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Oligotrophication from wetland epuration alters the riverine trophic network and carrying capacity for fish
Authors:Christiane Hudon  Antonella Cattaneo  Anne-Marie Tourville Poirier  Philippe Brodeur  Pierre Dumont  Yves Mailhot  Jean-Pierre Amyot  Simon-Pierre Despatie  Yves de Lafontaine
Institution:1. Fluvial Ecosystem Research Section, Environment Canada, St. Lawrence Centre, 105 McGill, 7th Floor, Montr??al, QC, H2Y 2E7, Canada
2. D??p. des Sciences Biologiques, Universit?? de Montr??al, Succursale Centre Ville, P.O. Box 6128, Montr??al, QC, H3C 3J7, Canada
3. Minist??re des Ressources naturelles et de la Faune du Qu??bec, Direction de l??expertise de la Mauricie et du Centre-du-Qu??bec, 100 rue Laviolette, Bureau 207, Trois-Rivi??res, QC, G9A 5S9, Canada
4. Minist??re des Ressources naturelles et de la Faune du Qu??bec, Direction de l??expertise de Laval, Lanaudi??re, Laurentides, Estrie, Montr??al et de la Mont??r??gie, Suite 4.05, 201 Place Charles-Le Moyne, Longueuil, QC, J4K 2T5, Canada
Abstract:Submerged aquatic vegetation (SAV) constitutes a major component of fish habitat, providing support for epiphytes and invertebrates as well as shelter from predators. The effects of wetland epuration from a mesotrophic to a nearly oligotrophic state were examined over a 15?km long reach of the St. Lawrence River under the direct influence of major farmland tributaries. We hypothesized that the nutrient-enriched zone would support a higher biomass of SAV, epiphytes, macroinvertebrates and fish than the nitrogen-deficient epurated zone located downstream of the wetland. Predictions included that the enriched habitat would support a richer fish assemblage, with higher biomass and growth of juvenile yellow perch than found in the epurated zone. Results supported these hypotheses, demonstrating the chain of effects of nutrient reduction on the biomass of SAV (fourfold drop), invertebrate prey (ninefold), small (threefold) and large (1.5-fold) fish between the two zones. In addition to the reduction in SAV biomass, the replacement of filamentous chlorophytes by benthic mats of filamentous cyanobacteria in the epurated zone resulted in a less complex 3-D habitat structure and a low invertebrate availability for fish. Oligotrophication by wetland epuration exerted negative effects on fish habitat quality, food quantity and availability, with an impairment of juvenile perch growth and recruitment. A generalized model of the changes in habitat carrying capacity occurring with epuration (oligotrophication) or eutrophication is presented, with examples of other aquatic systems in which strong linkages between trophic status, SAV, invertebrates and fish productivity were also demonstrated.
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