Worldwide, small lakes ( < 1 km²) are numerically dominant, yet the potential for interaction between physical and ecological processes therein has been largely ignored. High‐frequency time series of the thermal and current structures in a small dimictic lake (Lake Bromont, Quebec) revealed the occurrence of recurrent internal waves during the summer of 2007. Amplitudes and frequencies of the internal wave modes were characterized, along with wind and stratification conditions, during two focal periods of 5 days at the beginning and the end of the summer. Owing to a resonance with the daily wind, the second and third vertical mode seiches dominated over the first mode, which was observed only during larger wind events. Although the lake is small (0.41 km²) and shallow (mean depth of 4 m), the response of the thermal structure of the lake to wind forcing was very similar to that of alpine and other deep lakes. The phytoplankton community was controlled by the contrasting gradients of light and nutrients. Consequently, metalimnetic communities of cyanobacteria exposed to the recurrent internal waves, which occurred throughout the summer, formed the dominant phytoplankton biomass in the lake. The regular vertical excursion of the metalimnion influenced both light availability and nutrient fluxes and most likely contributed to an enhanced algal biomass.

Cyanobacterial blooms in eutrophic freshwater is a global threat to the functioning of ecosystems, human health and the economy. Parties responsible for the ecosystems and human health increasingly demand reliable predictions of cyanobacterial development to support necessary decisions. Long-term data series help with identifying environmental drivers of cyanobacterial developments in the context of climatic and anthropogenic pressure. Here, we analyzed 13 years of eutrophication and climatic data of a shallow temperate reservoir showing a high interannual variability of cyanobacterial development and composition, which is a less occurring and/or less described phenomenon compared to recurrant monospecific blooms. While between 2007–2012 Planktothrix agardhii dominated the cyanobacterial community, it shifted towards Microcystis sp. and then Dolichospermum sp. afterwards (2013–2019). The shift to Microcystis sp. dominance was mainly influenced by generally calmer and warmer conditions. The later shift to Dolichospermum sp. was driven by droughts influencing, amongst others, the N-load, as P remained unchanged over the time period. Both, climatic pressure and N-limitation contributed to the high variability of cyanobacterial blooms and may lead to a new equilibrium. The further reduction of P-load in parallel to the decreasing N-load is important to suppress cyanobacterial blooms and ameliorate ecosystem health.