Validation of SeaWiFS chlorophyll a concentrations in the Southern Ocean: A revisit

Surface chlorophyll a concentrations (Ca, mg m− 3) in the Southern Ocean estimated from SeaWiFS satellite data have been reported in the literature to be significantly lower than those measured from in situ water samples using fluorometric methods. However, we found that high-resolution (∼ 1 km2/pixel) daily SeaWiFS Ca (CaSWF) data (SeaDAS4.8, OC4v4 algorithm) was an accurate measure of in situ Ca during January–February of 1998–2002 if concurrent in situ data measured by HPLC (CaHPLC) instead of fluorometric (CaFluor) measurements were used as ground truth. Our analyses indicate that CaFluor is 2.48 ± 2.23 (n = 647) times greater than CaHPLC between 0.05 and 1.5 mg m− 3 and that the percentage overestimation of in situ Ca by fluorometric measurements increases with decreasing concentrations. The ratio of CaSWF/CaHPLC is 1.12 ± 0.91 (n = 96), whereas the ratio of CaSWF/CaFluor is 0.55 ± 0.63 (n = 307). Furthermore, there is no significant bias in CaSWF (12% and − 0.07 in linear and log-transformed Ca, respectively) when CaHPLC is used as ground truth instead of CaFluor. The high CaFluor/CaHPLC ratio may be attributed to the relatively low concentrations of chlorophyll b (Cb/Ca = 0.023 ± 0.034, n = 482) and relatively high concentrations of chlorophyll c (Cc/Ca = 0.25 ± 0.59, n = 482) in the phytoplankton pigment composition when compared to values from other regions. Because more than 90% of the waters in the study area, as well as in the entire Southern Ocean (south of 60° S), have CaSWF between 0.05 and 1.5 mg m− 3, we consider that the SeaWiFS performance of Ca retrieval is satisfactory and for this Ca range there is no need to further develop a “regional” bio-optical algorithm to account for the previous SeaWiFS “underestimation”.