Variability of biological production in low wind-forced regional upwelling systems: A case study off southeastern Australia

Temporal variability of upwelling activity and primary production is examined for a southeastern Australian upwelling system off the Bonney Coast (36.5°S-38.5°S, 138°E-142°E). Three indices of upwelling activity and primary production are developed based on alongshore wind stress, upwelling plume area (anomalous sea surface temperature), and primary productivity of the upwelling plume (approximated using anomalous chlorophyll a concentration). The majority of the upwelling activity occurs during the austral summer upwelling season from November to March. Interannual variability in the wind forcing for this region shows marginal correspondence to El Niño-Southern Oscillation variability. Intraseasonal variability followed four distinct phases within the upwelling season of "onset," "sustained," "quiescent," and "downwelling" periods. The Bonney Upwelling is a predictable regional system with temporal variability of its physical forcing that is similar to larger, more intensely studied coastal wind-driven upwelling systems. However, we find that the Bonney Coast is a low wind-forced upwelling region compared to other upwelling systems. Simple linear quantitative models developed between upwelling activity predictors (wind and upwelling plume area) and the biological response (chlorophyll a) explain 40-50% of the seasonal variability of approximated phytoplankton productivity along the Bonney Coast. As primary productivity in a low-wind system is not affected by the deleterious effects of turbulence and advection, these simple models are able to predict chlorophyll variability in the Bonney region. Models developed in this study provide a method of assessing the effect of decadal or longer variability of chlorophyll concentration in low wind-forcing upwelling systems at regional and global scales. © 2009, by the American Society of Limnology and Oceanography, Inc.