Biomechanical properties of the emergent aquatic macrophyte Sparganium erectum: Implications for fine sediment retention in low energy rivers

This paper is concerned with the biomechanical properties of the emergent aquatic macrophyte, Sparganium erectum. We present observations of adjustments in the physical characteristics and biomechanical properties of S. erectum during the growing season (April-November) from the River Blackwater, UK. When a pulling device is attached to plant stems to measure their resistance to uprooting, individual plants show remarkable strength in their above- and below-ground biomass (median stem strength when stems break away from the underground biomass, 78 N, median rhizome strength, 39 N) and high resistance to uprooting (median uprooting resistance when entire plants uproot, 114 N). This provides the potential for the species to protect and reinforce the generally soft, silty sediments that it often retains and within which its rhizomes and roots develop in lower energy river environments. There is a propensity for plant stems to break before the plant is uprooted at the beginning and end of the growth season, but for the stems to have sufficient strength in mid season for plant uprooting to dominate. This ensures that rhizome and root systems remain relatively undisturbed at times when the silty sediments in which they grow are poorly protected by above-ground biomass. In contrast, rhizome strength remains comparatively invariant through the growing season, supporting the plant's potential to have a protective/reinforcing effect on fine sediments through the winter when above ground biomass is absent.