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

[摘要] The western Indian continental shelf is one of the most productive coastalsystems of the world ocean. This system experiences extreme changes in itsoxygen regime, being normoxic from November to May and suboxic(denitrifying)/anoxic from June to October, owing to the biogeochemicalresponse to cyclical monsoonal influence. In order to understand the impactof the seasonally varying oxygen regime on benthic mineralization, nutrientexchange and, in turn, on the shelf ecosystem, we carried out the first everintact-core incubations during two contrasting seasons – spring intermonsoonand fall intermonsoon (late southwest monsoon) at a 28 m-deep fixed site onthe inner shelf off Goa, dominated by fine-grained cohesive sediments. Theresults showed that incomplete sediment oxygen consumption (SOC) occurredduring April as opposed to the complete SOC and subsequent sulfide fluxobserved in the fall intermonsoon incubations. The sediments acted as aperennial net source of DIN (dissolved inorganic nitrogen i.e. NO₃⁻ +NO₂⁻ +NH₄⁺), PO₄³⁻ and SiO₄⁴⁻ to theoverlying water column. The efflux of DIN increased from 1.4 to3.74 mmol m⁻² d⁻¹ from April to October, of which NH₄⁺flux comprised 59–100%. During the oxic regime, ∼75% ofdiffusing NH₄⁺ appeared to be nitrified(2.55 mmol m⁻² d⁻¹), of which ∼77% remained coupledto benthic denitrification. Consequently, 58% of NH₄⁺ flux waslost in active coupled nitrification–denitrification, resulting in substantialN loss (1.98 mmol m⁻² d⁻¹) in the sediments. The continentalshelf sediments switched over from being a NO₃⁻ source during the oxicregime to a NO₃⁻ sink during the anoxic regime. During suboxia,benthic denitrification that is fed by NO₃^- from the overlying watercaused N loss at the rate of 1.04 mmol m⁻² d⁻¹. Nitrogen losscontinued even under sulfidic conditions during October, possibly through thechemolithoautotrophic denitrification, at a potential rate of3.21 mmol m⁻² d⁻¹. Phosphate flux increased more than 4-foldduring October as compared to April, due to reductive dissolution of Fe- andMn oxides. The SiO₄⁴⁻ flux increased during October apparently due tothe higher availability of siliceous ooze from diatom blooms commonlyoccurring in the monsoon season.

Slow oxidation of organic carbon (C_org) under anoxia, lowertemperature and reduced benthic faunal activity appeared to decrease benthicmineralization by 25% as suggested by the drop in the C_orgoxidation rate from 63.8 mmol C m⁻² d⁻¹ in April to 47.8 mmolC m⁻² d⁻¹ in October. This indicated a higher preservation ofC_org during the late southwest monsoon. Sediment porosity,C_org content and nutrients did not show significant variationsfrom April to October. Porewaters were found to be enriched withNH₄⁺, PO₄³⁻ and SiO₄⁴⁻ but depleted in NO₃⁻and NO₂⁻ in these organic-rich sediments. Significant DIN,PO₄³⁻ and SiO₄⁴⁻ effluxes indicate the potential of benthicinput in meeting nutrient demand of the phytoplankton community in thisseasonally N-limited shelf system.

[学科分类] 地球化学与岩石