A common assertion of reach-scale restoration is that modifying in-stream morphology can foster nutrient removal and improve ecological community structure. However, the mechanisms driving structural responses, specifically nutrient processing and decomposition activity at the sediment-water interface are often overlooked in stream assessments. The primary aim of this PhD was to measure the effectiveness of urban river restoration through examination of reach-scale physical, chemical and biological processes which affect biogeochemical process rates and ecological community structure. Restored reaches in this study were generally characterised by in-stream structures including woody debris and gravel material aimed at improving hydromorphological diversity. A novel nitrogen flux assay was developed to simulate nutrient processing across the sediment-water interface in response to physical disturbances and biogeochemical activity. Results revealed equivocal responses to river restoration, a likely legacy effect of urban pollution. Decomposition assays were used to examine ecosystem function using a range of natural substrates (leaf litter, tea bags and cotton strips), although overall rates of breakdown between restored and unrestored sites were negligible. However, the benefits of using standardised, experimental substrates such as cotton strips as surrogates for traditional leaf litter were noted. Fish and macroinvertebrate surveys were used to measure abundance, biomass, richness and community structure. Fish abundance as well as macroinvertebrate biomass and richness were significantly higher at restored sites (i.e. particularly the River Wandle and Hogsmill) and highlighted improvements in ecological community structure. However, restoration failed to modify ecosystem function for either nitrogen processing or organic matter decomposition. These results highlight the need to decipher drivers of ecological recovery through functional stream assessments focused on processes such as primary production.
Stream ecosystem responses to restoration across urban tributaries of the River Thames, London
Lavelle, A. (Author). 1 Nov 2019
Student thesis: Doctoral Thesis › Doctor of Philosophy