Juan Li ¹ David Antoine ^1* Yannick Huot ²

• ¹ Curtin University, Perth, Australia
• ² Université de Sherbrooke, Sherbrooke, Quebec, Canada

The composition and size distribution of particles in the ocean control their optical (scattering and absorption) properties and a range of biogeochemical and ecological processes. Therefore, they provide important information about the pelagic ocean ecosystem structure and functioning, which can be used to assess primary production, particle sinking, and carbon sequestration. Due to its harsh environment and remoteness, the particulate bio-optical properties of the Southern Ocean (SO) remain poorly observed and understood. Here, we combined field measurements from hydrographic casts from two research voyages and from autonomous profiling floats (BGC-Argo) to examine particulate bio-optical properties and relationships among several ecologically and optically important variables, namely the phytoplankton chlorophyll 𝑎 concentration (Chl), the particulate absorption coefficient (𝑎 ! ), the particulate backscattering coefficient (𝑏 "! ) and the particulate organic carbon (POC) concentration. In the clearest waters of the SO (Chl < 0.2 mg m -3 ), we found a large contribution to absorption by non-algal particles (NAP) at 442 nm, up to 10 times the absorption of phytoplankton, making the particulate bio-optical properties there remarkably different from typical oceanic case 1 waters. A matchup analysis confirms the impact of this larger NAP absorption on the retrieval of Chl from satellite ocean colour observations. For waters with Chl > 0.2 mg m -3 , no significant differences are noticed between the SO and temperate waters. We also show consistency between predictions of phytoplankton carbon either from Chl or 𝑏 "! , suggesting that both approaches can be used on the SO.