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

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. NO₃⁻ + NO₂⁻ + NH₄⁺), PO₄³⁻ and SiO₄⁴⁻ to the overlying water column. The efflux of DIN increased from 1.4 to 3.74 mmol m⁻² d⁻¹ from April to October, of which NH₄⁺ flux comprised 59–100%. During the oxic regime, ∼75% of diffusing NH₄⁺ appeared to be nitrified (2.55 mmol m⁻² d⁻¹), of which ∼77% remained coupled to benthic denitrification. Consequently, 58% of NH₄⁺ flux was lost in active coupled nitrification–denitrification, resulting in substantial N loss (1.98 mmol m⁻² d⁻¹) in the sediments. The continental shelf sediments switched over from being a NO₃⁻ source during the oxic regime to a NO₃⁻ sink during the anoxic regime. During suboxia, benthic denitrification that is fed by NO₃^- from the overlying water caused N loss at the rate of 1.04 mmol m⁻² d⁻¹. Nitrogen loss continued even under sulfidic conditions during October, possibly through the chemolithoautotrophic denitrification, at a potential rate of 3.21 mmol m⁻² d⁻¹. Phosphate flux increased more than 4-fold during October as compared to April, due to reductive dissolution of Fe- and Mn oxides. The SiO₄⁴⁻ flux increased during October apparently due to the higher availability of siliceous ooze from diatom blooms commonly occurring in the monsoon season.

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 NH₄⁺, PO₄³⁻ and SiO₄⁴⁻ but depleted in NO₃⁻ and NO₂⁻ in these organic-rich sediments. Significant DIN, PO₄³⁻ and SiO₄⁴⁻ effluxes indicate the potential of benthic input in meeting nutrient demand of the phytoplankton community in this seasonally N-limited shelf system.