Thalassia Testudinum)-Dominated Systems Across the Northern Gulf of Mexico

The University of Southern Mississippi The Aquila Digital Community Dissertations Summer 8-1-2021 Patterns of Habitat Use and Trophic Structure in Turtle Grass (Thalassia testudinum)-Dominated Systems Across the Northern Gulf of Mexico Christian Hayes Follow this and additional works at: https://aquila.usm.edu/dissertations Part of the Marine Biology Commons Recommended Citation Hayes, Christian, "Patterns of Habitat Use and Trophic Structure in Turtle Grass (Thalassia testudinum)- Dominated Systems Across the Northern Gulf of Mexico" (2021). Dissertations. 1914. https://aquila.usm.edu/dissertations/1914 This Dissertation is brought to you for free and open access by The Aquila Digital Community. It has been accepted for inclusion in Dissertations by an authorized administrator of The Aquila Digital Community. For more information, please contact [email protected]. PATTERNS OF HABITAT USE AND TROPHIC STRUCTURE IN TURTLE GRASS (THALASSIA TESTUDINUM)-DOMINATED SYSTEMS ACROSS THE NORTHERN GULF OF MEXICO by Christian Hayes A Dissertation Submitted to the Graduate School, the College of Arts and Sciences and the School of Ocean Science and Engineering at The University of Southern Mississippi in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy Approved by: Dr. M. Zachary Darnell, Committee Chair Dr. Kelly M. Darnell, Research Advisor Dr. Kevin S. Dillon Dr. Mark S. Peterson August 2021 COPYRIGHT BY Christian Hayes 2021 Published by the Graduate School ABSTRACT Seagrass structural complexity is a primary driver of nekton recruitment and faunal community structure. Few studies, however, have quantified the role of seagrass complexity on habitat use and trophic structures over large spatial scales. A large-scale simultaneous survey was conducted to assess relationships of multiple seagrass morphological complexity metrics to nekton habitat use, trophic dynamics, and blue crab (Callinectes sapidus) growth and mortality across the Northern Gulf of Mexico. Seagrass morphological and nekton community characteristics depended on site and season, and regional variation in seagrass morphology was an important driver of juvenile nekton abundance, species richness, beta diversity, assemblage structure, and functional diversity across the Northern GOM. Results from a stable isotope survey indicate that food web structures across turtle grass (Thalassia testudinum)-dominated ecosystems are similar, although there was a clear trend of more depleted carbon isotopes in primary producers, fish, shrimp, and crabs at sites in the Eastern GOM and more enriched isotopes at sites in the Western GOM, which may be associated with site-specific differences in environmental conditions, such as freshwater inflow and nutrient inputs. Blue crab growth and mortality experiments revealed that growth and mortality rates varied across the six sites, but overall mortality rate declined with increasing seagrass leaf area index and crab size. Blue crab growth rates, however, had no measurable relationship with seagrass complexity metrics. Results from this work indicate that habitat complexity metrics such as shoot density, canopy height, and leaf area index are important factors that define the nursery functions of seagrass habitats and ii should be incorporated into monitoring programs, conservation initiatives, and fishery models. This study also demonstrates the utility of conducting large-scale comparative studies to reveal regional differences and similarities in trophic structures. Finally, this study highlights the need for additional regional and species-specific studies of environmental drivers of nekton community production throughout the GOM, and our results suggest that models of nekton production in seagrass habitats should be created at regional, as well as local, scales to identify broad patterns but also to account for site- specific differences in nekton responses to environmental and habitat characteristics. iii ACKNOWLEDGMENTS I am deeply grateful to my advisor, Kelly Darnell, for her tireless support and guidance to me throughout my entire graduate school experience. I would also like to thank Zachary Darnell, for serving as the chair of my committee, providing valuable insight on my blue crab experiments, and for piloting the boat for the many trips to field sites. I thank Mark Peterson and Kevin Dillon for serving on my committee, for providing a wealth of insight on ecological processes, and for giving me helpful edits on the dissertation. Kevin Dillon also provided lab space and equipment for me to run stable isotope analyses for which I am deeply grateful. I would also like to thank Kelly Darnell, Zachary Darnell, Penny Hall, Lee Smee, Charles Martin, and Brad Furman for their guidance as primary investigators on the NOAA RESTORE project and their constant support throughout my dissertation research. Brad Furman also provided useful advice and sample code. I also thank the project expert advisory panel members, including Jeff Marx, Ryan Gandy, Matt Bethel, Zach Olsen, Penny Hall, Savanna Barry, Luiz Barbieri, Jason Adriance, Jim Tolan, Larry Handley, Jeff Rester, Ann Lazar, Peyton Cagle, Jennifer Leo, and Glen Sutton for providing helpful advice and guidance throughout the project. I am also very grateful to the National Oceanic and Atmospheric Administration’s RESTORE Science Program (grant # NA17NOS4510093), the University of Southern Mississippi, the Tom McIlwain Gulf Coast Research Laboratory Fisheries Endowment, the Drs. Julia and Thomas Lytle Coastal Sciences Fund, and the Nick Baron Memorial Marine Science Education Endowment for graciously providing funding for this research. iv I wish to thank all the graduate students, post-docs, technicians, research scientists, lab managers, and undergraduate students who gave sacrificially of their time to make this project a reality, including Samantha Smith, Benjamin Belgrad, Kelly Correia, Scott Alford, Ashley McDonald, Jessica Lunt, Adam Kemberling, Paige Harrison, Pavel Dimens, Emelia Marshall, Zachary Lane, Helen Olmi, Lennah Shakeri, Talene Yeghissian, Rachel Mugge, Nicholas Murphy, Matthew Byrnes, Leanne Poussard, Cailyn Ashcraft, Donna O'Dell, Robin Jung, Mike Wheeler, Katie Toth, Manuel Merello, Randi Cannon, Joey Reustle, Meredith Diskin, Jillian Lunt, Emily Combs, Sarah Flores, Tatayana Foster, Carissa Pinon, Polly Straughn, Koray Akozbek, Donna O’Dell, Scott Alford, Whitney Scheffel, Shea Husband, Cassandra Key, Anthony Messina, Audrey Looby, Sawyer Downey, Dara (Leslie) Glass, Emilie Gunn Berge, Susanna Robinson, Michael Hydrick, Tessa Moody, Dallas Albritton, Mason Williams, Christian Ballenger, Sabrina Atkinson, Allison Clark, Anna Reimer, Alyssa Frank, Vigneulle Puliatti, Deborah McNeal, Christina Moreau, Marcus Drymon, and Ashley Steele. I also thank Caitlyn Slife and Morgan Frank for their help in processing, analyzing, and interpreting my stable isotope samples; Joyce Shaw, Maryanne Anthony, and Martha Brown for their help in accessing scientific articles; Darcie Graham for help understanding graduate forms and processes; Jennifer Doerr for providing the mesocosms for the crab experiments; Richard Heard, Eric Haffey and Carley Zapfe for help in identifying animals; and Jonathan Snyder, Irene McIntosh, Ed Cake, and Elizabeth Dossett for their thoughtful edits on the dissertation. I thank my parents, Paul and Tammy Hayes, and my siblings, Jordan Hayes, Heather Vedder, and Brittany Hayes, for their constant support throughout my academic v pursuits, and I thank Samantha Smith for teaching me how to identify marine organisms, for counting tens of thousands of shrimp, and for being a constant source of help in every aspect of this project. Finally, I thank my Lord and Savior Jesus Christ who has graciously given me the opportunity to study His creation. vi DEDICATION I dedicate this work to my parents Paul and Tammy Hayes for their constant support throughout my education and to my high school biology teacher, Ralph McLin, for inspiring me to pursue the study of marine ecosystems. vii TABLE OF CONTENTS ABSTRACT ........................................................................................................................ ii ACKNOWLEDGMENTS ................................................................................................. iv DEDICATION .................................................................................................................. vii LIST OF TABLES ........................................................................................................... xiv LIST OF ILLUSTRATIONS ........................................................................................... xxi LIST OF ABBREVIATIONS ......................................................................................... xxx CHAPTER I – INTRODUCTION ...................................................................................... 1 1.1 Importance of habitat structure in seagrass environments ........................................ 1 1.2 Regional variation in seagrass structural complexity may drive regional differences in nekton use of seagrass beds ........................................................................................ 2 1.3 Seagrasses in the Northern GOM ............................................................................. 4 1.4 Objectives ................................................................................................................. 6 CHAPTER II – NEKTON HABITAT USE OF TURTLE GRASS

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