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Could increased cyanobacterial biomass following forest harvesting cause a reduction in zooplankton body size structure?

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Changes in phytoplankton and zooplankton communities were studied in 16 lake-years during a large-scale experimental before–after study as part of the Terrestrial Riparian Organisms Lakes and Streams (TROLS) project. After forest harvesting, phytoplankton biomass and especially cyanobacterial biomass increased in some of the experimental lakes, up to an order of magnitude. Although the response of phytoplankton was not directly related to the intensity of forest harvesting, this large-scale experiment provided a unique opportunity for studying the effects of an increasing gradient in cyanobacterial biomass on zooplankton communities. Cladoceran size structure and biomass were negatively correlated with cyanobacterial biomass, suggesting that large cladocerans and especially daphnids were inhibited by the increase in cyanobacterial biomass. The increase in cyanobacterial biomass seems to have caused a significant reduction in the size structure of zooplankton communities. As no correlation was found between cladocerans and cyanotoxins, our results point more towards feeding inhibition by filamentous and colonial cyanobacteria as a possible cause for the decline in the size structure and biomass of zooplankton communities. These results suggest that the increase in cyanobacterial biomass is likely to cause a shift in the size of zooplankton communities, which will lead to a profound change in functioning of lake systems.

Nous avons étudié les changements dans les communautés de phytoplancton et de zooplancton dans 4 lacs durant 4 ans (16 lacs-années) lors d'une expérience à grande échelle dans le cadre du projet TROLS (« Terrestrial Riparian Organisms Lakes and Streams »). Après la coupe forestière, la biomasse phytoplanctonique et particulièrement celle des cyanobactéries a augmenté par un ordre de grandeur, dans certains des lacs expérimentaux. Bien que la réponse du phytoplancton ne soit par directement reliée à l'intensité de la récolte forestière, cette expérience à grande échelle a fourni une occasion unique d'étudier les effets d'un gradient croissant de biomasse de cyanobactéries sur les communautés zooplanctoniques. La structure en taille et la biomasse des cladocères étaient négativement corrélés avec la biomasse des cyanobactéries, ce qui suggère que les cladocères de grande taille, et en particulier les daphniidés, sont inhibés par l'accroissement de la biomasse des cyanobactéries. L'augmentation de la biomasse des cyanobactéries semble avoir causé une réduction significative dans la structure de taille de la communauté de zooplancton. Dans l'absence de corrélation entre les cladocères et les cyanotoxines, nos résultats appuient la thèse d'une inhibition de l'alimentation par les cyanobactéries filamenteuses et coloniales comme cause possible du déclin de la structure en taille et de la biomasse des communautés de zooplancton. Ces résultats indiquent que l'augmentation de la biomasse des cyanobactéries cause vraisemblablement une réduction de la taille des communautés zooplanctoniques, ce qui peut mener à un profond changement dans le fonctionnement des systèmes lacustres.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: October 1, 2006

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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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