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Assessing Habitat Suitability of Ribbed Mussels (Geukensia Demissa) in Georgia Salt Marshes by Examining Predicted Mussel Densities and Mussel Population Parameters

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Assessing Habitat Suitability of Ribbed Mussels (Geukensia Demissa) in Georgia Salt Marshes by Examining Predicted Mussel Densities and Mussel Population Parameters Georgia Southern University Digital Commons@Georgia Southern Electronic Theses and Dissertations Graduate Studies, Jack N. Averitt College of Spring 2021 Assessing Habitat Suitability of Ribbed Mussels (Geukensia Demissa) in Georgia Salt Marshes by Examining Predicted Mussel Densities and Mussel Population Parameters William K. Annis Jr Follow this and additional works at: https://digitalcommons.georgiasouthern.edu/etd Part of the Aquaculture and Fisheries Commons, Biodiversity Commons, Marine Biology Commons, Population Biology Commons, Terrestrial and Aquatic Ecology Commons, and the Zoology Commons Recommended Citation Annis, William K. Jr, "Assessing Habitat Suitability of Ribbed Mussels (Geukensia Demissa) in Georgia Salt Marshes by Examining Predicted Mussel Densities and Mussel Population Parameters" (2021). Electronic Theses and Dissertations. 2259. https://digitalcommons.georgiasouthern.edu/etd/2259 This thesis (open access) is brought to you for free and open access by the Graduate Studies, Jack N. Averitt College of at Digital Commons@Georgia Southern. It has been accepted for inclusion in Electronic Theses and Dissertations by an authorized administrator of Digital Commons@Georgia Southern. For more information, please contact [email protected]. ASSESSING HABITAT SUITABILITY OF RIBBED MUSSELS (GEUKENSIA DEMISSA) IN GEORGIA SALT MARSHES BY EXAMINING PREDICTED MUSSEL DENSITIES AND MUSSEL POPULATION PARAMETERS by WILLIAM ANNIS (Under the Direction of John Carroll) ABSTRACT The salt marshes of coastal Georgia are ecologically and economically important ecosystems that are threatened by erosion as a result of sea level rise. Naturally these marshes are stabilized by the mutualistic relationship between Spartina alterniflora (cordgrass) and Geukensia demissa (ribbed mussel). Together these two organisms hold sediment in place through the intertwined connections of mussel byssal threads and cordgrass’ roots. In addition, the nitrogenous waste of mussel has been found to increase the growth of cordgrass. Due to these interactions, mussels have the potential to contribute to the success of salt marsh restoration. However, little is known about the drivers of the spatial distribution of mussels in Georgia. Understanding the factors that contribute to the distribution of mussels in Georgia can help inform managers on choosing locations to optimize the survivorship of mussels in restoration projects. This study sought to model mussel densities across the coast of Georgia and to compare predicted mussel densities in local marshes with mussel population parameters as means to gauge habitat suitability. Mussel densities were collected through a series of field surveys across a range of salt marshes along the coast of Georgia. These data were then overlaid with spatial data such as distance to creek heads (the ends of intertidal creeks which flood the marsh platform), elevation, and slope. Highest predicted mussel densities occurred at an elevation of 0.7m relative to NAVD 88, close to creek heads and far from subtidal creeks. Using the predicted mussel densities from the model, low, medium and high density mussel sites were selected at two geographic locations, Cannon’s Point Preserve, St. Simons, Georgia and Dean Creak, Sapelo Island, Georgia, to conduct mussel growth, predation, and recruitment experiments. In areas with higher predicted mussel densities, mussel recruitment and growth rates were the highest. Despite being statistically significant, differences in growth rates may not be biologically meaningful. While not statistically significant, predation risk was lowest in areas of high predicted mussel densities and increased with decreasing density. This indicates that in areas of low predicted mussel densities, recruitment and predation risk are likely the limiting factor to mussel densities. INDEX WORDS: Ribbed mussels, Georgia, Species distribution model, Growth rates, Recruitment, Predation ASSESSING HABITAT SUITABILITY OF RIBBED MUSSELS (GEUKENSIA DEMISSA) IN GEORGIA SALT MARSHES BY EXAMINING PREDICTED MUSSEL DENSITIES AND MUSSEL POPULATION PARAMETERS. by WILLIAM ANNIS B.S., University of Florida, 2017 A Thesis Submitted to the Graduate Faculty of Georgia Southern University in Partial Fulfillment of the Requirements for the Degree MASTER OF SCIENCE © 2021 WILLIAM ANNIS All Rights Reserved 1 ASSESSING HABITAT SUITABILITY OF RIBBED MUSSELS (GEUKENSIA DEMISSA) IN GEORGIA SALT MARSHES BY EXAMINING PREDICTED MUSSEL DENSITIES AND MUSSEL POPULATION PARAMETERS. by WILLIAM ANNIS Major Professor: John M. Carroll Committee: Risa A. Cohen Elizabeth A. Hunter Electronic Version Approved: May 2021 2 ACKNOWLEDGMENTS I would like to thank my committee Dr. Carroll, Dr. Hunter, and Dr. Cohen for assistance in experimental design, writing, and data analysis. For assistance in field work and sample processing, I’d like to thank the members of the CRABEy lab: Raven Hurt, Sarah Batchelor, and Jada Daniels. I would also like to thank Rachel Guy at Sapelo Island National Estuarine Research Preserve and Stephanie Knox at Cannon’s Point Preserve for field site access and logistical support. Funding for these studies were provided by Georgia Sea Grant and Marine Extension, Georgia Department of Natural Resources, and Georgia Southern University’s Graduate Student Organization. 3 TABLE OF CONTENTS Page ACKNOWLEDGMENTS...............................................................................................................2 LIST OF TABLES...........................................................................................................................5 LIST OF FIGURES.........................................................................................................................7 CHAPTER 1 ELEVATION AND MARSH FEATURES STRUCTURE RIBBED MUSSEL (GEUKENSIA DEMISSA) DISTRIBUTION IN GEORGIA’S SALT MARSHES…......10 Introduction............................................................................................................10 Methods..................................................................................................................14 Study Sites.................................................................................................14 Estimating Mussel Densities......................................................................14 Spatial Data ...............................................................................................15 Scale Selection….......................................................................................15 Statistical Analysis.....................................................................................15 Results....................................................................................................................17 Discussion .............................................................................................................18 2 POPULATION PARAMETER OF RIBBED MUSSELS (GEUKENSIA DEMISSA) VARY ACROSS A GRADIENT OF PREDICTED MUSSEL DENSITIES……….......32 Introduction............................................................................................................32 Methods………………………………………………………………..…………36 Study Sites ……………………………....................................................36 4 Mussel Distribution and Experimental Plot Selection………...................37 Mussel Collection and Preparation………………………………………37 Growth Rate Experiments…......................................................................38 Predatory Mortality....................................................................................38 Recruitment………....................................................................................39 Site Characteristics.....................................................................................40 Statistical Analysis…………………………………………………...…..42 Results....................................................................................................................43 Discussion .............................................................................................................44 REFERENCES..............................................................................................................................69 APPENDICES...............................................................................................................................76 1 COMPETITIVE MODELS (∆AIC<2) FOR MUSSEL CONDITION (A) AND SCARRING (B)…….............................................................................................….76 2 ESTIMATES OF MUSSEL CONDITION INDEX (A) AND SHELL SCARRING (B) MODEL BETA PARAMETERS WITH 95% CONFIDENCE INTERVAL AFTER MODEL AVERAGING.………………………………………………...…77 5 LIST OF TABLES Page Table 1.1: Prior distribution and posterior means with 95% credible intervals for zero inflated negative binomial model of ribbed mussel distributions across Georgia using density data collected in 2019-2020………………………………….……………24 Table 2.1: Mixed effect model results with R2 values for fixed effects with 95% confidence intervals for ribbed mussel growth rates (mm/day) and mussel condition index (unitless).. ................................................................................................51 Table 2.2: Mixed effect model results with R2 values for fixed effects with 95% confidence intervals for probability of predation and blue crab abundance (crabs per trap). ............................................................................................................................52
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