Ocean Acidification and Marine Biogenic Trace Gas Production
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Ocean Acidification and Marine Biogenic Trace Gas Production A thesis submitted to the School of Environmental Sciences of the University of East Anglia in candidature for the degree of Doctor of Philosophy By Frances Elizabeth Hopkins March 2010 © This copy of the thesis has been supplied on condition that anyone who consults it is understood to recognise that its copyright rests with the author and that no quotation from the thesis, nor any information derived therefrom, may be published without the author’s prior consent. i ii Abstract The oceanic uptake of anthropogenic CO2 emissions is leading to an alteration of seawater carbonate chemistry, manifested as increasing [H+], falling [CO32-] and a drop in seawater pH. Over the coming centuries this process, termed “ocean acidification”, is expected to negatively impact marine biota, with implications for marine biological and biogeochemical processes. In this thesis, the impact that such changes may have on the net production of a range of climatically- and atmospherically-important marine biogenic trace gases, including halocarbons and dimethyl sulphide (DMS), is assessed through a mesocosm phytoplankton bloom CO2 perturbation experiment, two laboratory CO2 incubation experiments on natural seawater samples, and at a volcanically-acidified shallow marine fieldsite in Italy. Large and significant reductions in DMS and DMSP concentrations under future high CO2 conditions were observed during the mesocosm experiment (mean decreases of 57 percent and 24 percent, respectively), a finding in strong support of a previous study (Avgoustidi 2007). Furthermore, concentrations of iodocarbons showed large decreases, with mean decreases under high CO2 ranging from 59 to 93 percent. Results for the laboratory incubation experiments also showed a reduction in iodocarbon concentrations (when normalised to chlorophyll a) under high CO2. These changes may be the result of shifts in plankton community composition in response to the high CO2 conditions, and/or impacts on dissolved organic matter and the bacterial communities involved in the formation of these compounds. The response of bromocarbons was less clear cut during the experimental studies. Following investigations at a naturally-acidified fieldsite in Italy, it was concluded that this site was a poor natural analogue to the impact of future ocean acidification on marine trace gas production. Taking the results of the mesocosm and laboratory incubations into consideration, a combined decrease in both DMS and iodocarbons in response to ocean acidification may have considerable impacts on future atmospheric chemistry and global climate. iii Acknowledgements Many people have contributed to making my PhD experience both enjoyable and rewarding. Firstly, my supervisory team: Professor Peter Liss and Dr. Sue Turner at the University of East Anglia, Dr. Phil Nightingale at Plymouth Marine Laboratory (PML), and Dr. Michael Steinke at the University of Essex, all of whom have consistently provided invaluable guidance, advice, expertise and support. I am extremely grateful to Claire Hughes and Manuela Martino for providing training and advice on halocarbon analysis, and to Tom Bell and Dan Franklin for help and training in DMS/DMSP analysis. This work would not have been possible without the amazing technical assistance of Gareth Lee, both in the lab at UEA and during the mesocosm experiment in Norway. I also thank Gareth, as well as Andy Macdonald and Rob Utting of the UEA technical department, for organising the often tricky logistics involved in the transportation of equipment, analytical instruments, chemicals and gases to far-flung locations, including Norwegian fjords and Italian islands. I am indebted to all those involved in the mesocosm experiment in Bergen, Norway for making the experiment both a great experience and a success, in particular Ian Joint for his leadership of the experiment, Dorothee Bakker for the pCO2 measurements, Isabelle Mary for phytoplankton flow cytometry data and Claire Widdicombe for phytoplankton biomass data. I am also extremely grateful to Claire for taking the time to analyse plankton samples for my Plymouth incubation experiments. I gratefully acknowledge all those at PML who offered help, assistance, advice and friendship during the summer of 2007, in particular Rachael Beale, Laura Goldson, Susan Kimmance, Malcolm Woodward, Glen Tarran, Steve Archer and Malcolm Liddicoat, and made me feel part of the PML “family” during those months spent working at the lab. There were many people involved in the Ischia fieldwork that deserve lots of thanks: Matt Jones, Phil Kerrison, Jason Hall-Spencer, Riccardo Rodolfo-Metalpa, the staff at Institute Laboratorio di Ecologia del Benthos, (Stazione Zoologica Anton Dohrn, Ischia), Roger and Josie Phillips, Tony Dolphin and of course, Sue Turner. A special thank you goes to the ENV PhD gang, all of whom made the last 4 years some of the best of my life, especially my second family Helen Band and Simon Busby, and Nem Vaughan, Laura Bristow and Gemma Bates. Lots of love and thanks goes to my family (Hopkins and Pask!) for all their love and support, and for encouraging me to pursue a career in my love of all things marine. And finally, all my love to Edward Pask, for being more than I could ever have wished for. iv Frances E. Hopkins Table of Contents Table of Contents Page Abstract iii Acknowledgements iv List of Figures xi-xv List of Tables xv-xviii Chapter 1: Introduction and Literature Review 1 Introduction 1 1.1. The Global Carbon Cycle 1 1.1.1. Atmospheric CO2 in Earth’s Past 2 1.1.2. Anthropogenic CO2 emissions 3 1.1.3. Atmospheric C flux to the oceans 3 1.1.4. Seawater DIC, Total Alkalinity (TA) and carbonate equilibrium 4 1.1.5. Seawater pH Buffering Capacity 5 1.1.6. Biological Pump 6 1.2. Seawater CaCO3 Saturation and the Carbonate Compensation Depth 6 1.2.1. Forms of Carbonate 6 1.2.2. Factors controlling saturation states 7 1.3. Natural Variability of seawater pH 8 1.3.1. Average surface ocean pH 8 1.3.2. Natural variations in pH 9 1.3.3. Spatial pH variations 9 1.3.4. Seasonal variability 10 1.4. The Future of Atmospheric CO2 10 1.4.1. Emissions scenarios 10 1.4.2. Impacts on ocean chemistry and pH 12 1.5. Impacts of Anthropogenic CO2 Emissions on Oceanic Carbonate System 14 1.5.1. Impacts on seawater DIC 14 1.5.2. Future projections 16 1.6. Biological Impacts of Ocean Acidification 17 1.6.1. Introduction 17 1.6.2. Marine Phytoplankton 18 1.6.2.1. Introduction 18 1.6.2.2. Photosynthesis and growth rates 18 1.6.2.3. Calcification and C cycling 20 1.6.2.4. Organic C Export 22 1.6.2.5. Trace gas production 23 1.7. Marine Biogenic Trace Gases 24 1.7.1. DMS 24 1.7.1.1. Sources 25 1.7.1.2. Oceanic distribution 26 1.7.1.3. Sinks 28 1.7.1.4. Atmospheric consequences 29 1.7.1.5. DMS-CCN-Climate system 31 1.7.1.6. DMS-CCN-Climate interactions in a high CO2 world 32 1.7.2. Halocarbon Compounds 36 1.7.2.1. Sources 37 1.7.2.1.1. Macroalgae 37 v Frances E. Hopkins Table of Contents 1.7.2.1.2. Phytoplankton 38 1.7.2.1.3. Bacteria 41 1.7.2.1.4. Photochemistry 42 1.7.2.1.5. Reactions with ozone and organic matter 43 1.7.2.2. Sinks 43 1.7.2.2.1. Air-sea exchange 43 1.7.2.2.2. Hydrolysis and nucleophilic substitution 45 1.7.2.2.3. Photochemistry 46 1.7.2.2.4. Bacteria 46 1.7.2.3. Atmospheric consequences 47 1.7.2.3.1. Sea-to-land transfer 47 1.7.2.3.2. New particle formation 47 1.7.2.3.3. Interactions with ozone 48 1.8. Research Aims and Objectives 49 Chapter 2: Methods of Marine Trace Gas Analysis 2.1. Introduction 53 2.2. Analytical Procedures, Materials and Methods 53 2.2.1. Gas extraction and pre-concentration techniques 53 2.3. Halocarbon Analysis 54 2.3.1. Analytical techniques 54 2.3.1.1. Sample collection and preparation 54 2.3.1.2. Purge and cryogenic trap 54 2.3.1.3. Purge and trap on sorbent tubes 55 2.3.1.4. Markes UltrA autosampler and Unity TD platform 59 2.3.1.5. GC-MS system and methods 64 2.3.2. System sensitivity drift and surrogate analytes 68 2.3.3. Halocarbon system calibrations 71 2.4. Dimethylsulphide (DMS) and Dimethylsulphoniopropionate (DMSP) 76 Analysis 2.4.1. Analytical techniques 76 2.4.1.1. Sample collection and preparation 76 2.4.1.2. Purge and cryogenic trap 76 2.4.1.3. Headspace analysis 77 2.4.1.4. Gas chromatograph – Flame Photometric Detector (GC-FPD) 77 system and methods 2.4.2. GC-FPD system calibrations 78 2.5. Other measurements 80 2.5.1. Chlorophyll a and phaeopigments 80 2.5.2. Phytoplankton counts (Flow cytometry) 81 2.5.3. Phytoplankton enumeration (Microscopy) 82 2.5.4. Seawater pH measurements 82 2.6. Summary 85 Chapter 3: Marine biogenic trace gas production during a mesocosm CO2 perturbation experiment 3.1. Introduction 87 3.1.1. Mesocosm experiments in marine science research 87 3.1.2. Mesocosm experiments in ocean acidification research 88 vi Frances E. Hopkins Table of Contents 3.1.3. Results of past ocean acidification mesocosm studies 88 3.1.3.1. Photosynthesis and growth rate 89 3.1.3.2. Phytoplankton calcification and C cycling 89 3.1.3.3. Organic C Export 90 3.1.3.4. Trace gas production 90 3.1.4.