Journal
JOURNAL OF PHYCOLOGY
Volume 44, Issue 1, Pages 38-44Publisher
WILEY
DOI: 10.1111/j.1529-8817.2007.00429.x
Keywords
benthic algae; cyanobacteria; Lyngbya; macroalgae; nitrogen; river
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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 mu g P . L-1), and ammonium (8-21 mu g 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 mu g N . L-1. A mixed assemblage of chlorophytes and cyanobacteria characterized a 1 km transition zone where nitrate decreased to 40-80 mu g N . L-1. In the last section of the transect, nitrate concentrations dropped below 10 mu g 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.
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