Date of Award
Master of Science
Humans have altered flow regimes of river systems worldwide. Restoration efforts are growing in an attempt to rehabilitate these modified systems. The ecological effects of alteration and restoration are not yet well understood due to a scarcity of monitoring over a wide range of environmental conditions. I examined macroinvertebrate community structure across four summers, ranging from record flood conditions to extreme drought, in a heavily altered system that is being considered for a flow restoration project. Due to a diversion channel and leveeing, the Cache River in southern Illinois is currently divided into the upper Cache River (UCR), which suffers from erosion and channel incision, and the lower Cache River (LCR), which is impaired by sedimentation and hypoxia. Resource managers are investigating the possibility of restoration via partial reconnection of the UCR and LCR. The Cache River experienced a large flood in 2011 and extreme drought conditions in 2012. I compared macroinvertebrate abundance, biomass, diversity, and community structure during June, July, and August of 2010-2013. Dissolved oxygen, temperature, chlorophyll, and discharge were also measured to aid in mechanistic interpretation. The UCR community had lower abundance (p = 0.04) and higher individual invertebrate body size (p = 0.03) than the LCR, and total biomass did not differ between the two river segments (p = 0.34). Macroinvertebrate community structure differed between the UCR and LCR (p = 0.001) and among years (p = 0.001). The UCR had higher Shannon diversity (p = 0.02) and lower Hilsenhoff Biotic Index (HBI) scores (p < 0.0001) than the LCR. Diversity in the UCR was highest in 2012 (p < 0.0001). HBI in the UCR was lowest in 2010 (p = 0.003), indicating an increase in community tolerance during and after hydrologic disturbances. Spatial variation in the LCR largely precluded significant patterns in biomass of individual taxa associated with year-to-year flow variation. In contrast, differences in biomass of several individual taxa were evident across years in the more sensitive and spatially homogeneous UCR community. The LCR community had significantly more bivoltine and multivoltine taxa and fewer univoltine taxa than the UCR (p < 0.0001), suggesting the LCR community was more resilient to disturbance after a century of reduced flow. Community structure in the UCR was correlated with monthly discharge, mean and maximum 24-h dissolved oxygen concentration, and degree-days, while the LCR community structure was only marginally correlated with flow variability, further indicating the higher sensitivity of the UCR community to changing environmental conditions. This study improves our understanding of the impacts of hydrologic alteration on river community dynamics, and suggests flow restoration in the Cache River could affect resistance and resilience to disturbance, and may also enhance overall ecosystem integrity through increasing the availability of larger-bodied, more diverse invertebrate prey for in-stream and riparian wildlife. Similar pre-restoration monitoring across a range of environmental conditions can help predict and assess ecological responses to restoration in other modified systems.
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