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Water Mass Connectivity and Mixing Along the Southern Margin of Australia: Hydrographic and Stable Isotope Analyses

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Water Mass Connectivity and Mixing Along the Southern Margin of Australia: Hydrographic and Stable Isotope Analyses Water Mass connectivity and mixing along the southern margin of Australia: hydrographic and stable isotope analyses Laura Ellen Richardson A thesis submitted for the degree of Doctor of Philosophy Of The Australian National University April 2015 Statement of Authorship Unless otherwise acknowledged, this thesis is my own original work. It would not have been possible without the intellectual and scientific guidance of John Middleton, Bradley Opdyke, Noel James and Kurt Kyser, who supervised the research and are co-authors on the three manuscripts (chapters 2, 3 and 4). Laura Richardson April 2015 Acknowledgements Firstly, thanks to my supervisor John Middleton at SARDI Aquatic Sciences. John you allowed me to follow the path I wanted to take with this project. I am eternally grateful for the opportunity to be part of the physical oceanography group and learn so much, and to be able to go out to sea and collect my own data. The cruises were such a great experience! I thank you for providing the funding to send me to Canada to run the isotope samples, without which I would not have been able to go. You have been very supportive through the whole process both personally and scientifically, even when I needed to move to Kalgoorlie and then Canada. I think very fondly of my time at SARDI, it was a great environment to work in and I learnt a great deal. I hope I was able to return the favour and teach you a little about isotopes too! Thanks to my Canadian advisors Noel James and Kurt Kyser at Queen’s University. Noel you have always been so supportive, believing in me and directing me back on the water mass path when the physical oceanography around me was getting too much! Thanks for listening as I went through my ideas and data with you, and for editing my manuscripts so thoroughly. Kurt I am so grateful for having access to the QF1R stable isotope lab - thank you for giving me the chance to run so many samples. Your insight into isotope processes has been so valuable and I have learnt so much from you. A big thank you to Kerry Klassen and April Vuletich in the stable isotope lab for helping me run the machines, especially all the troubleshooting that needed to be done. I appreciate your time and help. Thanks very much to Charles James for all your patience teaching me the basics of Matlab and helping me with scripts, and for helping me define oceanography terms while I was correcting my thesis. I really appreciate you helping me remotely to make figures when my scripts were not working! And thanks to Mark Doubell for helping me with scripts when Charles’ explanations were a bit too technical for me! Last but not least, thank you to my supervisor on the ground, Brad Opdyke. Things didn’t really go as planned but thanks for being so supportive and letting me change what I wanted to study. I really appreciate all your help remotely, and for being a friendly face when I came to visit. You’ve been a wonderful mentor since undergrad. You were central to me finding what I loved to study and have been an important part of the journey since then. 1 would like to thank my wonderful husband Jonathan, for your continual support and guidance; for listening to me formulate ideas, problem solve and share my enthusiasm for the project, and for your help with all things technical! Thank you so much for being there when I needed support, and for working so hard to give me the chance to do what I needed to do. I would not have gotten to this point without you. Many thanks to my Mum and Dad, for always believing in me and supporting the path I have taken without question or judgment. I am so lucky to have such a strong foundation and constant encouragement. Thank you to my Aunty Riet and Uncle Bruce for being so interested and invested in my schooling! It has been a long road but I have finally gotten here! Thanks Aunty Riet for all your writing and editing help. And thank you to the rest of my family and friends who have supported me through this process. Finally to my son Liam, you made this process so much more challenging and interesting! I’m so glad you were part of this journey. Abstract This study is the first to characterise the hydrographic properties and depth range of the Flinders Current and confirm its influence on shelf ecosystems of the Kangaroo Island upwelling region. Four water masses are identified in the top 1000 meters water depth (mwd) from Cape Leeuwin to Tasmania, using hydrochemistry and stable isotopes of seawater. Three water masses are identified from previous literature on the southeast Indian Ocean: Subtropical Surface Water (STSW), Tasmanian Subantarctic Mode Water (TSAMW) and Tasmanian Intermediate Water (TIW), and one is newly identified and named: South Australian Basin Central Water (SABCW). STSW is transported east by the Leeuwin Current system and is modified by heating and evaporation along the subtropical continental shelf. SABCW is formed at the subtropical front within the South Australian Basin at -40°S, TSAMW is formed within the Subantarctic Zone southwest of Tasmania, and TIW is formed from mixing of two different types of Antarctic Intermediate Water west of Tasmania. The Flinders Current transports SABCW, TSAMW and TIW west along the Australian continental slope. The top surface of SABCW delineates the interface between subantarctic water transported by the Flinders Current and subtropical water transported by the Leeuwin Current system. This interface is typically -300 mwd during winter and -250 mwd during summer, but can be as shallow as 150 mwd during summer in the Kangaroo Island upwelling region and off western Tasmania. Stable isotope values show these water masses continue north along the Western Australian slope, identifying connectivity between the Flinders Current and Leeuwin Undercurrent. Deep upwelling events in the Kangaroo Island upwelling region source SABCW from depths of 300 m or more, which is the first evidence that upwelling supplies Flinders Current water to shelf ecosystems. Stable isotopes of seawater identify the formation of a mixed water mass as SABCW mixes with STSW on the shelf. Spatial distribution of this water mass suggests that upwelled water is transported west towards Eyre Peninsula and north into the mouth of Spencer Gulf, and vertical mixing allows upwelled nutrients to be brought into the photic zone to be utilised by primary producers. Strong upwelling events during February and March 2008 and February and March 2010 recorded temperatures/salinities as low as 10.4°C/34.85, and NOx/phosphate concentrations as high as 13.35/0.94 pmol/L, on the shelf. New results for nutrients show average values of NOx and phosphate during months of strong upwelling to be 6.1 times and 4.6 times greater, respectively, than during winter months, and that upwelled water can have nutrient concentrations up to 90 times higher than those in summer surface waters, which is higher than values recorded previously for the Bonney Coast. Upwelled water was also low in silicate, a signature of Southern Ocean water masses, which has implications for phytoplankton community structure and diatom abundance on the shelf. Identifying nutrient signatures of upwelled water, as well as water mass interactions during upwelling events, has implications for mixing of nutrient-rich upwelled waters with oligotrophic surface waters, a situation that supports greater levels of primary productivity on the shelf. Table of Contents STATEMENT OF AUTHORSHIP....................................................................................................................II ACKNOWLEDGEMENTS............................................................................................................................... Ill ABSTRACT......................................................................................................................................................... V LIST OF FIGURES.............................................................................................................................................XI LIST OF TABLES............................................................................................................................................XIII LIST OF ABBREVIATIONS..........................................................................................................................XIV CHAPTER 1. INTRODUCTION...................................................................................................................... 1 1.1 M o t iv a t io n a n d sc o pe of t h e s is .........................................................................................................3 1.2 T hesis o u t l in e a n d key q u e s t io n s ...................................................................................................... 4 1.3 M e t h o d o l o g y Ov e r v ie w .....................................................................................................................7 Hydrochemistry data............................................................................................................................ 7 Stable isotope data...............................................................................................................................7 Stable isotope analyses.....................................................................................................................10 1.4 Ref er e n c e s ............................................................................................................................................11
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