Variability in nitrogen uptake and fixation in the oligotrophic waters off the south west coast of Australia

The results presented in this study provide the first broad-scale regional characterisation of the phytoplankton-nitrogen dynamics across the continental shelf between the Abrolhos Islands and Cape Leeuwin, Western Australia. We demonstrate key features of N-uptake, including nitrate and ammonium uptake and N fixation, across the dominant current systems of the west coast of Australia during a 2-week period in spring–summer. The surface waters of the region were N impoverished with low phytoplankton biomass. Nitrate and ammonium uptake rates were among the lowest on record, and f-ratios were high compared to other oligotrophic systems (f-ratio ∼0.5 throughout), indicating an equal preference for nitrate and ammonium uptake. Areas of high phytoplankton biomass were most likely fuelled by nitrate transported into the euphotic zone during temporally short bursts of upwelling on ∼weekly, rather than monthly time scales. When upwelling had relaxed, phytoplankton productivity was largely maintained by microbial regeneration in the euphotic zone. A large proportion (20–60%) of the phytoplankton biomass was comprised of picoplankton and nanoplankton cyanobacteria. However, rates of N2 fixation were 2 orders of magnitude lower than dissolved inorganic N uptake, indicating that N2-fixation did not significantly contribute to phytoplankton productivity during the study.