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 mu m from the nutricline at the base of the Leeuwin Current's mixed layer, with total water column production reaching a maximum of similar to 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 delta(15)N ranging from similar to 4 to 5 parts per thousand under both upwelling and non-upwelling ( winter) conditions. Unexpectedly, significant nutrient enrichment within the Leeuwin Current ( up to 3.1 mu m 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.