Abstract
Intermittent river networks are highly dynamic freshwater ecosystems that experience low flows, no-flows, drying, and re-wetting phases. At a global scale, intermittent rivers represent 51-60% of all watercourses. There is a growing need to better understand how flow intermittency affects 1) macroinvertebrate diversity and composition in lotic and lentic habitats within intermittent rivers, and 2) the processes structuring these communities at a local and regional scale.This thesis had two main aims. The first aim was to determine whether spatial proximity between communities or local environmental conditions were the main drivers of α- and β-diversity in the main channel, secondary channels, floodplain pools of four intermittent Italian braided rivers experiencing low and high flows (2015-2016). This aim was explored using a metacommunity framework. The second aim was to investigate the response of the Drought Effect of Habitat Loss on Invertebrates (DEHLI) index across multiple river typologies within the UK. I used data from an Environment Agency national dataset (2014-2019) comprising one partially intermittent (> 20-60% NFY: years with at least one day of no-flow event), 18 intermittent (> 60% NFY), and 13 near-perennial rivers (> 0-20% NFY). The DEHLI index assesses the impact of drying on macroinvertebrates by giving weighted scores to taxa that are associated with key habitat drying stages, including flow cessation.
The results of the first part of this study indicated that during low flows in four Italian braided river systems, α- and β-diversity were driven by local environmental filtering and dispersal limitation. The loss of hydrological connectivity negatively affected the dispersal abilities of many macroinvertebrate taxa. However, high flows were a stronger driving force on α- and β-diversity than flow intermittency due to a high local environmental filtering, which may have re-set the communities to an early successional stage. During high flows, floodplain pools possessed the highest β-diversity compared to main and secondary channels, and the highest local contribution to β-diversity (LCBD) values when all mesohabitats were combined. Their high LCBD values reflected a high dissimilarity between floodplain pools communities and the other mesohabitat communities, and thus their ecological uniqueness that contributed to high β-diversity during high flows when all mesohabitats were considered. Findings from the second part of the study indicated that flow intermittency was the main driver of DEHLI scores across all seasons, but particularly in summer. Findings from this thesis confirm the importance of examining 1) multiple lotic and lentic habitats (i.e., floodplain ponds) when studying the effects of flow intermittency on macroinvertebrate communities, and 2) identifying the effects of low flows to determine the impacts of the successional drying stages of river networks on macroinvertebrate communities (versus pre/post drying stages that is currently the most common sampling method). Findings from this thesis be useful to help managers and practitioners identify the overall biodiversity in intermittent rivers and include rivers experiencing low flow for their long-term conversation strategies.
| Date of Award | 27 Sep 2023 |
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| Original language | English |
| Supervisor | Tory Milner (Main Supervisor) & Matthew Hill (Co-Supervisor) |