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

Abstract

The accuracy of spectral indices and derivative‐based approaches for quantifying chlorophyll‐a on emersed intertidal sediments is explored. Reflectance measurements (350 nm–1050 nm) 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 μm 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: R₅₆₂/R₆₄₇ and R₄₀₀/R₅₀₀. 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 = 0.45–0.50). NIR/green ratios had the weakest correlation with chlorophyll‐a (r ² = 0.03–0.18). Ratios of visible bands had the strongest relationships with chlorophyll‐a (r ² = 0.65–0.68). The derivative measure with the strongest relationship to chlorophyll was the first derivative reflectance at 607 nm (r ² = 0.78). The spectral indices and derivative‐based methods were evaluated by applying them to a test set of spectral data. R₅₆₂/R₆₄₇ was superior to other indices and estimated chlorophyll, on average, to within +/−1.71 μg 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.

Acknowledgements

We would like to thank Grant Kaplan, Andres Grigaliunas and René Reinfrank for assistance in the field and Craig Myers for the analysis of laboratory samples. Dr Robert Bryant, Geography Department, Sheffield University, UK, provided helpful comments on the earlier drafts of this paper. The research was supported by funds from the Australian Research Council through the Special Research Centres Programme.

Notes

†Integrated between 644 nm and 733 nm, baseline (λ₁ = 644 nm).

‡Integrated between 641 nm and 740 nm.