EMERITA TALPOIDA and DONAX VARIABILIS DISTRIBUTION THROUGHOUT CRESCENTIC FORMATIONS; PEA ISLAND NATIONAL WILDLIFE REFUGE a Thesi
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EMERITA TALPOIDA AND DONAX VARIABILIS DISTRIBUTION THROUGHOUT CRESCENTIC FORMATIONS; PEA ISLAND NATIONAL WILDLIFE REFUGE A thesis submitted in partial fulfillment of the requirements for the degree MASTER OF SCIENCE in ENVIRONMENTAL STUDIES by BLAIK PULLEY AUGUST 2008 at THE GRADUATE SCHOOL OF THE COLLEGE OF CHARLESTON Approved by: Dennis Stewart, Thesis Advisor Dr. Robert Dolan Dr. Scott Harris Dr. Lindeke Mills Dr. Amy T. McCandless, Dean of the Graduate School 1454471 1454471 2008 ABSTRACT EMERITA TALPOIDA AND DONAX VARIABILIS DISTRIBUTION THROUGHOUT CRESCENTIC FORMATIONS; PEA ISLAND NATIONAL WILDLIFE REFUGE A thesis submitted in partial fulfillment of the requirements for the degree MASTER OF SCIENCE in ENVIRONMENTAL STUDIES by BLAIK PULLEY JULY 2008 at THE GRADUATE SCHOOL OF THE COLLEGE OF CHARLESTON Pea Island National Wildlife Refuge is a 13-mile stretch of shoreline located on the Outer Banks of North Carolina, 40 miles north of Cape Hatteras and directly south of Oregon Inlet. This Federal Navigation Channel is periodically dredged and sand is placed on the north end of the Pea Island beach. While the sediment nourishes the beach in a particularly sand-starved environment, it also alters the physical and ecological conditions. Most affected are invertebrates living in the swash, the most dominant being the mole crab (Emerita talpoida) and the coquina clam (Donax variabilis). These two species serve as a major food source for shorebirds on the island. It is especially important to protect this food resource on the federal Wildlife Refuge, which operates under a mandate to protect resources for migratory birds. For this research, beach cusps of various sizes were sampled to determine whether there is a correlation between invertebrate populations and the physical characteristics associated with these crescentic features. In small cusps (<50m), where the expected physical parameters for cusps were seen, a relationship between invertebrate abundance and cusp location did exist. For larger cusps, the expected physical parameters were not met, and a significant macrofaunal relationship was not always found. The results support the theory that a beach nourishment design using “disposal nodes” that incorporate and mimic natural crescentic features would be an ecologically appropriate method of sediment disposal. Such a method would minimize initial mortality, leave maximum populations intact to repopulate affected areas, and would also facilitate a faster population recovery. ii ACKNOWLEDGEMENTS I would like to thank the members of my thesis committee: Dr. Robert Dolan, Professor Lindeke Mills, Dr. Scott Harris, and particularly my major advisor, Dennis Stewart. I am also indebted to many others including Pea Island Biologist Kris Fair for field work suggestions and a whole host of summer interns who, despite not having a choice in the matter, provided a tremendous amount of sampling help. To superstar interns Amanda Horning and Bradley Cramer I am especially grateful as they became quite adept at efficient cusp sampling. I would also like to thank the University of Virginia for providing laboratory equipment and undergraduates to assist with sediment analysis. Jeffery Freiberg was particularly helpful as manager and organizer of sediment samples. Thanks to the U.S. Fish & Wildlife Service who not only provided me the opportunity to do this research but also provided me with housing on an otherwise undeveloped beach. I would also like to thank the faculty, staff, and my fellow students in the MES Program with a special thanks to Mark McConnell who is always an invaluable resource. Finally, I thank my friends and family for their continued support, moral and otherwise, throughout this process. iii TABLE OF CONTENTS ABSTRACT………………………………………………………………………………ii ACKNOWLEDGEMENTS………………………………………………………………iii LIST OF FIGURES………………………………………………………………………vi LIST OF TABLES……………………………………………………………………...viii 1. INTRODUCTION……………………………………………………………………..1 1.1 Project Overview……………………………………………………………………..1 1.2 Objectives…………………………………………………………………………….3 2. BACKGROUND………………………………………………………………………5 2.1 Ecological Context……………………………………………………………………5 2.1.a Swash Zone Fauna………………………………………………………………5 2.1.b Emerita…………………………………………………………………………..7 2.1.c Donax…………………………………………………………………………..12 2.2 Geophysical Context………………………………………………………………...17 2.2.a Barrier Islands: The Outer Banks………………………………………………17 2.2.b Beach Cusps: Formation and Spacing…………………………………………19 2.2.c Beach Cusp Features…………………………………………………………...25 2.3 Relationship between Geophysical and Ecological Processes………………………26 2.3.a Faunal Relationship with Sediment Grain Size………………………………..26 2.3.b Faunal Relationship with Beach Cusps………………………………………...26 2.4 Beach Nourishment………………………………………………………………….28 2.4.a History on Pea Island…………………………………………………………..29 2.4.b Ecological Impacts……………………………………………………………..29 2.4.c Pea Island NWR Monitoring…………………………………………………...30 3. METHODS…………………………………………………………………………...32 3.1 Field Sampling………………………………………………………………………32 3.1.a Spatial and Temporal Considerations………………………………………….32 3.1.b Cusp Selection…………………………………………………………………33 3.1.c Transect Placement…………………………………………………………….34 3.1.d Sampling……………………………………………………………………….34 iv 3.2 Sand Analysis………………………………………………………………………..36 3.3 Statistical Analysis…………………………………………………………………..37 4. RESULTS…………………………………………………………………………….38 4.1 Sediment Grain Size Intercusp Relationship………………………………………..39 4.2 Sediment Grain Size Intracusp Relationship………………………………………..41 4.3 Sediment Grain Size across the Foreshore…………………………………………..45 4.4 Other Physical Parameters…………………………………………………………..47 4.4.a Slope……………………………………………………………………………47 4.4.b Uprush (Swash Length)………………………………………………………..50 4.5 Intercusp Macrofaunal Relationship…………………………………………………53 4.6 Intracusp Macrofaunal Relationship………………………………………………...54 4.6.a All Species……………………………………………………………………..54 4.6.b Emerita…………………………………………………………………………54 4.6.c Donax…………………………………………………………………………..58 4.7 Macrofaunal Abundance across the Foreshore……………………………………...62 4.8 Macrofaunal Abundance and Sediment Grain Size…………………………………67 5. DISCUSSION………………………………………………………………………...69 5.1 Cusp Geomorphology……………………………………………………………….69 5.2 Macrofaunal Abundance…………………………………………………………….72 5.3 Suggestions for Future Research……………………………………………………74 6. IMPLICATIONS…………………………………………………………………......75 7. REFERENCES……………………………………………………………………….77 v LIST OF FIGURES Figure 1. Pea Island National Wildlife Refuge boundary map…………………………...2 Figure 2. Satellite image of the Outer Banks…………………………………………....18 Figure 3. Series of small beach cusps…………………………………………………...20 Figure 4. Hierarchical structure of beach cusps…………………………………………23 Figure 5. Large crescentic formations on Pea Island……………………………………24 Figure 6. Beach cusp physical characteristics…………………………………………...25 Figure 7. Diagram of cusp transects…………………………………………………….35 Figure 8. Cusp size frequency…………………………………………………………...38 Figure 9. Mean grain size for each cusp size including sample (d)……………………..40 Figure 10. Mean grain size for each cusp size excluding sample (d)…………………...40 Figure 11. Mean grain size for small cusps……………………………………………..42 Figure 12. Mean grain size for medium cusps…………………………………………..43 Figure 13. Mean grain size for large cusps……………………………………………...45 Figure 14. Slope for small cusps………………………………………………………...47 Figure 15. Slope for medium cusps……………………………………………………..48 Figure 16. Slope for large cusps………………………………………………………...49 Figure 17. Slope for each size class……………………………………………………..50 Figure 18. Swash length for small cusps………………………………………………..51 Figure 19. Swash length for medium cusps……………………………………………..52 Figure 20. Swash length for large cusps………………………………………………...53 Figure 21. Average Emerita per transect for small cusps……………………………….55 Figure 22. Average Emerita per transect for medium cusps……………………………56 Figure 23. Average Emerita per transect for large cusps……………………………….57 Figure 24. Average Emerita per transect for all cusps………………………………….58 Figure 25. Average Donax per transect for small cusps………………………………...59 Figure 26. Average Donax per transect for medium cusps……………………………...60 Figure 27. Average Donax per transect for large cusps…………………………………61 Figure 28. Average Donax per transect for all cusps……………………………………62 Figure 29. Macrofaunal abundance across the foreshore for small cusps………………64 vi Figure 30. Macrofaunal abundance across the foreshore for medium cusps……………65 Figure 31. Macrofaunal abundance across the foreshore for large cusps……………….67 Figure 32. Mean grain size vs. Emerita correlation….………………………………….68 Figure 33. Mean grain size vs. Donax correlation………………………………………68 vii LIST OF TABLES Table 1. Mean grain size for small cusps………………………………………………..42 Table 2. Mean grain size for medium cusps…………………………………………….43 Table 3. Mean grain size for large cusps………………………………………………..44 Table 4. Mean grain size across the foreshore for small cusps………………………….45 Table 5. Mean grain size across the foreshore for medium cusps………………………46 Table 6. Mean grain size across the foreshore for large cusps………………………….46 Table 7. Slope for small cusps…………………………………………………………..47 Table 8. Slope for medium cusps……………………………………………………….48 Table 9. Slope for large cusps…………………………………………………………..49 Table 10. Swash length for small cusps…………………………………………………51 Table 11. Swash length for medium cusps……………………………………………...51 Table 12. Swash length for large cusps…………………………………………………52 Table 13. Average Emerita per transect for small cusps………………………………..55 Table 14. Average Emerita per transect for medium