Light penetration and some related characteristics in small forest lakes in Southern Finland

Summary

Penetration of total PAR was measured in 14 small forest lakes during early August at the time of most intense stratification of the water columns. The lakes were chosen to cover the widest possible range of water colour (5–335 mg Pt · l⁻¹). The extinction coefficient of total PAR, and hence the depth of the euphotic zone, was strongly correlated with water colour, which provided a better prediction of €PAR than either dissolved organic carbon or absorbance at 260 nm. Chlorophyll a concentration in the lakes had no significant influence on €PAR. The depth of the mixed layer showed a strong inverse relationship to water colour so that the effective light climate experienced by phytoplankton was much less variable between lakes than might be expected from the variability of water colour and €PAR. Most of the lakes contained a very high proportion of motile or buoyant forms in their phytoplankton, but there was no evidence for any trend of increasing representation of flagellated phytoplankton with increasing water colour. In these small lakes the lack of turbulence is probably of more importance in promoting the development of populations of motile phytoplankton.