Forecasting the magnitude and composition of phytoplankton blooms in a eutrophic lowland river (Riviere Yamaska, Que., Canada)

The mechanisms controlling phytoplankton biomass and composition in the lower reach of the eutrophic (total phosphorus > 100 mu g P.L-1) Riviere Yamaska were studied over six consecutive summers characterized by high (2008-2009) and low (2012-2013) discharge conditions. In the lower river reach, periods of low river discharge favoured planktonic blooms (> 20 mu g Chl-a.L-1) dominated by centric diatoms, in sharp contrast with numerous public reports of simultaneous occurrence of cyanobacterial proliferation in the upstream lakes and reservoirs. Daily cycles of hourly dissolved oxygen production were disrupted by floods but resumed in the days following flow abatement and persisted under low discharge conditions, suggesting that booms in the lower river reach resulted from local production rather than advection from upstream reservoirs. Motile, flagellate chlorophytes and cryptophytes were more important under extreme high and low discharge conditions, whereas cyanobacteria co-occurred with centric diatoms under intermediate discharge and highly illuminated conditions. Although the Riviere Yamaska's flow remained sufficient to avoid cyanobacterial proliferation even under lowest discharge conditions, our results suggest that increasing water residence time, either through severe drought or river damming, will amplify the risk of toxic algal blooms in the free-flowing reaches of the lower Riviere Yamaska.