Temporarily summer-stratified lakes are common: profile data from 436 lakes in lowland Denmark

Lakes that undergo temporary stratification in summer do not fit into the classic categorisation of polymictic or dimictic lakes, but how common are they and what are the effects of stratification on the development of anoxia? We used monthly and bimonthly (twice per month) temperature and oxygen profile data from 436 Danish lakes (area range 1-3954 ha, maximum depth range 1.3-45 m) and defined a stratification indicator based on the temperature difference between the upper and lower 2 m of the water column. The stratification indicator had values between -1.6 and 17.7 & DEG;C and was significantly and strongly related to lake maximum depth and significantly but less strongly related to lake area. The indicator was highly variable, especially in lakes with maximum depths between 4 and 10 m, where intermediate indicator values suggest one or several mixing events during summer. The dissolved oxygen concentration in summer at the bottom was often <1 mg/L, even when the difference between top and bottom temperature was as low as 0.5-1.0 & DEG;C. Temporarily stratifying lakes with frequent mixing events over the summer are probably common in temperate lowland areas but are easily overlooked in routine monitoring programs. Temporary stratification has pronounced implications for the oxygen concentrations and potentially also for the biota and interactions between sediment and water.