Unexpected quasi-independence of colored dissolved organic matter absorption from chlorophyll-a concentration in the Southern Ocean

Abstract. The absorption coefficient of colored dissolved organic matter (CDOM), a_y, plays a critical role in driving ocean optical properties and so light attenuation and light-dependent biogeochemical cycles. In the Southern Ocean (SO), however, a_y remains poorly documented because of the scarcity of in situ measurements and the absence of suitable bio-optical models. To address this gap, we derived a_y in surface waters from the diffuse attenuation coefficient (K_d) derived from radiometric measurements performed by Biogeochemical-Argo floats. Sensitivity analyses indicated that the uncertainty of our estimates is mainly driven by K_d, with an overall ∼ 18 % uncertainty of a_y at 380 and 412 nm based on a Monte Carlo approach. The relationships we obtained between a_y and Chl in low-latitude waters are consistent with previous studies but diverge in the SO, with a much weaker dependence on Chl and a larger relative contribution to the absorption budget for clear waters. Possible reasons for this different contribution include CDOM release by sea ice melting, CDOM enrichment of surface layers through deep winter mixing, adaptation of phytoplankton physiology to cold waters and reduced photo degradation during the polar winter.