Provided for non-commercial research and educational use only. Not for reproduction, distribution or commercial use. This chapter was originally published in the book Advances in Ecological Research, Vol. 47 published by Elsevier, and the attached copy is provided by Elsevier for the author's benefit and for the benefit of the author's institution, for non-commercial research and educational use including without limitation use in instruction at your institution, sending it to specific colleagues who know you, and providing a copy to your institution’s administrator. All other uses, reproduction and distribution, including without limitation commercial reprints, selling or licensing copies or access, or posting on open internet sites, your personal or institution’s website or repository, are prohibited. For exceptions, permission may be sought for such use through Elsevier's permissions site at: http://www.elsevier.com/locate/permissionusematerial From: Eoin J. O'Gorman, Doris E. Pichler, Georgina Adams, Jonathan P. Benstead, Haley Cohen, Nicola Craig, Wyatt F. Cross, Benoît O.L. Demars, Nikolai Friberg, Gísli Már Gíslason, Rakel Gudmundsdóttir, Adrianna Hawczak, James M. Hood, Lawrence N. Hudson, Liselotte Johansson, Magnus P. Johansson, James R. Junker, Anssi Laurila, J. Russell Manson, Efpraxia Mavromati, Daniel Nelson, Jón S. Ólafsson, Daniel M. Perkins, Owen L. Petchey, Marco Plebani, Daniel C. Reuman, Björn C. Rall, Rebecca Stewart, Murray S.A. Thompson and Guy Woodward, Impacts of Warming on the Structure and Functioning of Aquatic Communities: Individual to Ecosystem-Level Responses. In Guy Woodward, Ute Jacob and Eoin J. O'Gorman, editors: Advances in Ecological Research, Vol. 47, Burlington: Academic Press, 2012, pp. 81-176. ISBN: 978-0-12-398315-2 © Copyright 2012 Elsevier Ltd. Academic Press Author's personal copy Impacts of Warming on the Structure and Functioning of Aquatic Communities: Individual- to Ecosystem-Level Responses Eoin J. O'Gorman*,1, Doris E. Pichler*, Georgina Adams†,‡, Jonathan P. Benstead}, Haley Cohen*, Nicola Craig*, Wyatt F. Cross}, Benoît O.L. Demars||, Nikolai Friberg#, Gísli Már Gíslason**, Rakel Gudmundsdóttir**, Adrianna Hawczak*, James M. Hood}, Lawrence N. Hudson†, Liselotte Johansson*,#, Magnus P. Johansson††, James R. Junker}, Anssi Laurila††, J. Russell Manson‡‡, Efpraxia Mavromati*, Daniel Nelson}, Jón S. Ólafsson}}, Daniel M. Perkins*, Owen L. Petchey}}, Marco Plebani}}, Daniel C. Reuman†,|| ||, Björn C. Rall##, Rebecca Stewart*, Murray S.A. Thompson*,‡, Guy Woodward*,1 *School of Biological and Chemical Sciences, Queen Mary University of London, London, United Kingdom †Imperial College London, Silwood Park Campus, Ascot, Berkshire, United Kingdom ‡Natural History Museum, London, United Kingdom } Department of Biological Sciences, University of Alabama, Tuscaloosa, Alabama, USA } Department of Ecology, Montana State University, Bozeman, Montana, USA ||The James Hutton Institute, Aberdeen, Scotland, United Kingdom #Department of Bioscience, Aarhus University, Silkeborg, Denmark **Institute of Life and Environmental Sciences, University of Iceland, Sturlugata, Reykjavik, Iceland ††Population and Conservation Biology, Department of Ecology and Genetics, Uppsala University, Uppsala, Sweden ‡‡The Richard Stockton College, Computational Science, Pomona, New Jersey, USA }} Institute of Freshwater Fisheries, Keldnaholt, Reykjavik, Iceland }} Institute of Evolutionary Biology and Environmental Studies, University of Zurich, Winterthurerstrasse 190, Zurich, Switzerland || ||Laboratory of Populations, Rockefeller University, New York, New York, USA ##J.F. Blumenbach Institute of Zoology and Anthropology, Georg-August-University of Go¨ttingen, Go¨ttingen, Germany 1Corresponding authors: e-mail address: [email protected]; [email protected] Contents 1. Introduction 83 1.1 Climate change: Identifying the key drivers and responses 83 1.2 The need for multi-scale and multi-level approaches for dealing with multi-species systems 85 1.3 Individuals, populations and environmental warming 88 # Advances in Ecological Research, Volume 47 2012 Elsevier Ltd. 81 ISSN 0065-2504 All rights reserved. http://dx.doi.org/10.1016/B978-0-12-398315-2.00002-8 Author's personal copy 82 Eoin J. O'Gorman et al. 1.4 Environmental warming impacts on species traits and trophic interactions 89 1.5 Linking communities to ecosystems: Food web and size structure 91 1.6 Environmental warming and ecosystem processes 93 1.7 Testing hypotheses in the Hengill system 95 2. Materials and Methods 95 2.1 Study site 95 2.2 Biotic characterisation 99 2.3 Individuals to populations: Testing temperature–size rules 101 2.4 Quantifying population-level traits and interactions 102 2.5 Quantifying community-level properties 104 2.6 Quantifying the food web and size structure: Community-ecosystem linkages 105 2.7 Ecosystem processes: Energy and nutrient cycling 106 2.8 Ecosystem processes: Ecosystem metabolism measurements 106 3. Results 107 3.1 Structure: Individuals to populations 107 3.2 Structure: Population-level traits 109 3.3 Structure: Population-level interactions 111 3.4 Structure: Community-level properties 113 3.5 Structure: Communities to ecosystems: Food web and size structure 115 3.6 Ecosystem processes: Energy and nutrient cycling 121 3.7 Ecosystem processes: Ecosystem metabolism measurements 121 4. Discussion 124 4.1 Individuals to populations 125 4.2 Population-level traits 127 4.3 Population-level interactions 129 4.4 Community-level properties 130 4.5 Communities to ecosystems: Food web and size structure 132 4.6 Ecosystem process rates: Energy and nutrient cycling 133 4.7 Ecosystem process rates: Ecosystem metabolism measurements 134 4.8 Caveats and limitations 135 4.9 Looking forward: An international partnership at Hengill 136 4.10 Conclusion 138 Acknowledgements 138 Appendix A. Physical and Chemical Properties of the Streams in the Hengill Catchment Examined in This Study 139 Appendix B. Length–Mass Relationships and Biovolume Calculations for the Diatom, Ciliate, Flagellate, Meiofaunal and Macroinvertebrate Assemblages 141 Appendix C. Yield-Effort Curves to Validate the Efficiency of Diatom and Macroinvertebrate Sampling in All Streams in April 2009 151 Appendix D. Source of food web links 155 Appendix E. Supplementary Methods 157 References 161 Author's personal copy Impacts of Warming on the Structure and Functioning of Aquatic Communities 83 Abstract Environmental warming is predicted to rise dramatically over the next century, yet few studies have investigated its effects in natural, multi-species systems. We present data col- lated over an 8-year period from a catchment of geothermally heated streams in Iceland, which acts as a natural experiment on the effects of warming across different organisational levels and spatiotemporal scales. Body sizes and population biomasses of individual species responded strongly to temperature, with some providing evidence to support temperature–size rules. Macroinvertebrate and meiofaunal community com- position also changed dramatically across the thermal gradient. Interactions within the warm streams in particular were characterised by food chains linking algae to snails to the apex predator, brown trout. These chains were missing from the colder systems, where snails were replaced by much smaller herbivores and invertebrate omnivores were the top predators. Trout were also subsidised by terrestrial invertebrate prey, which could have an effect analogous to apparent competition within the aquatic prey assemblage. Top-down effects by snails on diatoms were stronger in the warmer streams, which could account for ashallowingofmass–abundance slopes across the community. This may indicate reduced energy transfer efficiency from resources to consumers in the warmer systems and/or a change in predator–prey mass ratios. All the ecosystem process rates investigated increased with temperature, but with differing thermal sensitivities, with important implications for overall ecosystem functioning (e.g. creating potential imbalances in elemental fluxes). Ecosystem respiration rose rapidly with temperature, lead- ing to increased heterotrophy. There were also indications that food web stability may be lower in the warmer streams. 1. INTRODUCTION 1.1. Climate change: Identifying the key drivers and responses Climate has always shaped the planet’s ecosystems, but as we move deeper into the Anthropocene (Steffen et al., 2007), the predicted rates of change are unprecedented in recorded human history. One of the most pressing challenges in ecology is to understand and predict the likely consequences of climate, yet we are still surprisingly poorly equipped to do so (Walther, 2010). This is partly because climate change operates at large spatiotemporal scales and is also likely to interact with the numerous other anthropogenic stressors that are already imposed across the planet (Friberg et al., 2011; Jeppesen et al., 2012; Mo¨llmann and Diekmann, 2012; Woodward et al., 2010a). It is also a compound stressor whose component parts (e.g. warming, drought, atmospheric CO2 change) interact with one another, and often in seemingly unpredictable ways. Given the almost overwhelming task we are faced with, we need to compartmentalise the problem, so we can grapple Author's personal copy 84 Eoin J. O'Gorman et al. with it in its simplest forms by exploring one component at a time before attempting to consider its full range of possible effects and potential synergies with other drivers. Considerable