The Effect of the Leeuwin Current on phytoplankton biomass and production off Southwestern Australia

Unlike most eastern boundary currents, the Leeuwin Current off the west coast of Australia flows poleward and is therefore warm, nutrient-poor and suppresses upwelling. As a result, the waters off Western Australia are relatively oligotrophic. Primary productivity and concentrations of chlorophyll are particularly low in summer, when the water column is stratified and most chlorophyll is concentrated in a layer above the nutricline at approximately 100 m depth. The phytoplankton blooms in late autumn and winter, coinciding with the period of strongest Leeuwin flow. The bloom in winter is maintained by cooling and storms, which promote convective mixing of the upper water column and a shoaling of the nutricline and chlorophyll maximum layer. However, the late autumn bloom coincides with the initial intensification of the Leeuwin Current. Eddies and meanders spin up just beyond the shelf break, and Leeuwin Current water was observed at this time to destratify the water column and flood the shelf with relatively nutrient-rich water. Several potential sources for the nutrients are proposed: upwelling by the eddies and meanders as they induce cross-shelf motions; advection from the north, where the nutricline is shallower; and seasonal cooling.