Recurrent internal waves in a small lake: Potential ecological consequences for metalimnetic phytoplankton populations

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.