ORCID

0000-0001-8819-189X (Mulholland)

Document Type

Article

Publication Date

2026

DOI

10.3389/fmars.2026.1899585

Publication Title

Frontiers in Marine Science

Volume

13

Pages

1899585

Abstract

Large blooms of the diazotrophic cyanobacteria, Trichodesmium, have been re-ported along the north coast of Australia and are readily evident in remote sensing images. During a research cruise in November 1999, we sampled from Townsville to Broome, examined Trichodesmium population densities and their rates of carbon and N₂ fixation alongside microscopy-based cell counts of them and other cyanobacte-rial diazotrophs. Additionally, we also enumerated the picophytoplankton community using flow-cytometry, measured bulk chlorophyll concentrations, carbon and N₂ fixation rates, and water column hydrography, enabling comparison of diazotrophic and picophytoplankton functional groups across the system. Agglomerative hierarchical clustering analysis of physicochemical oceanographic variables identified four distinct clusters that generally corresponded to the five geographic regions traversed and were supported by principal component analysis. In some cases, cluster analysis of physical parameters revealed close similarity of water masses in different, distal regions. High surface densities (up to 10,000 trichomes per L) and visible surface slicks of Trichodesmium were encountered at many of the stations where N₂ fixation could be directly measured in unconcentrated surface samples. Areal rates of N₂ fixation in excess of 1 mmol N m−²d−¹ were noted at several stations, with the highest average N₂ fixation rates found in the Timor Sea. Although spatial relationships between bulk phytoplankton biomass and carbon fixation rates versus contributions from Trichodesmium were complex, Trichodesmium contributed as much as 25% of total primary production and accounted for more than 50% of surface phytoplankton biomass in the Coral Sea. Lastly, Trichodesmium dominance was observed alongside diatom-diazotroph associations, unicellular cyanobacteria Crocosphaera sp., and picophytoplankton assemblages, which collectively indicated that nitrogen fixation and primary production were partitioned among distinct phytoplankton and diazotrophic functional groups across hydrographic gradients.

Rights

© 2026 Capone, Subramaniam, Raut, Montoya, Mulholland, Foster, Furnas and Carpenter.

This is an open-access article distributed under the terms of the Creative Commons Attribution 4.0 International License (CC BY 4.0). The use, distribution or reproduction in other forums is permitted, provided the original authors and the copyright owners are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

Data Availability

Article states: "The original contributions presented in the study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author. We have also established a project site on the BCO-DMO data repository and archive curated at Woods Hole Oceanographic. Project: Collaborative Research: Nitrogen Fixation as a Source of New Nitrogen in the Oligotrophic Tropical Oceans: https://www.bco-dmo.org/project/1000512."

Original Publication Citation

Capone, D. G., Subramaniam, A., Raut, Y., Montoya, J. P., Mulholland, M. R., Foster, R. A., Furnas, M., & Carpenter, E. J. (2026). Regional oceanographic controls on water column nitrogen fixation in northern Australian waters. Frontiers in Marine Science, 13, Article 1899585. https://doi.org/10.3389/fmars.2026.1899585

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