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The community structure of planktonic cyanobacteria was studied in a dimictic lake in which recurrent summer surface algal blooms have frequently occurred since the beginning of this millennium. In eutrophic-hypereutrophic lakes, epilimnetic cyanobacterial blooms are promoted by increased ambient temperatures and water column thermal stability, which favour the vertical migration of buoyancy-regulating cyanobacteria. Here we propose that intensified external energy (wind) that alters thermocline stability could explain the occurence of heavy blooms in the surface of lakes with low external nutrient loading. Specifically, we hypothesized that: i) in small stratified lakes with low external nutrient sources, cyanobacterial growth primarily occurs near the lake bottom, where phosphorus is more abundant and light is available; ii) we additionally hypothesized that turbulence induced by strong winds increases the amplitude and energy of metalimnetic internal waves and entrains meta- and hypolimnetic water, rich in nutrients and cyanobacteria, into the epilimnion. The study was done in a small lake (45 Ha, maximum and mean depth 7.2 m and 4.3 m, respectively) with mean epilimnetic dissolved phosphorus concentrations ≈ 4 μg L-1 and chlorophyll α ≈ 8 μg L-1. Vertical temperature profiles during the open season were continuously registered using thermistors. Weekly vertical profiles of light transmission, phytoplankton distribution and water chemistry were also taken. On one occasion, these variables were measured throughout a continuous 24 h cycle. Results demonstrated that summer cyanobacterial blooms were dominated by Plankthotrix spp., which began their cycle in late spring at the bottom of the lake, and grew to form dense metalimnetic biomass peaks. Time series analysis of isotherms and the Lake number indicated that internal metalimnetic waves (seiches) were present through the summer. During the diel sampling cycle, we found that medium to strong westerly wind gust events (~5 to >12 m s-1) induced large amplitude internal waves (mainly V2H1 mode) that vertically displaced the isotherms by more than 3.5 m. During this event the top of the metalimnetic algal peak was entrained through the epilimnion, bringing metalimnetic Plankthotrix spp. to the lake surface, modified the deep metalimnion and hypolimnion concentrations of dissolved oxygen, and caused an upsurge in phosphorus. We conclude that algal and nutrient upwelling linked to intermittent deep mixing events, play an important role in supporting summer cyanobacterial blooms in lake Bromont.
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