Regional oceanographic controls on water column nitrogen fixation in northern Australian waters

Large blooms of the diazotrophic cyanobacteria, Trichodesmium, have been reported 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 cyanobacterial 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, the 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.