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Title: Quantifying the effect of river impoundment on riverine ecosystems at multiple spatial scales
Author: Krajenbrink, Henk
ISNI:       0000 0004 8500 7382
Awarding Body: Loughborough University
Current Institution: Loughborough University
Date of Award: 2019
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River impoundment is considered one of the most important anthropogenic impacts on rivers globally. The growing body of literature on the effects of impoundment on abiotic and biotic factors tends to focus on individual large (hydropower) dams, while the effects on smaller water supply reservoirs across multiple spatial scales remain poorly understood. This thesis examines the effects of impoundment by water supply reservoirs on instream communities through four separate studies using a large-scale multi-year dataset derived from a routine biomonitoring network. The first study compared macroinvertebrate community structure and biomonitoring indices between monitoring sites downstream of water supply reservoirs (n = 80) and paired control sites in England. The results demonstrate consistent differences in community structure between downstream and control sites across spatial scales, but also highlight the influence of other spatio-temporal variables including large-scale biogeographical factors. Taxonomic richness was typically higher at downstream sites, while indices including the Lotic-invertebrate Index for Flow Evaluation (LIFE) and the proportion of Ephemeroptera, Plecoptera and Trichoptera taxa (%EPT) were lower. The second study compared benthic diatom community structure and indices at downstream and control sites associated with 77 water supply reservoirs and tried to quantify the factors influencing diatom community change. It was shown that community structure as well as diatom ecological guilds at downstream sites differed from control sites. Values for indices including the Trophic Diatom Index (TDI) and taxonomic richness were generally higher at downstream sites, with the strongest increases in richness recorded among motile taxa. Water quality gradients appeared the main driver of diatom communities, but the results suggested the influence of other factors including the river flow regime. The third study examined the effect of long-term discharge alteration on macroinvertebrate and diatom communities downstream of nine water supply reservoirs using discharge indices derived from modelled flow–duration curves. The results demonstrate that water supply reservoirs typically reduced high and median discharges and increased low discharges, although large differences between reservoirs occurred. The macroinvertebrate index LIFE was positively associated with higher discharges and more limited flow magnitude changes, while some diatom community indices displayed the opposite response, being positively associated with reduced discharge variability. The fourth study examined the effect of impoundment on the thermal regime of rivers and the potential implications of thermal alteration on biotic communities using near-continuous water temperature measurements at three water supply reservoirs. The results demonstrate that reservoirs decreased annual thermal variability and shifted the timing of thermal events, but also indicate marked differences caused by management operations. Macroinvertebrate indices were positively associated with higher maximum temperatures and with lower minimum temperatures, while diatom indices were positively associated with increased minimum and mean temperatures. The results presented in this thesis may improve our understanding of the spatially-extensive and long-term effects of water supply reservoirs on instream communities and provide a basis for the future implementation of management and mitigation measures such as environmental flows on impounded rivers and heavily modified water bodies.
Supervisor: Not available Sponsor: Natural Environment Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: reservoirs ; river regulation ; macroinvertebrates ; diatoms