Estimation of surface chlorophyll-a on an emersed mudflat using field spectrometry:: accuracy of ratios and derivative-based approaches

The accuracy of spectral indices and derivative-based approaches for quantifying chlorophyll-a on emersed intertidal sediments is explored. Reflectance measurements (350nm-1050nm) and collocated samples of sediment were collected during low tide. In the laboratory, the amount of chlorophyll in each sample was measured spectrophotometrically. Because the sediment grain-size influences the brightness of the sediment and this is known to have an influence on vegetation indices, the proportion of sediment with a grain-size of < 63 mu m was determined. Several spectral indices were evaluated, including ratios of visible bands, near-infrared (NIR)/red bands and NIR/green bands. Two new vegetation indices are proposed: R-562/R-647 and R-400/R-500. Several derivative-based approaches were explored, including derivative reflectance at individual wavelengths, the wavelength position of the red-edge inflection point (REIP), and the area under the derivative curve. Indices constructed from NIR/red bands were weakly correlated with chlorophyll-a (r(2)=50.45-0.50). NIR/green ratios had the weakest correlation with chlorophyll-a (r(2)=50.03-0.18). Ratios of visible bands had the strongest relationships with chlorophyll-a (r(2)=50.65-0.68). The derivative measure with the strongest relationship to chlorophyll was the first derivative reflectance at 607nm (r(2) = 50.78). The spectral indices and derivative-based methods were evaluated by applying them to a test set of spectral data. R-562/R-647 was superior to other indices and estimated chlorophyll, on average, to within +/- 1.71 mu g cm(-2) ( 9% of the observed range of chlorophyll present). Sediment grain-size did not appear to have a consistent impact on any of the analytical methods tested.