Bohlen, Lisa

Sommer, Stefan

OceanRep

Scientific Highlights- IFM-GEOMAR Report 2011 -26Scientific HighlightsEnhanced recycling of bioavailable nitrogen in sediments underlying the Peruvian oxygen minimum zoneLisa Bohlen, Stefan Sommer - Marine Biogeochemistry -  Marine GeosystemsOxygen minimum zones (OMZ) are key regions for pelagic and benthic nitrogen turnover. Dur-ing Meteor cruise M77-1/2, conducted within the SFB 754, the shelf and upper slope sediments off Peru were identified as major recycling sites for bioavailable nitrogen. This is in strong contrast to the common perception that continental margin sediments in upwelling regions generally represent a major sink for bioavailable nitrogen releasing dinitrogen gas, N2, into the environment. As part of the SFB 754 “Climate - Bioge-ochemistry Interactions in the Tropical Ocean”, redox-sensitive sedimentary nitro-gen (N) cycling was extensively studied along a transect traversing the Peruvian oxygen minimum zone (OMZ) at 11° S. This transect covered the transition from anoxic bottom water conditions to low oxygen concentra-tion of around 40 µM (Fig. 1). As a limiting element for biological productivity, the avail-ability of dissolved inorganic nitrogen, DIN, (nitrate, NO3-; nitrite, NO2- and ammonium, NH4+) occupies a central role in ocean bio-geochemistry, exerting a significant influence on cycles of many other elements, in particu-lar carbon. While DIN, also known as fixed nitrogen, can generally be taken up by phyto-plankton, dinitrogen gas (N2) is only accessi-ble for a small group of nitrogen fixing organ-isms called diazotrophs. Hence, the balance of DIN removal and production is linked to primary production and carbon cycling with its availability controlling not only the pro-ductivity of the oceans but also the seques-tration of carbon dioxide from the atmos-phere. Due to the redox–sensitivity of many processes involved in the marine N cycle a globally significant proportion of N turnover proceeds in the relatively restricted areas of the world ocean OMZs.Marine sediments have been perceived to represent a main sink for DIN (e.g. Gruber, 2004) with denitrification and anammox rep-resenting the main processes. During denitri-fication organic matter is decomposed using nitrate as electron acceptor instead of oxygen (O2), during the recently discovered anam-mox pathway NH4+ is oxidized using NO2-. Both processes convert DIN into N2 and thus lead to a loss of bioavailable N. Denitrifica-tion and anammox are generally inhibited by O2. Hence, these processes are restricted to areas where O2 concentrations are very low such as in OMZs or reactive sediments where O2 only penetrates a few millimeters deep into the sediment. In order to determine to what extent the sed-iments underlying the Peruvian OMZ serve as a sink for bioavailable N, DIN fluxes across the sediment water interface were measured in situ at six stations along the transect using benthic landers (Fig. 1). With regards to the net flux of DIN the transect can be separated into two sections. At the slope stations 3 to 6 the uptake of NO3- + NO2- was higher than the release of NH4+ and the sediments rep-resented a net sink for DIN. However, at the shelf and the upper slope (station 1 and 2) uptake of NO3- + NO2- almost balanced release of NH4+ indicating strong recycling of bioavail-able N. This finding is in great contrast to the common belief that OMZ sediments represent major sinks (e.g. Middelburg et al., 1996; Devol and Christensen, 1993). At the shelf Fig. 1: Cross–section of oxygen concentrations (µM) on the shelf and slope of the Peruvian OMZ at 11 °S. The vertical lines denote the CTD casts where O2 mea-surements were made during cruise M77 leg 1. Station locations 1 to 6 for benthic studies are indicated. 	 				 Scientific	Highlights- IFM-GEOMAR Report 2011 - 27Scientific Highlightsand upper slope extended mats of giant sul-fur bacteria such as Beggiatoa and Thioploca were observed (Fig. 2). These organisms can store high amounts of NO3- in their cells to oxidize sulfide and thereby release NH4+. This process is termed dissimilatory nitrate reduc-tion to ammonium (DNRA). In contrast to denitrification and anammox, DNRA retains bioavailable N in the ecosystem and opposes the “self-cleaning” effect of denitrification and anammox.To resolve the different N turnover processes the in situ DIN fluxes as well as measured porewater concentration profiles were used to constrain a 1-D reaction-transport model (Bohlen et al., 2011). The model allows a detailed simulation of the N cycle including simultaneous two-step denitrification and nitrification, DNRA, and anammox. Results show that the relative importance of the dif-ferent N turnover processes distinctively changes along the transect (Fig. 3). At sta-tions 1 and 2, DNRA indeed governed N turn-over and accounted for more than half of the benthic NO3- + NO2- uptake. At greater water depth, denitrification became dominant and the sediments turn into a sink for DIN. Anam-mox was of minor importance on the shelf and upper slope yet contributed up to 62% to total N2 production at the deepest station. These new findings illustrate that OMZ sedi-ments do probably not remove as much bio-available N as previously thought. High rates of DIN recycling by DNRA counteract the overall loss of bioavailable N from the sys-tem. Consequently, the role of OMZ sedi-ments as strong sinks for DIN need to be revised. Moreover, strong NH4+ release poten-tially stimulates primary production and O2 consumption in the water column. Such a positive feedback mechanism maintains or even promotes further expansion of OMZs (Stramma et al., 2008). ReferencesBohlen, L., Dale, A.W., Sommer, S., Mosch, T., Hensen, C., Noffke, A., Scholz, F. and Wallmann, K., 2011: Benthic nitrogen cycling traversing the Peruvian oxygen minimum zone. Geochim. Cosmochim. Ac., 75, 6094-6111.Devol A.H., and Christensen, J.P., 1993: Benthic Fluxes And Nitrogen Cycling In Sediments Of The Continental–Margin Of The Eastern North Pacific. J. Mar. Res., 51, 345–372.Gruber N., 2004: The dynamics of the marine nitro-gen cycle and its influence on the atmospheric CO2 variations. In The Ocean Carbon Cycle and Climate (eds. Follows M. and Oguz T.). NATO ASI Series, 97–148.Middelburg, J.J., Soetaert, K., Herman, P.M.J. and Heip C. H.R., 1996: Denitrification in marine sediments: A model study. Global Biogeochem. Cy., 10, 661–673.Stramma, L., Johnson, G.C., Sprintall, J. and Mohrholz, V., 2008: Expanding oxygen–mini-mum zones in the tropical oceans. Science, 320, 655–658.Fig. 2: Bacterial mats (white) on the sediment surface at the Peruvian shelf. The length of the ground-weight is 20 cm.Fig. 3: Model-predicted depth-integrated rates of nitro-gen turnover along the 11°S transect. Station numbers are indicated by the arrows at the top. The dashed line denotes the shift from DNRA to denitrification as the major N-turnover process. 

Enhanced recycling of bioavailable nitrogen in sediments underlying the Peruvian oxygen minimum zone

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Enhanced recycling of bioavailable nitrogen in sediments underlying the Peruvian oxygen minimum zone

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