The northern Indian Ocean is home to remarkable phenomena, including seasonally reversing monsoon winds and associated upwelling and productivity, as well as the planet’s largest expanses of oxygen-depleted waters. The upwelling, productivity and hypoxia (open-ocean and coastal) are associated with biogeochemical processes that are of exceptional importance to global bioelement cycles, ocean nutrient inventories, and future climate. Given the large human populations that inhabit the rim nations and depend on marine resources, processes affecting the region also have major societal relevance. However, the two main basins – the Bay of Bengal and the Arabian Sea – also represent striking contrasts, in terms of circulation, upwelling and degree of hypoxia, in riverine and terrestrial influence, and in the effects on associated biogeochemical processes. As a result, the Arabian Sea, and, more recently, the Bay of Bengal have been sites of extensive biogeochemical and ecosystem research. With the advent of new methods and technology, and recognition of fundamental gaps in our understanding of both pelagic and benthic processes, a special issue focused on the northern Indian Ocean region is timely.

The special issue will serve to highlight results of benthic and pelagic process studies in the Arabian Sea and, more recently, the Bay of Bengal and Andaman Sea, including sediment trap deployments and moorings with biogeochemical sensors. It will also be a venue for articles on coastal studies by scientists from northern Indian Ocean rim nations, as well as diverse physical and biogeochemical modelling studies. Finally, the issue will serve to illustrate some of the key research areas that are relevant to the wider Indian Ocean, which have recently become the focus of the Sustained Indian Ocean Biogeochemistry and Ecosystem Research (SIBER) initiative.