Effect of Under-Ice Light Intensity and Convective Mixing on Chlorophyll a Distribution in a Small Mesotrophic Lake

Abstract

Data of long-term measurements of under-ice solar radiation, water temperature, and chlorophyll a are analyzed in four phytoplankton groups (green, diatoms, blue-green, and cryptophyte algae) in a small mesotrophic Vendyurskoe Lake (Karelia) in the period of spring under-ice convection. It is shown that, after thawing away of snow cover from lake surface, under-ice illumination increases, water temperature rises, the depth of convectively mixed layer (CML) increases, and microalga photosynthesis intensifies. In the daytime, chlorophyll a extremums appear in the CML, and, unlike the homogeneous characteristics (water electric conductivity, mineralization, etc.), the cells of different phytoplankton species can be used as tracers in studying convective mixing. A prognostic equation is obtained, reflecting an inverse dependence of the coefficients of variation of chlorophyll a concentration in CML on solar radiation fluxes, penetrating under ice bottom surface. A direct relationship was shown to exist between the increase in chlorophyll concentration in CML and its thickness.