Unifying theories on the Southern Ocean bio-optical anomaly

Phytoplankton in the Southern Ocean play a major role in regulating global biogeochemical cycles, but the region's remoteness makes in-situ monitoring challenging. Satellite ocean colour is among the best tools to monitor phytoplankton dynamics synoptically through retrievals of the chlorophyll-a concentration (Chl-a). However, optical properties in the Southern Ocean deviate from global patterns, causing persistent ocean-colour anomalies and biased Chl-a retrievals. Theories have attributed these anomalies to differences in either phytoplankton composition or non-algal constituents relative to global norms, each capturing important facets but without a unified explanation. Here, using an extensive set of in-situ measurements, we demonstrate that phytoplankton size structure in the Southern Ocean is fundamentally different to other ocean basins. Incorporating this distinct size distribution into an ocean-colour model, with the same set of optical parameters, successfully reproduces the observed bio-optical anomaly. Model simulations further reveal that these size shifts affect bulk optical properties of both phytoplankton and non-algal constituents, providing a unified explanation that reconciles the observed regional anomalies within a global framework. Tracking changes in phytoplankton size structure is key to improving satellite Chl-a trends in the Southern Ocean, with important implications for improved monitoring of the regional biogeochemical cycles that sustain Earth's climate system.