Predicting the risk of proliferation of the benthic cyanobacteriumLyngbya wolleiin the St. Lawrence River

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Abstract
Lyngbya wollei is a toxin-producing cyanobacterium, forming mats of filaments that proliferate in rivers, lakes, and springs in North America. We determined its distribution over a 250 km stretch of the St. Lawrence River (SLR; Quebec, Canada) to elaborate predictive models of its presence and biomass based on chemical and physical characteristics. A 2008 survey revealed L. wollei was generally found downstream of the inflow of small tributaries draining farmlands. As enriched waters flowed slowly through dense submerged vegetation, dissolved inorganic nitrogen (DIN) concentration dropped but dissolved organic carbon (DOC) and total dissolved phosphorus (TDP) remained high, leading to a low DIN:TDP ratio. Models identified DOC (positive effect), TP (negative effect), and DIN:TDP (negative effect) as the most important variables explaining L. wollei distribution. The risk of L. wollei occurrence in the SLR was correctly forecasted in 72%–92% of all cases with an independent data set. Proliferation of L. wollei, a potentially heterotrophic, diazotrophic cyanobacterium, is favoured by organic enrichment and imbalance of DIN:TDP ratio. Its dominance coincided with low macrophyte biomass, yielding a simplified, less productive ecosystem.

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