Physical ecosystem engineering by emergent aquatic vegetation: the importance of biomechanical traits

Abstract

This thesis explores the potential of the emergent macrophyte Sparganium erectum to act as a physical ecosystem engineer and delivers an understanding of the vegetative processes that enable it to function in such a capacity. An ecosystem engineer is an organism that creates or modifies habitat; the habitats in question are rivers, particularly low energy sections, the modification relates to the capacity of the species to trigger geomorphological change via a process of flow velocity reduction, sediment accumulation, and reinforcement by underground biomass. The influence of S. erectum, and other aquatic species, on flow and sediment accumulation has been demonstrated before, but its changeable influence at different energy conditions and a detailed understanding of how its morphology influences physical processes has yet to be revealed. To address these gaps in understanding, the research conducted within this thesis is divided into three distinct results chapters; Chapter 4 investigates the influence of S. erectum on patterns of flow and sediment at three reaches of the River Blackwater, Surrey, UK; Chapter 5 explores the capacity of the species to resist mimicked hydraulic stress, and the biomechanical traits that underpin its influence on physical processes; Chapter 6 measures the changing belowground architecture and biomass of the species. The research demonstrates that growth of S. erectum significantly alters river habitats and physical processes, but the nature of its influence varies substantially at the three study reaches, which are indicative of different energy conditions. The species demonstrates a number of subtle biomechanical and morphological traits that cause it to function so efficiently as an ecosystem engineer; these include its long growth cycle, high resistance to uprooting, and tendency for underground mass to occupy surficial layers of sediment. The study concludes by assessing the management implications of the results, which include the potential of the species as a restoration tool, given its ability to create a diversity of river habitats.