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

Surface chlorophyll a concentrations (Ca, mg m ) 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 highresolution (∼1 km/pixel) daily SeaWiFS Ca (Ca) 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 (Ca ) instead of fluorometric (Ca ) measurements were used as ground truth. Our analyses indicate that Ca Fluor is 2.48±2.23 (n=647) times greater than Ca HPLC between 0.05 and 1.5 mg m and that the percentage overestimation of in situ Ca by fluorometric measurements increases with decreasing concentrations. The ratio of Ca /Ca HPLC is 1.12±0.91 (n=96), whereas the ratio of Ca /Ca Fluor is 0.55±0.63 (n=307). Furthermore, there is no significant bias in Ca SWF (12% and −0.07 in linear and log-transformed Ca, respectively) when Ca HPLC is used as ground truth instead of Ca . The high Ca /Ca HPLC 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 Ca SWF between 0.05 and 1.5 mg m, 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”. © 2006 Elsevier Inc. All rights reserved.