Emergent Macrophyte Root Architecture Controls Subsurface Solute Transport

Abstract

Emergent macrophytes (helophytes) grow in the active channel of fluvial ecosystems. Subsurface flow beneath this area (i.e., hyporheic zone) is considered critical for ecological processes. However, little is known about the influence of helophyte roots on subsurface solute transport. We investigated the effect of three helophyte species with different root architecture (Iris pseudacorus L., Phragmites australis L., and Scirpus lacustris L.) on solute transport along subsurface flow paths. We considered both the physical and the biological roles of the roots, expecting that (1) roots will act as structures that create heterogeneities in the sediment (physical role); thus, root architecture will alter subsurface flow paths; (2) roots will remove water via evapotranspiration (biological role), leading to slower flow velocity; and (3) both scenarios will result in longer water residence times. We performed conservative tracer pulse additions in 12 flow-through flumes subjected to four treatments: absence of helophytes (Control) and presence of helophytes (Iris, Scirpus, and Phragmites). Tracer breakthrough curves were used to compare solute transport patterns between the treatments by fitting a mobile-immobile model and by applying temporal moment analysis. Results showed that helophyte roots increase subsurface water residence time by creating heterogeneities in the substrate and by removing water. Furthermore, hydraulic retention increased with the percent volume of fine roots but decreased in the presence of thicker roots. Based on these results we suggest that the root architecture of helophytes and their capacity to remove water via evapotranspiration should be considered when planning stream restoration activities aimed to improve water quality.