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

Abstract

Abstract. The western Indian continental shelf is one of the most productive coastal systems of the world ocean. This system experiences extreme changes in its oxygen regime, being normoxic from November to May and suboxic (denitrifying)/anoxic from June to October, owing to the biogeochemical response to cyclical monsoonal influence. In order to understand the impact of the seasonally varying oxygen regime on benthic mineralization, nutrient exchange and, in turn, on the shelf ecosystem, we carried out the first ever intact-core incubations during two contrasting seasons -spring intermonsoon and fall intermonsoon (late southwest monsoon) at a 28 m-deep fixed site on the inner shelf off Goa, dominated by fine-grained cohesive sediments. The results showed that incomplete sediment oxygen consumption (SOC) occurred during April as opposed to the complete SOC and subsequent sulfide flux observed in the fall intermonsoon incubations. The sediments acted as a perennial net source of DIN (dissolved inorganic nitrogen i.e. NO Slow oxidation of organic carbon (C org ) under anoxia, lower temperature and reduced benthic faunal activity appeared to decrease benthic mineralization by 25 % as suggested by the drop in the C org oxidation rate from 63.8 mmol C m −2 d −1 in April to 47.8 mmol C m −2 d −1 in October. This indicated a higher preservation of C org during the late southwest monsoon. Sediment porosity, C org content and nutrients did not show significant variations from April to October. Porewaters were found to be enriched with NH

Present past and future of the OMZ in the northern Indian Ocean

“…It would also result in expanded areas of reducing sediments and potential changes to carbon sequestration, N and P cycling and N 2 O emissions . Further, the magnitudes and the dramatic intermonsoon/monsoon (oxic/hypoxic) changes in benthic processes and nutrient fluxes seen at sites on the western Indian shelf (Pratihary et al, 2014), imply that expanded or intensified hypoxia could, through benthic-pelagic coupling, have major influences on nutrient inventories and processes occurring in shallow overlying waters.…”

“…Thus, while seasonal contrasts in benthic community organic matter processing were reported on the Pakistan shelf (see above), it is not otherwise clear if or how benthic communities have adapted to the recurring, possibly intensifying, hypoxia. What is clear is that wholesale seasonal changes occur in benthic microbial processes and in the magnitudes and directions of sediment-water nutrient fluxes (e.g.,Pratihary et al, 2014).…”

Massive Nitrogen Loss Over the Western Indian Continental Shelf During Seasonal Anoxia: Evidence From Isotope Pairing Technique

“…+ is perhaps more important as Pratihary et al (2014) reported very high benthic NH 4 + efflux (3.74 mmol m −2 d −1 ) based on whole core incubations in this region.…”

Upwelling intensity modulates N2O concentrations over the western Indian shelf

“…High NH 1 4 (5 mM) associated with highly depleted NO 2 3 (<0.5 mM) concentrations in the anoxic subpycnocline waters of Mangalore ( Figure 5) indicate the anaerobic oxidation of sinking surface production while such signals are weak in the suboxic waters of Kochi. CEAS region has been found to exhibit high benthic denitrification rate of 0.58-1.04 mmol N m 22 d 21 [Pratihary et al, 2014] which are consistent with those of 0.23-1.25 mmol m 22 d 21 reported earlier in this region [Naik and Naqvi, 2002;Naqvi et al, 2006a]. The N 2 O concentrations in the subpycnocline waters of Mangalore during September are distinctly low in the inner shelf (9.5-11.7 nM) than at midshelf (65-202 nM) despite NO 2 3 is highly depleted (<0.5 mM) under anoxic conditions in the former region (Figures 5b and 5c).…”