Ecological divergence in estuarine transition zone fishes

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Estuaries are often depicted as hostile environments where steep gradients in physical characteristics and strong tidal mixing permit the co-existence of only a small number of tolerant, yet abundant, resident species. Observations in the estuarine transition zone (ETZ), where freshwater first mixes with seawater, cast doubt on the validity of this general paradigm. Firstly, fishes and zooplankton are abundant in the ETZ of a number of major estuaries, indicating that the ETZ is an important nursery zone. In the St. Lawrence River, the ETZ supports high concentrations of all life-history stages of smelt and tomcod, with the early-life history stages of smelt enjoying significant growth and survival advantages in the turbid waters of the ETZ. Secondly, the ETZ food web appears to be unique among aquatic ecosystems. Consumers show strong selectivity for local primary production that constitutes only a minor fraction of total carbon production. Thirdly, we have demonstrated a surprising degree of intraspecific habitat partitioning and divergence in spite of the dispersive nature of the ETZ. These observations suggest that the ETZ fosters the existence of a far greater degree of biodiversity than previously surmised. The ultimate objective of my long-term Discovery-funded research program is to assess the potential for intraspecific groups to exploit different resources, to adapt to distinct habitats and ultimately to diverge as distinct populations within the highly dynamic environment of the St. Lawrence ETZ. We will extend our studies of the smelt populations and expand the scope of the study to include other species of fish to verify the generality of our major hypothesis. In addition, we will test the hypothesis that these same environmental gradients serve to structure the entire fish community. Finally, stable isotope analyses of carbon, nitrogen and sulphur will also be used to evaluate intraspecific habitat partitioning. The results of this program will provide the first estimates of the strength of natural selection associated with the partitioning of habitat in estuaries and will be invaluable for the designation of critical habitat and the conservation of biodiversity in one of Canada's most important rivers.