Mesoscale eddies drive cross-shelf transport, particle and nutrient biogeochemistry, and the nutritional value of zooplankton

Mesoscale eddies drive a significant component of cross-shelf transport important in the ecology of coastal ecosystems. The Leeuwin Current off Western Australia has a high kinetic energy in southwest WA which peaks and becomes unstable in the austral autumn triggering the formation of eddies. We captured the dynamics of an evolving anticyclonic eddy in situ and we traced water masses as they were incorporated into the eddy. ADCP profiles confirmed periodic offshore movement of ~2 Sv of shelf waters into the forming eddy from the adjacent shelf, carrying a load of shelf-sourced organic particles. Oxygen and nutrient profiles suggested rapid remineralization of nitrate mid-depth in the isolated water mass as it rotated, with a total drawdown of oxygen of 3.6 mol m-2 to 350 m (~0.5 mol O2 m-2 d-1) on the timescale of ~1 week. This implies that nitrate is acting primarily as a regenerated nutrient rather than as a source of new nitrogen. Zooplankton isotopic signatures indicated that warm-core eddies carried animals of poorer nutritional value than cold-core eddies, and this was reflected in the lipid content of rock lobster larvae isolated in the two eddy types. We present a conceptual model of the potential bottom-up control of zooplankton lipid stores by the mesoscale eddy field.