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Divergent impacts of experimental lake-level drawdown on planktonic and benthic plant communities in a boreal forest lake

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Abstract:

Water-surface elevation in lake 226 (L226) of the Experimental Lakes Area in northwestern Ontario, Canada, was lowered experimentally by 2–3 m during each of three successive winters, and increased naturally but incompletely during the ensuing summers. Our objective was to compare the responses of the littoral and pelagic plant communities to this physical disturbance. Water-chemistry changes were muted, and neither nitrogen nor phosphorus concentration changed. Phytoplankton biomass, species assemblages, productivity, and nutrient status were largely unaffected except for small changes in species diversity and relative abundance of cyanobacteria and cryptophytes. Despite possible transient changes in functional and structural properties, the principal disruption for benthic algae was loss of colonizable surfaces. Floating-leaved and submersed macrophytes (hydrophytes) responded initially with large decreases in biomass and cover. The subsequent response of hydrophytes to drawdown varied: relative frequency of isoetids such as Eriocaulon septangulare decreased, while that of pondweeds such as Potamogeton spirillus increased. The trophic impacts of declining lake levels, whether due to hydroelectric reservoir manipulations or climate change, are likely to be much greater in the littoral zone than in the pelagic zone if major nutrients are unaltered.

Le niveau de la surface de l'eau du lac 226 (L226) de la Région des lacs expérimentaux (nord-ouest de l'Ontario, Canada) a été abaissé expérimentalement de 2–3 m au cours de chacun de trois hivers successifs; il s'est rétabli naturellement, mais partiellement, pendant chacun des étés suivants. L'objectif était de comparer les réactions des communautés de plantes littorales et pélagiques à cette perturbation physique. La chimie de l'eau a peu changé et l'azote et le phosphore sont demeurés stables. La biomasse du phytoplancton, les associations d'espèces, la productivité et le statut trophique sont demeurés inchangés, à l'exceptions de petites modifications de la diversité spécifique et de l'abondance relative des cyanobactéries et des cryptophytes. Malgré certains changements transitoires de leurs propriétés fonctionnelles et structurales, la principale perturbation des algues benthiques a été la perte de surfaces à coloniser. Les macrophytes (hydrophytes) à feuilles flottantes et submergées ont affiché au départ une importante perte de biomasse et de couverture végétale. La réaction subséquente des hydrophytes à l'abaissement du niveau a varié : l'importance relative des plantes de type isoétide, telles qu'Eriocaulon septangulare, a diminué, alors que celles des potamots, tels que Potamogeton spirillus, a augmenté. L'impact trophique des réductions de niveau des lacs, qu'elles soient causées par les manipulations des réservoirs hydroélectriques ou par le changement climatique, risque d'être beaucoup plus important dans la zone littorale que dans la zone pélagique, lorsque les nutriments principaux restent inchangés.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: May 1, 2005

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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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