Potential of optical and ecological proxies to quantify phytoplankton carbon in oligotrophic waters

Abstract. Satellite ocean color observations provide two proxies to estimate the phytoplankton carbon concentration, Cphyto, then used as input to models quantifying growth rates and primary production, namely the phytoplankton chlorophyll-a concentration, Chl-a, and the particulate backscattering coefficient, bbp. Variability in phytoplankton community composition, pigment assemblages and contribution of non-algal material all interplay in the relation between these proxies and Cphyto, so that no ubiquitous relationship exists between them. It is accordingly still unclear which of Chl-a or bbp is best suited to quantify Cphyto, or whether they both are yet each in specific trophic conditions, especially for low-productivity oligotrophic waters. Here we use a data set from the eastern Indian Ocean that includes phytoplankton cell counts, phytoplankton pigments, particulate organic carbon (POC) and inherent optical properties (IOPs) to perform a comparative assessment of Cphyto derived from either Chl-a or bbp or cell counts combined with allometric relationships. We found significant correlations (r2 > ~0.5–0.6) between the three Cphyto estimates and IOPs, Chl-a or POC when samples from all depths down to 150 m are included. When only the top 25 m are included (amenable to ocean color remote sensing), no significant relationships were found, except between the cytometry-derived Cphyto and both Chl and POC. The bbp-derived Cphyto showed the smallest variability across the entire data set. These results warn about applying to satellite ocean color observations relationships derived from data collected throughout the euphotic layer.