Masters Thesis

How small bugs tie down big rocks: measuring and modeling the forces acting between nets spun by Caddisfly larvae (Hydropsychidae) and gravel particles at the onset of motion

Many organisms, both plants and animals, influence geomorphic processes in rivers. Common aquatic insects, net-spinning caddisfly larvae (Tricoptera: Hydrospyche), build silk nets that can increase the threshold of sediment motion or critical shear stress t * by up to a factor of 2. Although previous research has shown nets increase t *, the magnitude of resisting force attributed to individual nets and the threshold of their effect as grain size increases is still unknown. To explore net strength, I conducted flume experiments at the Stroud Water Research Center in Avondale, Pennsylvania, where I made direct measurements of the forces acting between nets and individual sediment particles of various sizes using a strain gage during initial particle motion. 1 paired force readings with underwater video of each rock to make observations of the stretching, tearing and detachment of individual nets and measure the increase in force they contribute. Another way I detected the magnitude of force contributed by caddisfly nets is by comparing the measured peak forces to an abiotic model used to calculate the peak force without caddisflies to quantify the difference. Results from measurements made of the force contributed by individual nets and the difference of peak forces and an abiotic model for individual rock pulled from the bed suggest the threshold lies between 70 mm and 75 mm b-axis. Net measurements show larger rocks have nets that are contributing more force because larger rocks allow for larger nets to be built. The influence of caddisfly nets on bed stability has potentially significant implications for the timing and magnitude of bedload sediment transport in gravel-bedded rivers.

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