The cyanobacterium Gleotrichia pisum forms macroscopic spherical colonies epiphytic on submerged vascular plants. To assess and explain its distribution and abundance in a large, nutrient-rich fluvial lake of the St. Lawrence River (Lac Saint-Pierre; 400 k²), a 72–stations survey was carried out in August 2006. Physical (depth, clarity, current speed, conductivity, colour), chemical (TN, NO₂-NO₃, NH_4,TP, TDP, DOC) and biological (biomass of vascular plants and metaphyton) characteristics were measured simultaneously at each station. Gleotrichia occurred in shallow and clear-water stations characterized by markedly low nutrient concentrations, particularly nitrate (19 μg/L) and ammonium (4 μg/L). Condition of the host plants (chlorophyll, C, N, and P content) and presence of other epiphytes were inversely related with the abundance of Gloeotrichia. We hypothesize that the distribution and abundance of this cyanobacterium reflect the frequency and duration of nitrogen deficit episodes, which occur commonly in summer in areas of slow water transit time and intense biological activity (assimilation by macrophytes, bacterial denitrification). The interactions of this cyanobacterium with vascular plants and other benthic algae and its ability to fix nitrogen may affect primary production and nitrogen budget in this heavily-enriched sector of the St. Lawrence River.