Research Article: Enhanced Nitrogen Loss by Eddy-Induced Vertical Transport in the Offshore Peruvian Oxygen Minimum Zone

Date Published: January 25, 2017

Publisher: Public Library of Science

Author(s): Cameron M. Callbeck, Gaute Lavik, Lothar Stramma, Marcel M. M. Kuypers, Laura A. Bristow, Yiguo Hong.

http://doi.org/10.1371/journal.pone.0170059

Abstract

The eastern tropical South Pacific (ETSP) upwelling region is one of the ocean’s largest sinks of fixed nitrogen, which is lost as N2 via the anaerobic processes of anammox and denitrification. One-third of nitrogen loss occurs in productive shelf waters stimulated by organic matter export as a result of eastern boundary upwelling. Offshore, nitrogen loss rates are lower, but due to its sheer size this area accounts for ~70% of ETSP nitrogen loss. How nitrogen loss and primary production are regulated in the offshore ETSP region where coastal upwelling is less influential remains unclear. Mesoscale eddies, ubiquitous in the ETSP region, have been suggested to enhance vertical nutrient transport and thereby regulate primary productivity and hence organic matter export. Here, we investigated the impact of mesoscale eddies on anammox and denitrification activity using 15N-labelled in situ incubation experiments. Anammox was shown to be the dominant nitrogen loss process, but varied across the eddy, whereas denitrification was below detection at all stations. Anammox rates at the eddy periphery were greater than at the center. Similarly, depth-integrated chlorophyll paralleled anammox activity, increasing at the periphery relative to the eddy center; suggestive of enhanced organic matter export along the periphery supporting nitrogen loss. This can be attributed to enhanced vertical nutrient transport caused by an eddy-driven submesoscale mechanism operating at the eddy periphery. In the ETSP region, the widespread distribution of eddies and the large heterogeneity observed in anammox rates from a compilation of stations suggests that eddy-driven vertical nutrient transport may regulate offshore primary production and thereby nitrogen loss.

Partial Text

Oceanic oxygen minimum zones (OMZ) typically occur in regions where upwelling of nutrient rich waters fuels high surface primary productivity. The resulting export of organic matter stimulates microbial respiration, and combined with poor regional ventilation creates low oxygen concentrations [1]. Traditionally OMZ boundaries are defined by oxygen concentrations of less than 20 μM [2], although, oxygen is regularly observed to be < 10 nM in these regions [3, 4]. Under low oxygen concentrations the anaerobic processes anammox and denitrification contribute to nitrogen loss. Specifically, the former catalyzes the anaerobic oxidation of ammonium with nitrite, while the latter is the stepwise reduction of nitrate to N2. An estimated 30–50% of oceanic nitrogen loss occurs in OMZs, which represent roughly 0.1% of the global ocean volume [5]. These regions are primarily located within the Arabian Sea, the Bay of Bengal, off the coast of Namibia, the Eastern Tropical North Pacific, and the Eastern Tropical South Pacific (ETSP) [6]. In the majority of OMZ studies, anammox has been shown to be the main sink of fixed inorganic nitrogen (NO3-, NO2- and NH4+) [7–12]. The main source of inorganic nitrogen substrates for anammox comes from the remineralization of organic matter exported from the photic zone [13]. Based on in situ rate measurements, anammox activity is strongest over the upper shelf where the input of organic matter is highest [8, 9, 13]. Therefore, organic matter supply places constraints on nitrogen loss [13], which has been attributed to coastal upwelling [1]. In this study we provide the first rate measurements of nitrogen loss processes across cyclonic and anticyclonic mode-water eddies in the ETSP. Contrary to the recent ‘hotspot’ studies, which have suggested that the highest activity occurs in the eddy center [14, 36–38, 48], our 15N-labelling incubation experiments revealed that nitrogen loss activity was greatest at the periphery of mesoscale eddies. Although, highest chlorophyll concentrations were observed in the center [37], depth-integrated chlorophyll content was also highest at the eddy periphery. The observed lateral intrusions and deep chlorophyll pockets occurring along the eddy periphery [37], suggest that this area of the eddy was active in the generation and export of organic matter, in agreement with modeling studies [40, 41].   Source: http://doi.org/10.1371/journal.pone.0170059

 

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