Oceanography, primary production and dissolved inorganic nitrogen uptake in two Leeuwin Current eddies

The Leeuwin Current (LC) is an unusual poleward-flowing eastern boundary current that carries warm, low-salinity water southward along the coast of Western Australia. LC dynamics include the formation of a dynamic mesoscale eddy field whose biological dynamics have not been studied. Satellite altimetry indicates that the eddies studied in the 2003 field programme were dynamically typical of LC eddies, but the warm-core (WC) eddy was relatively large and long-lived. The WC eddy contained relatively elevated chlorophyll a concentrations thought to originate, at least in part, from the continental shelf/shelf break region and to have been incorporated during eddy formation. Primary production per unit volume in the WC eddy was ∼2× higher than in the cold-core (CC) eddy due to an historical accumulation of chlorophyll a over the period since eddy formation (5–6 months), though chlorophyll a-specific daily production was volumetrically ∼50% greater in the CC eddy. In the WC eddy, nitrate uptake rates were 4× greater than in the CC eddy, despite the fact that vertical diffusive fluxes of nitrate into the WC eddy were probably only 50% of those in the CC eddy. We therefore hypothesize that other nitrate sources were important, possibly including isopycnal mixing and/or lateral transport into the eddy from surrounding waters. In addition, a deep mixed layer favoured a large (>5 μm) diatom population within the centre of the WC eddy while the CC eddy was persistently stratified, with a shallower mean mixed-layer depth (∼100 m vs. ∼200 m for the WC eddy) and a well developed deep chlorophyll a maximum at ∼100 m composed of 30% prochlorophytes (not capable of taking up nitrate). Both factors probably contributed to higher f-ratios in the WC and in the >5 μm phytoplankton relative to total phytoplankton.