Carbon and nitrogen productivity during spring in the oligotrophic Indian Ocean along the GO-SHIP IO9N transect

Abstract There is limited biogeochemical rate data from the oligotrophic central Indian Ocean, but it is known that there are geographical gradients in the physical and chemical conditions that may lead to unique biogeochemical regimes. As participants on a GO-SHIP repeat hydrography cruise, a transect was completed in spring of 2016 from 28 °S to 18 °N in the Indian Ocean, roughly along the 95 °E meridion. Cell count samples (phytoplankton and heterotrophic bacteria) analyzed by flow cytometry, and samples for carbon and nitrogen productivity incubations, assessed by stable isotopic tracers, were obtained from 20 m at approximately every other degree of latitude. Microbial cell counts by flow cytometry indicate that Prochlorococcus was the principal autotroph, but with increasing contributions of Synechococcus around the equator. Large eukaryotes (> 20 μm), imaged using FlowCAM, were generally absent or in very low abundance ( −1 ) from near-surface waters. Heterotrophic bacteria contributed 28.4 ± 4.6% of the microbial community biomass and were positively correlated to autotrophic biomass. Dissolved nitrate and phosphate concentrations were below detection limits throughout the surface 50 m, while ammonium was undetectable at almost all stations and depths. In the 50–200 m depth range, nitrate and phosphate concentrations steadily increased from south to north, reaching a maximum of 31 and 2.2 μM respectively north of 6 °N. The ratio of dissolved NO 3 - :PO 4 - standing stocks was always below 15. Urea concentrations (0.02–0.18 μmol L −1 ) showed an increasing trend towards the north and dissolved organic phosphorus concentrations (0.25–0.37 μmol L −1 ) had a maxima between 5 and 15 °S. Absolute uptake rates of nitrate, ammonium and urea were all less than 1.5 nmol N L −1 h −1 south of 15 °S, and increased until local maxima of 3.4, 9.0, and 21 nmol N L −1 h −1 for nitrate, ammonium, and urea respectively, were reached between 1.5 and 6.5 °N. Ammonium and urea uptake rates were consistently 3–8 times higher than nitrate uptake rates, with a mean f-ratio of 0.23 for the entire transect. Primary production followed the same trend of nitrogen uptake, with a local maximum of 7.4–10.8 nmol C L −1 h −1 found 1.5–6.5 °N. 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.