Benthic Nitrogen Loss in the Arabian Sea Off Pakistan

Abstract

A pronounced deficit of nitrogen (N) in the oxygen minimum zone (OMZ) of the Arabian Sea suggests the occurrence of heavy N-loss that is commonly attributed to pelagic processes. However, the OMZ water is in direct contact with sediments on three sides of the basin. Contribution from benthic N-loss to the total N-loss in the Arabian Sea remains largely unassessed. In October 2007, we sampled the water column and surface sediments along a transect cross-cutting the Arabian Sea OMZ at the Pakistan continental margin, covering a range of station depths from 360 to 1430 m. Benthic denitrification and anammox rates were determined by using 15N-stable isotope pairing experiments. Intact core incubations showed declining rates of total benthic N-loss with water depth from 0.55 to 0.18 mmol N m−2 day−1. While denitrification rates measured in slurry incubations decreased from 2.73 to 1.46 mmol N m−2 day−1 with water depth, anammox rates increased from 0.21 to 0.89 mmol N m−2 day−1. Hence, the contribution from anammox to total benthic N-loss increased from 7% at 360 m to 40% at 1430 m. This trend is further supported by the quantification of cd1-containing nitrite reductase (nirS), the biomarker functional gene encoding for cytochrome cd1-Nir of microorganisms involved in both N-loss processes. Anammox-like nirS genes within the sediments increased in proportion to total nirS gene copies with water depth. Moreover, phylogenetic analyses of NirS revealed different communities of both denitrifying and anammox bacteria between shallow and deep stations. Together, rate measurement and nirS analyses showed that anammox, determined for the first time in the Arabian Sea sediments, is an important benthic N-loss process at the continental margin off Pakistan, especially in the sediments at deeper water depths. Extrapolation from the measured benthic N-loss to all shelf sediments within the basin suggests that benthic N-loss may be responsible for about half of the overall N-loss in the Arabian Sea.

Climate oscillations reflected within the microbiome of Arabian Sea sediments

“…Studies from several nearby coring locations showed that nitrate is depleted within the first 10 cm 62 (Fig. 2A).…”

“…62 . Star in map indicates location of the core from our study, and colored symbols indicate core locations from Sokoll et al .…”

Benthic mineralization and nutrient exchange over the inner continental shelf of western India

“…Our observation is similar to that reported by Rysgaard et al (1994), who noticed that under aerobic conditions, NO − 3 generated through nitrification within the sediment fed denitrification, while, under anaerobic conditions, NO − 3 diffused from the overlying water was found to be the main source for denitrification. Dominance of denitrification and low anaerobic ammonium oxidation (anammox) activity in the suboxic (denitrifying) water column and sediments was observed by Jayakumar et al (2009) and Sokoll et al (2012) within the oxygen minimum zone (OMZ) of the eastern Arabian Sea margin environments.…”

Seasonal and interannual variations in the nitrogen cycle in the Arabian Sea

“…Since the high denitrification rates were furthermore measured at water depths between 20 and 80 m, it is assumed that NO − 3 loss on the Indian shelf hardly affect the SNM, which is at water depths between 125 and 375 m in the open ocean. N losses measured by means of incubation experiments off Pakistan in surface sediment at water depth of 360 m revealed denitrification and anammox rates of 2.7 and 0.21 mmol N m −2 day −1 , respectively (Sokoll et al, 2012). This is within the range of those measured by Schwartz et al (2009) at the Pakistan continental margin (0.07-3.78 mmol N m −2 day −1 ) and exceeds those measured on the Indian shelf of 0.17-1.33 mmol N m −2 day −1 (Naik, 2006;Schwartz et al, 2009).…”