In the Southern Ocean, subsurface chlorophyll maxima (SCMs) can indicate deep accumulations of phytoplankton. Recent observations of subsurface chlorophyll fluorescence maxima (SFMs) from a large network of biogeochemical Argo (BGC-Argo) floats suggest that Southern Ocean SCMs are widespread. However, the attribution of SFMs to SCMs is not trivial, and SFMs are often observed without the presence of subsurface biomass maxima (SBMs), where biomass is quantified by particulate organic carbon. Consequently, it is questionable if these widespread SFMs represent increased phytoplankton biomass or if they are formed by intracellular processes that alter chlorophyll fluorescence, without a concurrent increase in biomass, such as photo-acclimation or non-photochemical quenching.

This study builds confidence in the interpretation of SFMs as SCMs and finds their widespread occurrence of SCMs in the Southern Ocean during summer. We identify SCMs from ship-based chlorophyll sampling and SFMs from fluorometers using a distributional shape-based clustering method which achieves consistent results between ship and BGC-Argo float datasets. Ship data reveal a 15 % disagreement in the identification of SFMs as SCMs. We attribute these uncertainties to non-photochemical quenching corrections and increases in chlorophyll fluorescence yields with depth. In the overlying waters above these SCMs we find increased non-algal contributions to bio-optical POC in the upper mixed layer. These non-algal stocks obscure deep accumulations of phytoplankton biomass and result in the decoupling of SBMs from SCMs in a way that cannot be explained by increases in intracellular chlorophyll fluorescence with depth.