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

Surface chlorophyll a concentrations (C-a, 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 (similar to 1 km(2)/pixel) daily SeaWiFS C-a (C-a(SWF)) data (SeaDAS4.8, OC4v4 algorithm) was an accurate measure of in situ C-a during January-February of 1998-2002 if concurrent in situ data measured by HPLC (C-a(HPLC)) instead of fluorometric (C-a(Fluor)) measurements were used as ground truth. Our analyses indicate that C-a(Fluor) is 2.48 +/- 2.23 (n = 647) times greater than C-a(HPLC) between 0.05 and 1.5 mg m(-3) and that the percentage overestimation of in situ C, by fluorometric measurements increases with decreasing concentrations. The ratio of C-a(SWF)/C-a(PLC) is 1.12 +/- 0.91 (n = 96), whereas the ratio of C-a(SWF)/Ca-Fluor is 0.55 +/- 0.63 (n = 307). Furthermore, there is no significant bias in C-a(SWF) (12% and -0.07 in linear and log-transformed C-a, respectively) when C-a(HPLC) is used as ground truth instead of C-a(Fluor). The high C-a(Fluor)/C-a(HPLC) ratio may be attributed to the relatively low concentrations of chlorophyll b (C-b/C-a = 0.023 +/- 0.034, n = 482) and relatively high concentrations of chlorophyll c (C-c/C-a = 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 degrees S), have C-a(SWF) between 0.05 and 1.5 mg m(-3), we consider that the SeaWiFS performance of C-a retrieval is satisfactory and for this C-a range there is no need to further develop a "regional" bio-optical algorithm to account for the previous SeaWiFS "underestimation". (c) 2006 Elsevier Inc. All rights reserved.