Lake morphometry and light in the surface layer

Mean irradiance in the surface layer was calculated as a percent of incident irradiance using Secchi depths and thermocline depths for two sets of lakes: (1) 20 nearly circular lakes in northwest Minnesota, USA, sampled at a similar season, and (2) 1140 lakes of heterogeneous shapes in Ontario, Canada, sampled throughout the ice-free season. For both, as lake area increased, the thickness of the surface layer increased. Consequently, lakes larger in area had lower mean irradiance in their surface layer. Mean irradiance is complexly related to lake size; when depth increases slowly with area, larger lakes may have lower mean irradiance in their surface layers. But, when depth increases quickly with area, mean irradiance can be unrelated to area. -from Author