Award: OCE-1559021

The Indian Ocean accounts for 15-20% of global ocean primary production and the northern regions (Arabian Sea and Bay of Bengal) contain major oxygen minimum zones, and thus it is a key component in the global ocean/climate system. There has been extensive work done on the northern, eastern, and southern margins of the Indian Ocean, including the Arabian Sea, the Bay of Bengal, the Leeuwin Current around Australia and the Indian section of the Southern Ocean, yet we currently have a relative limited understanding of the variation in phytoplankton diversity and productivity in the central Indian Ocean and linkages to the ocean/climate system. Previous studies have described gradients in the physical and chemical conditions of the eastern tropical Indian Ocean that indicate unique biological and biogeochemical regions. Changes in temperature and nutrient supply to the surface ocean vary across a north/south gradient. The Southern Indian Ocean gyre, extending from approximately 30°S - 10°S, has relatively cooler surface waters (ca. 25°C) and very low nutrients, while the equatorial region of the Indian Ocean (10°S - 5°N) is an area of weak upwelling, resulting in somewhat higher nutrient concentrations and productivity. Further to the north, the Bay of Bengal is characterized by even warmer waters and elevated nutrient supply from the surrounding margins, leading to higher productivity. The objectives of our project were two-fold. The first objective was to characterize the phytoplankton community via direct observational measurements across this north/south gradient and to compare those observations with simultaneous measurements of primary and nitrogen production. The second objective was to experimentally evaluate the role of different nutrients in limiting primary production through directed nutrient manipulation incubations. Direct phytoplankton counts show that the cyanobacterium Prochlorococcus was the principal autotroph along the entire transect, but that a related cyanobacterium, Synechococcus, had increasing contributions near the equator. Heterotrophic bacteria comprised about 25% of the microbial community and were positively correlated to cyanobacteria abundance. Large eukaryotic phytoplankton were generally absent or at very low abundance in the near-surface waters. Uptake rates of nitrogen-containing compounds nitrate, ammonium and urea were all low but increased along the transect from south to north. Ammonium and urea uptake rates were consistently 3-8 times higher than nitrate uptake rates. Primary production followed the same trend of nitrogen uptake, with a local maximum at the north end of the transect. These data represent some of the first reported measurements of primary production, nitrogen uptake, and phytoplankton diversity across biogeochemical provinces in the central oligotrophic Indian Ocean, a large but understudied region of the global ocean. Nutrient addition experiments indicated nitrogen was the primary limiting nutrient of autotrophic biomass accumulation, but specific phytoplankton groups pointed to a more complex nutrient limitation mosaic. Prochlorococcus and small eukaryotic phytoplankton were both limited by multiple nutrients (nitrogen, phosphorus and/or iron) in the Bay of Bengal. Comparisons of nutrient ratios in cells and underlying source waters indicate a gradient of nitrogen and iron stress along the transect. These data demonstrate that autotroph communities are poised near multiple nutrient limitation horizons in extremely oligotrophic waters far from micronutrient sources. The results of this research will be used to contrain rates within biogeochemical models of the Indian Ocean, as well as the interactions of phytoplankton and nutrients in these models. Data collected as part of this project are archived and available through the Biological and Chemical Oceanography Data Management Office (https://www.bco-dmo.org/dataset/723191). Last Modified: 03/25/2019 Submitted by: Michael Lomas