An exploration of associations between assemblages of aquatic plant morphotypes and channel geomorphological properties within British rivers

Riparian vegetation, particularly trees and shrubs, can play a crucial role in the construction and turnover of fluvial landforms, but aquatic plants may also act as river ecosystem engineers.Macrophyte and environmental data from 467 British river reaches are used to explore associations between aquatic plant morphotypes and the physical characteristics of the reaches. The abundance of five plant morphotypes (mosses, linear-submerged, patch-submerged, linear emergent, branched emergent) is estimated for each river reach. Cluster analysis is applied to the abundances of the five morphotypes across the 467 reaches to identify six typical assemblages or clusters of the morphotypes. These clusters are found to be associated with statistically significantly different values of seven physical variables (altitude, slope, median annual flood discharge, channel width, mean bed sediment size, percentage cover of sand and silt on the river bed, and unit stream power). Associations between the morphotype clusters and combinations of the physical variables are explored using Canonical Correspondence Analysis and standard slope–discharge–sediment calibre–channel style graphs.Several of the morphotype clusters are discriminated by unit stream power and bed sediment size. In particular, morphotype clusters dominated by emergent and submerged macrophytes are associated with granules, sand, and finer bed sediments and are rarely found where unit stream power exceeds 100 W m− 2. One cluster characterised by branched emergent species with relatively low cover of submerged morphotypes is confined to sites with unit stream power below 20 W m− 2; and another cluster characterised by linear emergents with low cover of submerged morphotypes is associated with particularly extensive, fine bed sediments, suggesting possible smothering of submerged plants. In contrast, mosses reach their highest abundance in two clusters associated with the highest unit stream power and coarsest bed sediments, with the patch-submerged morphotype reaching relatively high abundance in the slightly lower energy cluster of these two.British rivers have been modified over hundreds of years such that the sample of study reaches have predominantly single-thread channels. However, the plotting positions of these reaches on established graphs describing slope–discharge–sediment calibre–channel style associations, illustrates the potential of some of these sites to develop wandering or braided forms and, in lower energy situations, the potential for aquatic plants to trap fine sediments and contribute to landform building and channel change if maintenance (cutting and dredging) of the emergent and submerged morphotypes were reduced.