Estimation of chlorophyll-a concentration in estuarine waters: case study of the Pearl Riverestuary, South China Sea

Abstract

The objective of this work is to estimate chlorophyll-a (chl-a) concentration in the PearlRiver estuary in China. To test the performance of algorithms for the estimation of thechl-a concentration in these productive turbid waters, the maximum band ratio (MBR) andnear-infrared–red (NIR–red) models are used in this study. Specific focus is placedon (a) comparing the ability of the models to estimate chl-a in the range 1–12 mg m − 3, which is typical for coastal and estuarine waters, and (b) assessing the potential of theModerate Resolution Imaging Spectrometer (MODIS) and Medium Resolution ImagingSpectrometer (MERIS) to estimate chl-a concentrations. Reflectance spectra andwater samples were collected at 13 stations with chl-a ranging from 0.83 to 11.8 mg m − 3 and total suspendedmatter from 9.9 to 21.5 g m − 3. A close relationship was found between chl-a concentration and totalsuspended matter concentration with the determining coefficient (R2) above0.89. The MBR calculated in the spectral bands of MODIS proved to be a good proxy for chl-a concentration(R2 > 0.93). On the other hand, both the NIR–red three-band model, with wavebands around665, 700, and 730 nm, and the NIR–red two-band model (with bands around 665and 700 nm) explained more than 95% of the chl-a variation, and we wereable to estimate chl-a concentrations with a root mean square error below 1 mg m − 3. The two- and three-band NIR–red models with MERIS spectral bands accounted for 93%of the chl-a variation. These findings imply that the extensive database of MODIS andMERIS images could be used to quantitatively monitor chl-a in the Pearl River estuary.