Hydraulic controls on river biota and the consequence for ecosystem processes.

Disturbance by flooding can dramatically disrupt population and community structure in stream ecosystems. My dissertation research focused on two species that are vulnerable to high flow events: the caddisfly Dicosmoecus gilvipes and the western pearlshell mussel Margaritifera falcata. I investigated their influence on energy and nutrient dynamics in a Northern California coastal river.In Chapter 1, I examined the impact of the limnephilid caddisfly Dicosmoecus gilvipes on periphyton structure and ecosystem processes. Dicosmoecus larvae reduced periphyton accrual, chlorophyll a, gross primary productivity, and ammonium uptake in experimental channels, and their impact persisted 46 days after the larvae were removed. Given Dicosmoecus vulnerability to high flow events, any change in flood timing, frequency, and/or magnitude due to river regulation or climate conditions may significantly alter ecosystem processes in Northern California streams.In Chapter 2, I investigated whether flood timing would have a differential impact on Dicosmoecus gilvipes populations. Specifically, I measured how critical flow thresholds and habitat use varies with larval size. Critical flow velocity and dimensionless flow threshold indices increased with larval size, as did their flow velocity preference. The results suggest early flood events will have a greater impact on Dicosmoecus populations than later flood events of a similar magnitude, and during low-flow periods the interaction between Dicosmoecus distribution, periphyton composition and productivity, and flow velocity may significantly impact ecosystem processes on smaller scales.In Chapter 3, I investigated the functional role of cases built by Dicosmoecus gilvipes. The larvae collect thin plant material and Douglas-Fir needles and build arrow-shaped lateral extensions on their case. Larvae with lateral extensions experienced fewer revolutions and regained their footing faster in experimental trials than those without. The results suggest lateral extensions provide stability against overturning in fast flow and may improve their ability to forage efficiently in turbulent flow conditions.In Chapter 4, I manipulated the presence and absence of the mussel Margaritifera falcata in stream mesocosms. I measured their impact on organic matter accrual, microbial activity in the sediment, and the growth of larval Pacific lamprey, Lampetra tridentata. Margaritifera presence increased microbial activity in the sediment and larval lamprey growth. Organic matter accrual was not significantly affected. The results suggest that lamprey larvae benefit from native mussels, and that lamprey populations may decrease with the rapid decline of native freshwater mussels.In summary, the presence of both Dicosmoecus gilvipes and Margaritifera falcata had significant affects on ecosystem processes. Knowledge of species impacts on energy and nutrient dynamics and the physical conditions that control species abundance and distribution is essential to predicting both small- and large-scale consequences of an altered hydrograph, whether due to river regulation or climate change