Abstract
The present study made evaluations of SeaWiFS-derived and MODIS-derived Chlorophyll a (Chl a) concentrations in the Northern South China Sea (NSCS), using in situ data collected during two research cruises which were conducted during the summer of 2004 (September 18 to October 8) and 2007 (August 10 to 29). The data of ±48 h and 3 × 3 pixels were used for the comparison between satellite and in situ Chl a data, and the results reveal a systematic overestimation of Chl a concentration by National Aeronautics and Space Administration (NASA) global algorithms (OC2v4, OC4v4, and OC3M). The RMSEs of the selected algorithms are larger than 0.35 except OC2_D’Ortenzio (one regional algorithm for the Mediterranean Sea). The overestimation seems to correlate with numerous (≈77%) low Chl a concentration (< 0.1 mg m -3 ) due to the oligotrophic characteristics of the South China Sea (SCS) in summer, and to correlate with the error in atmosphere correction introduced by aerosols. Therefore, the OC2 and OC4 algorithms for SeaWiFS and OC3M algorithm for MODIS are adapted to NSCS by fitting the satellite data set to in situ Chl a data in NSCS. With the new coefficients based on our field data, the regional version of the three algorithms (TP series) showed good performance with RMSE values of 0.245, 0.245, and 0.288 respectively, which were slightly higher than the algorithm “noise” (0.222 in RMSE). Those TP series algorithms may be considered preliminary due to the relatively small number of available in situ data, and they are suitable in summer season in NSCS.
Highlights
Chlorophyll a (Chl a) concentration, a proxy for phytoplankton abundance, is a valuable indicator of the marine ecosystem, and satellite remote sensing is the only way at present to take frequent measurements of Chl a at regional and ocean-basin scales (Richardson et al 2004)
Two in situ stations located in northwest Luzon show the lowest Chl a values of 0.003 and 0.005 mg m-3 (Figs. 2c, d), which are lower than 0.008 mg m-3 (O’Reilly et al 2000), and two in situ stations located in the costal and upwelling area show higher Chl a values of 0.601, 0.507 mg m-3 (Figs. 2c, d)
There are 17 matching pairs according to the time difference of ±24 h for both Sea-viewing Wide Field-of-view Sensor (SeaWiFS)/in situ and Moderate Resolution Imaging Spectroradiometer (MODIS)/ in situ, and 36 pairs for SeaWiFS/in situ and 35 pairs for MODIS/in situ according to the time difference of ±48 h (Figs. 2c, d)
Summary
Chlorophyll a (Chl a) concentration, a proxy for phytoplankton abundance, is a valuable indicator of the marine ecosystem, and satellite remote sensing is the only way at present to take frequent measurements of Chl a at regional and ocean-basin scales (Richardson et al 2004). Studies on Chl a concentrations in the South China Sea (SCS) have been carried out using satellite sensors including the Coastal Zone Color Scanner (CZCS)(Tang et al 1998), Ocean Color and Temperature Scanner (OCTOBERS)(Tang et al 2002, 2003), Sea-viewing Wide Field-of-view Sensor (SeaWiFS) (Tang et al 2004a, b, 2005; Zhao and Tang 2007; Zheng and Tang 2007). Previous work showed that both SeaWiFS and MODIS Chl a data agreed with in situ measurements in most area of SCS, but to be noted that in situ Chl a values were higher (> 0.1 mg m-3) in these study areas, i.e., along coastal area and near upwelling area (Tang et al 2003; Zhang et al 2006). There are few comparisons between SeaWiFS, MODIS products and in situ data in oligotrophic area in NSCS (Chl a < 0.1 mg m-3)
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