Hydrological control of filamentous green algae in a large fluvial lake (Lake Saint-Pierre, St. Lawrence River, Canada)

•We assessed the hydrological control of filamentous green algae (FGA) occurrence.•Surveys were performed over 6 y in a fluvial lake of the St. Lawrence River.•Water level and its change prior to sampling were the best explanatory variables.•We identified a critical water level limit for FGA occurrence.•Models forecasted with 74% success FGA occurrence in satellite images.

Filamentous green algae (FGA) often proliferate in nutrient-rich rivers under low flow conditions. We assessed the hydrological and meteorological variables controlling the occurrence and composition of FGA in Lac Saint-Pierre (LSP), the largest of the St. Lawrence River fluvial lakes. We hypothesized that, under nutrient-rich conditions, hydrology and climate would be the main drivers of FGA occurrence and composition. We tested this hypothesis during the 2005 growing season, by fortnightly sampling at 7 stations within Anse-du-Fort, a 9 km2 LSP embayment chronically exposed to the inflow of two tributaries draining farmland. Cladophora was the prevalent genus (89% occurrence), but Oedogonium, Hydrodictyon, and Spirogyra were at times dominant. Hydrological (depth, water level change) and meteorological variables (hours of sunshine, wind velocity) were the best predictors of FGA occurrence (r2 = 0.55), but no water quality variable was significant in any model. The generality of our findings was then assessed using lake-wide FGA occurrence from surveys conducted over 6 years (2000–2001 and 2005–2008) of widely different hydrological and climatic conditions. Years of low water levels (2001 and 2007) coincided with early occurrence, highest prevalence and longest seasonal FGA persistence. Validation of model predictions using remote sensing images yielded a 74% success rate in forecasting FGA. We identified a water level limit for FGA occurrence (< 4.07 m above sea level). This limit provides a simple operational guideline for managers, improving our ability to predict FGA proliferation under increasingly low discharge and warming climatic conditions forecasted for the St. Lawrence River.