Marine iodine speciation has emerged as a potential tracer of primary productivity, sedimentary inputs, and ocean oxygenation. The reaction of iodide with ozone at the sea surface has also been identified as the largest deposition sink for tropospheric ozone and the dominant source of iodine to the atmosphere. Accurate incorporation of these processes into atmospheric models requires improved understanding of iodide concentrations at the air-sea interface. Observations of sea surface iodide are relatively sparse and are particularly lacking in the Indian Ocean basin. Here we examine 127 new sea surface (10 m depth) iodide and iodate observations made during three cruises in the Indian Ocean and the Indian sector of the Southern Ocean. The observations span latitudes from ~12oN to ~70oS, and include three distinct hydrographic regimes: the South Indian subtropical gyre, the Southern Ocean and the northern Indian Ocean including the southern Bay of Bengal. Concentrations and spatial distribution of sea surface iodide follow the same general trends as in other ocean basins, with iodide concentrations tending to decrease with increasing latitude (and decreasing sea surface temperature). However, the gradient of this relationship was steeper in subtropical waters of the Indian Ocean than in the Atlantic or Pacific, suggesting that it might not be accurately represented by widely used parameterisations based on sea surface temperature. This difference in gradients between basins may arise from differences in phytoplankton community composition and/or iodide production rates. Iodide concentrations in the tropical northern Indian Ocean were higher and more variable than elsewhere. Two extremely high iodide concentrations (1241 and 949 nM) were encountered in the Bay of Bengal and are thought to be associated with sedimentary inputs under low oxygen conditions. Excluding these outliers, sea surface iodide concentrations ranged from 20 to 250 nM, with a median of 61 nM. Controls on sea surface iodide concentrations in the Indian Ocean were investigated using a state-of-the-art iodine cycling model. Multiple interacting factors were found to drive the iodide distribution. Dilution via vertical mixing and mixed layer depth shoaling are key controls, and both also modulate the impact of biogeochemical iodide formation and loss processes.

The community composition of zooplankton with an emphasis on copepods was assessed in the frontal zones of the Indian sector of the Southern Ocean (SO) during summer 2013. Copepods were the dominant group in both the bongo net and multiple plankton sampler across the entire region. High zooplankton abundance was recorded along each transect in the Polar Front (PF). Community structure in this front was dominated by common taxa, includingCtenocalanus citer,Clausocalanusspp.,Calanoides acutus,Calanus propinquus,Calanus australisandRhincalanus gigas, which together accounted for > 62% of the total abundance.Calocalanusspp.,Neocalanus tonsusandC. propinquuswere indicator species in the Sub-Tropical Front (STF), Sub-Antarctic Front and PF, respectively. A strong contrast in population structure and biovolume was observed between then PF and the STF. The community structure of smaller copepods was associated with the high-temperature region, whereas communities of larger copepods were associated with the low-temperature region. Thus, it seems probable that physical and biological characteristics of the SO frontal regions are controlling the abundance and distribution of zooplankton community structure by restricting some species to the warmer stratified zones and some species to the well-mixed zone.

We report measurements of strontium concentrations and Sr/Ca ratios in the Indian and Southern Oceans, which show that subtle geochemical variations along the main thermocline are the product of CaCO₃ (calcium carbonate) and SrSO₄ (celestite) precipitation and dissolution. Our calculations suggest that celestite skeletons precipitated by Acantharia contribute up to 10 mol % of the combined amount of carbonate and celestite shells precipitated in the Indian Ocean. The data suggest that the distribution of the concentration of strontium in the global ocean is governed by the different modes of deep-water formation in the Southern Ocean and North Atlantic. The formation of Antarctic Bottom Waters from strontium-rich, upwelled deep waters forms a southern end member contrasted with the strontium-depleted North Atlantic Deep Water. The difference in strontium concentrations and Sr/Ca ratios of the different water masses here reported is maintained by precipitation, export and dissolution of CaCO₃ and SrSO₄. These preformed strontium concentrations correlate linearly with nitrate and phosphate concentrations in the Indian Ocean but this correlation is weaker in low latitudes where the mixotrophic life style of Acantharia allows them to thrive in nutrient-depleted environments.

The western Indian continental shelf houses the world’s largest naturally formed coastal low-oxygen zone that develops seasonally during the summer monsoon. We investigated multiple reductive nitrogen transformation pathways and quantified their rates in this system through anaerobic incubations with additions of ¹⁵N-labeled substrates during the anoxic period for three consecutive years (2008–2010). Addition of ¹⁵N labeled ammonium (¹⁵NH₄⁺) resulted in low to moderate anaerobic ammonia oxidation (Anammox) rates in about half of our incubations from the oxygen depleted waters. In contrast, incubations with labeled nitrite (¹⁵NO₂^–) led to large production of ³⁰N₂ over ²⁹N₂ in all incubation experiments, indicating denitrification to be the dominant N-loss pathway. Rates of dissimilatory nitrate/nitrite reduction to ammonium (DNRA) were found to be highly variable and were lower by an order of magnitude than the denitrification rates. Extrapolation of average rates over the sampling periods and volume of anoxic waters showed large nitrogen removal (3.70–11.1 Tg year^–1) which is about three times as high as the previously reported estimate (1.3–3.8 Tg year^–1). Despite the small area it occupies, this shallow seasonal anoxic zone may account for as much as 20–60% the of the total annual fixed nitrogen loss in the perennial oxygen minimum zone of the Arabian Sea.