Écologie et implications trophiques de la cyanobactérie Lyngbya wollei dans le fleuve Saint-Laurent

Harmful proliferations of the filamentous cyanobacterium L. wollei forming conspicuous benthic mats on the bottom sediment have been reported with increasing frequency in the last 30 years in rivers, lakes, and springs in North America. It is a known producer of neurotoxins and volatile organic compounds (geosmin, 2-methylisoborneol) thus exerting socioeconomic and public health impacts. Lyngbya wollei is also considered a poor nutritional source for invertebrates because of its robust sheath and toxin production. Proliferation of L. wollei in St. Lawrence River (SLR; Quebec, Canada) was first noticed in 2005. We deemed important to determine its distribution over a 250 km stretch of the SLR to elaborate predictive models of its presence and biomass based on chemical and physical characteristics. Lyngbya wollei was generally found downstream of the inflow tributaries draining farmlands. As enriched waters flowed slowly through 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. We then examined if hydrodynamic conditions, namely currents generated by waves and river flow, control spatial and temporal variations of L. wollei biomass in a large river system. We measured L. wollei biomass together with meteorological, physical, and chemical variables over three years at 10 sites along a gradient of exposure to current and wind in a large (148 km2) fluvial lake of SLR. Wave exposure and current velocity controlled spatial and temporal biomass variations. Biomass increased from May to November and persisted during winter. Interannual variations were primarily controlled by river flow (water level) with spring discharge dislodging mats from the previous year. As anticipated under climate change scenarios, drops in water level and rising storm intensity may lead to an increase in the areas colonized by L. wollei, together with more frequent episodes of mat disruption and beach fouling. Additionally, we evaluated the relative importance of L. wollei with respect to macrophytes and epiphytes. We assessed the influence of the spatial scale in structuring environmental variables and biomass of these benthic primary producers (PP). We also test to which extent their biomass reflected the nature of patches based on ecogeomorphology or the river continuum. To address these two questions, we used a nested design at 3 spatial scales within the SLR: 1) along a 250-km-long upstream-downstream river stretch, 2) among three fluvial lakes located within that river stretch and 3) within each fluvial lake. Environmental factors (conductivity and TP) and spatial structure together explained 59% of the variability in biomass of all three benthic PP. Spatial variability of biomass was best explained by conductivity (+) for macrophytes, DIN:TDP ratio (+) and water extinction coefficient (+) for epiphytes and DOC (+) and NH4+ (-) for L. wollei mats. Within-lake structure was the most important spatial component for all benthic PP, suggesting that local effects, such as enrichment by the inflow of tributaries, rather than upstream-downstream gradients, determined the biomass and composition of benthic PP. Therefore patch dynamics represents a general framework which adequately covers the spatial variability and wide variety of environmental conditions experienced by aquatic organisms found in large rivers. Finally, we investigated the ecological role of L. wollei mats in aquatic ecosystems, especially as a food source and shelter for the amphipod Gammarus fasciatus. We offered amphipods a choice between mats of L. wollei and either chlorophytes or an artificial mat made of acrylic wool in laboratory experiment. Moreover, we reconstructed in situ amphipod diet using mixing model (δ13C and δ15N). Gammarus fasciatus selected the substratum offering the best light refuge (Acrylic > Lyngbya = Rhizoclonium > Spirogyra). Presence of saxitoxins, tissue elemental composition and epiphyte abundance had no significant effect on substratum choice. Lyngbya wollei and its epiphytes constituted 36 and 24% of the in situ diet of G. fasciatus whereas chlorophytes, macrophytes and associated epiphytes represented a less important fraction of its diet. Benthic cyanobacterial mats should be considered a good shelter and food source for small omnivorous invertebrates such as amphipods.

Keyphrases: Macrophytes, Nutriments, Hydrologie, Amphipodes gammarides, Epiphytes, Lyngbya wollei cyanobacterial mats, St. Lawrence River, River ecology, Nutrients, Dissolved organic carbon, Hydrology, Gammarid amphipods