Abstract
Abstract. The discovery that foraminifera are able to use nitrate instead of oxygen as an electron acceptor for respiration has challenged our understanding of nitrogen cycling in the ocean. It was thought before that only prokaryotes and some fungi are able to denitrify. Rate estimates of foraminiferal denitrification have been very sparse and limited to specific regions in the oceans, not comparing stations along a transect of a certain region. Here, we present estimates of benthic foraminiferal denitrification rates from six stations at intermediate water depths in and below the Peruvian oxygen minimum zone (OMZ). Foraminiferal denitrification rates were calculated from abundance and assemblage composition of the total living fauna in both surface and subsurface sediments, as well as from individual species specific denitrification rates. A comparison with total benthic denitrification rates as inferred by biogeochemical models revealed that benthic foraminifera probably account for the total denitrification in shelf sediments between 80 and 250 m water depth. The estimations also imply that foraminifera are still important denitrifiers in the centre of the OMZ around 320 m (29–50% of the benthic denitrification), but play only a minor role at the lower OMZ boundary and below the OMZ between 465 and 700 m (2–6% of total benthic denitrification). Furthermore, foraminiferal denitrification has been compared to the total benthic nitrate loss measured during benthic chamber experiments. The estimated foraminiferal denitrification rates contribute 2 to 46% to the total nitrate loss across a depth transect from 80 to 700 m, respectively. Flux rate estimates range from 0.01 to 1.3 mmol m−2 d−1. Furthermore we show that the amount of nitrate stored in living benthic foraminifera (3 to 3955 μmol L−1) can be higher by three orders of magnitude as compared to the ambient pore waters in near-surface sediments sustaining an important nitrate reservoir in Peruvian OMZ sediments. The substantial contribution of foraminiferal nitrate respiration to total benthic nitrate loss at the Peruvian margin, which is one of the main nitrate sink regions in the world ocean, underpins the importance of the previously underestimated role of benthic foraminifera in global biogeochemical cycles.
Highlights
Nitrate is a limiting nutrient in surface ocean primary productivity and takes a central role in marine biogeochemistry (Gruber, 2004; Arrigo, 2005; Lam et al, 2009)
Foraminiferal assemblages in sediments of the centre of the Peruvian oxygen minimum zones (OMZs) were characterized by low diversities and very high population densities (Mallon et al, 2012)
Our data highlights the importance of foraminiferal denitrification in one of the major regions of nitrate loss of the world ocean, the Peruvian OMZ, and that there are strong variations in the rate of foraminiferal denitrification even on a regional scale
Summary
Nitrate is a limiting nutrient in surface ocean primary productivity and takes a central role in marine biogeochemistry (Gruber, 2004; Arrigo, 2005; Lam et al, 2009). In the oceanic nitrogen cycle, N2 SfroomlidtheEaatmrothsphere becomes bioavailable by N2 fixation. The loss of nitrogen from the marine ecosystem to the atmosphere as N2 gas is dominated by two major pathways. About 20 to 40 % of the global nitrogen loss in the oceans is estimated to take place in oxygen minimum zones (OMZs), these zones occupy only 0.1 % of the global ocean volume (Gruber and Sarmiento, 1997; Gruber, 2004; Lam et al, 2009).
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