Wave exposure and current regulate biomass accumulation of the benthic cyanobacterium Lyngbya wollei in a large fluvial lake

Harmful proliferations of the mat-forming cyanobacterium Lyngbya wollei are increasingly reported in North American lakes, springs, and reservoirs. We examined the hypothesis that currents generated by waves and river flow control spatial and temporal variations of L. wollei biomass in a large river system. We measured L. wollei biomass together with meteorological, physical, and chemical variables during 2009-2011 at 10 sites along a gradient of exposure to current and wind in Lake Saint-Louis, a large (148 km(2)) fluvial lake of St Lawrence River. Where water chemistry was conducive to L. wollei growth, wave exposure and current velocity controlled spatial and temporal biomass variations. Biomass increased from May to November and persisted during winter. Interannual variations were primarily controlled by river flow (water level), and high spring discharge dislodged mats from the previous growing season. Under climate-change scenarios, anticipated declines in water level and rising storm intensity may lead to an increase in the areas colonized by L. wollei, more frequent than present-day episodes of mat disruption, water-use impairment, and beach fouling.