Phytoplankton community structure and nitrogen nutrition in Leeuwin Current and coastal waters off the Gascoyne region of Western Australia

Within the coastal waters of the eastern Indian Ocean adjacent to Western Australia, we tested the hypothesis that regenerated production (and, by inference, the microbial food web) would predominate in oligotrophic Leeuwin Current (LC) and offshore (OS) surface waters. Conversely, we expected that new production would be more important within the ∼5 times more productive shelf countercurrents (Ningaloo and Capes Currents; NC&CC) and the LC&OS deep chlorophyll maximum (DCM). Phytoplankton species composition and abundance were assessed using both light microscopy and chemotaxonomic methods, and isotopic nitrogen uptake experiments (15NO3−, 15NH4+) were performed at trace (0.05 μM) and saturating (5.0 μM) levels. Phytoplankton community structure was statistically distinct between LC&OS and countercurrent regions. Picoplankton (unicellular cyanobacteria and prochlorophytes) accounted for a mean of 55–65% of pigment biomass in LC&OS waters, with haptophytes as the other primary contributor (21–32%). Conversely, within countercurrent and shelf regions, diatoms (up to 22%) and haptophytes (up to 57%) were more abundant, although cyanobacteria still played an important role (up to 40% of pigment biomass). Absolute NO3− uptake rates for all samples ranged between 0.5 and 7.1 nmol L−1 h−1, and in countercurrent waters were not significantly different at the surface (3.0±2.1 nmol L−1 h−1; mean±SD) compared to the DCM (2.7±2.3 nmol L−1 h−1). However, in LC&OS waters, rates were significantly lower at the surface (1.2±0.7 nmol L−1 h−1) than the DCM (3.9±2.5 nmol L−1 h−1; p=0.05). These values represent conservative estimates for the region due to methodological difficulties encountered with nitrogen uptake experiments in these oligotrophic waters. In contrast with the distinct community composition between different water types, mean estimates of the f-ratio were similar across sampling depths and water types: 0.17±0.07 at the surface and 0.16±0.06 at the DCM of shelf countercurrent waters, and 0.14±0.05 at the surface and 0.14±0.09 at the DCM of LC&OS waters. These results demonstrate the importance of ammonium-based production in both oligotrophic LC&OS waters and the more productive upwelling-influenced countercurrents. They also highlight the utility of chemotaxonomic methods in studying pelagic ecosystem structure along the Gascoyne region of Western Australia.