The spectral distribution of downwelling light in northern Wisconsin lakes.
The special characteristics of downwelling light were determined for seven lakes in the Trout Lake region of \ Visconsin ( 46 ° N, 89 o W) during late summer. Most of the lakes were subjects of early studies in limnological optics during the era of BIRGE \& JUDAY. They are mesotrophic to oligotrophic systems that vary in organic color from 0 to \textgreater 260 PCU. Scans were made with an underwater spectroradiometer at 1M to 5 M depth intervals to measure the wavelength-specific quantum flux between 350 nM and 850 nM. The readings were integrated over 2 nm steps in this wavelength range. The results clearly show how the spectral distribution of light in a given lake is progressively compressed into a relatively narrow region with depth. Increasing concentrations of dissolved and/ or colloidal humic material between lakes is accompanied by a shift in this spectral region from green to red light; and the ratio, rc =(quantum flux49o to 62onM)/(quantum flux620 to 750nM), effectively separates lakes on the basis of organic staining. Although the selectively attenuating effects of phytoplankton were secondary to the gelbstoff effect, absorption by two photosynthetic pigments, chlorophyll-a and phycocyanin, was indicated beneath thin plankton layers in some lakes with red light at depth.