The Underwater Photic Environment of a Small Arctic Lake

Abstract

The underwater light field in a small arctic lake on Victoria Island, Northwest Territories, was examined. Downward radiance was found to be bimodal, with transmission peaks at 480 and 640 nanometres (nm, or .000000001 m). Upward radiance was similar near the surface, with peaks at 480 and 620 nm, but became unimodal with depth and shifted to 580 nm near the bottom. Diurnal variation in the underwater downward and upward irradiance of PAR (photosynthetically active radiation) was approximately two orders of magnitude. The spectral quality of light transmission also changed over this 24 hour period. Unimodal transmission of red light occurred in the early morning (1:00 and 5:00) and late evening (22:00), while bimodal transmission of blue-green and red light was observed during the day (9:00-17:30). Kd(Zm), the vertical attenuation coefficient for downward irradiance at the midpoint of the euphotic zone, was relatively insensitive to changes in solar elevation. Diurnal variation in the reflectance of PAR differed from the predicted by previous simulation models, while the inverse relationship between reflectance and the absorption coefficient was in agreement with these same models. Gilvin, humic material-dissolved iron complexes, algal fucoxanthin, chlorophyll a and tripton all contribute to the attenuation of light and are responsible for the unique underwater spectral transmission in Keyhole Lake.