Dreissenid mussels have been exceptionally successful invaders in North American lakes and rivers, especially in the lower Laurentian Great Lakes. As benthic filter feeders capable of attaching to hard substrates, the magnitudes of their biomass and filtering activity in nearshore waters are without precedent. The dreissenid colonization has implications for the removal and fate of materials filtered from the water by the mussels and for the longer-term development of the nearshore benthic community and lake ecosystem. A conceptual model, the nearshore shunt, seeks to describe a fundamental redirection of nutrient and energy flow consequent to dreissenid establishment. The model explains some emergent problems in the Great Lakes, such as reemergence of Cladophora in some coastal zones while offshore P concentrations remain low, and highlights areas in need of more research. The source of particulate nutrient inputs to dreissenids and the fate of materials exported from the benthic community are critical to understanding the role of dreissenids in the lakes and assessing the applicability of current models for managing nutrients and fisheries. The nearshore shunt would require even more stringent P management for lakes strongly impacted by dreissenids to maintain nearshore water quality.Résumé : Les moules dreissenidées ont envahi avec un succès exceptionnel les lacs et rivières d'Amérique du Nord, et en particulier les Grands Lacs inférieurs. Des animaux benthiques se nourrissant par filtration et capables de se fixer à des substrats durs, ils ont atteint des biomasses et des niveaux d'activité de filtration sans précédent dans les eaux côtières. La colonisation des dreissenidés a des conséquences sur le retrait et le sort des matériaux filtrés de l'eau par les moules, ainsi que sur le développement à plus long terme de la communauté benthique près du rivage et sur l'écosystème lacustre. Un modèle conceptuel, la « déviation côtière », cherche à décrire l'altération fondamentale du cheminement des nutriments et du flux d'énergie depuis l'établissement des dreissenidés. Le modèle explique certains des problèmes qui surgissent dans les Grands Lacs, comme la ré-emergence de Cladophora dans certaines zones côtiè-res, bien que les concentrations de phosphore au large restent faibles et il identifie des domaines qui nécessitent des recherches supplémentaires. Une connaissance des sources des apports de particules nutritives pour les dreissenidés et du sort des matériaux exportés de la communauté benthique est essentielle pour comprendre le rôle des dreissenidés dans les lacs et pour évaluer l'applicabilité des modèles courants de gestion des nutriments et des pêches. La « dévia-tion côtière » exigerait une gestion encore plus rigoureuse du phosphore dans les lacs fortement affectés par les dreissenidés afin de maintenir la qualité de l'eau près du rivage.[Traduit par la Rédaction] Hecky et al. 1293

Cladophora glomerata (L.) Kütz. is, potentially, the most widely distributed macroalga throughout the world's freshwater ecosystems. C. glomerata has been described throughout North America, Europe, the Atlantic Islands, the Caribbean Islands, Asia, Africa, Australia and New Zealand, and the Pacific Islands. Cladophora blooms were a common feature of the lower North American Great Lakes (Erie, Michigan, Ontario) from the 1950s through the early 1980s and were largely eradicated through the implementation of a multibillion-dollar phosphorus (P) abatement program. The return of widespread blooms in these lakes since the mid-1990s, however, was not associated with increases in P loading. Instead, current evidence indicates that the resurgence in blooms was directly related to ecosystem level changes in substratum availability, water clarity, and P recycling associated with the establishment of dense colonies of invasive dreissenid mussels. These results support the hypothesis that dreissenid mussel invasions may induce dramatic shifts in energy and nutrient flow from pelagic zones to the benthic zone.

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The algal taxonomy and community metabolism of epilithon and epiphyton in a lake experimentally acidified to pH 5 were compared with those in a nearby neutral lake to interpret the effect of acidification on periphyton. Acidification altered the algal community structure. Acidified periphyton had fewer filamentous cyanobacteria, whereas green algae (especially Mougeotia) were relatively more abundant. Periphytic metabolism also differed substantially. Rates of dark respiration were higher and compensation irradiances were greater in all acidified benthic communities. Photosynthesis was C limited and enhanced by experimentally increasing turbulence to simulate rates of water movement in the shallow littoral zone. These findings are consistent with the hypothesis that the filamentous green algae of acid lakes can take up CO, faster than the adnate algae usually dominant in neutral lakes, thus (partially) overcoming the C limitation found in acidified lakes.