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Benthic N cycling in the Peruvian oxygen minimum zone (OMZ) was investigated at ten stations along 12 °S from the middle shelf (74 m) to the upper slope (1024 m) using in situ flux measurements, sediment biogeochemistry and modeling. Middle shelf sediments were covered by mats of the filamentous bacteria Thioploca spp. and contained a large ‘hidden’ pool of nitrate that was not detectable in the porewater. This was attributed to a biological nitrate reservoir stored within the bacteria to oxidize sulfide during ‘dissimilatory nitrate reduction to ammonium’ (DNRA). The extremely high rates of DNRA on the shelf (15.6 mmol m −2 d −1 of N), determined using an empirical steady-state model, could easily supply all the ammonium requirements for anammox in the water column. The model further showed that denitrification by foraminifera may account for 90% of N 2 production at the lower edge of the OMZ. At the time of sampling, dissolved oxygen was below detection limit down to 400 m and the water body overlying the shelf had stagnated, resulting in complete depletion of nitrate and nitrite. A decrease in the biological nitrate pool was observed on the shelf during fieldwork concomitant with a rise in porewater sulfide levels in surface sediments to 2 mM. Using a non-steady state model to simulate this natural anoxia experiment, these observations were shown to be consistent with Thioploca surviving on a dwindling intracellular nitrate reservoir to survive the stagnation period. The model shows that sediments hosting Thioploca are able to maintain high ammonium fluxes for many weeks following stagnation, potentially sustaining pelagic N loss by anammox. In contrast, sulfide emissions remain low, reducing the economic risk to the Peruvian fishery by toxic sulfide plume development. [ABSTRACT FROM AUTHOR]

Abstract Benthic fluxes of total dissolved phosphate (TPO 4 3-), dissolved iron (Fe2+), and dissolved inorganic carbon (DIC) were determined in situ using benthic chambers at nine stations along a depth transect between 47 and 1108 m water depth at 18°N off Mauritania (NW Africa) during the upwelling season in 2014 (RV Meteor cruise M107). Bottom water oxygen (O 2) concentrations were always ≥ 25 µM, and all fluxes (TPO 4 3-, Fe2+, DIC) were consistently directed from the sediments into the bottom water. The highest benthic TPO 4 3- release of 0.2 ± 0.07 mmol m2 d−1 was found at 47 m water depth (50 µM O 2). The highest diffusive Fe2+ flux of 0.03 mmol m2 d−1, determined from porewater Fe2+ concentrations, occurred at 67 m water depth (27 µM O 2). This was much lower than the detrital Fe supply as indicated by constant Fe/Al ratios along the depth transect. TPO 4 3- release rates decreased concurrently with DIC flux and water depth. A difference of up to one order of magnitude between benthic chamber and diffusive TPO 4 3- fluxes indicated that the total TPO 4 3- release was strongly enhanced by bioirrigation. The observed fluxes were similar to those measured during an earlier cruise in 2011, generally indicating comparable release rates during both upwelling seasons. Furthermore, ex situ oxygen manipulation experiments showed an increase of the nutrient release (e.g. TPO 4 3-, Fe2+) after seven days of anoxic bottom water conditions. The fluxes were enhanced by a factor of 1.4 for P and 7.3 for Fe compared to the measured release under natural conditions and reached values as high as those measured in the anoxic oxygen minimum zone off Peru. Our observations support the hypothesis that increasing deoxygenation of the oceans will likely enhance sedimentary TPO 4 3- and Fe2+ release and thus contribute to a positive feedback mechanism with increasing nutrient levels and increased ocean productivity. Highlights • Bioirrigation and bottom water percolation through permeable sediments enhance benthic TPO 4 3- and Fe2+ release additionally. • Changes in bottom water oxygenation induce slight changes benthic TPO 4 3- and Fe2+ release rates measured in 2011 and 2014. • Deoxygenation experiments imply enhanced TPO 4 3- and Fe2+ release at ongoing deoxygenation in the Mauritanian OMZ. [ABSTRACT FROM AUTHOR]

Oxygen minimum zones (OMZ) are key regions for fixed nitrogen loss in both the sediments and the water column. During this study, the benthic contribution to N cycling was investigated at ten sites along a depth transect (74–989 m) across the Peruvian OMZ at 12°S. O 2 levels were below detection limit down to ~500 m. Benthic fluxes of N 2 , NO 3 – , NO 2 – , NH 4 + , H 2 S and O 2 were measured using benthic landers. Flux measurements on the shelf were made under extreme geochemical conditions consisting of a lack of O 2 , NO 3 – and NO 2 – in the bottom water and elevated seafloor sulphide release. These particular conditions were associated with a large imbalance in the benthic nitrogen cycle. The sediments on the shelf were densely covered by filamentous sulphur bacteria Thioploca , and were identified as major recycling sites for DIN releasing high amounts of NH 4 + up to 21.2 mmol m −2 d −1 that were far in excess of NH 4 + release by ammonification. This difference was attributed to dissimilatory nitrate (or nitrite) reduction to ammonium (DNRA) that was partly being sustained by NO 3 – stored within the sulphur oxidizing bacteria. Sediments within the core of the OMZ (ca. 200–400 m) also displayed an excess flux of N of 3.5 mmol m −2 d −1 mainly as N 2 . Benthic nitrogen and sulphur cycling in the Peruvian OMZ appears to be particularly susceptible to bottom water fluctuations in O 2 , NO 3 − and NO 2 − , and may accelerate the onset of pelagic euxinia when NO 3 − and NO 2 − become depleted. [ABSTRACT FROM AUTHOR]