On the upwelling-driven zonation of nitrogen, phytoplankton, and zooplankton in the eastern Great Australian Bight, Australia: A coupled physical-biological modelling study

This study uses a fully coupled physical-biological model to study nutrient enrichment and plankton dynamics in a seasonal coastal upwelling system. This upwelling system, located in the eastern Great Australian Bight, Australia, provides the feeding ground for a range of predatory species including tuna, sea lions, sharks, and whales. The biological model describes the interactions between dissolved nitrogen, phytoplankton, zooplankton, and detritus in response to changes in the physical environment predicted by a standard three-dimensional hydrodynamic model. This study tests a “zonation hypothesis” claiming that, in coastal upwelling systems, zones of maximum nitrogen, phytoplankton and zooplankton develop spatially separated along the coast due to the advective effect of coastal upwelling currents and biological delays in the formation of phytoplankton and zooplankton. While the physical process of wind-driven coastal upwelling is well understood and predictable, several aspects of the biological response simulated in this study are surprising. (i) During the upwelling phase, maximum phytoplankton and zooplankton production occurs in shallower waters alongside the upwelling zone of maximum surface nitrogen. In this upwelling shadow, recycled nitrogen contributes the same amount as physical processes to the local nutrient flux. (ii) Conversely, physical effects offset most of the local phytoplankton growth in the upwelling zone. After wind relaxation, the shutdown of the upwelling process eventually also triggers phytoplankton blooms in this zone. (iii) Wind relaxation creates a narrow coastal countercurrent that operates to maintain the plankton biomass near the upwelling center. For these reasons, the zonation hypothesis does not hold for the coastal upwelling system studied in this work.