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 v elocity was not the structuring factor. Patterns exhibited by zooplankton t axa and fish were investigated at different spatial scales of habitat heter ogeneity. One habitat type excepted, macrophyte types did not differ in zoo plankton composition, but they differed in their total zooplankton abundanc e. Zooplankton abundance was higher in vegetated areas and showed clear dif ferences among macrophyte types, whereas no difference was apparent among c hannel and side-arm habitats. No distinct fish assemblages appeared to be a ssociated with the different macrophyte types. However, these macrophyte ty pes differed in their total juvenile fish abundance, which was higher in ve getated areas and in side-arm habitats. Zooplankton and juvenile fish abund ance were positively correlated, and morphological characteristics of the d ifferent macrophytes studied were hypothesized to result in different habit at quality (protection against predators, food availability). Our results e mphasize the influence of physical and chemical conditions (water temperatu re, dissolved oxygen and structural complexity) for zooplankton abundance, whereas trophic resources appeared to be the main determinant of juvenile f ish abundance. The importance of submerged macrophytes and trophic resource s for habitat use in large rivers is discussed.