The role of the Leeuwin Current and mixed layer depth on the autumn phytoplankton bloom off Ningaloo Reef, Western Australia

On a seasonal cycle, the phytoplankton biomass off the North West Cape region of Australia surrounding Ningaloo Reef increased during the austral autumn and peaked in winter. In this study, historical field data sets, satellite-derived ocean color observations and output from a data-assimilating numerical ocean model are used to investigate physical mechanisms that influence the seasonal variability in phytoplankton biomass off this coast. The mixed layer depth (MLD) off Ningaloo deepened to ∼100 m in autumn, partly due to the acceleration of the Leeuwin Current, which induced an increase in nutrient concentrations and is the key mechanism responsible for the seasonal enhancement in chlorophyll a concentrations observed in the waters off Ningaloo Reef. By comparing the MLDs and the critical depth it is suggested that while average irradiance throughout the MLD was low, this had less impact on net growth than grazing.