Physical and chemical consequences of artificially deepened thermocline in a small humic lake - a paired whole-lake climate change experiment

Climate change with higher air temperatures and changes in cloud cover, radiation and wind speed alters the heat balance and stratification patterns of lakes. A paired whole-lake thermocline manipulation experiment of a small (0.047 km(2)) shallow dystrophic lake (Halsjarvi) was carried out in southern Finland. A thermodynamic model (MyLake) was used for both predicting the impacts of climate change scenarios and for determining the manipulation target of the experiment. The model simulations assuming several climate change scenarios indicated large increases in the whole-lake monthly mean temperature (+1.4-4.4 degrees C in April-October for the A2 scenario), and shortening of the length of the ice covered period by 56-89 days. The thermocline manipulation resulted in large changes in the thermodynamic properties of the lake, and those were rather well consistent with the simulated future increases in the heat content during the summer-autumn season. The manipulation also resulted in changes in the oxygen stratification, and the expansion of the oxic water layer increased the spatial extent of the sediment surface oxic-anoxic interfaces. In addition, the experiment affected several other chemical constituents; concentrations of organic carbon, TotN, and NH(4) showed a statistically significant decrease, likely due to both changes in hydrological conditions during the experiment period and increased decomposition and sedimentation. In comparison with the results of a similar whole-lake manipulation experiment in a deep, oligotrophic, clear-watered lake in Norway, it is evident that shallow dystrophic lakes, common in the boreal region, are more sensitive to physical perturbations. This means that projected climate change may modify their physical and chemical conditions in the future.