The zoogeomorphology of case-building caddisfly larvae

Caddisfly (Trichoptera) are an abundant and widespread aquatic insect group. Caddisfly larvae of most species build cases from silk and fine sediment at some point in their lifecycle. Case-building caddisfly have the potential to modify the distribution and transport of sediment by: 1) altering sediment properties through case construction, and 2) transporting sediment incorporated into cases over the riverbed. This thesis investigates, for the first time, the effects of bioconstruction by case-building caddisfly on fluvial geomorphology. The research was conducted using two flume experiments to understand the mechanisms of caddisfly zoogeomorphology (case construction and transporting sediment), and two field investigations that increase the spatial and temporal scale of the research. Caddisfly cases varied considerably in mass between species (0.001 g - 0.83 g) and grain sizes used (D50 = 0.17 mm - 4 mm). As a community, caddisfly used a wide range of grain-sizes in case construction (0.063 mm – 11 mm), and, on average, the mass of incorporated sediment was 38 g m-2, in a gravel-bed stream. This sediment was aggregated into biogenic particles (cases) which differed in size and shape from their constituent grains. A flume experiment determined that empty cases of some caddisfly species (tubular case-builders; Limnephilidae and Sericostomatidae) were more mobile than their incorporated sediment, but that dome shaped Glossosomatidae cases moved at the same entrainment threshold as their constituent grains, highlighting the importance of case design as a control on caddisfly zoogeomorphology. A second flume experiment found that crawling Glossosomatidae larvae transport sand vertically upwards, from sheltered interstices below gravel particles, onto the surface of armoured gravel-beds, increasing the hydraulic exposure of this sediment. As a result of both larvae crawling and case entrainment, case-building caddisfly were responsible for over 30% of coarse sand transport during low to medium discharge conditions in a gravel-bed stream. Tubular case-builders (especially Limnephilidae) and dome case-builders (Glossosomatidae) were particularly important zoogeomorphic agents, using and transporting substantial amounts of coarse sand and fine gravel. This research has shown that case bioconstruction by caddisfly larvae may have a destabilising effect on sand in gravel-bed rivers. The thesis uses case design to conceptualise and understand caddisfly zoogeomorphology under differing biotic and abiotic controls. Future research should consider sediment stabilisation associated with caddisfly pupation, and the relative importance of destabilisation and stabilisation across spatial and temporal gradients. The broad geographic spread, functional diversity, and commonly high abundances of case-building caddisfly mean that they may have important and widespread implications for sediment dynamics in rivers.