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Summary 1. A simulation study of the feedback of phytoplankton biomass on temperature stratification in the large, monomictic Lake Constance was undertaken. Phytoplankton biomass affects the light extinction coefficient (LEC) of the water and, in turn, the vertical distribution of short‐wave radiation, which shapes the temperature stratification in the lake. 2. A sensitivity study of the variation in LEC using the hydrodynamic model DYRESM showed that a high LEC is associated with stronger stratification, shallower thermoclines, higher surface temperatures and reduced heat content during the heating phase of the lake. During the cooling phase, a shallower thermocline at high LEC leads to a faster decrease in water temperature so that during autumn, a high LEC is associated with lower surface temperatures. Thermal structure was particularly sensitive to changes in LEC when its value was below 0.5 m−1. 3. When LEC is simulated dynamically with the coupled hydrodynamic–ecological model DYRESM‐CAEDYM, its value is a function of phytoplankton dynamics that change vertically and temporally. Comparing simulations with and without dynamic LEC (i.e. with and without phytoplankton dynamics) produced a complex picture: during the vegetation period, we often found a warmer surface layer and colder water beneath in the simulations with dynamic LEC, as expected from the higher LEC when phytoplankton is abundant. However, since phytoplankton biomass (as LEC) fluctuates and because of occasional cooling phases, the patterns were comparatively weak and not consistent over the whole growing season. 4. The most obvious patterns emerged by comparing simulations of oligotrophic and eutrophic conditions. In the eutrophic state, with its higher LEC, stratification was stronger and characterized by higher surface water temperatures, a shallower thermocline and colder water temperatures between 5 and 10 m depth. 5. Statistical analysis of long‐term data of water temperatures in Lake Constance, corrected for external forcing by air temperature, revealed a significant tendency towards warmer temperatures at 7.5 and 10 m depths with decreasing LECs over the course of reoligotrophication. This finding is consistent with our model results.

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Field data were used to estimate the wavelength, phase speed, direction of propagation, frequency, and the vertical structure of high-frequency internal waves observed on the crests of basin-scale waves of Lake Kinneret during periods of strong wind. Shear stability analysis indicates that these waves were generated by shear in the surface mixing layer. The characteristics of the high-frequency internal waves changed within a wind event as the result of the evolution of the background flow conditions following the deepening of the surface layer and the propagation of the basin-scale internal waves. When the background conditions were appropriate, the vertical structure of the unstable mode was such that the perturbations generated visible sinuous internal waves that in turn modified the density profile in the metalimnion in such a way that secondary shear instabilities were triggered. The high-frequency internal waves were observed over larger distances, but poor coherence in temperature records from stations 200 m apart indicated that individual high-frequency internal waves were dissipated locally; these waves are thus a local mechanism allowing energy to be drawn from the energized surface layer and transported to the metalimnion, where it sustains turbulence. Part of the energy extracted from the surface layer was also returned to the mean flow in the metalimnion; high-frequency internal waves are therefore also a source of momentum for the metalimnetic currents. The vertical excursions of the waves also indicate that they could potentially play a role in phytoplankton growth by significantly altering the light regime at relatively high frequencies.

Large cities around the world source their drinking water from forested catchments, which deliver water that is treatable at minimal cost. These forests are often prone to wildfire, which tend to increase surface runoff, destabilize soils, and trigger increased sediment delivery into water reservoirs (