Distribution of invertebrates within beds of two morphologically contrasting stream macrophyte species

Increases in the morphological complexity of stream macrophytes are often shown to be linked to higher macroinvertebrate abundance and diversity, but a number of studies have demonstrated, however, that this is not always the case. The objective of this study was to compare the macroinvertebrate community associated with two macrophyte species with contrasting structural complexity and bed shoot density (Sparganium emersum = simple and Callitriche platycarpa = complex), and subsequently relate the observed fine-scale distribution and habitat characteristics within the macrophyte beds to their feeding functional groups (FFG). We hypothesized that a structurally more complex macrophyte community would support higher macroinvertebrate abundance and that the distribution of different FFGs within macrophyte patches would be related to current velocity, thus reflecting changes in food source availability. To test our hypotheses we sampled the macroinvertebrate community at various locations within patches of two different macrophyte species. Contrary to our expectations, we found that the two morphologically different macrophytes supported similar macroinvertebrate densities, whereas the macroinvertebrate community composition was significantly different. Filter feeders were more abundant in S. emersum than in C. platycarpa, whereas the remaining FFGs exhibited higher abundance of C. platycarpa. Individuals belonging to the shredder FFG were not found in S. emersum. We found that the abundance of filter feeders increased with increasing current velocity within patches of C. platycarpa, but not within patches of S. emersum, and that the abundance of predators, scrapers and gatherers increased with increasing macrophyte biomass. We infer that the differences in macroinvertebrate distribution both between the two macrophyte species and within patches of the same macrophyte species are likely to be driven by differences in current velocity and food availability rather than structural complexity per se. This study adds to existing understanding of the mechanisms driving higher diversity in morphologically more complex habitats.