Influence of chlorophyll and colored dissolved organic matter (CDOM) on lake reflectance spectra: Implications for measuring lake properties by remote sensing

Light reflected from lake surfaces can convey much information about water quality, especially algal abundance, humic content, turbidity and suspended solids. Light reflectance from lakes is complicated, and detailed spectra are needed for analysis of controlling factors. We obtained detailed reflectance spectra from the water surfaces of 15 lakes in east-central Minnesota and found patterns related to chlorophyll a (ch1 a), turbidity and humic matter (colored dissolved organic matter, CDOM). Increasing ch1 a and turbidity generally resulted in higher reflectance across the visible and near-infrared spectrum. Increasing CDOM led to low reflectance, especially below similar to 500 nm. Spectra of lakes with high ch1 a were distinguishable from those of lakes low in ch1 a, and lakes with low or high CDOM had readily distinguishable spectra. Several optical characteristics of lake water can be estimated from reflectance intensities measured over narrow wavelength bands. The ratio of reflectance at 700 nm to that at 670 nm was the best predictor of ch1 a over a wide range of conditions, including high turbidity and CDOM. Several relationships involving reflectance at 412, 443, 488, and 551 nm, the wavelengths used to calculate oceanic ch1 a from MODIS satellite data, also yielded a high R-2. The ratio of reflectance at 670 nm to 571 nm provided the best estimates of humic color despite the low absorbance of CDOM at these wavelengths. Relationships involving reflectance for all 15 lakes in the range 400-500 nm, where CDOM absorbs light, had low r(2) values; none was high enough for reliable estimates of lake color. For 10 lakes with low to medium ch1 a levels (<= 10 mg m(-3)), regressions involving 412 and 443 nm yielded moderately good relationships. Airborne and satellite remote sensing thus might be used to identify lakes high in CDOM, and may provide reasonable estimates of humic color in lakes with low ch1 a levels.