‘We still say that NASA satellite algorithms under-estimate chlorophyll in the Southern Ocean’

Chlorophyll fluorescence measurements taken using Biogeochemical-Argo (BGC-Argo) profiling floats offer unique potential for validating Southern Ocean satellite ocean color products. The spatial and temporal distribution of observations is far superior to ship-based in-situ chlorophyll measurements, derived from High Performance Liquid Chromatography (HPLC) methods which have historically been used to validate and re-calibrate ocean color algorithms. A 2013 HPLC-based study suggested that the NASA OCM4 global ocean color algorithms significantly under estimate chlorophyll concentrations in the Southern Ocean. Contradictory to this, a recent study using BGC-Argo derived chlorophyll has suggested that the NASA OCI global ocean color algorithms agree with in-situ measurements, and that corrected OCM4 algorithms overestimate chlorophyll in the Southern Ocean.

We explore why assessments from BGC-Argo floats and ship-based HPLC measurements yield different conclusions about satellite ocean colour algorithms. A new HPLC data set with increased spatial coverage for the Southern Ocean is employed, alongside the SOCCOM BGC-Argo dataset, to re-assess systematic satellite underestimation of chlorophyll. We find that the OCI, like the OCM4 global ocean colour algorithm underestimates Southern Ocean chlorophyll when compared to HPLC chlorophyll. It is also revealed that phytoplankton communities near the Polar Front (a High Nutrient Low Chlorophyll region) have fluorescence yields per unit chlorophyll up to double that of surrounding waters, which may explain the BGC-Argo/HPLC differences.