Extensive nitrogen loss from permeable sediments off North‐West Africa

Abstract

The upwelling area off North-West Africa is characterized by high export production, high nitrate and low oxygen concentration in bottom waters. The underlying sediment consists of sands that cover most of the continental shelf. Due to their permeability sands allow for fast advective pore water transport and can exhibit high rates of nitrogen (N) loss via denitrification as reported for anthropogenically eutrophied regions. However, N loss from sands underlying naturally eutrophied waters is not well studied, and in particular, N loss from the North-West African shelf is poorly constrained. During two research cruises in April/May 2010/2011, sediment was sampled along the North-West African shelf and volumetric denitrification rates were measured in sediment layers down to 8 cm depth using slurry incubations with 15 N-labeled nitrate. Areal N loss was calculated by integrating volumetric rates down to the nitrate penetration depth derived from pore water profiles. Areal N loss was neither correlated with water depth nor with bottom water concentrations of nitrate and oxygen but was strongly dependent on sediment grain size and permeability. The derived empirical relation between benthic N loss and grains size suggests that pore water advection is an important regulating parameter for benthic denitrification in sands and further allowed extrapolating rates to an area of 53,000 km 2 using detailed sediment maps. Denitrification from this region amounts to 995 kt yr À1 (average 3.6 mmol m À2 d À1 ) which is 4 times higher than previous estimates based on diffusive pore water transport. Sandy sediments cover 50-60% of the continental shelf and thus may contribute significantly to the global benthic N loss.

Benthic Dinitrogen Fixation Traversing the Oxygen Minimum Zone Off Mauritania (NW Africa)

“…However, in the study by Sokoll et al (2016) denitrification rates along the 18 • N transect were found to be higher at similar water depths (53-787 m) and significantly correlated with grain size, a parameter that was not investigated during our sampling survey. Sokoll et al, 2016) with the corresponding N 2 fixation rate, results in a lower compensation when compared to denitrification rates determined by Dale et al (2014).…”

“…Sokoll et al, 2016) with the corresponding N 2 fixation rate, results in a lower compensation when compared to denitrification rates determined by Dale et al (2014). According to the findings of Sokoll et al (2016), benthic N 2 fixation would compensate for only 4-10% of the N loss by denitrification. In summary, diazotrophs in the Mauritanian OMZ sediments may have, at certain sites, a considerable attenuating effect on the loss of fixed N from the benthic environment.…”

“…In order to determine the relevance of benthic N 2 fixation in the Mauritanian OMZ, we compared N 2 fixation rates determined in this study with denitrification rates from Dale et al (2014) and Sokoll et al (2016). Denitrification by Dale et al (2014) was investigated along a similar depth transect (18 • N) and in similar water depth.…”

Does denitrification occur within porous carbonate sand grains?

“…It has been reported that nitrification within the sediment may be a significant source of nitrate to fuel denitrification (Marchant et al, 2016;Rao et al, 2007), although modelling studies suggest that, even in the presence of nitrification, flow fields in permeable sediments may lead to little coupling between nitrification and denitrification (Kessler et al, 2013). In systems with high bottom-water nitrate concentrations, high rates of denitrification have been reported (Gao et al, 2012;Sokoll et al, 2016). Previous studies have also reported high rates of denitrification in carbonate sands (Eyre et al, , 2013b, which is surprising given the low nitrate concentrations in the over-lying water and their highly oligotrophic nature.…”

“…One of the dominant habitat types on coral reefs are carbonate sands formed from the breakdown of carbonate produced by calcifying organisms (Eyre et al, 2014). Quantifying denitrification in sandy sediments is complicated by the fact that these sediments are permeable, allowing water movement through the sediment, which can both enhance and reduce denitrification (Cook et al, 2006;Kessler et al, 2012;Sokoll et al, 2016). Reproducing these conditions while measuring denitrification is difficult; thus, models combined with a mechanistic understanding of the primary controls on denitrification offer a promising approach to quantifying denitrification in these environments (Evrard et al, 2013;Kessler et al, 2012).…”