Relationships between optical backscattering, particulate organic carbon, and phytoplankton carbon in the oligotrophic South China Sea basin

Abstract

The particulate backscattering coefficient ( b bp ) provides effective proxies for particulate organic carbon (POC) and phytoplankton carbon ( C phy ); however, their bio-optical relationships in the oligotrophic ocean are rarely reported. In this work, based on the in-situ synchronous optical and biogeochemical measurements in the oligotrophic South China Sea (SCS) basin, we refined the regional relationships between POC (and C phy ) and b bp and investigated the impacts of phytoplankton community compositions and size classes on the b bp variability. The observations showed that: 1) POC and C phy exhibited good linear relationships with b bp ; 2) the relationship between C phy and POC could also be fitted in a linear function with a positive POC intercept, and the POC contributed by phytoplankton-covarying non-algal particles was nearly two-fold of C phy ; and 3) the POC-specific b bp ( b* bp ) was positively correlated with the fraction of the phytoplankton groups haptophytes (Type 8) and diatoms to total Chla, but negatively correlated with the fraction of pico-phytoplankton to Chla ( f pico ). These findings suggest that in oligotrophic waters, the variability of b* bp was mainly determined by the variability in the relative contribution of large phytoplankton with complex structures.