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Transfert de méthylmercure et structure des réseaux trophiques chez les macroinvertébrés littoraux

Abstract : First chapter: Total mercury and methylmercury concentrations ([THg], [MeHg]) were measured in littoral macroinvertebrates from Lake St. Pierre, Quebec, Canada. Functional groups (detritivore, grazer, edible predator, inedible predator) explained the greatest fraction of [MeHg] variation compared to time (year, month), and space (station and shore). Greatest [THg] and [MeHg] were found in inedible predators mostly from families of heteropterans and coleopterans. Detritivores and grazers exhibited the lowest Hg concentrations while edible predators were intermediate. Inedible predators also had the highest percentage of MeHg ([MeHg]/[THg]), with some taxa close to 100%. Such high percentages are seldom observed in freshwater organisms other than piscivorous fish. MeHg burden (concentrations × biomass) in inedible predators accounted for 10% of the MeHg pool for the whole invertebrate community. These relatively significant quantities of MeHg are sequestrated in aquatic “trophic dead-ends” and could partly explain the low [MeHg] measured in fish, compared to [MeHg] of macroinvertebrates from Lake St. Pierre and other freshwater ecosystems with large littoral zones. We recommend taking into account the inedible organisms in Hg cycling models in order to avoid a possible overestimation of the MeHg pool available to fish.

Second chapter: 1. The main objective of this study was to assess sources of methylmercury (MeHg) for freshwater littoral macroinvertebrates primary consumers.
2. Isotopic ratios of carbon and nitrogen stable isotopes (δ13C, δ15N) of sources (epiphytes, macrophytes, suspended particulate matter _SPM) and of macroinvertebrates consumers were measured in a fluvial lake with an extensive macrophytes beds (emergent and submerged).
3. To determine the relative contribution of each OM source to macroinvertebrate diets we used the IsoSource model that examines all possible combinations of solutions for each source
4. Results show that autochthonous littoral sources (epiphytes and to a lesser extent, macrophytes) are preponderant in the diet of macroinvertebrates, especially in early summer (July). Later (August), allochthonous SPM constitutes a non-negligible OM-source to the primary consumers.
5. The proportion of epiphytes in macroinvertebrate diet was positively correlated with the percentage of MeHg in their tissues. There was no relationship between SPM assimilation and Hg contamination in macroinvertebrate consumers. These results suggest that epiphytes constitute the privileged path of entry for MeHg in littoral food webs.

Third chapter: The δ15N enrichment in littoral food webs is not well-known despite the importance of macroinvertebrates in lacustrine energy fluxes. We wanted to assess the influence of functional group (grazer, collector, shredder, predator, predator-hematophagous, predator-sucker) and spatiotemporal variables (year, month, station of sampling) on littoral macroinvertebrate δ15N signatures. For two years, during the ice-free period phytophilous littoral macroinvertebrates were sampled in Lake St. Pierre, a large fluvial lake of the St. Lawrence River, Quebec, Canada. δ15N analyses have shown that station was the most important factor for explaining δ15N variation, followed by sampling month and functional group. The organisms sampled on the south shore, which is influenced by heavy inputs of agricultural organic matter and fertilizers exhibited higher δ15N valuers than those sampled on the north shore which watershed is mostly constituted by Canadian Shield boreal forest. Grazer-to-predator enrichment valued 1.6‰, which is inferior to the 3.4‰ generally admitted in food-web research. Shredders exhibited the lowest δ15N values and predators-hematophagous the highest ones. δ15N signature of invertebrates increased monthly during summer, with an amplitude of 3‰ between May and September. We recommend better taking into account station, month and functional group in future research about littoral food webs and δ15N signature analyses.

Fourth chapter: The influence of macrophyte habitat and architecture on macroinvertebrate biomass, abundance, and richness was investigated in Lake St. Pierre, a large fluvial lake of the St. Lawrence River (Quebec, Canada). A lake-wide estimate of macroinvertebrate biomass associated with different macrophyte habitats was also calculated in order to assess the quantitative effects of vegetation changes on macroinvertebrate communities. For two years during the ice-free period, phytophilous macroinvertebrates were sampled in macrophyte beds comprising more than ten species of plants and three habitats (emergent, floating-leaved, submerged), and in three submerged macrophyte architectures based on plant morphology (simple, intermediate, and complex). Invertebrate sub-samples were classified into four functional groups (detritivore, grazer, crawling predator, diving predator). Biomass and density of invertebrates were expressed per unit of plant dry weight. The main findings are that macroinvertebrate biomass, abundance and richness were significantly greater in submerged than in emergent and floating-leaved habitats. However, macrophytes with a complex architecture did not host significantly greater macroinvertebrate biomass than plants with a simpler architecture. This could be related to substrate preferences of herbivores (mostly Gastropoda) toward the tape grass Vallisneria americana. Differences in macroinvertebrate abundance and biomass were found between the two years associated with variations in the river water level. During the year with average water level, total macroinvertebrate biomass was 16% greater than in the year with a lower water level. We conclude that a reduction in the water level of Lake St. Pierre, predicted to occur with climate change, could lead to a decrease in benthos biomass which constitutes a crucial food source for fish.
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Fabien Cremona. Transfert de méthylmercure et structure des réseaux trophiques chez les macroinvertébrés littoraux. Ecologie, Environnement. Université du Québec à Montréal, 2007. Français. ⟨tel-00365591⟩

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