Post-eruption recovery and secondary production of grazing insects in two streams near Mt. St. Helens Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/s7526h694

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  • The eruption of Mt. St. Helens provided the opportunity to study secondary production of grazing insects in the context of disturbance. Two stream sites were chosen that significantly differed in how their watersheds were impacted by the eruption. Clearwater Creek was catastrophically disturbed (physical alteration of habitat, loss of riparian vegetation, and abundant ashfall); Elk Creek was disturbed only by heavy ashfall. Secondary production of the insect community was estimated for 1985 and 1986. The relative importance of disturbance history and between-site habitat differences in determining secondary production was assessed by placing results in the context of ten years (1980-1989) of summer data from both streams. Few insects were found in Clearwater Creek four months after the eruption. In contrast, the community at Elk Creek was diverse and dominated by long-lived taxa. From 1980 to 1989 at Clearwater Creek there was a gradual shift from dominance by Chironomidae to an increasingly diverse community with an abundance of mayflies, stoneflies, and caddisflies. Disturbance from ash scour during the 1980-81 winter reset the benthos of Elk Creek. However, the recovery process in that stream after 1982 was rapid. The annual production of insects in 1985 was 9.7 g dry wt/m2 at Elk Creek and 8.6 g/m2 at Clearwater Creek. In 1986, production was higher at Clearwater Creek (27.4 g/m2) than at Elk Creek (16.3 g/m2). Aufwuchs grazers were the most important insects (ca. 75% of total insect production) of both streams in both years. After differences in assimilation efficiencies of food types were considered, it was found that 84.3% of grazer production at Clearwater Creek depended on algae as compared to 74.3% at Elk Creek. The disturbance history of each site was an important factor determining the insect community structure and the importance of grazers at each site. Consequently, secondary production in 1985-1986 was influenced to a large degree by the 1980 eruption. However, between-site differences in basin dimensions, substrate size, and riparian vegetation likely controlled the productive capacity of each stream.
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