Continental shelves host 20-30% of global primary production and 90% of the global fish catch, with South-East Australia providing the bulk of fresh fish to Melbourne and Sydney markets. However, this region is a hotspot of global ocean-warming with the intensification of the East Australian Current (EAC). CSIRO’s SEA-MES project aims to understand changes in fish assemblages and species abundances through a comprehensive bio-physical assessment of the shelf slope.
Our July 2023 RV Investigator voyage revealed an offshore EAC eddy near the Bass-Strait entrance, creating a very sharp thermal and density front along the shelf edge that facilitated strong ageostrophic vertical transport and cross-shelf exchanges, resulting in: (i) injection of cool, low O2 and salinity, high nutrient water from deep ocean layers of the Tasman Sea onto the shelf at 100-200m and (ii) export and cascade of dense , high O2, shelf Bass Strait water off the continental shelf towards a 300-500m deep layer. Observations of sub-mesoscale fronts and associated ageostrophic vertical transport are rare and highlight efficient pathways for vertical exchange between the surface and deep ocean. Our findings suggests that the intensification of the EAC poleward extension may increase the occurrences of these sub-mesoscale nutrient pathways.