Pacifastacus Leniusculus) and the Implications for Sediment Recruitment to Rivers
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Biotic and abiotic controls of burrowing by signal crayfish (Pacifastacus leniusculus) and the implications for sediment recruitment to rivers. by Harry Sanders A Doctoral thesis submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University (June 2020) © by Harry Sanders (2020) Abstract The signal crayfish (Pacifastacus leniusculus) is a globally invasive species, and since its introduction to Great Britain has developed a behaviour of burrowing into riverbanks, which has not been documented or observed in its native range. These have the potential to recruit sediment into river systems both directly, through burrow construction, and indirectly, by promoting accelerated erosion and the occurrence of mass failure events. This thesis investigates, for the first time, the drivers of crayfish burrowing, and the effects of crayfish burrows on fluvial geomorphology. The research was conducted using two field investigations that considered the spatial extent and temporal dynamics of crayfish burrows and associated sediment dynamics, and three laboratory studies that considered the physical and biotic mechanisms behind the associations and processes observed in the field. Crayfish burrows were distributed throughout Great Britain across all sampled habitats, and were, on average, 205 mm deep (range 20 – 870 mm), and excavated 1.15 kg of sediment per burrow directly into river systems. In rivers where burrows were present, an average burrow density of 0.39 burrows m- 1 of riverbank (maximum = 1.13 burrows m-1) was observed, directly contributing 0.93 t km-1 of fine sediment (maximum = 4.14 t km-1) to the channel. However, accelerated erosion caused by the presence of burrows recruited 29.5 times more sediment than burrowing alone, with burrowing and the associated acceleration of retreat and collapse recruiting an additional 25.4 t km-1 a-1 of sediment at one field site; an estimated 29.8% of total bank sediment yield. This was supported by physical modelling, which showed that the spatial distribution of burrows was important for determining retreat rates and mechanisms. Numerical modelling successfully predicted the presence, density, and geomorphic impacts of crayfish burrows. Burrow presence was attributed to associations between crayfish population densities and sediment grain size distributions, and burrow density was dependent on river flow velocity. This modelling was supported by mesocosm and flume experiments with live crayfish. Further, mesocosm experiments undertaken in the UK and the USA showed that all signal crayfish have the capacity to burrow, suggesting that burrowing and associated geomorphic change may occur in any invasive signal crayfish population. This thesis has shown that signal crayfish burrows can have substantial geomorphic impact, and is the first research to quantify the geomorphic role of signal crayfish burrows, and more broadly to quantify the relative importance of biotic and abiotic forcing of fluvial sediments. Future research should better consider animals as geomorphic agents, both directly, but also indirectly, by considering the facilitative role that zoogeomorphology may have on wider system processes. ii Acknowledgements Thank you to everyone who has been a part of my PhD experience. It would not have been possible without the support, help, and friendship of so many people. To my supervisors, Steve Rice, thank you for your trust and belief in me, and providing this opportunity that I have enjoyed so much. Thank you for all the time you’ve spent teaching me to write proper, your constant enthusiasm, guidance, and dedication to making this research happen, and supporting me as researcher and friend throughout. To Paul Wood, thank you all the statistics help, for your ever-present support, advice and good jokes at the stressful times, and for your care, friendship, and bad jokes always. You have both made this a thoroughly enjoyable experience; I am forever indebted. This study required a considerable amount of field and lab work which would not have been possible without the assistance of many people. In particular, special thanks go to Richard Mason and Milly Bulcock for your continued and dedicated field and pastoral support throughout this thesis. To Richard, whatever ridiculous situation I have thrown you in, from eating spaghetti hoops with nothing but a cable tie in the boot of a department car a Scottish layby, to being my low speed getaway driver for sand heists, you have always been there, and without your supportive energy, advice, and mountainous adventures this thesis would not be finished. To Milly, thank you for injecting so much fun and laughter to our months of travels across the country, meeting a whole new cross section of society at Ilham Hall and beyond (Thorpe Cloud still awaits!). Thank you for being my shoulder to cry on when routinely finding field sites to be navigable by shipping liner, or not to exist at all. To you both, I am eternally grateful. Thank you also to a host of other people who have helped with fieldwork; to Beth Worley, Ciara Dwyer, Daniel Mills, Ellen Goddard, Hazel Wilson, Jess Green, Kate Mathers, Laura Crawford, Rebecca McKenzie, Sam Wilby and Sarah Sanders for assistance in the field. Also, I must thank the catchment managers, land owners and facilitators for taking time to permit and arrange access; my thanks to Clare Warren, Gill and Richard Trace, Nathan Hall, Julian Payne, and all at the Environment Agency, Paul Bradley and all at PBA Applied Ecology, Mr & Mrs Jones, Tony Booker and Paul Sansom-Timms, Willy Yeomans and David McColl, and all at the Clyde River Foundation, and the Clyde angling community. iii Thank you also to the gy.labs team, in particular to Richard Harland and Rebecca McKenzie, for your laboratory assistance in constructing flumes, and allowing me to shovel copious amounts of clay in your laboratories for the last three years. Particular thanks go to Bas Bodewes for all your help in constructing, running, analysing, and solving all the problems the Friedkin flume threw at us. Thanks also to Beth Worley, Daniel Mills, Davide Vettori, Guy Tallentire, James Smith, Keechy Akkerman and Richard Mason for laboratory assistance throughout. Thank you also to Lindsey Albertson, and all at the AlberCross lab for hosting me for a two month stay at Montana State University. My thanks also to Ben Tumolo, Eric Scholl, Holden Reinhart, Mike MacDonald, and in particular to Zach Maguire for crayfish collection, and to Zach Maguire for laboratory assistance in Montana. I would also like to thank Loughborough University, the British Society for Geomorphology, the Royal Geographical Society and Santander for your financial support of this project. Lastly, but my no means least, thank you to everyone in the Loughborough Geography community, to all past and present members of Loughborough Student’s Mountaineering Club, Klimmen climbing club, Loughborough Geography cricket team, The Jackals quiz team, and everyone else who has kept me sane throughout this thesis. Here’s to many more years of high- octane geomorphology! iv Table of Contents Abstract ii Acknowledgements iii Table of Contents v List of Tables xi List of Figures xvi Chapter 1 Introduction 1 1.1 Research Context and Development of Research Theme 2 1.2 Aims and Research Objectives 3 1.3 Thesis Structure 4 Chapter 2 Zoogeomorphology: Crayfish and Riverbanks 7 2.1 Chapter Structure 8 2.2 Zoogeomorphology and Ecosystem Engineering 8 2.2.1 Zoogeomorphology: Biotic Considerations 12 2.2.2 Zoogeomorphology: Abiotic Considerations 15 2.2.3 Biogeomorphology and Zoogeomorphology of Riverbanks 16 2.3 Fine Sediment, River Ecology, and River Geomorphology 21 2.3.1 Channel Sources of Fine Sediment 21 2.3.2 Non-Channel Sources of Fine Sediment 22 2.3.3 Deposition and Storage of Fine Sediment 23 2.3.4 Geomorphological Impacts of Fine Sediment 24 2.3.5 Chemical Impacts of Fine Sediment 24 2.3.6 Ecological Impacts of Fine Sediment 25 2.4 Signal Crayfish as Zoogeomorphic Agents 30 2.4.1 Crayfish as Geomorphic Agents 31 2.4.2 The Natural History of Crayfish Burrows 37 2.4.3 Types of Crayfish Burrows 38 2.4.4 Where Crayfish Burrow 39 2.4.5 Why Crayfish Burrow 40 2.4.6 Geomorphic Implications of Crayfish Burrows 41 2.4.7 Modelling Crayfish Burrowing 42 2.5 Justification of Research Objectives 43 v Chapter 3 Quantifying the biological, hydrological, and geophysical controls of riverbank burrowing by signal crayfish 45 3.1 Introduction 46 3.2 Aims 46 3.3 Methods 47 3.3.1 Overall description of approach used 47 3.3.2 Field Site Selection 49 3.3.3 Sampling Strategy 49 3.3.4 Crayfish Burrow Survey 50 3.3.5 Candidate variables for characterising crayfish burrows 53 3.3.6 Statistical Analyses 66 3.3.7 Predictive Modelling 69 3.4 Results 77 3.4.1 Quantifying excavated sediment and burrow characteristics 77 3.4.2 Univariate Associations Between Variables and Burrow Characteristics 82 3.4.3 Modelling Burrows and Sediment Input 86 3.5 Discussion 102 3.5.1 The Geomorphological Significance of Burrow Characteristics 102 3.5.2 Modelling Burrow Presence and Absence: Ideal Models 105 3.5.3 Modelling Burrow Density: Ideal Models 109 3.5.4 Ideal Versus Operational Models 111 3.5.5 Model Contributions and Univariate Associations of Discussion Environmental Variables 112 3.5.6 Discussion Summary 124 Chapter 4 The effects of flow velocity on signal crayfish burrowing and associated sediment dynamics 127 4.1 Introduction 128 4.2 Aims 129 4.3 Methods 130 4.3.1 Physical