Date of Award
Master of Science
The Cache River, located in southern Illinois, faces a unique set of restoration challenges due to multiple anthropogenic modifications to the watershed. In 1915 the Cache River was disconnected and divided into two sub-watersheds, the upper Cache River (UCR) and the lower Cache River (LCR). This alteration has led to impairments in both sections of the Cache River. The UCR currently suffers from channel incision and wetland loss due to an increase in channel slope, while the LCR experiences decreased flows and related habitat degradation. Currently, watershed managers are proposing to restore a more natural flow regime to the LCR through some degree of hydrologic reconnection. I quantified the effects of small-scale differences in flow velocities on snag-dwelling aquatic macroinvertebrates in the UCR and LCR. My study was designed to provide critical information on potential ecological responses to proposed reconnection of the Cache River by examining the effects of flow on elements of ecosystem structure (macroinvertebrate community structure, diversity, richness, evenness, biomass, and abundance) and function (secondary production). Total snag-dwelling macroinvertebrate abundance was higher in the LCR (p < 0.001), and both total biomass and total production did not differ between the UCR and LCR. Passive filter-feeders (families Hydropsychidae and Simuliidae), EPT taxa (orders Ephmeroptera Plecoptera and Trichoptera), and Elmidae were more abundant and had higher biomass on snags in the UCR compared to the LCR. Due to high variability in estimates, only Elmidae production was higher in the UCR (p < 0.05), with non-significant trends of higher production of passive filter-feeders and EPT taxa in the UCR. Non metric multidimensional scaling (NMDS) ordinations showed different communities on snags in the UCR and LCR, and analysis of similarity indicated these differences were significant (ANOSIM Global R = 0.98, p < 0.05). This UCR community was more diverse (p < 0.05) and composed of larger-sized individuals than the LCR (p < 0.001). In contrast, the LCR community was composed mainly of taxa that are associated with very low flows (e.g., zooplankton), tolerant of degraded conditions (e.g., Isopoda and Chironomidae), and generally smaller in body size. Although not statistically significant, there was a trend for higher total production in the LCR (28.90 ± 54.51) (mean ± 95% CI) than the UCR (10.22 ± 6.43). This trend was driven primarily by a patchily distributed Chironomidae, which were highly productive on some LCR snags. However, given the degraded water quality in the LCR, the ultimate fate of this production is not clear. The more natural flow regime in the UCR is likely driving many of the differences between the UCR and LCR that I observed, but relationships may be complex, as flow influences biota in numerous direct and indirect ways. This study is the first I know of to examine the structure and function of macroinvertebrate communities prior to hydrologic restoration of a river. Results will assist resource managers in the justification, planning, and execution of hydrologic restoration in the Cache watershed. Given that many river restoration projects are not based on sound ecological information and principles, this project can also serve as a model for future river restoration efforts.
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