Meridional eddy transport across the Antarctic Circumpolar Current is an essential component of the global meridional overturning circulation and the transport of climate relevant tracers. Challenges in comparing model and observational estimates of the transport arise from varying methodologies describing ‘eddy’ processes. We reconcile the approach used in shipboard surveys of eddies, complemented by satellite eddy tracking, with Reynolds decomposition applied to model outputs. This allows us to estimate the fraction of total meridional tracer transport attributed to coherent eddies in a global 0.1$^\circ$ ocean model. The model realistically simulates observed eddy kinetic energy and three-dimensional characteristics, particularly in representing an observed cyclonic eddy near 150 \degrees E, a hotspot for poleward heat flux. Annual meridional transports due to coherent eddies crossing the Subantarctic Front are estimated by vertically and radially integrating the tracer contents of all eddies. Notably, only cyclonic eddies moving equatorward across the Subantarctic Front contribute to the coherent eddy transport, with no anticyclonic eddies found to cross the front poleward in this region. Applying Reynolds decomposition, our study reveals predominantly poleward meridional transports due to all transient processes in a standing meander, particularly between the northern and southern branches of the Subantarctic Front. Coherent, long-lived eddies tracked from satellite data contribute less than 20\% to transient poleward heat transport, and equatorward nitrate transport in the model. Furthermore, we demonstrate that the integrated surface elevation of mesoscale eddies serves as a reliable proxy for inferring subsurface eddy content.

In the Southern Ocean, subsurface chlorophyll maxima (SCMs) can indicate deep accumulations of phytoplankton. Recent observations of subsurface chlorophyll fluorescence maxima (SFMs) from a large network of biogeochemical Argo (BGC-Argo) floats suggest that Southern Ocean SCMs are widespread. However, the attribution of SFMs to SCMs is not trivial, and SFMs are often observed without the presence of subsurface biomass maxima (SBMs), where biomass is quantified by particulate organic carbon. Consequently, it is questionable if these widespread SFMs represent increased phytoplankton biomass or if they are formed by intracellular processes that alter chlorophyll fluorescence, without a concurrent increase in biomass, such as photo-acclimation or non-photochemical quenching. This study builds confidence in the interpretation of SFMs as SCMs and finds their widespread occurrence of SCMs in the Southern Ocean during summer. We identify SCMs from ship-based chlorophyll sampling and SFMs from fluorometers using a distributional shape-based clustering method which achieves consistent results between ship and BGC-Argo float datasets. Ship data reveal a 15 % disagreement in the identification of SFMs as SCMs. We attribute these uncertainties to non-photochemical quenching corrections and increases in chlorophyll fluorescence yields with depth. In the overlying waters above these SCMs we find increased non-algal contributions to bio-optical POC in the upper mixed layer. These non-algal stocks obscure deep accumulations of phytoplankton biomass and result in the decoupling of SBMs from SCMs in a way that cannot be explained by increases in intracellular chlorophyll fluorescence with depth.

Subsurface accumulations of chlorophyll, also known as deep chlorophyll maxima (DCMs), have been studied in the tropical and temperate oceans for decades, but they have received less attention in the Southern Ocean. Their formation and maintenance are still under debate, as is their contribution to phytoplankton biomass and net primary productivity (NPP). Recently, the application of satellite-based NPP algorithms to data from biogeochemical (BGC)-Argo floats has improved vertically-resolved NPP estimates. Using this new approach on 247 BGC-Argo floats, we report (1) subsurface (below the mixed layer) estimates of NPP, (2) the contribution of subsurface NPP to total NPP, and (3) the influence of DCMs and deep biomass maxima (DBMs, based on particulate backscatter) on (1) and (2). We compare and contrast trends in adjacent latitudinal bands in the southern hemisphere, south of 30°S, from nitrate-limited oligotrophic waters to iron-limited high-nutrient, low-chlorophyll (HNLC) regions. This comparison of pervasive DCMs in oligotrophic waters with the same features in HNLC waters reveals differences in seasonality of DCM occurrence and their contribution to total NPP. Unlike oligotrophic DCMs, HNLC DCMs occur only during spring and summer, and their contribution to total NPP decreases from ~40% to ~25% through the productive season, likely linked to the availability of iron and silicate. When DCMs are present but not accounted for, up to 45% of NPP is not quantified. Our results highlight the importance of understanding the vertical structure of phytoplankton stocks and productivity, with direct impacts on global NPP estimates and, ultimately, the biological carbon pump.

Southern Ocean eddies shape the foraging ecology of marine apex predators such as marine mammals and seabirds. A growing number of animal tracking studies show that predators alter their swimming, diving, and foraging behavior in mesoscale eddies. However, little is known about how Southern Ocean eddies influence the distribution of mesopelagic micronekton (fish, squid, and crustaceans), which are major prey items of megafauna. Studies in other parts of the world have found that eddies can impact the abundance and community composition of micronekton. Here, we analyze acoustic observations from a 14-day survey of a mesoscale eddy, its surrounding waters, and the Sub-Antarctic frontal waters where the eddy originated. We report and interpret spatial patterns of acoustic backscattering at 18 kHz, a proxy indicating combined changes in species, size, and abundance of micronekton. We find that the vertical distribution of Deep Scattering Layers matched the underwater light conditions characteristic of the eddy core, periphery, and surrounding waters, at scales smaller than 10 km. Furthermore, the average water-column integrated acoustic backscattering values in the eddy core were only half of the values measured in the Sub-Antarctic Zone waters surrounding the eddy. By contrast, the acoustic properties of the eddy core were similar to those measured in the Polar Front Zone, where the eddy originated 27 days before our sampling. These results show that, as for physical and chemical tracers, the eddy maintained its biological characteristics from its source waters creating a unique habitat compared to its surrounding waters.