Lake and Reservoir Management

Abstract

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 (chl a), turbidity and humic matter (colored dissolved organic matter, CDOM). Increasing chl a and turbidity generally resulted in higher reflectance across the visible and near-infrared spectrum. Increasing CDOM led to low reflectance, especially below ~500 nm. Spectra of lakes with high chl a were distinguishable from those of lakes low in chl 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 chl 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 chl a from MODIS satellite data, also yielded a high R2. 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 r2 values; none was high enough for reliable estimates of lake color. For 10 lakes with low to medium chl-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 chl-a levels.

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