Bi-dimensional feeding ecologies and food web structure: A conceptual framework linking trophic and spatial processes in aquatic ecosystems, with application to mercury bioaccumulation in fish

This thesis presents the results of a study of the ecology of fish communities of lakes and reservoirs in Labrador, Canada. The first objective of the thesis was to evaluate the relevance and implications of a bi-dimensional framework to integrate spatially-explicit energy flows in consumer feeding ecology and food web structure. Second objective was to assess whether spatially-explicit bi-dimensional feeding ecologies and food web structures can improve the assessment of mercury (Hg) bioaccumulation in fish. A bidimensional framework consists in the integration of horizontal (spatially-explicit) and vertical (trophic) energy flows. This approach is demonstrated in comparative studies of fish populations and communities from four pristine lakes. Horizontal energy flows here distinguished between littoral and pelagic carbon contributions to fish consumers in lakes, quantified by variations in stable isotope ratios of carbon in fish tissue. Vertical energy flows described variations in fish trophic position quantified by variations in stable isotope ratios of nitrogen. Comparing life-stage and among-lake patterns of trophic position and littoral-pelagic resource use in four fish species demonstrated that bidimensional feeding strategies are determined by species ontogeny and ecosystem characteristics. Comparisons of lake-specific bi-dimensional food web structures estimated from community averages of fish trophic position and littoral-pelagic foraging demonstrated that spatially-explicit community organization is related to consumer species composition, to primary productivity constraints and to physical ecosystem structure. Applied to the understanding of Hg bioaccumulation, a bi-dimensional framework integrated spatial influences on Hg exposure and trophic transfers in aquatic ecosystems and improved predictions of fish Hg concentrations. Horizontal energy flows in this case further distinguished between allochthonous-autochthonous carbon contributions to primary producer-consumer. A bi-dimensional approach revealed that aquatic ecosystems with longer food chains, greater assimilation of autochthonous carbon at lower trophic levels and greater reliance on littoral carbon by fish, reach greater Hg contamination. The influence of littoral-pelagic resource use on fish Hg levels varied with the type and size of prey consumed across habitat boundaries. Autecological types distinguish consumers bi-dimensional feeding ecologies as shaped mainly by ontogeny (Quis type), by ecosystem features (Ubi type) or both (Aequivocus type). This classification provided species-specific distinctions for the importance of biological and environmental determinants of Hg bioaccumulation in fish. A bi-dimensional framework provides a solid conceptual basis for spatially-explicit ecological understanding that can improve the assessment of environmental issues.;Keywords. spatial influences, feeding ecology, food web structure, stable isotopes, fish consumers, boreal lakes, reservoir, mercury bioaccumulation, littoral-pelagic habitats, carbon source, trophic position, ontogeny, autecological types.