Biogeochemical Argo Floats Reveal the Evolution of Subsurface Chlorophyll and Particulate Organic Carbon in Southeast Indian Ocean Eddies

Eddies modulate open ocean productivity, and this influence depends on both eddy source and evolution. Southeast Indian Ocean eddies are important pathways for the westward transport of biogeochemical anomalies from the Leeuwin current into the central oligotrophic South Indian Ocean (SIO). Eddy processes at the base of the mixed layer may stimulate and sustain phytoplankton, allowing these eddy impacts to persist over thousands of kilometers. We present 4 months of high-frequency profiles from autonomous floats in one anticyclonic and one cyclonic eddy in the SIO. At the start of observations, from September to October, particulate organic carbon (POC) and especially chlorophyll were higher in the cyclone, and evenly distributed throughout the mixed layers in both eddies. As spring progressed and the eddies were transported westward, chlorophyll and POC concentrated at the base of the mixed layer at depths invisible to satellites, likely reflecting nutrient depletion in overlying waters. In the anticyclone, the increased chlorophyll at depth occurred as POC decreased, suggesting photo-acclimation and thus both light and nutrient stress. In contrast, in the cyclone chlorophyll to POC ratios remained close to constant as their subsurface maxima formed. In both eddies, the subsurface biomass maxima exhibited no significant change in oxygen saturation state over several months suggesting these communities are sustained by low ongoing productivity in balance with community respiration. Thus, deep biomass layers may represent a mechanism for long-distance transfer of eddy plankton communities which is not reflected in satellite remote sensing.Plain Language Summary Eddies are spinning vortices of water about 100-200 km across. They are everywhere in the ocean, and they move heat, salt, nutrients, and biomass both vertically and horizontally. In the southeast Indian Ocean, eddies are created in the Leeu win Current along the coast of Western Australia. These eddies are unusual because the clockwise-spinning (cyclonic) eddies have lower surface productivity than their anticyclonic counterparts. Both types of eddies last for months to years and move westward over time. Satellite measurements of surface chlorophyll, a proxy for ocean productivity, indicate decreased chlorophyll in both types of eddies, during the transition to summer. Our measurements from robotic floats show that the satellites see decreased chlorophyll because it is redistributing deeper (>30-50 m) out of view of the satellites. This redistribution appears to also affect other components of the ecosystem, because particulate organic carbon levels, estimated from light scattering sensors, also become focused into subsurface layers. At the base of the ocean surface mixed layer, about 100-150 m, these layers of plankton biomass exhibit a balance between oxygen production from photosynthesis and oxygen consumption by other trophic levels, suggesting the layers remain active over long periods.