1. The one-dimensional equation solver, PROgram for Boundary layers in the Environment, was used to simulate the temperature structure of Lake Erken, a medium-sized Swedish lake, assuming differing extinction coefficients for a series of modelled years driven by observed meteorological data and by a set of idealized meteorological data.
2. Results suggested that, as expected, larger extinction coefficients initially led to surface waters becoming warmer. The reverse was true late in the summer, however, as the warming induced by greater absorption of solar radiation was outweighed by the cooling effects of entrained colder hypolimnetic water.
3. There was between a two- and fourfold inter-annual variation in the effects on key physical lake parameters, induced by changing extinction coefficient, such as maximum heat flux, heat content and Schmidt stability.
4. The change in surface heat flux induced by a change in extinction coefficient was up to almost 50 W m−2.
5. In the summer, changes in extinction coefficient from 0.5 to 0.2 m−1 led to a dramatic shift in the duration of the stratified period as well as to enormous changes in Schmidt stability and hypolimnetic temperature.
6. Future changes to extinction coefficients of small and medium-sized lakes are likely to have wide-ranging effects on lake thermal structure and ecology.