Changement climatique - Recherche française (avec laboratoires CNRS) - 2010-2015

Vertical mixing plays a major role in functioning of seasonally stratified aquatic systems. In this study, we employ a 1-D stratification model and a 9 year forcing data set to simulate the thermal dynamics in a large, but shallow reservoir that regularly displays a polymictic character with complete mixing events during summer. Such mixing dynamics is typical for many water bodies in the temperate zone having an intermediate depth. In many cases summer-mixing events were documented to induce severe water quality deteriorations (e.g., cyanobacterial blooms). We examined and quantified the response of summer-mixing behavior to combinations of hydrological regimes, i.e., water level fluctuations and withdrawal depth, and changes in meteorological variables, i.e., air temperature and wind speed. According to our findings: (i) increasing summer air temperatures considerably increase the resistance of the water column against mixing; (ii) while the combination of maintenance of a high and constant water depth and implementation of epilimnetic discharge results in almost complete resistance to mixing, their individual effects are also substantial, being roughly comparable to the effects of 4-6 K increase in air temperatures; (iii) wind is a critical variable, 30% increase of which can compensate up to 5.5 K increase in air temperatures; and (iv) effects of changes in air temperature, wind speed, and water depth are inter-dependent, as indicated by enhanced importance of wind and temperature in response to decreasing water depth, as well as reduced importance of depth in response to decreasing wind speed and increasing temperature.