SHIFT FROM CHLOROPHYTES TO CYANOBACTERIA IN BENTHIC MACROALGAE ALONG A GRADIENT OF NITRATE DEPLETION(1).

A survey of the spatial distribution of benthic macroalgae in a fluvial lake of the St. Lawrence River (Lake Saint-Pierre, Quebec, Canada) revealed a shift in composition from chlorophytes to cyanobacteria along the flow path of nutrient-rich waters originating from tributaries draining farmlands. The link between this shift and changes in water quality characteristics was investigated by sampling at 10 sites along a 15 km transect. Conductivity, current, light extinction, total phosphorus (TP; >25 μg P · L(-1) ), and ammonium (8-21 μg N · L(-1) ) remained fairly constant along the transect in contrast to nitrate concentrations, which fell sharply. Filamentous and colonial chlorophytes [Cladophora sp. and Hydrodictyon reticulatum (L.) Bory] dominated in the first 5 km where nitrate concentrations were >240 μg N · L(-1) . A mixed assemblage of chlorophytes and cyanobacteria characterized a 1 km transition zone where nitrate decreased to 40-80 μg N · L(-1) . In the last section of the transect, nitrate concentrations dropped below 10 μg N · L(-1) , and cyanobacteria (benthic filamentous mats of Lyngbya wollei Farl. ex Gomont and epiphytic colonies of Gloeotrichia) dominated the benthic community. The predominance of nitrogen-fixing, potentially toxic cyanobacteria likely resulted from excessive nutrient loads and may affect nutrient and trophic dynamics in the river.