Seasonal production regimes off south-western Australia: influence of the Capes and Leeuwin Currents on phytoplankton dynamics

Temporal primary production dynamics were investigated off south-western Australia, where the summer upwelling regime of the Capes Current was compared with early winter conditions characterised by strengthened near-shore Leeuwin Current flow. Seasonal upwelling in this region sourced nitrate levels of =1 µm from the nutricline at the base of the Leeuwin Current’s mixed layer, with total water column production reaching a maximum of ~950 mg C m-2 day-1 in the Capes Current. Stable isotope signatures of particulate matter indicated that productivity off south-western Australia was heavily reliant on nitrate as a nitrogen source, with mean d15N ranging from ~4 to 5 ‰ under both upwelling and non-upwelling (winter) conditions. Unexpectedly, significant nutrient enrichment within the Leeuwin Current (up to 3.1 µm nitrate) occurred during winter, likely as a result of the meandering Leeuwin Current flooding the inner shelf north of the study area and entraining relatively high-nutrient shelf waters in its southwards flow. However, early winter production under these nutrient-replete conditions (mean ± s.d. 310 ± 105 mg C m-2 day-1) was significantly lower than in summer (695 ± 140 mg C m-2 day-1) due to light limitation, both as a result of reduced surface irradiance characteristic of the winter months and significantly higher light attenuation within the water column as compared with summer conditions.