Description: Biogeochemical cycles of carbon, nutrients, and oxygen transmit mean states, trends and variations of the physical realm in coastal upwelling systems to their food webs and determine their role in regional budgets of greenhouse gases. This contribution focuses on biogeochemical processes in the northern Benguela Upwelling System (NBUS), where low oxygen levels in upwelling source water are a major influence on carbon and nutrient cycles. Based on measurements during numerous expeditions and results of 3-D regional ecosystem modeling (project GENUS; Geochemistry and Ecology of the Namibian Upwelling System) we here examine source water character, effects of low oxygen conditions on nutrient masses and ratios, and of diazotrophic N2-fixation on productivity of the system and its transition to the adjacent eastern South Atlantic. In available observations, the effects of denitrification in water and sediment and phosphate release from sediments are minor influences on nitrate:phosphate ratios of the system, and excess phosphate in aged upwelling water is inherited from upwelling source water. Contrary to expectation and model results, the low N:P ratios do not trigger diazotrophic N2-fixation in the fringes of the upwelling system, possibly due to a lack of seeding populations of Trichodesmium. We also examine the flux of carbon from the sea surface to either sediment, the adjacent sub-thermocline ocean, or to regenerated nutrients and CO2. Observed fluxes out of the surface mixed layer are significantly below modeled fluxes, and suggest that regeneration of nutrients and CO2 is unusually intense in the mixed layer. This contributes to very high fluxes of CO2 from the ocean to the regional atmosphere, which is not compensated for by N2-fixation. Based on observations, the NBUS thus is a significant net CO2 source (estimated at 14.8 Tg C a− 1), whereas the CO2 balance is closed by N2-fixation in the model. Methane concentrations were low in surface waters in on-line measurements during 1 expedition, and based on these our estimate for the emission of methane for the entire Benguela system is below 0.2 Tg CH4 a− 1.

Description: Rising stable nitrogen isotope ratios (δ15N) in dated sediment records of the German Bight/SE North Sea track river-induced coastal eutrophication over the last 2 centuries. Fully exploiting their potential for reconstructions of pristine conditions and quantitative analysis of historical changes in the nitrogen cycle from these sediment records requires knowledge on processes that alter the isotopic signal in non-living organic matter (OM) of sinking particles and sediments. In this study, we analyze the isotopic composition of particulate nitrogen (PN) in the water column during different seasons, in surface sediments, and in sediment cores to assess diagenetic influences on the isotopic composition of OM. Amino acid (AA) compositions of suspended matter, surface sediments, and dated cores at selected sites of the German Bight serve as indicators for quality and degradation state of PN. The δ15N of PN in suspended matter had seasonal variances caused by two main nitrate sources (oceanic and river) and different stages of nitrate availability during phytoplankton assimilation. Elevated δ15N values (〉 20‰) in suspended matter near river mouths and the coast coincide with a coastal water mass receiving nitrate with elevated isotope signal (δ15N 〉 10‰) derived from anthropogenic input. Particulate nitrogen at offshore sites fed by oceanic nitrate having a δ15N between 5 and 6‰ had low δ15N values (〈 2‰), indicative of an incipient phytoplankton bloom. Surface sediments along an offshore–onshore transect also reflect the gradient of low δ15N of nitrate in offshore sites to high values near river mouths, but the range of values is smaller than between the end members listed above and integrates the annual δ15N of detritus. Sediment cores from the coastal sector of the gradient show an increasing δ15N trend (increase of 2.5‰) over the last 150 years. This is not related to any change in AA composition and thus reflects eutrophication. The δ15N signals from before AD 1860 represent a good estimation of pre-industrial isotopic compositions with minimal diagenetic overprinting. Rising δ13C in step with rising δ15N in these cores is best explained by increasing productivity caused by eutrophication.