Pioneer macrophyte species engineer fine-scale physical heterogeneity in a shallow lowland river

- We assessed interactions between macrophytes, hydraulics and substrate type.
- Macrophyte species enhance hydraulic and substrate heterogeneity.
- Macrophytes manipulate channel morphology and hydraulics by stabilising sediment.
- Pioneer macrophyte species are a key response to altered fine-sediment loads.
- These ecosystem engineers can stimulate recovery of physical processes.

Macrophyte responses to disturbance provide a short-term opportunity to document and interpret plant succession and related hydrogeomorphological interactions relative to the long-term, concomitant evolution of river morphology. Macrophyte colonisation may drive changes, e.g., increasing heterogeneity in physical characteristics that define broad-scale hydrogeomorphology. Relationships between macrophyte presence and key physical descriptors were investigated within channelized river reaches of a low gradient river. Plant species within two morphotypes sharing similar traits were the most abundant colonisers of the wetted channel. The standard deviation of depth, flow velocity and substrate were used in turn as response variables expressing physical heterogeneity across the channel. A second substrate response variable was the summed coarseness score based on substrate size (fines = 1, gravel = 2 and cobble = 3) across the channel. Macrophyte presence showed strong fine-scale relationships to heterogeneity in flow velocity and depth but not to heterogeneity in substrate. Substrate coarseness score was positively related to increasing heterogeneity in flow velocity, with increased presence of coarse substrate indicative of greater heterogeneity in velocity. Certain macrophyte species appear to play an important pioneer role in initiating and driving physical changes that underlie hydrogeomorphological processes. Likely mechanisms are flow manipulation, sediment interception and sorting.