ABSTRACT Title of Dissertation: THE SIGNIFICANCE OF SEA ICE ALGAE IN THE PACIFIC ARCTIC DETERMINED BY HIGHLY BRANCHED ISOPRENOID BIOMARKERS Chelsea Wegner Koch, Doctor of Philosophy, 2021 Dissertation directed by: Dr. Lee Cooper Professor, University Maryland Center for Environmental Science Our current understanding of ice algae as a carbon source at the base of the Arctic food web is limited because of difficulties unequivocally distinguishing sympagic (sea ice) from pelagic primary production once assimilated by consumers. For this study, I tested the utility of highly branched isoprenoids (HBI), which are unusual lipids produced by diatoms. This includes a biomarker found exclusively in Arctic sea ice termed the ice proxy with 25-carbon atoms (IP25) and two other HBIs with sea ice and pelagic sources. HBI measurements in the Pacific Arctic (the northern Bering and Chukchi seas) were sparse compared to the rest of the Arctic prior to this investigation. Analysis of surface sediments and cores collected across the continental shelf revealed a latitudinal gradient of increasing sympagic HBIs. Some of the highest concentrations of IP25 recorded in the Arctic were found in the Chukchi Sea. Fluxes of IP25 indicated year- round export of ice algal lipids in this region. Persistent diatom fluxes and rapid burial of sympagic carbon are likely a sustaining resource for infaunal communities throughout the year. As such, HBIs were measured in benthic primary consumers and indicated an elevated utilization of ice algae by surface and subsurface deposit feeders, while suspension feeders by contrast showed greater pelagic organic carbon utilization. Sympagic organic carbon signatures were largely influenced by the HBI content in local sediments. This led to the identification of two species with possible dependencies on ice algae. This method was extended to transient, higher trophic organisms by measurement of HBIs in Pacific walrus livers harvested during subsistence hunting activities. Relative HBI proportions were shown to relate to foraging location and revealed a higher reliance on sympagic organic carbon by female and juvenile Pacific walruses relative to males. This is likely due to a greater requirement for sea ice habitat by females and calves in the Bering and Chukchi seas. This study showed that HBI biomarkers can robustly track sea ice organic carbon contributions through the Pacific Arctic food web and should be considered alongside other trophic markers in future monitoring efforts in response to climate change. THE SIGNIFICANCE OF SEA ICE ALGAE IN THE PACIFIC ARCTIC DETERMINED BY HIGHLY BRANCHED ISOPRENOID BIOMARKERS by Chelsea Wegner Koch Dissertation submitted to the Faculty of the Graduate School of the University of Maryland, College Park, in partial fulfillment of the requirements for the degree of Doctor of Philosophy 2021 Advisory Committee: Professor Lee W. Cooper, Chair Professor Jacqueline M. Grebmeier Professor Andrew Heyes Professor Ryan Woodland Dr. Thomas A. Brown © Copyright by Chelsea Wegner Koch 2021 Dedication At the time of writing this dissertation, we were in the midst of a global pandemic with the novel coronavirus (COVID-19) causing over half a million deaths in the United States. This was the backdrop to months of political instability leading to an insurrection at the US Capitol and a summer of civil unrest driven by racial inequities, injustices and police brutality. This may all sound pretty depressing – and indeed it was. However, during all of this chaos and trying to finish my PhD, there was a silver lining. I brought a new life into this world. I dedicate this dissertation to my daughter, Thalia. I was three months pregnant when Jackie, Lee and I traveled to St. Lawrence Island in the middle of winter, to discuss our research with the tribal councils in Savoonga and Gambell. We arrived in Savoonga on a tiny plane from Nome after waiting out blizzard conditions on the island. We had intended on visiting Gambell but never managed to make it due to weather. I will never forget that you were along for the adventure – as we rode on snow machines and trudged through the dark and blustery snow. I hope that this spirit of exploration stays with you. I also realize that the Arctic we know today will be unrecognizable when you grow up. I hope this will serve as a reminder to you that no matter how small you think your contributions are to the world, every little bit you can do counts. ii Acknowledgements None of this would have been possible without the support of my advisor, Lee Cooper. Lee is always willing to help other researchers improve their manuscripts or give advice, myself included. I have valued his support out at sea (helping me sample mud), in the lab, and improving my manuscripts. Lee has helped me become a more seasoned researcher. I also could not have done this without Jackie Grebmeier. Lee and Jackie have been great mentors that have helped me thrive throughout my PhD program. Jackie and Lee are tireless in their pursuits and have taught me firsthand all the hard work and grit required to get your fieldwork done. Their leadership in the Arctic research community is admirable – serving on so many different international research and planning committees – and have inspired me to pursue early career leadership positions in several Arctic organizations myself. I feel lucky to have worked alongside this dynamic duo. Our lab is a fantastic group of people that have made this work enjoyable. Christina Goethel has been a supportive lab-mate, shipmate and travel buddy. Together we have learned that Astarte clams can survive anything and have had a good laugh or two about the priapulid worms we bring up in the Van Veen grabs. Needless to say, we have spent many hours together spraying mud off the back deck of the Laurier. Cédric Magen was instrumental in analyzing my stable isotope samples and generally anything else I needed help with in the lab. Laura Gemery was critical to getting this project together by sharing some of her mud from prior cruises and has also been a wonderful colleague and friend. I could always count on Alynne Bayard to discuss GIS and what shows were worth watching. I also thank Stephanie Soques for taking the time to teach me in the sorting lab. My committee members supported my growth and development as a scientist. Dr. Tom Brown has been a sounding board throughout my PhD program. He taught me everything I needed to know about HBI analysis when I visited his lab in Scotland and was always willing to answer any question I had iii along the way. Dr. Andrew Heyes and Cheryl Clark made it possible for me to set up this research at the Chesapeake Biological Laboratory and were always available to help. Cheryl taught me the skills to maintain and operate the gas chromatograph – mass spectrometer detector for which I am so grateful. Dr. Ryan Woodland was integral in setting up our clam HBI depuration experiment at the lab and providing the lawn chairs needed to hold it all together. While not officially committee members, Dr. Catherine Lalande and Dr. Karen Frey, have been important mentors for me on this PhD journey and it has been enriching to have collaborated with them on this research. I had the privilege to sail with some amazing scientists that were helpful in a number of ways and I hope to continue working with. This group includes Luisa Young, Shea Wyatt, Sarah Ann-Questel, Dr. John Nelson, Sophie Spiliotopoulos, Clare Gaffey and Sarah Zimmerman. Our time on the CCGS Sir Wilfrid Laurier were some of my favorite memories from my PhD and I will never forget them. A special thanks to these people for always having a good sense of humor even though we were all tired and sore. I also have to thank my dear friend, Dr. Emily Osborne, for all of her words of encouragement and our discussions about my research over coffee. Our science adventures together began in 2012 in South Carolina, the Knauss Fellowship reunited us in Washington, D.C. years later and then again we both wound up in the Arctic. Having had someone to navigate through all of this with and share these experiences has made it so much more meaningful. I was so grateful we had the opportunity to sail together on the Healy. She continuously exemplifies a strong, dedicated and professional female scientist and I sincerely hope our science careers continue to intersect. And of course, none of this would have been possible without my husband, Dave. He has been unwavering in his support of me throughout this entire journey. He gave me the encouraging nudge I needed to leave my job at the US Antarctic Program to pursue this pesky idea that I wanted to get my PhD. He has also listened to me run through enough practice talks that he is able to almost accurately tell iv you what IP25 is. Finding his little notes of encouragement hidden in my suitcase every time I had to do fieldwork always reminded me that I could count on him and made our time apart easier. He has helped me achieve everything I wanted and more. I am so grateful to have such a supportive partner in all of my aspirational pursuits. I also acknowledge the Aleut/Unangan, Iñupiat and Yupik lands we visited during our field work in Unalaska, Utqiaġvik, Nome and Savoonga. Co-production of knowledge and Indigenous-led research is necessary to achieve truly inclusive work that meets the urgent needs and challenges Arctic residents face due to climate change. I have been learning what this means and trying to build these bridges during my PhD program, but recognize that additional efforts are needed to provide these communities stability and equity.