Chlorophyll-a Estimation in Productive Waters: the Effect of Backscattering on Algorithm Accuracy
Yacobi, Yosef1; Gitelson, Anatoly2
1Israel Oceanographic and Limnological Research, ISRAEL; 2University of Nebraska, UNITED STATES
Chlorophyll a is, by high probability, the most frequently studied biological water quality determinant in aquatic environments. It is characterized by highly specific spectral signatures which can be easily measured after extraction or in situ. With the implementation of sensors like MERIS, and the soon to-be launched Sentinel, which combine high spatial resolution and high-frequency overpass, tools for monitoring of water quality determinants in coastal and inland waters became feasible in these often productive water bodies. Algorithms developed for chlorophyll a assessment in turbid, productive waters make use of the reflectance output in several bands of the red-NIR domain of the electromagnetic spectrum. Using data collected in 2012 in Lake Kinneret (Israel) we explore the effect of particle light backscattering on performance of algorithms based on information from the red-NIR range of the spectrum. The range of variation of our database was typical for coastal waters - chlorophyll a varied from 6.6 to 17.5 mg m-3, total suspended matter from 2.9 to 6.5 g m-3 and organic matter comprised from 38 to 95% of particles. We found that backscattering coefficient declines exponentially towards longer wavelength and the interaction between particle backscattering, absorption by chlorophyll a and by water molecules affects the resulting reflectance. This is likely to affect the reliability of 3-band model for the remote estimation of chlorophyll a concentrations in inland and coastal waters, in which a wavelength independent backscattering coefficient in the range from 660 through 750 nm was assumed. Our results indicate that the 2-band red-NIR model, in which the ratio of the reflectance centered on 708 nm (Sentinel band Oa11) to that centered on 665 nm (Sentinel band Oa8) is utilized, provides sufficient, and often the best estimate of chlorophyll a concentration, and thus sustain the conclusion emanating from many productive water bodies.