Effects of fetch and dissolved organic carbon on epilimnion depth and light climate in small forest lakes in southern Sweden

We sampled small (0.03 to 3.3 km(2) surface area) forest lakes in southern Sweden during two summers to investigate how concentration of dissolved organic carbon (DOC) and lake size influence epilimnion depth (z(e)) and light conditions in the epilimnion. z(e) increased with increasing fetch (square root of lake area) but did not decrease with increasing DOC concentration. This suggests greater importance of wind mixing in our study area than in areas with more continental climate, i.e., epilimnion-deepening wind mixing overrides the tendency for epilimnion depth to decrease with increasing DOC concentration. Extinction of photosynthetically active radiation was mainly caused by DOC. The euphotic zone was shallower in the first year than in the second, probably because of higher precipitation and lower solar irradiance in 2007, which together led to higher water color. Mean epilimnetic irradiance was low, especially in 2007, mainly because the increase in DOC did not lead to shallower epilimnia. Browning of lakes in southern Sweden, in combination with a predicted increase in the number of storm events, may lead to more severe light limitation of phytoplankton in small, nutrient-poor lakes, since it may not be accompanied by a compensatory shallowing of the epilimnion. As a consequence, lake ecosystems will become more heterotrophic, CO2-evasion to the atmosphere will increase, and fish production decrease.