Description: 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°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 N2 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 N2 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.

Description: Within the BMBF funded project GENUS (Geochemistry and Ecology of the Namibian Upwelling System) the mole fraction of CO2 (xCO2) was measured in surface waters by using an underway pCO2 system (SUNDANS) duirng seven cruises. SUNDANS was developed by "marine analytics and data" (MARIANDA, Germany, www.marianda.com) according to the recommendations of the 2002 underway pCO2 system workshop in Miami. It was equipped with a shower type equilibrator, an open pre-equilibrator and a non-dispersive dual cell infrared gas analyzer (LI-7000). The LI-7000 was calibrated by using nitrogen gas (zero CO2) and a two additional standard gas for CO2. The standard gases were obtained from the company Deuste Steininger GmbH, Germany and revealed CO2 concentrations of 350 to 480 ppm (Std1) and around 800 ppm (Std2). The CO2 standard gases were calibrated against the standard gases provided by NOAA at the Institute for Baltic Sea Research in Warnemünde, Germany (Ref. No. CA07600 and CC311968) and the Centre for Tropical Marine Research in Bremen, Germany (Ref. No. CB08923 and CA06265). The xCO2 data were recorded each 6 seconds and subsequently averaged minute by minute. Minute by minute data on atmospheric pressure, wind speed, seawater temperature and salinity were measured by underway systems mounted on board the research vessels. xCO2 was converted into pCO2 by using the CO2 sys program. The difference between the equilibrator and the sea water temperature was taken into account as suggested by Dickson et al. (2007, SOP5, page 8). During the RV Metoer cruise M67/2 between May 15 and June 05 2008 the pCO was measured by using PSI CO2-ProTM underwater carbon dioxide sensor designs by Pro-Oceanus Systems In., USA.