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

Abstract

The absorption coefficient of coloured dissolved organic matter (CDOM), ay, plays a critical role in driving ocean optical properties and thereby light attenuation and light-dependent biogeochemical cycles. In the Southern Ocean (SO), however, ay 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 ay in surface waters from the diffuse attenuation coefficient (Kd) derived from radiometric measurements performed by Biogeochemical-Argo floats. Sensitivity analyses using Monte Carlo simulations indicated that the uncertainty of our estimates is mainly driven by the uncertainty in Kd, and is overall ∼ 18 % for ay at 380 and 412 nm. Our derived ay vs. chlorophyll-a concentration (Chl) relationships for low-latitude waters are consistent with previously published relationships. They, however, diverge in the SO, with a larger relative contribution of ay to the absorption budget for clear waters (Chl < ∼ 0.2 mg m−3) and the opposite for greener waters, leading to a weaker dependence of ay on Chl. Lower-than-expected CDOM absorption mostly happens during the austral summer, suggesting significant photobleaching or lower biologically-mediated production. The relative contributions of CDOM and phytoplankton to the absorption budget are also found to diverge from what bio-optical models predict, with implication for interpretation of satellite ocean colour observations in the SO.