Predicting diversity versus community composition of aquatic plants at the river scale

We tested the relative importance of physical versus chemical factors in explaining aquatic plant species diversity and community composition within a temperate lowland river. A total of 38 macrophyte species were identified at 33 sites along the 104 km length of the Rideau River, a National Heritage River of Canada. Species richness ranged from 0 to 15 species per site, and Shannon diversity from 0 to 2.98. Macrophyte species richness and Shannon diversity were significantly related to the physical characteristics of sites. For Shannon diversity, 77% of the increase was explained by an increase in sediment organic content and a decrease in water velocity. For species richness, 70% of the increase was explained by the latter factors in addition to an increase in the littoral zone (0–2 m depth contour) width and planktonic chlorophyll concentrations. River water chemistry did not explain any observed variation in either Shannon diversity or species richness in this moderately enriched system. In contrast to species richness, the physical and chemical variables measured failed to explain variation in community composition. Cluster analysis did not reveal any grouping of species into distinct communities. Canonical correlation analysis showed that environmental variables had minimal effect on the distribution of most species, with only floating-leaved species responding to water velocity. We conclude that physical factors can predict species diversity at the within river scale but not the species composition at a given site, underlying the need to preserve the geomorphological diversity of rivers to maintain plant diversity.