Lagrangian characterization of nitrate supply and episodes of extreme phytoplankton blooms in the Great Australian Bight

Abstract. Phytoplankton growth is the foundation for energy transfer into higher trophic levels, influences climate by the uptake of atmospheric CO₂, and plays an important role in nutrient cycling. Here we use a novel lagrangian approach to characterize the nitrate supply to the Great Australian Bight, identify episodes of extreme phytoplankton blooms and ascertain the origin of the nitrate sources that fuel them. We find that 55 % of nitrate used by phytoplankton enters the GAB in the upper 100 m and that 88 % originates locally from a region between the GAB and the Sub Antarctic Front, rather than from more remote oceans; thus, most of the nitrate is recycled locally. Our results show extreme phytoplankton blooms have an annual periodicity, peaking in the Austral autumn when the mixed layer deepens. This suggests that stratification erosion is key supplying nutrients into the euphotic zone and triggering these episodes.