The downward transport of organic particles produced by marine organisms is a key control on the ocean's carbon storage. Measurements of particle attenuation through the water column have historically been used to infer the sequestration potential of the biological carbon pump. While often modelled using a single power-law fit, this simplification may obscure important depth-dependent processes. Using data from biogeochemical-Argo floats in the Southern Ocean, here we show that splitting the water column into two distinct regimes captures depth-dependent variability in particle attenuation more effectively. Compared to the single power-law fit the model reveals greater attenuation just below the productive layer, reducing particulate organic carbon flux into the mesopelagic (200 - 400 m) by 40 - 60 %. Our findings suggest a more mechanistic representation of particle attenuation is needed to improve estimates of carbon transport and the durability of biologically-based marine carbon dioxide removal technologies, and to reduce uncertainties in ocean-climate feedbacks and future climate projections.

Here, we hypothesize that Fe uptake rates by sea-ice algae and under-ice phytoplankton are higher than the rates reported for open ocean phytoplankton in the SO. We performed 55Fe and carbon (14C) short-term uptake field measurements in, on and under Antarctic sea ice. We collected under ice seawater, melted snow and sea-ice cores. We then spiked them with 14C or 55Fe radiotracers to measure Fe and C uptake rates by sea-ice algae. Samples were then filtered, and residual radioactivity on the filters measured liquid scintillation counter (Packard).

The Southern Ocean spring phytoplankton bloom impacts regional food webs and the marine carbon cycle, but we do not fully understand which drivers – environmental, ecological, or biological – control the timing or productivity of the spring bloom. Nutrients, and particularly iron, are likely replete in the austral winter, but the importance of underwater light availability and grazing pressure are topics of ongoing discussion. Furthermore, in the extreme polar winter, phytoplankton physiology may impart additional constraints on phytoplankton variability when ocean mixing decreases. Of particular interest is the impact of highly variable sea ice on the Southern Ocean environment, where over the last decade the Antarctic sea ice extent (SIE) has recorded record highs and lows. The anomalous low in 2023 suggested a new reduced sea-ice state, with unknown impacts on phytoplankton bloom dynamics, including bloom phenology and magnitude. Such changes in SIE will alter the physical environment, and in turn will have profound implications for the Southern Ocean ecosystem. Phytoplankton are especially sensitive to such changes in the physical environment, but understanding and predicting future changes resulting from a reduced sea-ice state remains challenging.

We dissect the spokes of the ferrous wheel associated with Fe demand by quantifying the uptake rates of heterotrophic bacteria and phytoplankton in the subantarctic Southern Ocean during summer and situate these findings within a seasonal context. To do so, we conducted bioassays in which the effects of light on Fe photochemistry and uptake physiology were studied by comparing light and dark incubations, and the effects of DOC supply and competition between phytoplankton and heterotrophic bacteria were examined by isolating bacteria from the larger members of the ferrous wheel by pre-incubation size fractionation.