Determination of subsurface spectral irradiance reflectance by use of a hand-held spectroradiometer and then estimation of chlorophyll a (Chl-a) concentration facilitate assessment of ecological change in turbid lakes, rivers, and estuaries. The method was applied under widely varying solar elevation and weather. Noise due to reflected sky light was evaluated by theory on surface waves and by field data. A strong relationship was established for the shallow and eutrophic freshwater IJssel Lagoon including the River IJssel (The Netherlands) between corrected Chl-a concentrations (3-185 mg m-3) and the reflectance ratio for 704 and 672 nm with backscattering derived from the reflectance at 776 nm (R2 > 0.95; N = 114; standard error of estimate <9 mg m-3). This relationship adequately predicted Chl-a concentrations in well-mixed and optically deep water in lakes in The Netherlands, the Scheldt Estuary in Belgium and The Netherlands, and Lake Tai in China (N = 40; standard error of estimate <9 mg m-3). Floating layers of the cyanobacterium Microcystis in Lake Chao in China were distinguished from homogeneously distributed phytoplankton by reflectance in the violet, blue, and near-infrared wavebands.
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