IMPACT OF DROUGHT ON STREAM ECOSYSTEM STRUCTURE AND FUNCTIONING Gavin Mark David Williams A thesis submitted to the University of Birmingham for the degree of DOCTOR OF PHILOSOPHY School of Geography, Earth and Environmental Sciences College of Life and Environmental Sciences University of Birmingham October 2016 i University of Birmingham Research Archive e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder. University of Birmingham Research Archive e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder. ii Abstract Climate change is projected to increase the frequency and severity of extreme events, adding to the plethora of existing pressures that streams and rivers already face. Compound events such as drought may comprise numerous stressors that occur in concert to elicit ecological change. However the causal mechanisms of such impacts remain unknown, and research attempting to disentangle impacts of compound events, or link effects across levels of ecological organisation, remains in its infancy. This research investigates impacts of key drought stressors – sedimentation, dewatering and warming – across multiple ecological, hierarchical levels. At the individual level, macroinvertebrates displayed differential thermal sensitivity to warming which may explain idiosyncratic ecological responses reported elsewhere, whilst sedimentation intensified predator-prey interactions. Mesocosms were effective tools for studying drought stressors independently and in combination at the community and functional level. Dewatering main effects reduced the density of a common taxon and functional feeding group biomass, whilst all three stressors sometimes interacted together in complex ways. Stressors also had quantifiable effects at the whole-system level, e.g. stream metabolism. This study provides initial findings pertaining to drought impact causative mechanisms across multiple levels of ecological complexity, highlighting the importance of an experimental approach to predict future effects of compound events. iii For Mum and Dad iv ACKNOWLEDGEMENTS This piece of work would not be what it is without the help and support of a multitude of people. Thank you to my friends and family for your support and advice throughout the last 4 years. In particular, a huge thank you to my parents David and Anne Williams for everything you have done, including towing your ‘Cromer Cruiser’ two wheeled accommodation to Hampshire and back, allowing me to live in the field and carry out my fieldwork. Thank you to Sarah Brown for your continued support, encouragement and perseverance in me through this journey, and to my very good friend Andrew Witty for all of your support and practical help in the field. Thank you to my supervisors Mark Ledger and Lesley Batty and to Scott Haywood for providing comments on chapters and to Matt O’Callaghan and Kris Hart for your invaluable support throughout the PhD and excellent company in the field. Thank you to Jon Sadler, Kieran Khamis and colleagues from room 411 for help with R; Mel Bickerton and Andy Moss for taxonomic support; Richard Johnson and Sajid Awan for technical and laboratory support; Gareth Jenkins, Bjorn Rall and Eoin O’Gorman for functional response advice; Jamie and other Vitacress farm staff at Fobdown who offered their support; and all undergraduates and postgraduates who provided assistance in the field and laboratory as part of their dissertations. This PhD was part of a larger project entitled ‘DriStream’ involving the University of Birmingham, Queen Mary University of London and Imperial College London, and was fully funded by the Natural Environment Research Council (NERC). v “And from his alder shades and rocky falls, And from his fords and shallows sent a voice” The River Derwent, William Wordsworth vi TABLE OF CONTENTS Page Table of Contents i List of Figures iv List of Tables vi Chapter 1. Introduction 1 1.1 Lowland chalk streams as model systems 10 susceptible to drought 1.2 Ecosystem functioning 14 1.3 Drought as a compound disturbance 15 1.3.1 Sedimentation 15 1.3.2 Dewatering 16 1.3.3 Warming 17 1.4 Thesis overarching aims 19 1.5 Thesis outline 20 1.6. References 21 Drought as a compound disturbance: community Chapter 2. structure 33 2.1 Abstract 34 2.2 Introduction 35 2.3 Methodology 39 2.3.1 Study site 39 2.3.2 Experimental design 43 2.3.3 Sample processing 44 2.3.4 Data analysis 45 2.4 Results 46 2.4.1 Treatments 46 2.4.2 Treatment responses 52 2.5 Discussion 67 2.6 Conclusion 78 i 2.7 References 79 Drought as a compound disturbance: ecosystem Chapter 3. functioning 88 3.1 Abstract 89 3.2 Introduction 90 3.3 Methodology 97 3.3.1 Study site 97 3.3.2 Experimental design 97 3.3.3 Sample processing 98 3.3.4 Data analysis 106 3.4 Results 107 3.5 Discussion 130 3.6 Conclusion 142 3.7 References 143 Sedimentation intensifies predator-prey interactions Chapter 4. in rivers: a comparative functional response 155 experiment 4.1 Abstract 156 4.2 Introduction 157 4.2.1 Taxa selection 161 4.3 Methodology 162 4.3.1 Data analysis 166 4.4 Results 167 4.5 Discussion 174 4.6 Conclusion 180 4.7 References 181 Ecological implications of macroinvertebrate Chapter 5. physiological responses to warming 190 5.1 Abstract 191 5.2 Introduction 192 5.3 Methodology 199 5.3.1 Macroinvertebrate collection and housing 199 ii 5.3.2 Critical Thermal Method (CTM) 201 5.3.3 Water temperature variation in lowland 205 streams 5.3.4 Comparing taxa activity threshold to river 207 temperatures 5.4 Results 207 5.4.1 Macroinvertebrate activity thresholds 207 5.4.2 Lowland stream water temperature 211 5.4.3 Stream community structure and functioning 216 vulnerability 5.5 Discussion 224 5.6 Conclusion 237 5.7 References 237 Chapter 6. General discussion 250 6.1 Utility of experiments in drought-stressor 251 research 6.1.1 Drought stressors as causal mechanisms 251 6.1.2 Drought stressors across multiple ecological 256 levels 6.2 River restoration 262 6.3 Suggestions for further research 268 6.4 Conclusion 270 6.5 References 270 APPENDICES 278 Appendix A: Chapter 2 supplementary information 279 Appendix B: Chapter 3 supplementary information 288 Appendix C: Chapter 4 supplementary information 299 Appendix D: Chapter 5 supplementary information 304 iii LIST OF FIGURES No. Description Page 1.1 Critical stages of drought…………………………………………. 8 1.2 Conceptualisation of drought research to date……………….. 9 1.3 Photographs of chalk streams near to the mesocosm facility in Hampshire, U.K…………………………………………. 12 1.4 Photographs of iconic chalk stream flora and fauna………… 13 2.1 Geographic location of the mesocosm facility………………... 41 2.2 Photograph of mesocosms………………………….................... 42 2.3 Schematic diagram of a mesocosm channel………………….. 42 2.4 Mean diel water temperature in the experiment………………. 48 2.5 Physical treatment characterisation following stressor application……………………………………………………………. 49 2.6 Mesocosm treatment effects on temperature and dissolved oxygen………………………………………………………………… 50 2.7 Mesocosm treatment effects on pH……………………………... 51 2.8 Community level treatment effect responses…………………. 55 2.9 RDA ordination diagram of square root transformed taxa abundance……………………………………………………………. 56 2.10 RDA ordination diagrams of relative taxa abundance……….. 57 2.11 Mean (±1SE) density of 12 core taxa in treatments…………… 59 2.12 Mean (±1SE) density of 12 core taxa in treatments…………… 60 2.13 Mean (±1SE) density of 12 core taxa in treatments…………… 61 2.14 Mean (±1SE) density of 12 core taxa in treatments…………… 62 2.15 Mean (±1SE) density of 12 core taxa in treatments…………… 63 2.16 Mean (±1SE) density of 12 core taxa in treatments…………… 64 2.17 Mean (±1SE) biofilm biomass among treatments…………….. 66 3.1 Photographs of the mesocosm channels………………………. 105 3.2 Photographs taken of two contrasting mesocosm treatments……………………………………………………………. 111 3.3 Macroinvertebrate standing stock……………………………….. 112 3.4 Mean (± 1SE) functional feeding group biomass responses to treatments………………………………………………………… 113 3.5 Photographs of harvested macrophytes at the end of the experiment…………………………………………………………… 115 3.6 Mean (± 1SE) Relative Growth Rate (RGR) of two contrasting macrophyte taxa……………………………………... 116 3.7 Mean macrophyte leaf chlorophyll concentration (mg g -1; ± 1SE) across treatments……………………………………………. 118 3.8 Dissolved oxygen and light (PAR) diel curves………………… 119 3.9 Dissolved oxygen and light (PAR) diel curves………………… 120 3.10 Dissolved oxygen and light (PAR) diel curves………………… 121 3.11 Dissolved oxygen and light (PAR) diel curves………………… 122 iv 3.12 Ecosystem metabolism responses among treatments……… 123 3.13 Ecosystem metabolism responses among treatments……… 124 3.14 Ecosystem metabolism responses among treatments……… 125 3.15 Decay coefficient (-K) comparison across channel treatments……………………………………………………………. 127 4.1 Photograph of mesocosms……………………………………….. 164 4.2 Physical treatment characterisation of the mesocosms…….. 165 4.3 Bullhead type II functional response curves…………………... 170 4.4 Proportional mortality of G. pulex following 24 hours feeding by bullhead………………………………………………… 171 4.5 Logistic regression model 4: Partial residual visualisation… 173 5.1 Map of macroinvertebrate collection sites at Fobdown Farm, Alresford, U.K………………………………………………..