Shift from chlorophytes to cyanobacteria in benthic macroalgae along a gradient of nitrate depletion

• Published in: Journal of phycology 2008-02, Vol.44 (1), p.38-44
• Main Authors: Vis, Chantal, Cattaneo, Antonella, Hudon, Christiane
• Format: Article
• Language: English
• Subjects: benthic algae; Cyanobacteria; Lyngbya; macroalgae; nitrogen; river; rivers
• ISSN: 0022-3646; 1529-8817
• DOI: 10.1111/j.1529-8817.2007.00429.x

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.