Shift in wetland plant composition and biomass following low-level episodes in the St. Lawrence River: looking into the future

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Abstract:

The effects of a 1-m drop in average water levels in 1999 on species composition and biomass were documented for a St. Lawrence River wetland and compared with a similar episode in 1931. These observations highlight the manifold effects of past and future water level fluctuations on St. Lawrence River wetlands and faunal habitats, resulting from natural hydrologic variability, climate change, and (or) human intervention. In 1931 and 1999, waters were 2–3 °C warmer than the previous 10-year average. Low water levels markedly altered wetland vegetation: various Graminea (including Phalaris arundinacea and Phragmites australis) and facultative annual species invaded previously marshy areas. Submerged species previously found in shallow waters were replaced on dry ground by annual terrestrial plants; Alisma gramineum colonized emergent waterlogged mudflats. The low water levels of 1999 induced a spatially discontinuous plant biomass that was richer in terrestrial material than in previous years (1993–1994). In comparison with the 1930s, recent surveys indicate a decline of assemblages dominated by Equisetum spp. and Najas flexilis and a rise of those dominated by Lythrum salicaria, Potamogeton spp., and filamentous algae. These shifts reveal the additional effects of nutrient enrichment, alien species, and shoreline alteration accompanying a change from a mostly agricultural to a mostly urbanized and industrialized landscape.

Les effets sur la composition spécifique et la biomasse des milieux humides d'une baisse de niveau du Saint-Laurent en 1999 ont été comparés avec un épisode semblable en 1931. Ces observations soulignent l'importance du niveau d'eau sur les milieux humides et les habitats fauniques du Saint-Laurent, résultant de la variabilité hydro logique naturelle, des changements climatiques et (ou) des interventions humaines. En 1931 et en 1999, la température de l'eau était plus élevée de 2–3 °C que la moyenne des 10 années précédentes. Les bas niveaux ont modifié dramatiquement la végétation des milieux humides : diverses graminées (incluant Phalaris arundinacea et Phragmites australis) et espèces annuelles facultatives ont envahi les anciens marais. Les plantes submergées ont été remplacées par des plantes annuelles terrestres croissant sur les vasières asséchées; Alisma gramineum a germé et fleuri sur la vase humide. Les bas niveau de 1999 ont généré un couvert littoral discontinu pour la faune, comportant une biomasse terrestre supérieure à celle de 1993–1994. En comparaison avec les années 30, les données récentes indiquent une diminution des assemblages dominés par Equisetum spp. et Najas flexilis et un accroissement de ceux dominés par Lythrum salicaria, Potamogeton spp. et les algues filamenteuses. Ces changements révèlent les effets additionnels de l'eutrophisation, des espèces exotiques et de l'altération des rives accompagnant le passage de la vocation principalement agricole à la vocation urbaine et industrielle des terres avoisinantes.

Document Type: Research Article

Publication date: April 1, 2004

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  • Published continuously since 1901 (under various titles), this monthly journal is the primary publishing vehicle for the multidisciplinary field of aquatic sciences. It publishes perspectives (syntheses, critiques, and re-evaluations), discussions (comments and replies), articles, and rapid communications, relating to current research on cells, organisms, populations, ecosystems, or processes that affect aquatic systems. The journal seeks to amplify, modify, question, or redirect accumulated knowledge in the field of fisheries and aquatic science. Occasional supplements are dedicated to single topics or to proceedings of international symposia.
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