Determination of chlorophyll and dissolved organic carbon from reflectance data for Colorado reservoirs

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Abstract
Relationships between phytoplankton abundance, concentration of dissolved organic carbon (DOC) and reflectance spectra were investigated in eight reservoirs of the Colorado Front Range. The purposes of the study were:(1)to determine the degree to which chlorophyll and DOC could be determined from spectral data of the type that can be acquired through remote sensing; and(2)to determine whether equations developed for other waters are applicable to Colorado reservoirs. Supplementary information was obtained from water selectively enriched in dissolved organic carbon (Georgia blackwater lakes) and in phytoplankton (phytoplankton cultures). Reflectance for the lakes ranged from 1 to 5 per cent across the spectrum. All lakes showed a peak of reflectance near 570 nm (green range) corresponding to the minimum absorption for chlorophyll. For both the reservoirs and the phytoplankton cultures, a peak near the red-infrared boundary (ca 700 nm) increased in size and shifted toward longer wavelengths with increasing concentration of chlorophyll a. Georgia blackwater lakes dominated by DOC yielded flat spectra combined with low overall reflectance (usually below 1 per cent). With the presence of tripton in addition to DOC, reflectance was higher and showed peaks near 710 and 810 nm caused by selective scattering and absorption. For the eight reservoirs, chlorophyll showed a very close relationship to ratios of reflectance at a number of wavelengths. The strongest relationship was for the ratio of reflectance in the near-infrared to reflectance in the green spectral range (r2 = 0·98): Chlorophyll a, μg 1−1 =407 (R 806/R 571)2·52. Single ratios of reflectance did not show such strong relationships with concentrations of dissolved organic carbon (r2 2 =0·95): DOC, mgl−1 =0.55 [(R 716/R 670)−9·60][(R 706/R 670)12·94]. Reflectance ratios or red to green wavelengths and ratios containing reflectance near 800 nm work well for predictions of chlorophyll in Colorado reservoirs, although equations of this type have not been previously reported in the literature. Equations that have been developed for other areas do not consistently fit the data for Colorado reservoirs. Spectral analysis of reflectance in relation to water quality will be required from numerous geographic regions before universal equations or universal typologies can be considered.