MEPS 516:85-102 (2014)  -  DOI: https://doi.org/10.3354/meps11009

ABSTRACT: The contribution of planktonic diazotrophs to the overall N budget is a key unknown in the eastern Indian Ocean. Here we investigated the relationships between dissolved inorganic nutrients, phytoplankton pigment composition, microbial community structure, nitrogen fixation rates and the δ¹⁵N of fractionated zooplankton samples along the shelf break of Western Australia (32° to 12°S) in September 2012. Bulk nitrogen fixation rates declined from 4.8 nmol l^-1 h^-1 in the colder and more saline sub-tropical waters at higher latitudes to 1.5 nmol l^-1 h^-1 in the warmer and fresher Timor Sea at lower latitudes. A regional bloom of Trichodesmium was identified between 13° and 9°S in the Timor Sea. Trichodesmium-specific N₂ fixation rates were 0.05 ± 0.01 nmol colony^-1 h^-1. Highest dissolved inorganic nitrogen (DIN) concentrations occurred at the highest NH₄⁺:NO₃^- ratios, thereby deviating from the paradigm that greater DIN concentrations come primarily from increased NO₃^- through advection, mixing or upwelling. Both the microplankton and nanoflagellate fraction declined significantly in warmer waters, with higher DIN concentrations but decreasing % NO₃^-. A clear increase in the prokaryotic diagnostic pigment zeaxanthin was seen with increasing temperatures from higher to lower latitudes. The microbial community, measured using automated ribosomal intergenic spacer analysis (ARISA), clustered strongly according to the water mass biogeochemistry including temperature, salinity, DIN and phosphate concentrations (p < 0.001). Isotope analysis suggested that injections of low δ¹⁵N from N₂ fixation lowered the zooplankton δ¹⁵N signature of animals up to ~500 µm in size and that nearly 47% of the fixed nitrogen was used by zooplankton (≤500 µm fraction) in the Timor Sea.

KEY WORDS: Indian Ocean · Nitrogen fixation · Phytoplankton community · Trophic transfer · ARISA · Trichodesmium

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Cite this article as: Raes EJ, Waite AM, McInnes AS, Olsen H, Nguyen HM, Hardman-Mountford N, Thompson PA (2014) Changes in latitude and dominant diazotrophic community alter N2 fixation. Mar Ecol Prog Ser 516:85-102. https://doi.org/10.3354/meps11009 Export citation Share:    Facebook - - linkedIn