Chapter 12 - Oxygen, carbon, and pH variability in the Indian Ocean

Abstract

This chapter provides an overview of the physical and biogeochemical factors that control spatial and temporal patterns of oxygen minimum zones (OMZs) and carbon and pH variability in the Indian Ocean. Oxygen concentrations decline to nearly zero in the Arabian Sea intermediate water with profound biogeochemical impacts on the nitrogen cycle as a result of denitrification. These impacts can hardly be observed in the Bay of Bengal, where oxygen concentrations are poised just above the threshold below which denitrification becomes significant. Hypoxic/anoxic conditions in the open ocean waters of the northern Indian Ocean have not dramatically changed over past decades, but evidence is now emerging that oxygen concentrations are starting to decline, with significant biogeochemical and ecological impacts. The Indian Ocean accounts for ∼1/5 of the global oceanic uptake of atmospheric CO2, with the Arabian Sea as a source of CO2 to the atmosphere and the southern subtropical gyre as a CO2 sink. Net CO2 flux in the Bay of Bengal is uncertain due to sparse sampling. Surface pH values in the Indian Ocean are anomalously low and projected to decline further with negative impacts on calcifying organisms. Dissolved organic carbon (DOC) concentrations in the Indian Ocean tend to be high in near-surface (sub) tropical waters where autotrophic production of DOC exceeds heterotrophic consumption and vertical stability of the water column favors accumulation. In contrast, the highest particulate organic carbon (POC) concentrations in the Indian Ocean are observed in the northwestern part of the basin and the lowest in the southern subtropical gyre, reflecting primary production patterns. POC export flux patterns in the Indian Ocean are similar to the patterns in POC concentration, though carbon flux is also strongly influenced by lithogenic matter content in river-influenced regions like the Bay of Bengal. Observational and modeling research should target an improved understanding of northern Indian Ocean OMZ and carbon system variability, as such is needed to predict the impacts of anthropogenic influence and global warming on Indian Ocean biogeochemistry and ecosystems.