The concentration of the saxitoxin analogue LWTX-1 was quantified in samples of the benthic
filamentous cyanobacterium Lyngbya wollei (Farlow ex Gomont) Speziale and Dyck collected in two
fluvial lakes of the St. Lawrence River (Canada) over the 2006–2013 period. The study was aimed at
documenting the spatial (between fluvial lakes, between sites within each lake) and temporal (interannual,
monthly) variations of toxin concentration in relation with hydrological (water level), physical
(water temperature, conductivity, transparency), chemical (nutrients in overlying water) and biological
(L. wollei biomass and mat condition) characteristics. Toxin concentration was hypothesized to vary
seasonally with biomass accumulation and environmental conditions. Toxin concentrations measured in
Lake Saint-Louis (51 +/- 40 mg LWTX-1 g-1 DM, N = 29 days in 2007, 2009–2011) were double those in Lake
Saint-Pierre (25 +/- 31 mg LWTX-1 g-1 DM, N = 26 days in 2006–2008, 2012–2013); however, August 2007
measurements taken from both lakes did not differ significantly. Ten of the twelve highest values (>100 mg
LWTX-1 g-1 DM) were obtained from Lake Saint-Louis, between April and October in 2007, 2010 or 2011.
Under ice samples showed intermediate concentrations of LWTX-1 (42 +/- 9 mg LWTX-1 g-1 DM, N = 2).
Concentrations of LWTX-1 were positively correlated with Secchi depth (r = 0.59, p < 0.001), L. wollei
biomass (Spearman r = 0.31, p < 0.01) and %N in filaments (r = 0.48, p < 0.001), suggesting toxin production
was linked to mat growth and metabolism rather than water quality. Although LWTX-1 has been reported to
have a low toxicity, monitoring of L. wollei abundance is required to assess the environmental and human
health risks posed by this taxon in the St. Lawrence - Great Lakes system.