LAUSR

Abstract Mesoscale eddies are ubiquitous in the East Australian Current (EAC), a nutrient poor western boundary current, and exert considerable impact on the biogeochemical characteristics of the region. However, in-situ biogeochemical studies of these eddies are relatively rare. Here we combine ship-based and satellite measurements to characterize the physical and biogeochemical properties of three EAC eddies – one cyclone and two anticyclones – in the austral fall of 2018. Deep Chlorophyll Maxima (DCM) were observed in the two northernmost eddies, with Chlorophyll-a concentrations ranging from 0.4 to 1.3 mg m-3 and generally decreasing with increasing depth of the DCM. The anticyclone farthest south, which was also the oldest observed eddy, was more productive and contained higher biomass than the northern eddies, including the cyclone. Deeper mixed layers in this anticyclone suggest that surface cooling resulting from poleward travel and seasonal cooling might have led to increased convection and an injection of nutrients. All eddies were nitrogen limited, and nitrogen fixation likely occurred in the surface waters of the two northernmost eddies. Where DCM were present, their depth was well explained as a consequence of an optimisation of light and nitrogen supply by phytoplankton. We observed prominent primary nitrite maxima (PNM) in the eddies. Their depths were highly correlated with, and consistently below, the depths of DCM, indicating that incomplete nitrate reduction by phytoplankton under low-light conditions was likely the main driver for their production. To our knowledge, this is the first description of PNM in eddies of the EAC.