Linking zooplankton and juvenile fish assemblages in a large lowland river: influence of submerged macrophytes

Zooplankton and juvenile fish were sampled among different macrophyte beds in a large lowland river, during a very low discharge period when current velocity was not the structuring factor. Patterns exhibited by zooplankton taxa and fish were investigated at different spatial scales of habitat heterogeneity. One habitat type excepted, macrophyte types did not differ in zooplankton composition, but they differed in their total zooplankton abundance. Zooplankton abundance was higher in vegetated areas and showed clear differences among macrophyte types, whereas no difference was apparent among channel and side-arm habitats. No distinct fish assemblages appeared to be associated with the different macrophyte types. However, these macrophyte types differed in their total juvenile fish abundance, which was higher in vegetated areas and in side-arm habitats. Zooplankton and juvenile fish abundance were positively correlated, and morphological characteristics of the different macrophytes studied were hypothesized to result in different habitat quality (protection against predators, food availability). Our results emphasize the influence of physical and chemical conditions (water temperature, dissolved oxygen and structural complexity) for zooplankton abundance, whereas trophic resources appeared to be the main determinant of juvenile fish abundance. The importance of submerged macrophytes and trophic resources for habitat use in large rivers is discussed.