Riverscale distribution of zooplankton in the St. Lawrence River in relation to hydrological networks, hydroperiods and local environmental gradients.

We studied the community structure of crustacean zooplankton along the biogeographical zones of the St. Lawrence River (Québec, Canada), to evaluate how the riverscale hydrological network formed by water masses, and local-scale aquatic environment, influenced the distribution of crustacean groups (cladocerans, calanoids, cyclopoids and harpacticoids) during the spring (high discharge) and summer (low discharge) hydroperiods. Zooplankton and environmental data were sampled at 52 sites forming 16 transects along the fluvial section zone (FSZ), the fluvial estuary zone (FEZ) and the estuarine transition zone (ETZ) of the St. Lawrence River in May and August 2006. We compared zooplankton community composition among biogeographical zones and fluvial lakes and across the fluvial estuarine continuum. Analyses were carried out using asymmetric eigenvector maps (AEM), redundancy analysis (RDA), and variation partitioning. Spatial distribution patterns revealed a complex river model. Riverscale discontinuities between the fluvial and estuarine transition zones, and the hydrological network of water masses in the fluvial zones, explained better the spatial zooplankton distribution patterns along the fluvial estuarine continuum than the local environment. Spatial variation in the flow network and environmental conditions were the main drivers of zooplankton distributions in spring whereas the flow network of water masses was the most influential factor in summer. [ABSTRACT FROM AUTHOR]