mmol N m− 2 h− 1), and decreased abruptly towards older waters (3.9 and 2.3 mmol N m− 2 h− 1 in waters with a pseudo-age of 13 to 55 d, and > 55 d, respectively). Preg rates were similar in < 13 d and 13–55 d waters (10.9 and 11.1 mmol N m− 2 h− 1, respectively), and decreased to 6.24 mmol N m− 2 h− 1 in waters with a pseudo-age > 55 d. Measuring nitrogen regeneration and DON release fluxes allowed us to correct Pnew and Preg rates. NO3− regeneration rates were low (< 0.5 mmol N m− 2 h− 1), while NH4+ regeneration rates were in the range of NH4+ uptake rates (~ 2 to 5 mmol N m− 2 h− 1), thus influencing significantly Preg rates. Parallel studies presented in this volume indicate a relatively high abundance of dinoflagellates and mixotrophic microflagellates, which may be partly responsible for the high Preg rates observed. Our results suggest that nitrogen regeneration plays an important role in sustaining primary production in this upwelling system.