Callinectes Sapidus) Response to Altered Nursery Habitat

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Dissertations, Theses, and Masters Projects Theses, Dissertations, & Master Projects

Spring 2017

Juvenile Blue Crab (Callinectes Sapidus) Response to Altered Nursery Habitat

Megan Wood College of William and Mary - Virginia Institute of Marine Science, [email protected]

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Recommended Citation Wood, Megan, "Juvenile Blue Crab (Callinectes Sapidus) Response to Altered Nursery Habitat" (2017). Dissertations, Theses, and Masters Projects. Paper 1499449868. http://dx.doi.org/10.21220/M2GD1Q

This Dissertation is brought to you for free and open access by the Theses, Dissertations, & Master Projects at W&M ScholarWorks. It has been accepted for inclusion in Dissertations, Theses, and Masters Projects by an authorized administrator of W&M ScholarWorks. For more information, please contact [email protected]. Juvenile blue crab (Callinectes sapidus) response to altered nursery habitat

______

A Dissertation

Presented to

The Faculty of the School of Marine Science

The College of William and Mary in Virginia

In Partial Fulfillment

of the Requirements for the Degree of

Doctor of Philosophy

______

Megan A. Wood

May 2017 APPROVAL SHEET

This dissertation is submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy

______Megan A. Wood

Approved, by the Committee, April, 2017

______Romuald N. Lipcius, Ph.D. Committee Chairman/ Advisor

______Rochelle D. Seitz, Ph.D.

______Kenneth A. Moore, Ph.D.

______Carl T. Friedrichs, Ph.D.

______Sarah L. Day, Ph.D. College of William and Mary Williamsburg, VA

______Karen J. McGlathery, Ph.D. University of Virginia Charlottesville, VA

ii DEDICATION

For my mom, who made sure her kids knew how to swim because she couldn’t. I miss you every day.

iii

TABLE OF CONTENTS ACKNOWLEDGEMENTS ...... vi LIST OF TABLES ...... vii LIST OF FIGURES ...... ix LIST OF APPENDICES ...... xi DISSERTATION ABSTRACT...... xii CHAPTER 1 Dissertation Introduction...... 2 LITERATURE CITED ...... 5 CHAPTER 2 Exotic red alga Gracilaria vermiculophylla substitutes for seagrass as blue crab nursery habitat in the emerging Chesapeake Bay ecosystem ...... 7 ABSTRACT ...... 8 INTRODUCTION ...... 10 MATERIALS AND METHODS ...... 14 Study area...... 14 Survey ...... 15 Analysis ...... 16 RESULTS ...... 17 Spatial distribution ...... 17 Seasonal and annual distribution patterns ...... 17 DISCUSSION ...... 19 Gracilaria distribution and biomass patterns ...... 19 Interactions with native seagrasses ...... 21 Implications for juvenile blue crabs ...... 22 LITERATURE CITED ...... 24 TABLES ...... 30 FIGURES ...... 33 CHAPTER 3 Utilization of native seagrass and exotic algae by juvenile blue crabs and macrofaunal prey communities ...... 39 ABSTRACT ...... 40 INTRODUCTION ...... 41 MATERIALS AND METHODS ...... 44 Epifaunal communities ...... 45 Infaunal communities ...... 45 Analysis ...... 46 RESULTS ...... 47 Juvenile blue crab density and size ...... 47 Epifaunal communities ...... 48 Infaunal communities ...... 48 DISCUSSION ...... 50 LITERATURE CITED ...... 52 TABLES ...... 55 FIGURES ...... 61

iv CHAPTER 4 In situ growth of juvenile blue crabs in native seagrass and the exotic alga Gracilaria vermiculophylla in lower Chesapeake Bay ...... 65 ABSTRACT ...... 66 INTRODUCTION ...... 67 MATERIALS AND METHODS ...... 70 RESULTS ...... 72 Physical variables ...... 72 Growth...... 73 DISCUSSION ...... 73 LITERATURE CITED ...... 75 TABLES ...... 81 FIGURES ...... 85 APPENDIX I ...... 88 CHAPTER 5 Simulated effects of benthic habitat on juvenile blue crab recruitment in the York River ...... 91 ABSTRACT ...... 92 INTRODUCTION ...... 93 MATERIALS AND METHODS ...... 97 Blue crab nursery habitat model ...... 97 Numerical simulations ...... 98 Sensitivity analysis ...... 99 RESULTS ...... 100 Three-habitat model ...... 100 Two-habitat model ...... 101 Sensitivity analysis ...... 102 DISCUSSION ...... 103 Model simplification ...... 103 Habitat changes and implications for juvenile blue crabs ...... 104 LITERATURE CITED ...... 106 TABLES ...... 112 FIGURES ...... 119 CHAPTER 6 Summary and Conclusions ...... 123 LITERATURE CITED ...... 126 VITA...... 127

v ACKNOWLEDGEMENTS

So many people have made this dissertation possible through their help and encouragement. I would first like to thank my committee: my advisor, Dr. Romuald Lipcius, who was always there to help me see the forest through the trees; Dr. Rochelle Seitz for always having an encouraging word ready during the tough times; Dr. Karen McGlathery for teaching me the basics of aquatic ecology in undergrad; Dr. Carl Friedrichs for being excited about every result but especially excited if it involved sediment; Dr. Kenneth Moore for his pep talks; and Dr. Sarah Day who taught me everything I ever wanted to know (and more) about nonlinear dynamics. I truly appreciate all of your helpful comments and support over the years. The members of the Marine Conservation Biology and Community Ecology laboratories are like a family, and this dissertation would not have gotten done without the mentorship, help, and laughs provided by my labbies. Thank you to Katie Knick, Alison Smith, Mike Seebo, Danielle McCullough, Mandy Bromilow, Allison Colden, Theresa Davenport, Melissa Karp, Bruce Pfirrmann, Gabrielle Saluta, Gina Ralph, Cassie Bradley, Diane Tulipani, Dave Schulte, and Russ Burke. I also want to thank the many interns and volunteers who have helped me out over the years. I have made so many friends over the years at VIMS, and I appreciate all of you. There are a few people who have been especially supportive. First, I want to thank my fellow “masterminds” Britt Dean and Julia Moriarty for helping to keep me on track with my productivity goals. Alison Smith and Mandy Bromilow, thank you for being the sounding boards for all of my frustrations. I want to thank Katie May Laumann for helping me navigate the sometimes treacherous waters of finding a job. Cassie Glaspie deserves a special shout-out for being my biggest cheerleader and supporter ever since we met on our first day at VIMS. Thank you for lending an ear during all of my freak- outs and panic attacks and celebrating with me after every milestone, big or small. I appreciate your friendship more than you may ever know. Thank you to my family for believing in me, especially my dad, Terry Wood, who always supported my dreams and never doubted that I could reach them. And finally, thank you to my best friend and partner in life Sam Bowling. I am and will always be so grateful for your unwavering confidence in me even when I am filled with self-doubt.

vi LIST OF TABLES

CHAPTER 2

Table 1. Summary of environmental data collected at sampled sites, including means of each variable during each sampling year; 95% confidence intervals (CI) are in parentheses...... 30

Table 2. Information theoretic analysis (Anderson 2008) of twelve logistic models (gi) using salinity (S), base habitat (BH), region (R), temperature (T), dissolved oxygen (DO), month (M), and year (Y) as predictors of Gracilaria vermiculophylla presence, where k is the number of parameters in a model, AIC is the Akaike information criterion, AICc is the corrected AIC, ∆i is the difference between any model and the best model in the set, and wi is the model probability...... 31

Table 3. Summary of ANOVA for model g5, including parameter estimates, standard errors, and p-values...... 32

CHAPTER 3

Table 4. Juvenile crab (< 30 mm CW) densities (ind. m-2) in seagrass and Gracilaria vermiculophylla in June and August with standard error and 95% confidence interval...... 55 Table 5. Mean juvenile crab size (mm carapace width) in seagrass and Gracilaria vermiculophylla in June and August with standard error and 95% confidence interval...... 56 Table 6. Mean species richness (no. species) and abundance (no. ind. m-2) in epifaunal and infaunal habitats with standard error and 95% confidence interval...... 57 Table 7. Contributions of abundance-dominant species in epifauna (no. ind. m-2, % contribution, and cumulative % contribution) to dissimilarities between Gracilaria vermiculophylla and seagrass...... 58 Table 8. Contributions of abundance-dominant species in infauna (no. ind. m-2, % contribution, and cumulative % contribution) to dissimilarities between (A) Gracilaria vermiculophylla and seagrass, (B) unvegetated habitat and seagrass, and (C) G. vermiculophylla and unvegetated habitat...... 59 Table 9. Percent sediment grain size composition by component in each habitat with 95% confidence intervals in parentheses...... 60

CHAPTER 4

Table 10. Recovered tagged crabs relative to crabs placed initially...... 81

vii Table 11. Percent sediment grain size composition by component a) inside and outside of cages and b) downriver and midriver. 95% confidence intervals are in parentheses...... 82 Table 12. Pair-wise comparisons of mean weekly growth rate using Tukey’s HSD test (confidence level = 0.95). Values are p-values...... 83 -1 Table 13. Mean proportional growth rate (mm CW week /mm CWI) in each habitat with standard errors and 95% confidence intervals...... 84

CHAPTER 5

Table 14. Table of parameters used in the simulations with Equations (1), (2), and (4). 112 Table 15. Model scenarios and parameter values used with the three-habitat model (Equations (1) and (2))...... 113 Table 16. Table of model scenarios and parameter values used with the two-habitat model (Equation (4))...... 114 Table 17. Simulation results for the three-habitat model, where values are the number of individuals (millions) remaining at time t = 31 with percent change from simulation 1 in parentheses...... 115 Table 18. Simulation results for the two-habitat model, where values are the number of individuals (millions) remaining at time t = 31 with percent change from simulation 1 in parentheses...... 116 Table 19. Table of model sensitivity analysis for the three-habitat model (Equation (4)), where values are the number of individuals (millions) remaining at time t = 31 with percent change from the standard run in parentheses...... 117 Table 20. Table of model sensitivity analysis for the two-habitat model (Equation (4)), where values are the number of individuals (millions) remaining at time t = 31 with percent change from the standard run in parentheses ...... 118

viii LIST OF FIGURES

CHAPTER 2

Figure 1. Map of the York River, a tributary of Chesapeake Bay (d), locations of downriver (i), midriver (ii), and upriver (iii) sampling regions along the river axis, and, for each year of this study: (a) locations of all sites where Gracilaria vermiculophylla was present (filled) or absent (open); (b) G. vermiculophylla biomass (g dry weight m-2) at locations where it was present; and (c) seagrass beds (Orth et al. 2014, Orth et al. 2015)...... 33 Figure 2. Relationship between Gracilaria vermiculophylla volume and biomass with linear regression...... 34 Figure 3. Mean percent presence of Gracilaria vermiculophylla during 2013 and 2014 over: (a) the entire York River; (b) the downriver region, where base habitats are seagrass (filled) and unvegetated (open); and (c) midriver region. Gracilaria vermiculophylla was not observed upriver. Vertical bars represent 1 standard error of the mean. Note differing scales on y-axes...... 35 Figure 4. Mean Gracilaria vermiculophylla biomass (g dry weight [DW] m-2) during 2013 and 2014 over: (a) the entire York River; (b) the downriver region, where base habitats are seagrass (filled) and unvegetated (open); and (c) midriver region. Gracilaria vermiculophylla was not observed upriver. Vertical bars represent 1 standard error of the mean. Note differing scales on y-axes...... 36 Figure 5. Mean Gracilaria vermiculophylla percent cover during 2013 and 2014 over: (a) the entire York River; (b) the downriver region, where base habitats are seagrass (filled) and unvegetated (open); and (c) midriver region. Gracilaria vermiculophylla was not observed upriver. Vertical bars represent 1 standard error of the mean. Note differing scales on y-axes...... 37 Figure 6. Locations within the York River of (a) seagrass cover in 1971 (Orth and Gordon 1975), (b) seagrass cover in 2014 (Orth et al. 2015), and (c) areas where Gracilaria vermiculophylla was present in 2013 and 2014...... 38

CHAPTER 3

Figure 7. Map of sampling locations in June and August in each sampling region: a) downriver, b) midriver, and c) upriver...... 61 Figure 8. Juvenile blue crab size frequencies in (A) seagrass and (B) Gracilaria vermiculophylla in August...... 62 Figure 9. Multidimensional scaling plots for (A) epifaunal communities in Gracilaria vermiculophylla and seagrass and (B) infaunal communities in G. vermiculophylla, seagrass, and unvegetated habitat. Stress estimates are from ANOSIM tests for differences between habitat types...... 63

ix Figure 10. Limecola balthica density vs. (A) percent sand and (B) percent silt in surface sediments with Loess curves (solid black) and 95% confidence intervals (dashed). 64

CHAPTER 4

Figure 11. Locations of (i) the York River, a tributary of Chesapeake Bay (inset); (ii) study areas midriver (a) and downriver (b, c); and (iii) all caging locations in 2012 (black) and 2014 (gray)...... 85 Figure 12. Final carapace width versus initial carapace width in 2012 (filled) and 2014 (shaded) with linear regressions from an additive model with year as a covariate (2012: y = 10.616 + 0.837x; 2014: y = 13.526 + 0.837x; R2 = 0.85)...... 86 Figure 13. Proportional weekly growth rate relative to initial carapace width for all crabs with linear regression (y = 0.224 - 0.004x; R2 = 0.48)...... 87

CHAPTER 5

Figure 14. Schematic of the blue crab habitat model, where N is the initial abundance of juveniles placed in a particular habitat at time t = 0...... 119 Figure 15. Values of the dispersal function...... 120 Figure 16. Results of simulations of the three-habitat model with the number of juvenile blue crabs (in millions) remaining in seagrass (solid), Gracilaria (dashed), and unvegetated (dotted) habitat after t = 30 days...... 121 Figure 17. Results of simulations of the two-habitat model with the number of juvenile blue crabs (in millions) remaining in vegetated (solid) and unvegetated (dotted) habitat after t = 30 days...... 122

x LIST OF APPENDICES

APPENDIX I

Supplementary Figure 1. (A) Mean number conspecifics (untagged crabs that recruited after cage deployment) found cages in each habitat type with standard error (SE) bars. (B) Mean number of conspecifics found in cages in seagrass (DS), downriver Gracilaria vermiculophylla (DG), downriver unvegetated (DU), midriver G. vermiculophylla (MG), and midriver unvegetated (MU) habitats with SE bars. Letters denote significant differences based on Tukey’s HSD test (confidence level = 0.95)...... 89

Supplementary Table 1. Mean size (mm CW) of untagged conspecifics found in cages with tagged crabs...... 90

xi DISSERTATION ABSTRACT

Habitats of Chesapeake Bay have been altered due to anthropogenic impacts and climate change. Due to these human disturbances, seagrasses have been extirpated from many areas in lower Chesapeake Bay and persisting beds face future losses as water temperatures continue to rise. Further loss of seagrass habitat will negatively impact juvenile blue crabs (Callinectes sapidus) that use seagrass beds as nursery grounds.

Habitat degradation allows for more successful introductions of exotic species, and the communities formed from the mixing of native and exotic species are known as emerging ecosystems. Gracilaria vermiculophylla, an exotic macroalga, may be an emerging nursery habitat for juvenile blue crabs in Chesapeake Bay; however the extent to which the alga is present and used as a nursery by juvenile blue crabs are largely unknown. I investigated algal distribution in the shallow littoral areas of the York River, a subestuary of Chesapeake Bay, over two years (2013 – 2014) and found that G. vermiculophylla presence correlated with salinity and that algal presence and biomass increased with seagrass presence, although biomass was generally low. The alga was present in areas where seagrasses have been lost, and is therefore likely providing nursery habitat in these areas of high megalopal recruitment. Benthic epifaunal communities had lower species richness and were less abundant in G. vermiculophylla relative to seagrass, while benthic infaunal communities had lower species richness but similar abundance in the alga relative to seagrass. Juvenile blue crab densities were similar in the alga and seagrass, although seagrass supported about 3 times as many first and second instar crabs than G. vermiculophylla. Young juvenile blue crabs preferred seagrass, which may be due to epifaunal prey preference, and G. vermiculophylla likely represents a secondary nursery

xii habitat. Juvenile blue crab growth rates of crabs 15 – 50 mm carapace width were similar in the alga, native seagrass, and unvegetated habitat, indicating that growth does not drive ontogenetic shifts in habitat use by larger (20 – 30 mm carapace width) juveniles. Similar growth rates also suggest that G. vermiculophylla performs similarly to seagrass as a nursery habitat in terms of providing resources for growth. Simulations of density- dependent migration of young juvenile blue crabs between habitat types suggest that G. vermiculophylla may mediate continued seagrass loss, at least in part. Together, these results increase our understanding of an emerging Chesapeake Bay ecosystem and the impacts that changes to nursery habitats have on the juvenile component of the blue crab population.

xiii

Juvenile blue crab (Callinectes sapidus) response to altered nursery habitat

CHAPTER 1

Dissertation Introduction

2 Anthropogenic factors and changing climate are altering ecosystems worldwide.

Over the past half-century, Chesapeake Bay has undergone drastic changes to its benthic habitats, including declines of native seagrass since the late 1960s and 1970s (Orth and

Moore 1983) largely due to increased anthropogenic nutrient and sediment inputs (Pugh

2005) as well as natural disturbances (Orth et al. 2010). Climate change has also driven ecosystem change in the Bay. As the average water temperature continues to rise in shallow areas of the Bay, this will have direct and indirect effects on the species present, especially seagrasses (Höffle et al. 2011). Changing environmental conditions and disturbances may also present ideal circumstances for the introduction of an exotic species, and mixed communities of native and exotic species, known as novel or emerging ecosystems, are increasingly replacing co-evolved native species assemblages

(Hobbs et al. 2006). Novel or emerging ecosystems are systems formed due to anthropogenic impacts with unique species compositions, and possibly altered ecosystem functions, that have not previously occurred (Milton 2003, Hobbs et al. 2006). Estuaries, like Chesapeake Bay, are of particular interest to those studying emerging ecosystems because anthropogenic activities, such as shipping, recreational boating, and fishing industry, that facilitate introduction or invasion of exotic species commonly occur in these areas (Williams and Smith 2007).

Submerged aquatic vegetation (SAV), such as seagrasses, provides important habitats within Chesapeake Bay that offer protection, nursery habitat, and other functions for fishery species like the blue crab Callinectes sapidus (Duffy and Baltz 1998).

Fragmentation of seagrass beds and decreased shoot density within seagrass beds in

Chesapeake Bay may have negative implications for juvenile blue crabs that use seagrass

3 as a nursery habitat, especially for recruitment of blue crab larvae (Stockhausen and

Lipcius 2003).

Macroalgae can be abundant locally but are generally not widespread in

Chesapeake Bay. These nonvascular plants may outcompete and replace seagrasses in eutrophied areas (Valiela et al. 1997) and may provide substitute habitat for organisms like the blue crab. Gracilaria vermiculophylla is an exotic, coarsely branching, red macroalgae originating from the Western Pacific (Ohmi 1956) that has colonized shallow coastal areas of North America and Europe (Bellorin et al. 2004, Thomsen et al. 2005,

2006, Freshwater et al. 2006, Thomsen et al. 2007). It is possible that this exotic alga may fill some of the ecological roles of seagrasses in areas where beds have retreated

(Rodriguez 2006). The combination of seagrass decline and spread of G. vermiculophylla in Chesapeake Bay indicates that G. vermiculophylla may represent an emerging ecosystem within the Bay. Gracilaria vermiculophylla may ameliorate the loss of seagrass as a nursery habitat for juvenile blue crabs and other organisms in Chesapeake

Bay. However, the extent to which this alga is present in shallow water areas of tributaries is unknown. Understanding temporal changes in the distribution of G. vermiculophylla, as well as its impacts on community structure, is key to determining whether this species represents an emerging ecosystem.

This dissertation used multiple lines of evidence, including field and modeling studies, to address basic and applied questions regarding the suitability of G. vermiculophylla as a nursery habitat for juvenile blue crabs using the York River as a representative system for Chesapeake Bay. In Chapter 2, I quantified the distribution and abundance of G. vermiculophylla to determine the extent to which it might provide an

4 alternative habitat for juvenile blue crabs in this region. In Chapter 3, I determined the alga’s ability to act as habitat for small juvenile blue crabs and their prey communities relative to native seagrass. In Chapter 4, I conducted an in situ growth experiment, which built on previous work on juvenile crab survival, to determine if tradeoffs between growth and survival drive ontogenetic habitat shifts in juvenile blue crabs. In Chapter 5, I simulated the impacts of altered habitats such as G. vermiculophylla on juvenile blue crab recruitment using a density-dependent model of juvenile blue crab migration. In Chapter

6, I summarized the results of this dissertation and made suggestions for future research topics.

LITERATURE CITED

Bellorin, A. M., M. C. Oliveira, and E. C. Oliveira. 2004. Gracilaria vermiculophylla: A western Pacific species of Gracilariaceae (Rhodophyta) first recorded from the eastern Pacific. Phycological Research 52: 69–79. Duffy, K. C., and D. M. Baltz. 1998. Comparison of fish assemblages associated with native and exotic submerged macrophytes in the Lake Pontchartrain estuary, USA. Journal of Experimental Marine Biology and Ecology 223: 199–221. Freshwater, D. W., F. Montgomery, J. K. Greene, R. M. Hamner, M. Williams, and P. E. Whitfield. 2006. Distribution and identification of an invasive Gracilaria species that is hampering commercial fishing operations in southeastern North Carolina, USA. Biological Invasions 8: 631–637. Hobbs, R. J., S. Arico, J. Aronson, J. S. Baron, P. Bridgewater, V. A. Cramer, P. R. Epstein, J. J. Ewel, C. A. Klink, A. E. Lugo, D. Norton, D. Ojima, D. M. Richardson, E. W. Sanderson, F. Valladares, M. Vilà, R. Zamora, and M. Zobel. 2006. Novel ecosystems: theoretical and management aspects of the new ecological world order. Global Ecology and Biogeography 15: 1–7. Höffle, H., M. S. Thomsen, and M. Holmer. 2011. High mortality of Zostera marina under high temperature regimes but minor effects of the invasive macroalgae Gracilaria vermiculophylla. Estuarine, Coastal and Shelf Science 92: 35–46. Milton, S. J. 2003. “Emerging ecosystems”: a washing-stone for ecologists, economists and sociologists? South African Journal of Science 99: 404–406. Ohmi, H. 1956. Contributions to the knowledge of Gracilariaceae from Japan: Ⅱ. On a new species of the genus Gracilariopsis, with some considerations on its ecology.

5 Bulletin of the Faculty of Fisheries Hokkaido University 6: 271–279. Orth, R. J., and K. A. Moore. 1983. Chesapeake Bay: An unprecedented decline in submerged aquatic vegetation. Science 222: 51–53. Orth, R. J., S. R. Marion, K. A. Moore, and D. J. Wilcox. 2010. Eelgrass (Zostera marina L.) in the Chesapeake Bay region of mid-Atlantic coast of the USA: challenges in conservation and restoration. Estuaries and Coasts 33: 139–150. Pugh, J. G. 2005. Blue crab habitat and management in Chesapeake Bay. Doctoral dissertation, Virginia Polytechnic and State University. natrespro.nvgc.vt.edu. Rodriguez, L. F. 2006. Can invasive species facilitate native species? Evidence of how, when, and why these impacts occur. Biological Invasions 8: 927–939. Stockhausen, W. T., and R. N. Lipcius. 2003. Simulated effects of seagrass loss and restoration on settlement and recruitment of blue crab postlarvae and juveniles in the York River, Chesapeake Bay. Bulletin of Marine Science 72: 409–422. Thomsen, M. S., C. F. D. Gurgel, S. Fredericq, and K. J. McGlathery. 2005. Gracilaria vermiculophylla (Rhodophyta, Gracilariales) in Hog Island Bay, Virginia: a cryptic alien and invasive macroalga and taxonomic correction. Journal of Phycology 42: 139–141. Thomsen, M. S., K. J. McGlathery, and A. C. Tyler. 2006. Macroalgal distribution patterns in a shallow, soft-bottom lagoon, with emphasis on the nonnative Gracilaria vermiculophylla and Codium fragile. Estuaries and Coasts 29: 465–473. Thomsen, M. S., P. A. Stæhr, C. D. Nyberg, S. Schwærter, D. Krause-Jensen, and B. R. Silliman. 2007. Gracilaria vermiculophylla (Ohmi) Papenfuss, 1967 (Rhodophyta, Gracilariaceae) in northern Europe, with emphasis on Danish conditions, and what to expect in the future. Aquatic Invasions 2: 83–94. Valiela, I., J. McClelland, J. Hauxwell, P. J. Behr, D. Hersh, and K. Foreman. 1997. Macroalgal blooms in shallow estuaries: controls and ecophysiological and ecosystem consequences. Limnology and Oceanography 42: 1105–1118. Williams, S. L., and J. E. Smith. 2007. A global review of the distribution, taxonomy, and impacts of introduced seaweeds. Annual Review of Ecology, Evolution, and Systematics 38: 327–359.

CHAPTER 2

Exotic red alga Gracilaria vermiculophylla substitutes for seagrass as blue crab nursery habitat in the emerging Chesapeake Bay ecosystem

7 ABSTRACT

Exotic species have colonized estuarine and marine ecosystems worldwide. They can become deleterious and invasive or potentially beneficial as components of novel ecosystems. The exotic red macroalga Gracilaria vermiculophylla may be beneficial in providing nursery habitat where eelgrass is in decline in Chesapeake Bay. A shallow- water survey of the York River, a western tributary of Chesapeake Bay, was conducted monthly from May through October 2013 and 2014 to determine the extent to which G. vermiculophylla presence may potentially provide nursery habitat lost by seagrass decline. To do this, G. vermiculophylla presence, percent cover, and biomass data were collected in three regions (upriver, midriver, and downriver) using stratified random sampling. At each site, G. vermiculophylla presence, percent cover, and biomass were assessed using 20-m transects and 0.0625-m2 quadrats. Gracilaria vermiculophylla found in quadrats was removed, and its volume was measured and converted to ash-free dry weight. The effects of region, salinity, month, year, seagrass presence, and environmental variables were analyzed using logistic regression and the Akaike Information Criterion

(AIC), which identified the additive effect of region and seagrass presence as the model best predicting G. vermiculophylla presence. In general, G. vermiculophylla presence, percent cover, and biomass were highest downriver, and the alga occurred in midriver areas of the York River from which seagrass has been extirpated. The alga currently has no widespread negative impacts on seagrass in the York River, likely because percent cover and biomass are relatively low, although continued increases in water temperature and nutrient loading may both further harm seagrasses and benefit the alga. Where

8 seagrasses have been lost midriver, G. vermiculophylla is providing additional subtidal nursery habitat for juvenile blue crabs (Callinectes sapidus) and other structure-reliant species.

9 INTRODUCTION

Coastal and estuarine systems are often the most degraded systems worldwide due to increased human activity along coastlines (Hassan et al. 2005, Lotze et al. 2006), which renders these systems susceptible to colonization by non-native (= exotic) species

(Carlton and Geller 1993). Exotic species are often harmful to the ecosystems they colonize, both ecologically and economically (Schaffelke and Hewitt 2007, Davis et al.

2011, Schlaepfer et al. 2012). However, exotic species may benefit degraded systems by restoring lost functions (Schlaepfer et al. 2011). For instance, the green crab Carcinus maenas has facilitated salt marsh recovery in some areas where it is not native by reducing consumption of cordgrass by native species (Bertness and Coverdale 2013).

Introduced plants can increase structural heterogeneity and provide novel habitat that can profit native species (Crooks 2002). For example, the green alga Codium fragile ssp. tomentosoides increased recruitment of native mussels in the Adriatic Sea (Bulleri et al.

2006). The red alga Gracilaria vermiculophylla, facilitated by the polychaete Diopatra cuprea, attracted epifaunal colonizers by adding structure to previously unvegetated, intertidal mudflats along the southeaster Atlantic coast of North America (Byers et al.

2012). This increased epifaunal species richness when the alga was entangled in seagrass beds (Zostera marina) in Denmark (Thomsen 2010).

Globally, seagrasses are in decline (Orth et al. 2006), and the fauna that use seagrasses as nursery habitats are thus threatened (Beck et al. 2001). While seagrass habitats are susceptible to disturbances, both natural and anthropogenic, eutrophication is one of the primary causes of seagrass decline that has led to macroalgal blooms around

10 the world (Duarte 1995, Burkholder et al. 2007). Retreating seagrass beds leave unvegetated substrate behind that may then be colonized by macroalgae (Valiela et al.

1997), which may fill some of the ecological roles of seagrasses in these areas

(Rodriguez 2006).

In lower Chesapeake Bay, eelgrass Zostera marina is the dominant seagrass along the shallow shoals, and, along with widgeon grass Ruppia maritima, provides resources and protection to early life history stages of many animals including the blue crab

Callinectes sapidus (Orth and van Montfrans 1987, Pardieck et al. 1999, Hovel and

Lipcius 2002, Lipcius et al. 2005, Seitz et al. 2005). However, seagrasses have been in decline in Chesapeake Bay since the 1960s due to anthropogenic and natural disturbances

(Orth and Moore 1983). Increased fragmentation and decreased areal cover of seagrass beds may negatively impact the recruitment of blue crab postlarvae that use seagrass as primary nursery habitat (Hovel and Lipcius 2001, Hovel and Lipcius 2002, Stockhausen and Lipcius 2003).

Similar to what has occurred in marine ecosystems worldwide (Rodriguez 2006), macroalgae may provide nursery habitat for blue crabs and compensate for the loss of seagrasses. Gracilaria vermiculophylla is an exotic, coarsely branching, red macroalga originating from the Western Pacific (Ohmi 1956) that has colonized shallow coastal areas of the Atlantic Ocean along North America and Europe (Bellorin et al. 2004,

Thomsen et al. 2005, Freshwater et al. 2006, Thomsen et al. 2006, Thomsen et al. 2007,

Byers et al. 2012). Species within the genus Gracilaria are often morphologically similar and therefore difficult to differentiate (Oliveira et al. 2000, Gurgel et al. 2004, Thomsen

2010). In Chesapeake Bay and the seaside lagoons of Virginia and Maryland, the initial

11 introduction and subsequent spread of G. vermiculophylla was overlooked due to its cryptic morphological characteristics (Orth et al. 2006, Thomsen et al. 2006, Thomsen et al. 2009). The alga has become ubiquitous in shallow areas and coves in the tributaries of

Chesapeake Bay and seaside lagoons (Duarte 1995, Thomsen et al. 2006, Burkholder et al. 2007) and ranges along the east coast of North America from Georgia (Byers et al.

2012) to Newfoundland (Mathieson et al. 2008).

Gracilaria vermiculophylla may act as a nursery habitat by providing both refuge from predation and increased food resources. Survival of juvenile blue crabs is enhanced in G. vermiculophylla compared to both seagrass and unvegetated substrate (Johnston and

Lipcius 2012). Unattached algae also modify soft-bottom habitat (Pihl et al. 1996,

Wallentinus and Nyberg 2007, Thomsen et al. 2010, Byers et al. 2012) and can change the structure of associated communities by altering the physical, chemical, and biological processes within those habitats. At intermediate levels of algal biomass, this coarsely branching macroalgae creates structural heterogeneity in colonized soft-bottom habitats, and may also provide new habitats and food resources for other organisms (Thomsen et al. 2007, Thomsen 2010, Byers et al. 2012). Thus, local species diversity may be enhanced by G. vermiculophylla (Nyberg et al. 2009). Drifting, unattached G. vermiculophylla can also become entangled in seagrass beds, creating a mixed habitat that supports a higher diversity and abundance of invertebrate fauna by increasing heterogeneity or by improving habitat quality (Thomsen 2010).

At high biomass, G. vermiculophylla may be detrimental to both seagrasses and other organisms. Dense mats are formed at high G. vermiculophylla biomass, which can decrease light availability for seagrasses and cause hypoxia or anoxia (Gray et al. 2002,

12 Bell and Eggleston 2005, Thomsen et al. 2006). Mats of G. vermiculophylla may smother and kill the seagrass in which they are entangled, leaving the alga without protection from tidal currents and waves that may remove it from the area, and thus cause a “habitat cascade” that is detrimental to fauna associated with both seagrass and G. vermiculophylla (Thomsen 2010). While dense mats are common in the seaside lagoons adjoining lower Chesapeake Bay (Thomsen et al. 2006), high densities of G. vermiculophylla are generally limited to areas with low water flow in tributaries within

Chesapeake Bay (Johnston and Lipcius 2012).

Chemical signals from structured nursery habitats like seagrass beds, macroalgae, and marshes cue megalopae (blue crab postlarvae) to the location of nursery habitats in lower Chesapeake Bay (Wolcott and DeVries 1994, Forward et al. 1997). Megalopae ride flood tide currents upstream towards nursery habitats and rest near the bottom during ebb tides (Olmi et al. 1990, Tankersley et al. 1995, Forward et al. 2003). When megalopae reach nursery habitats, they metamorphose into the first benthic instar (J1) (Metcalf and

Lipcius 1992, Etherington and Eggleston 2000). Metamorphosis from the megalopal stage to J1 (about 3 mm carapace width, CW) and settlement are accelerated when cues from structured nursery habitats or lowered salinity are present (McConaugha 1988,

Metcalf and Lipcius 1992, Forward et al. 1997). Juveniles typically remain in these habitats until they reach about 20-30 mm CW, after which they emigrate to unvegetated secondary nursery habitats like shallow mud coves (Pile et al. 1996, Lipcius et al. 2007).

Structured habitats provide refuge from predation as well as abundant prey resources for early juvenile crabs (Heck et al. 2003, Lipcius et al. 2005, Seitz et al. 2005, Lipcius et al.

2007). Emigration from structured habitats may be due to a lack of suitable refuges for

13 larger juveniles (Lipcius et al. 2005, Seitz et al. 2005, Lipcius et al. 2007, Johnston and

Lipcius 2012), or it may be density dependent in the case of smaller juveniles (Pile et al.

1996, Reyns and Eggleston 2004).

If G. vermiculophylla is present in areas from which seagrasses have been extirpated due to environmental change or where juvenile blue crab recruitment is higher than seagrasses can support, it may represent an alternative, emerging primary or secondary nursery habitat. Unfortunately, there is little data on the availability of G. vermiculophylla in shallow habitats of lower Chesapeake Bay. Thus, the objective of this study was to determine if G. vermiculophylla is present in shallow habitats when juvenile crabs are recruiting and where structured habitat is now absent by assessing the distribution and abundance of G. vermiculophylla in the York River, a tributary of lower

Chesapeake Bay.

MATERIALS AND METHODS

Study area

Studies were conducted in the York River, a tributary of lower Chesapeake Bay, in summer and early fall 2013 and 2014. The study sites extended from the mouth of the

York River where it meets Chesapeake Bay to ~ 42 km upriver. The river was stratified along its salinity gradient into downriver, midriver, and upriver regions (Figure 1).

Downriver, seagrass is currently present, while seagrasses were present historically midriver but have since been lost, and upriver areas were above areas of historical

14 seagrass abundance (Moore 2009). Environmental data including salinity, dissolved oxygen, and water temperature were recorded at most sites at the time of sampling using a YSI (Model 85, Yellow Spring Instruments). Mean salinity ranged from 19.6 to 20.6 downriver, 17.9 to 18.3 midriver, and 14.4 to 15.3 upriver. Mean water temperature ranged from 22.8 to 24.0°C downriver, 23.0 to 24.5°C midriver, and 24.9 to 25.0°C upriver. Hypoxic conditions (dissolved oxygen < 2 mg L-1) were not observed at any sampling sites over the study period (Table 1), although all measurements were daytime readings. Subtidal habitats vary across the region: downriver, there are large continuous seagrass beds dominated by Zostera marina with Ruppia maritima scattered throughout and unvegetated substrate (mostly sand with some mud); both midriver and upriver are dominated by unvegetated substrate, although seagrass beds were common midriver until

1972 (Orth and Moore 1983). All sites were located at < 1.5 m depth MLLW and ranged from the low intertidal to shallow subtidal.

Survey

This survey aimed to assess the distribution and biomass of Gracilaria vermiculophylla in the York River over two years. The survey occurred from May to

October in 2013 and 2014. Sites were selected using a stratified random sampling design with the three regions serving as strata. In 2013, 10 sites were selected in each region each month. In 2014, 7 – 8 sites were selected downriver and midriver, while 4 sites were selected upriver, each month.

15 At each site, three 20-m transects were set parallel to shore approximately 3 m apart. Transects were marked every meter, at which the vegetation present was noted.

Five haphazard quadrats (0.0625 m-2) were set along each transect. Within each quadrat, the percent cover of any vegetation was recorded, and, if it was present, G. vermiculophylla was removed from the quadrat and its volume measured to the nearest mL. Gracilaria vermiculophylla volume was converted to biomass (dry weight, DW) using a linear regression (y = 0.138x; R2 = 0.998; Figure 2).

Analysis

Twelve logistic regression models (g1 – g12) were developed to predict G. vermiculophylla presence, recorded as either 1 (present) or 0 (absent) at each site, as a function of salinity (continuous), base habitat (2 levels: 1 [seagrass] or 0 [unvegetated]), stratum (2 levels: downriver or midriver), temperature (continuous), dissolved oxygen

(continuous), month (6 levels: May – October), and year (2 levels: 2013 and 2014; Table

2). For six instances where environmental data could not be collected, salinity was estimated from the linear relationship between observed salinity and latitude and longitude during the sampling period (y = 3820.1 + 33.9 latitude - 32.6 longitude; R2

= 0.929). Each model produced a log-likelihood value, which was then used to calculate

Akaike’s information criterion (AIC) (Anderson 2008). AICc values were used to correct for bias due to low sample size (Anderson 2008). From these, Δi values and model probabilities (wi) were generated to compare the fit of the candidate models (gi) with the model having the lowest AICc. A model was eliminated if its wi was less than 0.10

16 (Anderson 2008); the individual parameter estimates of the best model were then evaluated. Changes in percent cover and biomass of G. vermiculophylla were analyzed with ANOVA models using log-transformed data to meet statistical assumptions.

RESULTS

Spatial distribution

In both years, Gracilaria vermiculophylla was typically present at more sites downriver than midriver (2013: p = 0.002; 2014: p = 0.001), and never present upriver

(Figure 1). In the midriver zone, it was present primarily along the north shore of the

York River. Similarly, G. vermiculophylla biomass was highest downriver and along the north shore midriver. In contrast, seagrass only occurred in the downriver zone.

Seasonal and annual distribution patterns

Gracilaria vermiculophylla was present on average at 30.8% of sites in 2013 and at 30.3% of sites in 2014 (Figure 3a). Algal presence was greatest overall in June 2014 at

45% of sites, downriver in June and October (75%), and midriver in August 2013 (50%).

Of the twelve candidate models, model g5 had the highest wi, although models g2, g4, and g11 deserved consideration because their wi values exceeded 0.1 (Table 2). Model g5 included the additive effects of region and base habitat. Since model g2 only included base habitat, and had a lower wi than g5 (wi = 0.15), it was not considered further. Model

17 g4, which included the additive effects of salinity and base habitat, had a relatively low wi

(= 0.17) and negligible difference to g2, indicating that the effects of salinity were small and eliminating it from further consideration. Model g11, which incorporated the interactive effects of region and base habitat, also had a relatively low wi (= 0.18) and no significant interaction between the predictor variables (p = 0.45), so this model was also removed from consideration.

Gracilaria vermiculophylla presence in the York River was influenced by region and whether seagrass was present. By region, G. vermiculophylla was 53% less likely to be present midriver than downriver (p = 0.045; Table 3). Similarly, algal presence increased with salinity (p = 0.003); for every 1 unit increase in salinity, it was 20% more likely that G. vermiculophylla was present. Salinity differed significantly between regions

(p < 0.001) but not between years (p = 0.701). The alga was more likely to be present at sites with seagrass than at those without (p = 0.014; Table 3). Gracilaria vermiculophylla was present at 63.6% of sites where seagrass was also present (Figure 1a, c) and, at those sites, occurred in 31.6% of quadrats. When seagrass was absent, G. vermiculophylla, was present at 30.3% of unvegetated sites and, at those sites, occurred in 19.5% of quadrats.

While there was interannual variability in G. vermiculophylla presence, with greater algal presence in 2013 than 2014 (p = 0.035; Figure 3), it also varied by month. Presence tended to decrease from May to October, particularly in September (p = 0.044) and

October (p = 0.012).

Overall, there was a trend towards higher algal biomass downriver, lower biomass midriver, and no G. vermiculophylla biomass upriver (Figure 1b, Figure 4). As with presence, algal biomass was greater in 2014 than 2013 (p = 0.005; Figure 4a). In 2013,

18 there was a plateau of relatively higher biomass from July – October; whereas in 2014, biomass was highest from June – July. Within regions, algal biomass was greater downriver in 2014 than 2013 (p = 0.013; Figure 4b), likely because salinity downriver was higher in 2014 than in 2013 (p < 0.001). Algal biomass was greater midriver in 2013 than 2014 (p = 0.014; Figure 4c). Downriver, biomass was greatest in July 2014 (11.9 g

DW m-2), and, while algal biomass was relatively stable in 2013, it was variable in 2014.

Midriver, algal biomass was greatest in October 2013 (3.9 g DW m-2), and patterns of biomass were similar between years. Trends in G. vermiculophylla percent cover were similar to those in biomass, with generally greater cover downriver, lower cover midriver, and no cover upriver, and similar temporal patterns (Figure 5).

DISCUSSION

Gracilaria distribution and biomass patterns

Unlike previous studies (Thomsen et al. 2007, Weinberger et al. 2008, Sfriso et al.

2012), salinity was a driving factor in the distribution of Gracilaria vermiculophylla, with greatest abundance at higher salinities. In addition, it was more abundant in the presence of seagrass, which may have created favorable conditions for the alga. The increased presence, biomass, and cover of the alga in areas with seagrass is likely due to the propensity algal fragments and mats to become entangled in or along the edges of seagrass beds. In unvegetated areas where there is little structure to encourage entanglement, the alga has a patchy distribution and large floating mats are rare;

19 typically, the alga is attached by holdfast to shell fragments and gravel or fragments may be incorporated into polychaete tubes (Wood, pers. obs.). While the upriver region falls within the salinity tolerance of G. vermiculophylla (5 – 60) (Yokoya et al. 1999, Raikar et al. 2001, Rueness 2005, Weinberger et al. 2008), the alga has not yet colonized the this region. In Sweden, G. vermiculophylla expanded its range by ~150 km over two years

(Nyberg et al. 2009), indicating that this alga is an efficient colonizer. It is therefore possible that algal propagules have not entered the upriver region in this study due to physical constraints like currents or other environmental factors like pulsed low salinity events or turbidity. It seems likely that the alga could spread farther upriver where the subtidal habitat is predominantly unvegetated via fragmentation, spore release, or entanglement in fishing gear or boat anchors.

The presence, biomass, and percent cover of G. vermiculophylla varied over time, which is consistent with a fast-growing alga that easily fragments. It is likely that differences in biomass between years are due to differences in environmental variables like storm activity, temperature, salinity, and nutrient inputs. For instance, increased storm activity may increase the likelihood of algal fragmentation, and winds may push these fragments into very shallow areas of low flow that were not captured well in this study. Fragments and mats may also be pushed into deeper water and, due to the negative buoyancy of the alga, be removed from the system. In the Baltic Sea, G. vermiculophylla biomass increased 3-fold over two years, while algal biomass increased by a factor of

18.5 in in situ experiments, indicating a potentially large sink for algal biomass in deep water (> 2 m depth) (Weinberger et al. 2008). In this study, average G. vermiculophylla

20 biomass increased 41% from 2013 to 2014, suggesting that the alga is becoming more prevalent in areas where it has already successfully colonized.

Interactions with native seagrasses

While G. vermiculophylla presence and biomass covaried with seagrass presence, algal biomass in this study was low and likely below the level at which negative impacts on seagrass would begin to occur (e.g., Hauxwell et al. 2001). It is likely that the possible negative impacts of G. vermiculophylla on seagrass will be exacerbated due to climate change and other anthropogenic impacts. Increased sea surface temperatures and eutrophication will likely cause reduced growth and increased mortality of seagrasses with concurrent increases in growth rates of algal species (Pedersen and Borum 1996,

Valiela et al. 1997, McGlathery 2001). In Chesapeake Bay, Zostera marina is already experiencing periodic mass mortality events due to above average summer water temperatures combined with other environmental stressors like increased turbidity

(Moore and Jarvis 2008). Thus, it is likely that G. vermiculophylla will increasingly impact seagrasses due to their interaction. For instance, G. vermiculophylla tends to exacerbate the negative effects of elevated temperature regimes (26 - 30°C) on Z. marina, which experiences decreased growth and increased mortality (Martínez-Lüscher and

Holmer 2010, Höffle et al. 2011), while growth of G. vermiculophylla is positive over a range of temperatures (5 - 30°C) with maximum growth from 15 - 25°C (Yokoya et al.

1999, Raikar et al. 2001) and decreased growth and increased mortality only at temperatures exceeding 32.5°C (Raikar et al. 2001).

21 Unlike eelgrass, widgeon grass (Ruppia maritima) is more tolerant to increased water temperature, experiencing increased growth with temperature increasing from 8°C to 30°C (Evans et al. 1986), suggesting that this species could potentially replace eelgrass where it has declined (Moore et al. 2014). In San Diego, CA, widgeon grass replaced eelgrass after a period of increased water temperature during an El Niño event (Johnson et al. 2003). However, R. maritima is limited to shallower areas than Z. marina and is more susceptible to physical disturbances like waves and storms (Orth and Moore 1988), which indicates that it likely will not be able to fully replace eelgrass in Chesapeake Bay.

Additionally, widgeon grass has not recolonized areas upriver where seagrasses have been lost (Moore 2009, Moore et al. 2014) and where G. vermiculophylla has become the dominant subtidal vegetation.

Differences in optimal growth conditions between Z. marina and both R. maritima and G. vermiculophylla suggest that the shallow subtidal structural habitat of lower Chesapeake Bay may shift from one dominated by eelgrass to one dominated by algae and widgeon grass as sea surface temperatures increase with climate change.

Implications for juvenile blue crabs

Gracilaria vermiculophylla is present when juvenile blue crabs (Callinectes sapidus) are using nursery habitat in the late summer and fall when megalopae are recruiting to Chesapeake Bay (van Montfrans et al. 1995), and the alga is also present in the late spring when crabs that recruited in late fall have overwintered and are still of a size (< 30 mm carapace width) to use structured habitat. However, variable algal biomass

22 indicates that G. vermiculophylla may not represent a stable nursery habitat, so juvenile blue crabs may use it opportunistically when it is available, especially downriver where juvenile densities can be quite high (Lipcius et al. 2005) and density-dependent dispersal from nursery habitats is more likely (Etherington and Eggleston 2000, Etherington and

Eggleston 2003). Gracilaria vermiculophylla is providing additional habitat midriver where seagrass has been lost (Figure 6). This change from unvegetated to vegetated substrate adds structural complexity to these shallow subtidal areas and may represent an important emerging nursery habitat for juvenile crabs in these lower salinity areas where megalopal settlement rates can be high (Stockhausen and Lipcius 2003).

Because G. vermiculophylla is likely to have increasingly large ecological impacts in Chesapeake Bay, it is important to understand how it might impact ecologically and economically important species like the blue crab. There is already some evidence that G. vermiculophylla is acting as a nursery habitat in Chesapeake Bay for juvenile crabs by increasing survival rates over those in both seagrass and unvegetated habitat (Johnston and Lipcius 2012). However, it is unclear whether this is a function of the structure of the alga decreasing predator search efficiency or if some other attribute of the alga, like the associated faunal assemblage, is creating favorable conditions. Future work should focus on the ability of the alga to provide adequate nursery habitat by: a) supporting juvenile crabs at increased densities both pre- and post-megalopal settlement; b) sustaining a large and diverse community of epifauna with similar species composition to native seagrass; and c) allowing for increased growth rates of juvenile crabs using the alga as a habitat comparable to those in native seagrass nurseries.

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28 Gulbransen, Karen J McGlathery, Marianne Holmer, and Brian R Silliman. 2010. Habitat cascades: the conceptual context and global relevance of facilitation cascades via habitat formation and modification. Integrative and Comparative Biology 50: 158–175. Thomsen, Mads Solgaard. 2010. Experimental evidence for positive effects of invasive seaweed on native invertebrates via habitat-formation in a seagrass bed. Aquatic Invasions 5: 341–346. Thomsen, Mads Solgaard, Carlos Frederico Deluqui Gurgel, Suzanne Fredericq, and Karen J McGlathery. 2005. Gracilaria vermiculophylla (Rhodophyta, Gracilariales) in Hog Island Bay, Virginia: a cryptic alien and invasive macroalga and taxonomic correction. Journal of Phycology 42: 139–141. Thomsen, Mads Solgaard, Karen J McGlathery, A Schwarzschild, and BR Silliman. 2009. Distribution and ecological role of the non-native macroalga Gracilaria vermiculophylla in Virginia salt marshes. Biological Invasions 11: 2303–2316. Thomsen, Mads Solgaard, Karen J McGlathery, and Anna Christina Tyler. 2006. Macroalgal distribution patterns in a shallow, soft-bottom lagoon, with emphasis on the nonnative Gracilaria vermiculophylla and Codium fragile. Estuaries and Coasts 29: 465–473. Valiela, Ivan, James McClelland, Jennifer Hauxwell, Peter J Behr, Douglas Hersh, and Kenneth Foreman. 1997. Macroalgal blooms in shallow estuaries: controls and ecophysiological and ecosystem consequences. Limnology and Oceanography 42: 1105–1118. van Montfrans, J, C E Epifanio, D M Knott, R N Lipcius, D J Mense, K S Metcalf, E J Olmi III, R J Orth, M H Posey, E L Wenner, and T L West. 1995. Settlement of blue crab postlarvae in Western North Atlantic estuaries. Bulletin Of Marine Science 57: 834–854. Wallentinus, Inger, and Cecelia D Nyberg. 2007. Introduced marine organisms as habitat modifiers. Marine Pollution Bulletin 55: 323–332. Weinberger, F, B Buchholz, R Karez, and M Wahl. 2008. The invasive red alga Gracilaria vermiculophylla in the Baltic Sea: adaptation to brackish water may compensate for light limitation. Aquatic Biology 3: 251–264. Wolcott, Donna L, and Mona C DeVries. 1994. Offshore megalopae of Callinectes sapidus: depth of collection, molt stage and response to estuarine cues. Marine Ecology Progress Series 109: 157–163. Yokoya, Nair S, Hirotaka Kakita, Hideki Obika, and Takao Kitamura. 1999. Effects of environmental factors and plant growth regulators on growth of the red alga Gracilaria vermiculophylla from Shikoku Island, Japan. Hydrobiologia 398/399: 339–347.

29 TABLES

Table 1. Summary of environmental data collected at sampled sites, including means of each variable during each sampling year; 95% confidence intervals (CI) are in parentheses.

Downriver Midriver Upriver 2013 2014 2013 2014 2013 2014 Temperature 24.7 24.0 24.6 24.5 25.2 24.9 (°C) (23.8, 25.5) (22.9, 25.2) (23.7, 25.4) (23.5, 25.5) (24.5, 25.9) (23.3, 26.4)

19.4 20.6 17.6 17.8 14.4 14.7 Salinity (19.0, 19.9) (20.1, 21.1) (17.2, 18.1) (16.9, 18.8) (13.7, 15.0) (12.9, 16.4)

Dissolved 8.1 8.0 7.0 7.8 7.0 7.5 Oxygen (mg/L) (7.4, 8.9) (7.4, 8.5) (6.7, 7.4) (7.5, 8.1) (6.5, 7.5) (7.1, 7.9)

30 Table 2. Information theoretic analysis (Anderson 2008) of twelve logistic models (gi) using salinity (S), base habitat (BH), region (R), temperature (T), dissolved oxygen (DO), month (M), and year (Y) as predictors of Gracilaria vermiculophylla presence, where k is the number of parameters in a model, AIC is the Akaike information criterion, AICc is the corrected AIC, ∆i is the difference between any model and the best model in the set, and wi is the model probability.

Model k AIC AICc Δi wi

g1: S 3 285.062 285.176 15.839 <0.01

g2: BH 3 271.127 271.241 1.904 0.15

g3: R 3 273.270 273.383 4.047 0.05

g4: S + BH 4 270.710 270.901 1.564 0.17

g5: R + BH 4 269.146 269.337 0.000 0.38

g6: T + DO 5 274.703 274.990 5.653 0.02

g7: R + T + DO 9 279.984 280.862 11.525 <0.01

g8: R + T + DO + Y + M + BH 12 277.864 279.408 10.071 <0.01

g9: R + Y + M + BH 10 274.859 275.938 6.601 0.01

g10: M + BH + S 9 274.093 274.972 5.635 0.02

g11: BH * R 5 270.530 270.817 1.481 0.18

g12: 1 2 292.187 292.243 22.906 <0.01

31 Table 3. Summary of ANOVA for model g5, including parameter estimates, standard errors, and p-values.

Coefficients Estimate SE z P Intercept -0.367 0.311 -1.18 0.238 Stratum = midriver -0.751 0.374 -2.009 0.045* Base habitat = seagrass 0.949 0.387 2.455 0.014*

32 FIGURES

Figure 2. Relationship between Gracilaria vermiculophylla volume and biomass with linear regression.

Figure 3. Mean percent presence of Gracilaria vermiculophylla during 2013 and 2014 over: (a) the entire York River; (b) the downriver region, where base habitats are seagrass (filled) and unvegetated (open); and (c) midriver region. Gracilaria vermiculophylla was not observed upriver. Vertical bars represent 1 standard error of the mean. Note differing scales on y-axes.

Figure 4. Mean Gracilaria vermiculophylla biomass (g dry weight [DW] m-2) during 2013 and 2014 over: (a) the entire York River; (b) the downriver region, where base habitats are seagrass (filled) and unvegetated (open); and (c) midriver region. Gracilaria vermiculophylla was not observed upriver. Vertical bars represent 1 standard error of the mean. Note differing scales on y-axes.

Figure 5. Mean Gracilaria vermiculophylla percent cover during 2013 and 2014 over: (a) the entire York River; (b) the downriver region, where base habitats are seagrass (filled) and unvegetated (open); and (c) midriver region. Gracilaria vermiculophylla was not observed upriver. Vertical bars represent 1 standard error of the mean. Note differing scales on y-axes.

Figure 6. Locations within the York River of (a) seagrass cover in 1971 (Orth and Gordon 1975), (b) seagrass cover in 2014 (Orth et al. 2015), and (c) areas where Gracilaria vermiculophylla was present in 2013 and 2014.

CHAPTER 3

Utilization of native seagrass and exotic algae by juvenile blue crabs and macrofaunal prey communities

39 ABSTRACT

The exotic macroalga Gracilaria vermiculophylla may represent an emerging nursery habitat for juvenile blue crabs Callinectes sapidus in Chesapeake Bay, which may help to ameliorate the decline of native seagrass habitat. We compared prey communities associated with G. vermiculophylla, seagrass (eelgrass Zostera marina and widgeon grass

Ruppia maritima) and unvegetated bottom to determine whether the exotic alga provides suitable nursery habitat for blue crabs. Additionally, we assessed juvenile blue crab density in G. vermiculophylla and seagrass to determine carrying capacity of these habitats. In summer 2013, suction sampling was used to quantify crab density and prey community structure in habitats in the York River in lower Chesapeake Bay. Using multidimensional scaling (MDS), analysis of similarity (ANOSIM), and similarity percentage analysis (SIMPER), we show that epifaunal communities differed between G. vermiculophylla and seagrass, whereas infaunal communities were more similar between

G. vermiculophylla and seagrass than between G. vermiculophylla and unvegetated habitat. Seagrass and G. vermiculophylla had similar carrying capacities for juvenile blue crabs in both June and August. While juvenile crab densities were similar between the alga and seagrass, approximately 3 times more first and second benthic instar crabs inhabited seagrass compared to G. vermiculophylla, indicating that megalopae preferentially settle in seagrass and that juveniles use the alga opportunistically as a secondary habitat.

40 INTRODUCTION

As seagrasses continue to decline due to eutrophication, sedimentation, and climate effects (Najjar et al. 2010), other submerged aquatic vegetation (SAV) may provide adequate subtidal nursery habitat for juvenile crabs. The exotic red macroalga

Gracilaria vermiculophylla is one such potential alternative nursery habitat in lower

Chesapeake Bay. Although the alga may negatively impact native seagrasses at high biomass (Hauxwell et al. 2001, Gray et al. 2002, Bell and Eggleston 2005, Thomsen et al.

2006), G. vermiculophylla biomass in areas of lower Chesapeake Bay has been relatively low (mean = 1.54 g DW m-2; Chapter 2). In the York River, a tributary of Chesapeake

Bay, G. vermiculophylla is present in some areas where seagrasses have been lost and is most abundant in areas of higher salinity where seagrasses are also present (Chapter 2).

Early summer juvenile crab densities are similar in G. vermiculophylla and seagrass, and juvenile crabs experience higher survival in G. vermiculophylla relative to both seagrass and unvegetated habitat (Johnston and Lipcius 2012). While there is some evidence that juvenile blue crabs are using G. vermiculophylla as habitat, as noted above, how the crabs are using the alga remains unclear. For instance, crabs may use the alga as a structural refuge and forage elsewhere or, alternatively, the alga may serve as a nursery that acts as both refuge and feeding area. Megalopae may preferentially settle in seagrass and small juveniles may then emigrate to nearby algal mats if crab density is high, or megalopae may have no settlement preferences for either vegetation type. The alga may also impact the communities associated with the habitats in which it colonizes. For example, mixed communities of G. vermiculophylla and Z. marina supported a more diverse invertebrate

41 epifaunal assemblage than either vegetation alone along the Danish coast (Thomsen

2010), and the alga provided novel habitat in previously unvegetated mudflats in South

Carolina and Georgia when incorporated as tube decoration by the polychaete Diopatra cuprea (Byers et al. 2012). This is the first study to examine both epifaunal and infaunal communities associated with G. vermiculophylla in Chesapeake Bay.

Ontogenetic shifts in habitat use are common in many marine species and are an important part of the life history of the blue crab Callinectes sapidus. The blue crab starts life in the pelagic zone as part of the zooplankton, metamorphoses into the postlarval

(megalopal) stage to invade coastal and estuarine areas, and then metamorphoses into the first benthic stage (J1 instar; approx. 3 mm carapace width, CW) when it settles in appropriate primary nursery habitats (Heck and Thoman 1984, Orth and van Montfrans

1987, Etherington and Eggleston 2000). In the mid-Atlantic region of North America, primary nursery habitats are typically seagrass beds (Orth and van Montfrans 1987, 1990,

Pile et al. 1996, Etherington and Eggleston 2000, 2003, Reyns and Eggleston 2004). This invasion of estuarine nursery habitats by megalopae is known as primary dispersal. Small juvenile crabs tend to emigrate from structural refuges (e.g. seagrasses) to size refuges

(e.g. oyster reefs and unvegetated bottom) when the suitability of the nursery is reduced

(i.e. when survival or growth rates are reduced relative to those in another habitat). This secondary dispersal typically occurs when juvenile crabs reach approximately 20 - 30 mm CW (Pile et al. 1996, Hovel and Lipcius 2001, Lipcius et al. 2005, 2007, Johnston and Lipcius 2012). However, secondary dispersal may also occur when juvenile densities are very high due to increased competition for space or resources (Reyns and Eggleston

42 2004) or increased density-dependent cannibalism by larger juvenile and adult blue crabs

(Moksnes et al. 1997), causing juvenile crabs to leave nurseries earlier.

In Chesapeake Bay, the loss of seagrasses (Zostera marina and Ruppia maritima) since the 1960s has reduced the primary nursery habitat available for juvenile blue crabs

(Orth and Moore 1983). Thus, initial crab densities upon recruitment are likely very high in remaining seagrass beds (Stockhausen and Lipcius 2003), and early secondary dispersal to less favorable habitats may become increasingly common. While early secondary dispersal alleviates the immediate threat of density-dependent cannibalism in seagrass beds (Moksnes et al. 1997), survival rates of small juvenile crabs in secondary habitats such as shallow unvegetated bottom and marsh edges are typically much lower

(Wilson et al. 1987, Heck et al. 2003, Johnston and Lipcius 2012). For instance, in unvegetated bottom, the habitat for large juvenile and adult crabs, small juvenile crab mortality is 20% greater than that in seagrass (Johnston and Lipcius 2012). Intertidal marshes provide variably available habitat due to tides, and, consequently, only marsh edges are used by small juvenile crabs (Orth and van Montfrans 1990). Crabs in marsh edges also experience elevated mortality relative to those in seagrass (Orth and van

Montfrans 2002).

To begin to understand the role of the alga in mitigating the impacts of seagrass loss on blue crab recruitment through secondary dispersal and survival, the objectives of this study were to: 1) determine the density of juvenile blue crabs in Gracilaria vermiculophylla relative to that in seagrass; and 2) establish whether the alga supports similar epifaunal and infaunal communities as those in seagrass and unvegetated habitats.

Because juvenile crabs experience changes in diet as they grow, differences in blue crab

43 habitat use may be related to differences in the prevalence of preferred prey resources within each habitat. We hypothesized that: infaunal communities would be similar between G. vermiculophylla and unvegetated habitat; epifaunal communities would be similar between G. vermiculophylla and seagrass; and a wider size range of juvenile blue crabs would use G. vermiculophylla than either seagrass or unvegetated habitat. Larger juvenile crabs (~30 mm CW) prefer bivalve prey, particularly the clam Limecola balthica, in Chesapeake Bay. Thus, we expected a greater number of larger juvenile crabs to frequent G. vermiculophylla than seagrass, because the alga colonizes unvegetated substrate where L. balthica is more common.

MATERIALS AND METHODS

A survey of infaunal and epifaunal benthic community composition was conducted in the York River, a subestuary of lower Chesapeake Bay (Figure 7). Sampling occurred in June and August 2013. To ensure adequate sampling of subtidal habitats, three regions with varying habitat types were identified: downriver (3 habitats: seagrass,

Gracilaria vermiculophylla, and unvegetated), midriver (2 habitats: G. vermiculophylla and unvegetated), and upriver (1 habitat: unvegetated). Seagrass beds were comprised predominantly of Zostera marina with some Ruppia maritima and ranged from 30 to

100% cover. Gracilaria vermiculophylla also ranged from 30 to 100% cover in epifaunal samples.

44 Epifaunal communities

During each sampling event, epifaunal samples were collected at random sites in downriver vegetated habitats (n = 8 – 10 in each habitat), and no epifaunal samples were collected midriver or upriver, because seagrass is only present downriver and G. vermiculophylla is sparse (< 5% cover; Chapter 2). Epifaunal communities were assessed downriver in G. vermiculophylla and seagrass using the standard drop-net suction methods (Orth and van Montfrans 1987). Juvenile blue crabs (Callinectes sapidus) were removed from all samples, counted, and measured to the nearest 0.1 mm using Vernier calipers. Crab densities were calculated for crabs < 30 mm carapace width (CW) and corrected for 78% suction efficiency (Orth and van Montfrans 1987). A subsample of epifaunal suctions, with equal allocation from each month and habitat type over a range of vegetation cover, was sorted for all epifauna (n = 16), and species were identified to the lowest taxonomic level possible (usually species). All animals were counted and then dried at 60°C for at least 48 hours before being combusted at 550°C for 5 hours to determine ash-free dry weight (AFDW).

Infaunal communities

Infaunal samples were taken immediately adjacent to epifaunal samples in vegetated habitats. Infaunal suctions were also taken at unvegetated sites downriver (n =

7 - 8). Midriver and upriver, infaunal suctions were taken at 10 sites in each region,

45 including both unvegetated and G. vermiculophylla habitats midriver and unvegetated habitat upriver.

Infaunal communities were assessed in all regions and habitat types using a 0.11 m2 cylinder suctioned to 30 cm sediment depth. All samples were sorted and organisms were identified to the lowest possible taxonomic level (usually species). Animals were counted and then dried at 60°C for at least 48 hours before being combusted at 550°C for

5 hours to determine ash-free dry weight (AFDW). Adjacent to each infaunal sampling site and prior to suctioning, a surface sediment core (2.5 cm diameter) was taken for grain-size analysis. Standard wet sieve and pipette analysis were used to determine the percentages of gravel (> 2 mm), sand (> 62.5 μm), silt (4 phi; phi = – [ln(particle diameter) × ln(2)-1]), and clay (8 phi) (Folk 1980).

Analysis

Juvenile crab densities (CW < 30 mm) were compared between vegetated habitats and sampling periods using ANOVA tests, pairwise t-tests, and size-frequency distributions. Both epifaunal and infaunal abundances were corrected for suction area and square root-transformed to downweight extremely abundant species while increasing the relative weight of intermediately abundant species (Clarke and Warwick 2001). Analysis of variance tests and Student’s t-tests were used to assess differences in epifaunal abundance, biomass (AFDW), and species richness between sampling periods and habitat type, while ANOVA tests and Tukey’s HSD tests were used to assess differences in these metrics for infaunal communities due to unequal sampling size. Epifaunal and infaunal

46 abundances were analyzed separately and compared between habitat types, but not sampling period, using non-metric multidimensional scaling (nMDS; PRIMER v6) with

Bray-Curtis similarity matrices. Further, the analysis of similarity (ANOSIM) function tested for differences between habitat types, and similarity percentages (SIMPER) analysis identified species that cause dissimilarity between communities in the habitats.

Densities of the clam Limecola balthica, a primary prey resource for larger juvenile and adult blue crabs, were also analyzed across sediment type using Loess curves.

RESULTS

Juvenile blue crab density and size

In both June and August, juvenile blue crab densities (ind. m-2) were similar in

Gracilaria vermiculophylla and seagrass (Table 4; pJune = 0.835; pAugust = 0.080). Blue crab densities increased in both vegetated habitats over the two summer months of sampling (pGracilaria = 0.012; pseagrass < 0.001), and, subsequently, CW decreased over time

(pGracilaria < 0.001; pseagrass < 0.001). However, crabs were significantly smaller in seagrass than in the alga (p = 0.021; Table 5), and there tended to be fewer individuals in most size classes in G. vermiculophylla compared to seagrass (Figure 8). There were 2.8 times more crabs between 2.5 to 4 mm CW (J1 and J2 crabs) in seagrass than in the alga in

August.

47 Epifaunal communities

Neither epifaunal richness nor abundance changed with sampling time (prichness =

0.884; p abundance = 0.637), and therefore data were pooled by habitat type for subsequent analysis. Species richness was greater in seagrass than in G. vermiculophylla (Table 6; p

= 0.007), and epifaunal abundance was greater in seagrass than in G. vermiculophylla (p

= 0.018). In G. vermiculophylla, communities were 51.7% similar across samples, while communities were 46.2% similar across seagrass samples. Overall, epifaunal communities were different in G. vermiculophylla than in seagrass (Figure 9A; one-way

ANOSIM; Global R = 0.422; p = 0.003) when pooled across sampling times.

Communities in seagrass and G. vermiculophylla were 60.32% dissimilar on average, and the isopod Erichsonella attenuata and skeleton shrimp Caprella penantis contributed the most to the dissimilarity between the communities in these habitats (Table 7). In seagrass,

E. attenuata and C. penantis were 2.0 to 7.4 times more abundant than in G. vermiculophylla.

Infaunal communities

Like epifaunal communities, infaunal richness and abundance did not change with sampling period (prichness = 0.644; pabundance = 0.880), and further analysis pooled data by habitat type. While infaunal richness was significantly greater in seagrass than in G. vermiculophylla (p < 0.001) and unvegetated habitat (p < 0.001), there was no difference in richness between G. vermiculophylla and unvegetated habitat (p = 0.156). Infaunal

48 abundance was comparable between the alga and both seagrass (p = 0.080) and unvegetated habitat (p = 0.791). Infaunal communities were marginally different across the three habitats (Figure 9B; one-way ANOSIM; Global R = 0.346; p = 0.001); however, communities in seagrass and G. vermiculophylla were similar to each other (R = 0.166; p

= 0.002), while the community in unvegetated habitat was relatively different than those in both seagrass (R = 0.457; p = 0.001) and G. vermiculophylla (R = 0.319; p = 0.001).

Within habitats, communities were 27.1% similar in G. vermiculophylla, 48.8% similar in seagrass, and 38.4% similar in unvegetated habitat. The infaunal community in unvegetated habitat was 73.96% dissimilar to the G. vermiculophylla community and

73.93% dissimilar to the seagrass community on average, and the clam Limecola balthica contributed most to the dissimilarities (Table 8A, B). Communities in seagrass and G. vermiculophylla were 69.65% dissimilar with the polychaetes Clymenella torquata and

Alitta succinea contributing most to the dissimilarity (Table 8C). Both infaunal abundance and richness were similar across sampling periods (pabunance = 0.880; prichness =

0.644). Sediment grain size differed among habitats, with seagrass having more sand and less silt and clay than either G. vermiculophylla or unvegetated habitat, and G. vermiculophylla having less clay than unvegetated habitat while other sediment components were similar (Table 9). Densities of the clam Limecola balthica were stable across most sediment types, but tended to decrease when % sand content was high

(Figure 10A) and, correspondingly, when % silt was low (Figure 10B).

49 DISCUSSION

While this study supports previous work that blue crab megalopae preferentially settle in seagrass (Orth and van Montfrans 1987, 1990, Forward et al. 1994, Moksnes and

Heck 2006), it suggests that small juvenile blue crabs may also use Gracilaria vermiculophylla as an early secondary habitat if densities are high in seagrass. Although there was no difference in the average density of juvenile blue crabs in G. vermiculophylla and seagrass, there were almost 3 times more newly recruited juvenile crabs (J1 and J2) in seagrass as compared to the other habitats in August, indicating that megalopae likely preferentially settled in seagrass rather than in the alga. Juvenile crab densities (< 30 mm CW) in June reported here are similar to those observed by Johnston and Lipcius (2012) in both seagrass and G. vermiculophylla; however, they also reported almost 10 times more crabs 5 – 10 mm CW in the alga compared to seagrass, whereas crabs < 12 mm CW occurred more frequently in seagrass than in G. vermiculophylla here. While both studies were conducted in the York River, the spatial distribution of vegetated samples was broader in this study, compensating for any location-related, rather than habitat-related, differences in crab density.

Juvenile crabs may use algal habitat more opportunistically to access preferred prey items. As crabs increase in size, their diet shifts from one dominated by small crustacean epifauna like amphipods and isopods to one dominated by bivalves (Minello and Wooten 1993, Seitz et al. 2011). Substantially fewer crustaceans, especially the isopod Erichsonella attenuata and skeleton shrimp Caprella penantis, inhabited G. vermiculophylla compared to seagrass, likely due to differences in habitat preference of

50 these species. This suggests that small juvenile crabs may face increased competition for their preferred prey species in the alga. Small crustaceans were more abundant in seagrass and, since they coevolved with seagrasses, may have preferences for microalgae growing on Zostera marina and Ruppia maritima (Orth and van Montfrans 1984).

Gracilaria vermiculophylla supported fewer epifaunal prey animals than seagrass overall, indicating that this habitat may be less suitable for juvenile blue crabs that form dense aggregations due to megalopal settlement patterns.

Contrary to the hypothesis, G. vermiculophylla does not seem to provide additional benefit to larger juvenile crabs (> 15 mm CW; J7 – J9) by increasing access to preferred infaunal prey, leading to low frequency of occurrence of larger juvenile crabs in both seagrass and G. vermiculophylla. In particular, Limecola balthica abundance was similarly depressed in both vegetated habitats relative to unvegetated habitat, which is typical (Seitz et al 2005, Bonsdorff 1995). However, recruitment of L. balthica into vegetated structural habitats is likely high (Boström and Bonsdorff 2000). For instance, juvenile L. balthica are a dominant species in drifting algal mats in the Baltic Sea

(Boström and Bonsdorff 2000), indicating that macroalgae may be efficient at trapping clam recruits. This suggests that recruits settling in vegetated habitat experience increased predation in seagrass and G. vermiculophylla, which leads to lower adult L. balthica densities relative to unvegetated habitat. Thus, juvenile blue crabs likely benefit from G. vermiculophylla colonizing unvegetated habitat if the alga increases juvenile clam recruitment in these areas. Future studies should include benthic cores in these habitats to determine differences in meiobenthic species abundance and richness between habitats.

While juvenile crabs use G. vermiculophylla as habitat, it seems to represent a

51 secondary nursery rather than a primary nursery like seagrass. The alga affords juveniles increased survival relative to unvegetated habitat (Johnston and Lipcius 2012), a typical secondary habitat, and thus is likely a superior secondary nursery for blue crabs. In the

York River, G. vermiculophylla has colonized areas where seagrass has been extirpated midriver; however the alga is sparse in these areas, while algal cover is highest in and near seagrass beds (Chapter 2). Thus, it is plausible that G. vermiculophylla area will decline as seagrass continues to decline. It is therefore unlikely that the alga will fully compensate for the continued loss of seagrass habitat in Chesapeake Bay for important commercial species like the blue crab. However, it is difficult to predict how blue crab settlement preferences or behavior will change as benthic habitats continue to shift.

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53 on the role of micrograzing: a review. Aquatic Botany 18: 43–69. Orth, R. J., and J. van Montfrans. 1987. Utilization of a seagrass meadow and tidal marsh creek by blue crabs Callinectes sapidus. 2. Seasonal and annual variations in abundance with emphasis on post-settlement juvneiles. Marine Ecology Progress Series 41: 283–294. Orth, R. J., and J. van Montfrans. 1990. Utilization of marsh and seagrass habitats by early stages of Callinectes sapidus: a latitudinal perspective. Bulletin of Marine Science 46: 126–144. Orth, R. J., and J. van Montfrans. 2002. Habitat quality and prey size as determinants of survival in post-larval and early juvenile instars of the blue crab Callinectes sapidus. Marine Ecology Progress Series 231: 205–213. Orth, R. J., and K. A. Moore. 1983. Chesapeake Bay: An unprecedented decline in submerged aquatic vegetation. Science 222: 51–53. Pile, A. J., R. N. Lipcius, J. van Montfrans, and R. J. Orth. 1996. Density-dependent settler-recruit-juvenile relationships in blue crabs. Ecological Monographs 66: 277– 300. Reyns, N. B., and D. B. Eggleston. 2004. Environmentally-controlled, density-dependent secondary dispersal in a local estuarine crab population. Oecologia 140: 280–288. Seitz, R. D., K. E. Knick, and M. Westphal. 2011. Diet selectivity of juvenile blue crabs (Callinectes sapidus) in Chesapeake Bay. Integrative and Comparative Biology 51: 598–607. Seitz, R. D., R. N. Lipcius, and M. S. Seebo. 2005. Food availability and growth of the blue crab in seagrass and unvegetated nurseries of Chesapeake Bay. Journal of Experimental Marine Biology and Ecology 319: 57–68. Stockhausen, W. T., and R. N. Lipcius. 2003. Simulated effects of seagrass loss and restoration on settlement and recruitment of blue crab postlarvae and juveniles in the York River, Chesapeake Bay. Bulletin of Marine Science 72: 409–422. Thomsen, M. S. 2010. Experimental evidence for positive effects of invasive seaweed on native invertebrates via habitat-formation in a seagrass bed. Aquatic Invasions 5: 341–346. Thomsen, M. S., K. J. McGlathery, and A. C. Tyler. 2006. Macroalgal distribution patterns in a shallow, soft-bottom lagoon, with emphasis on the nonnative Gracilaria vermiculophylla and Codium fragile. Estuaries and Coasts 29: 465–473. Wilson, K. A., K. L. Heck Jr, and K. W. Able. 1987. Juvenile blue crab, Callinectes sapidus, survival: an evaluation of eelgrass, Zostera marina, as refuge. Fishery Bulletin 85: 53–58.

54 TABLES

Table 4. Juvenile crab (< 30 mm CW) densities (ind. m-2) in seagrass and Gracilaria vermiculophylla in June and August with standard error and 95% confidence interval. June August Mean SE 95% CI Mean SE 95% CI Seagrass 4.0 1.1 2.2 19.4 3.7 7.3 Gracilaria 3.3 1.1 2.1 12.9 3.2 6.3

55 Table 5. Mean juvenile crab size (mm carapace width) in seagrass and Gracilaria vermiculophylla in June and August with standard error and 95% confidence interval. June August Mean SE 95% CI Mean SE 95% CI Seagrass 18.0 0.7 1.4 7.9 0.2 0.4 Gracilaria 17.8 0.9 1.7 9.1 0.3 0.5

56 Table 6. Mean species richness (no. species) and abundance (no. ind. m-2) in epifaunal and infaunal habitats with standard error and 95% confidence interval. Richness Abundance Mean SE 95% CI Mean SE 95% CI Seagrass 23.5 1.2 2.3 4361.0 1042.6 2043.4 Epifauna Gracilaria 17.4 1.5 2.9 1147.5 210.7 412.9

Seagrass 15.0 0.7 1.5 1274.2 343.4 673.1 Infauna Gracilaria 7.7 0.9 1.9 623.3 260.6 510.9 Unvegetated 6.2 0.4 0.7 465.7 47.9 93.8

57 Table 7. Contributions of abundance-dominant species in epifauna (no. ind. m-2, % contribution, and cumulative % contribution) to dissimilarities between Gracilaria vermiculophylla and seagrass.

Contribution to Average Abundance Dissimilarity Contribution Cumulative Gracilaria Seagrass (%) (%) Erichsonella 12.49 32.12 15.13 15.13 attenuata Caprella penantis 3.07 22.6 13.86 28.99 Cymadusa compta 17.56 14.32 8.36 37.34 Edotia triloba 4.65 14.59 6.85 44.19 Gammarus 13.72 10.86 6.69 50.88 mucronatus Bittiolum varium 6.93 12.19 6.14 57.02 Palaemon pugio 5.31 0.75 3.25 60.28

58 Table 8. Contributions of abundance-dominant species in infauna (no. ind. m-2, % contribution, and cumulative % contribution) to dissimilarities between (A) Gracilaria vermiculophylla and seagrass, (B) unvegetated habitat and seagrass, and (C) G. vermiculophylla and unvegetated habitat. Contribution to A Average Abundance Dissimilarity Contribution Cumulative Gracilaria Unvegetated (%) (%) Limecola balthica 2.96 13.28 21.84 21.84 Alitta succinea 8.22 6.13 11.84 33.68 Owenia fusiformis 1.62 2.97 7.29 40.97 Capitellid spp. 2.69 2.57 6.14 47.1 Clymenella torquata 2.55 1.54 5.25 52.36 Ameritella mitchelli 0.81 2.4 4.99 57.35 Spiochaetopterus oculatus 2.22 1.71 4.43 61.78

B Contribution Cumulative Unvegetated Seagrass (%) (%) Limecola balthica 13.28 2.46 13.05 13.05 Clymenella torquata 1.54 8.6 8.66 21.71 Alitta succinea 6.13 10.62 8.35 30.05 Parasabella micropthalmus 0.06 6.24 6.26 36.31 Spiochaetopterus oculatus 1.71 6.72 5.99 42.31 Leitoscoloplos spp. 1.17 5.97 5.85 48.16 Capitellid spp. 2.57 4.62 5.8 53.96 Tagelus plebeius 1.1 5.19 5.34 59.3 Kelliopsis elevata 0.54 3.75 4.06 63.36

C Contribution Cumulative Gracilaria Seagrass (%) (%) Clymenella torquata 2.55 8.6 9.71 9.71 Alitta succinea 8.22 10.62 8.56 18.26 Parasabella micropthalmus 0.75 6.24 6.72 24.98 Spiochaetopterus oculatus 2.22 6.72 6.49 31.47 Capitellid spp. 2.69 4.62 6.42 37.89 Leitoscoloplos spp. 2.22 5.97 6.25 44.14 Tagelus plebeius 2.05 5.19 5.85 49.99 Limecola balthica 2.96 2.46 4.55 54.54 Glycera dibranchiata 1.03 3.92 4.53 59.07 Kelliopsis elevata 0.12 3.75 4.32 63.39

59 Table 9. Percent sediment grain size composition by component in each habitat with 95% confidence intervals in parentheses.

% Gravel % Sand % Silt % Clay Seagrass 0.169 90.457 6.085 3.165 (-0.031, 0.370) (87.932, 93.225) (3.981, 8.190) (2.456, 3.874) 0.505 73.022 20.552 5.922 Gracilaria (-0.151, 1.160) (64.287, 81.757) (13.118, 27.985) (4.294, 7.549) 0.345 64.261 18.178 17.216 Unvegetated (-0.079, 0.769) (54.626, 73.896) (13.224, 23.133) (12.096, 22.335)

60 FIGURES

Figure 7. Map of sampling locations in June and August in each sampling region: a) downriver, b) midriver, and c) upriver.

61 A) B)

Figure 8. Juvenile blue crab size frequencies in (A) seagrass and (B) Gracilaria vermiculophylla in August.

62 A)

Figure 9. Multidimensional scaling plots for (A) epifaunal communities in Gracilaria vermiculophylla and seagrass and (B) infaunal communities in G. vermiculophylla, seagrass, and unvegetated habitat. Stress estimates are from ANOSIM tests for differences between habitat types.

63 A)

Figure 10. Limecola balthica density vs. (A) percent sand and (B) percent silt in surface sediments with Loess curves (solid black) and 95% confidence intervals (dashed).

CHAPTER 4

In situ growth of juvenile blue crabs in native seagrass and the exotic alga Gracilaria vermiculophylla in lower Chesapeake Bay

65 ABSTRACT

Nursery habitats increase survival and growth of juvenile animals, allowing more juveniles to recruit to the adult population. In Chesapeake Bay, the primary nursery habitat of young juvenile blue crabs, seagrass, has declined; however, the exotic red alga

Gracilaria vermiculophylla may ameliorate the loss of seagrass and fulfill some ecological functions of seagrass, such as acting as a nursery habitat. While in situ studies of survival are common, similar studies of growth are rare. This study assessed growth rates of juvenile crabs in G. vermiculophylla, native seagrass, and unvegetated habitats.

Juvenile crabs (15 – 50 mm carapace width) were tagged with microwires for identification and placed in plastic and VEXAR cages (0.34 m2 surface area) deployed near the mouth of the York River and in midriver replicate locations for four weeks.

Growth did not differ significantly between habitat type nor between midriver and downriver areas. Abundance of untagged conspecifics in the cages did not affect growth of tagged crabs. Crabs grew similarly in all habitats, which suggests that G. vermiculophylla provides sufficient prey resources for blue crab growth, has the capacity to act as a nursery habitat, and represents a non-detrimental exotic species in the

Chesapeake Bay ecosystem.

66 INTRODUCTION

Exotic species are becoming increasingly common additions to coastal and estuarine habitats worldwide, and exotic algae in particular tend to modify the structure of the benthos in introduced areas by acting as foundation species. Most studies of exotic algae that form structured benthic habitat focus on their impacts on community structure and faunal abundance (Britton-Simmons 2004, Wikström and Kautsky 2004, Bulleri et al.

2006, Buschbaum et al. 2006, Thomsen and McGlathery 2006, Norkko and Bonsdorff

2006, Schmidt and Scheibling 2007, Gribben et al. 2008, Thomsen 2010, White and

Shurin 2011, Gribben et al. 2012, Thomsen et al. 2013), the behavior of native grazers and foragers (Trowbridge 1995, Boudouresque et al. 1996, Trowbridge and Todd 2001,

Scheibling and Anthony 2001, Longepierre et al. 2005, Sumi and Scheibling 2005,

Gollan and Wright 2006, Scheibling et al. 2008, Cacabelos et al. 2010, Tomas et al.

2011), or the survival of native fauna in these novel ecosystems (Bulleri et al. 2006,

Byers et al. 2010, Hernández Cordero and Seitz 2010, Johnston and Lipcius 2012,

Gribben et al. 2012, Carroll and Peterson 2013, Bishop and Byers 2014). In contrast, few studies have examined the impact of exotic algae on growth of native fauna, especially in situ (Bulleri et al. 2006, Carroll and Peterson 2013), and most focus on growth of direct consumers (Scheibling and Anthony 2001, Lyons and Scheibling 2007). Thus, experimental evidence of the effects of exotic algae on native fauna is sparse.

Small juvenile blue crabs (Callinectes sapidus) are often associated with vegetated habitats in estuarine areas (Hovel and Lipcius 2001), in which they typically reside until they reach 20 – 30 mm carapace width (CW) before dispersing to unvegetated

67 bottom (Pile et al. 1996, Lipcius et al. 2007). Structured habitats like seagrasses increase survival and growth of juvenile animals by decreasing predator search efficiency and providing diverse and abundant prey assemblages (Heck et al. 2003, Lipcius et al. 2005,

Seitz et al. 2005, Lipcius et al. 2007). These positive effects may decrease as juveniles get larger, resulting in ontogenetic shifts to other, less structured habitats due to tradeoffs between survival and growth (Werner and Gilliam 1984). Historically, vegetated habitats in Chesapeake Bay were extensive subtidal seagrass beds comprised of eelgrass (Zostera marina) and widgeon grass (Ruppia maritima); however, seagrasses in the Bay have been in decline since the 1960s (Orth and Moore 1983). Recovery of seagrasses has been hampered by increased nutrients, sedimentation, and water temperatures (reviewed in

Orth et al. 2010). The disturbances that negatively impact native seagrasses in

Chesapeake Bay also likely make the estuary more amenable for macroalgae (Duarte

1995, Burkholder et al. 2007). For instance, the exotic red alga Gracilaria vermiculophylla, which has become ubiquitous in shallow subtidal areas of lower

Chesapeake Bay and the seaside lagoons of the Eastern Shore (Thomsen et al. 2006,

2009), is tolerant to increased water column nutrient concentration, burial by sediment, low light conditions (Thomsen and McGlathery 2007), and elevated water temperature

(Yokoya et al. 1999, Raikar et al. 2001, Martínez-Lüscher and Holmer 2010, Höffle et al.

2011).

Gracilaria vermiculophylla may represent an emerging nursery habitat for juvenile crabs in lower Chesapeake Bay (Johnston and Lipcius 2012). In field experiments, juvenile blue crabs had increased survival rates in the alga relative to both seagrass and unvegetated habitat, and survival in G. vermiculophylla did not decrease

68 with increasing crab size as it did in seagrass (Johnston and Lipcius 2012). Predation may be reduced in Gracilaria vermiculophylla due to increased search time, and density- dependent cannibalism may be reduced in G. vermiculophylla due to an increased carrying capacity of the alga. In areas where seagrasses have been lost, the alga may be the only structured habitat present subtidally. For instance, in the York River, a subestuary of lower Chesapeake Bay, G. vermiculophylla is not only present at relatively low biomass in and around seagrass beds near the mouth of the river, but also occurs farther upriver (Chapter 2) where seagrasses have been absent since the 1970s (Orth and Moore

1983). While the alga can be detrimental to seagrasses and fauna at high biomass

(Hauxwell et al. 2001, Gray et al. 2002, Bell and Eggleston 2005, Thomsen et al. 2006),

G. vermiculophylla also provides structural heterogeneity in the shallow, soft-bottom areas it colonizes when at low to moderate biomass (Thomsen et al. 2007, Thomsen

2010, Byers et al. 2012).

While predation pressure on juvenile blue crabs is moderated by Gracilaria vermiculophylla (Johnston and Lipcius 2012), the effect of habitat choice on growth rate has not been investigated. Therefore, the objectives of this study were to determine if juvenile blue crabs experience similar growth in a presumed nursery habitat (G. vermiculophylla) compared to a known nursery habitat (seagrass), and, from these results, determine if G. vermiculophylla represents an emerging nursery habitat for juvenile blue crabs in lower Chesapeake Bay.

69 MATERIALS AND METHODS

To assess growth rates of juvenile blue crabs in native seagrass, exotic Gracilaria vermiculophylla, and unvegetated (control) habitats, an in situ growth study was conducted in the York River, a tributary of lower Chesapeake Bay (Figure 11). Growth rates were evaluated during two periods: late summer 2012 (August 22 – September 21) and early summer 2014 (June 9 – July 8). Cages (height = 0.25 m; area = 0.06 m2) were constructed from VEXAR mesh (8 mm diagonal mesh size) attached to plastic cylinders

(height = 15 cm). Cages were placed in a habitat by fully inserting the plastic cylinder into the sediment to prevent crab escape through burrowing, with two PVC stakes to secure each cage. Cages were deployed downriver and midriver, with 9-10 cages placed in each habitat type (Table 10). Downriver, cages were placed in seagrass, G. vermiculophylla, and unvegetated habitats; midriver, cages were placed in G. vermiculophylla and unvegetated habitats, because seagrasses do not occur midriver

(Orth and Moore 1983). After placement, cages were cleaned every 7 – 10 d to remove epiphytes and other encrusting organisms. Temperature, dissolved oxygen (DO) and salinity were measured at each site at the start and end of the experiments (Model 85,

Yellow Springs Instruments). Mean temperature and salinity were compared between years and areas using Student’s t-tests. Mean DO was compared between habitats and areas using Student’s t-tests. Sediment cores were taken inside and outside unvegetated cages before suctioning to test for cage impacts on flow-related sediment alterations; standard wet sieve and pipette analysis determined the percentages of gravel (> 2 mm),

70 sand (> 62.5 μm), silt (4 phi; phi = – [ln(particle diameter) × ln(2)-1]), and clay (8 phi)

(Folk 1980).

Crabs were collected from the York River using a dip net approximately one week prior to being placed in cages. All crabs were tagged with one 0.5 mm microwire in the basal muscle of one of the 5th pereiopods for identification at the end of the study and held in tanks with unfiltered York River water for at least 48 h prior to placement in cages. Microwire tags do not significantly affect growth or mortality rates in juvenile crabs, and the retention rate is high (88 – 98%) after the first post-tagging molt (van

Montfrans and Orth 1986, Fitz and Wiegert 1991). Crabs were measured to the nearest

0.1 mm using Vernier calipers immediately prior to placement in cages. Solitary crabs were introduced into each cage through a trap door at the top after the cage had been secured in a given habitat. Only crabs > 15 mm CW were used due to the mesh size. In

2012, experimental crabs were 15 - 30 mm CW, whereas in 2014 they were 15 – 50 mm

CW.

After approximately 4 weeks, crabs were recovered from cages by placing a PVC cylinder (0.11 m2) with an attached net (3 mm mesh) that extended to the surface around the cage. The cage was removed from the cylinder, the interior of the cylinder was suctioned to a depth of approximately 20 cm, and a dip net was used to ensure no animals remained in the cylinder. Samples were held on ice until returned to the lab for sorting.

Crabs recovered with microwires (Table 10) were measured using Vernier calipers, and daily growth rates were calculated for each crab. Untagged conspecifics found in cages with tagged crabs were counted and measured using Vernier calipers (Appendix I).

Final crab size (mm CW) was related to initial size using a linear model with year

71 (2 levels: 2012, 2014) as an additive covariate. Weekly growth rates were analyzed using single-factor ANOVA models with habitat, site (downriver or midriver), and year as predictor variables. Tukey’s HSD tests (α = 0.05) were used to assess differences between mean growth rate pairs. Differences in conspecific abundance by habitat and site were assessed with Tukey’s HSD tests (α = 0.05). Single-factor ANOVA models were used to determine the impacts of conspecific abundance on tagged crab growth and differences in conspecific size (mm CW) between habitats.

RESULTS

Physical variables

Temperature differed significantly between the start and end of both experiments.

In 2012, mean temperature declined from 27.8C at the start to 24.1C at the end of the experiment (p = 0.045). In 2014, mean temperature increased from 25.0C at the start to

29.0C at the end of the experiment (p = 0.047). Mean temperature was 1.1°C greater in

2014 compared to 2012 (27.0C ± 0.32 SE vs. 25.9C ± 0.14 SE; p = 0.003) and greater midriver than downriver (25.8C ± 0.23 SE vs. 27.3C ± 0.26 SE ; p < 0.001). While hypoxic conditions were never observed during either experiment, dissolved oxygen was lower midriver than downriver (p < 0.001) but was similarly high across habitat types (p

= 0.140). Salinity was lower midriver than downriver (p < 0.001) and differed between years (p < 0.001), with higher mean salinity in 2012 (21.7 ± 0.22 SE) than in 2014 (18.6

± 0.21 SE). Sediment grain size (Table 11) was similar inside and outside of cages (p =

72 0.687) but was composed of more fine particles (silt and clay) relative to coarse particles

(sand and gravel) midriver than downriver (p < 0.001).

Growth

Final crab size (mm CW) was greater in 2014 than 2012 (p < 0.001; Figure 12), and this difference was attributed to significantly higher water temperature in 2014.

Higher water temperatures decrease the intermolt period (Tagatz 1968, Leffner 1972,

Cadman and Weinstein 1988, Fitz and Wiegert 1991) and likely allowed more crabs in

2014 to molt more than once during the experiment. Weekly growth rates were standardized by adding the difference in final size (mm CW) between crabs from 2012 and 2014 (2.9 mm) to crabs from 2012, and further analyses pooled crabs from both years. Proportional growth rates of juvenile crabs decreased significantly with initial size but were highly variable (R2 = 0.48; p = < 0.001; Figure 13). Crab growth did not differ between sites (downriver or midriver; p = 0.139) or among habitat types (p = 0.746;

Table 13). Mean temperature at each site did not affect growth rates (p = 0.518).

Abundance of untagged conspecifics did not impact tagged crab growth (p = 0.979;

Appendix I).

DISCUSSION

The major finding of this experimental study was that growth of juvenile blue crabs did not differ between seagrass and the exotic Gracilaria vermiculophylla, nor

73 between vegetated and unvegetated habitats. Hence, G. vermiculophylla provides adequate prey resources for growth similar to native habitats, including unvegetated bottom. Similar growth rates of crabs in exotic G. vermiculophylla and native seagrass, combined with enhanced survival that juvenile crabs experience in G. vermiculophylla compared to seagrass, as seen in a previous study (Johnston and Lipcius 2012), demonstrates that the alga is likely providing at least an equivalent nursery habitat for crabs as compared to native seagrasses.

Because growth is similar across benthic habitats, shifts in juvenile crab habitat preference are likely due to changes in survival with size. Although larger juvenile crabs also grow and survive well in unvegetated habitat (Lipcius et al. 2005, Seitz et al. 2005), the smallest juveniles occur at much lower densities in unvegetated habitats due to predation (Johnston and Lipcius 2012). Previous studies have shown that survival of juvenile blue crabs < 50 mm CW is positively correlated with size in unvegetated habitat but negatively correlated in seagrass (Johnston and Lipcius 2012), and corresponds to a shift in juvenile blue crab density from seagrass to unvegetated habitat around 20 – 30 mm CW (Pile et al. 1996, Hovel and Lipcius 2001, Lipcius et al. 2005, 2007). Therefore, changes in juvenile blue crab habitat preference with ontogeny maximize survival.

The results of this study add to the growing evidence that some exotic species may enhance ecosystems to create novel ecosystems by performing a crucial role as structured habitat for many species by reducing predator search efficiency. Like juvenile blue crabs, bay scallops (Argopecten irradians concentricus) also survived equally well in seagrass and G. vermiculophylla (Hernández Cordero et al. 2012), suggesting that the alga may be beneficial for many species in Chesapeake Bay. Other exotic algae provide benefits to

74 native fauna like the exotic green alga Codium fragile ssp. tomentosoides, which enhanced survival of mussels (Mytilus galloprovincialis) relative to unvegetated habitat in the Adriatic Sea (Bulleri et al. 2006). In general, exotic macroalgae that positively impact their introduced ecosystems lack typical invasive characteristics in those ecosystems (i.e. outcompeting native vegetation and dominating the benthos) and increase structural complexity (Schaffelke and Hewitt 2007). For instance, in parts of their introduced ranges, the exotic green alga Caulerpa taxifolia increased biomass and diversity of fish and invertebrates in Italy (Relini et al. 1998a, 1998b, 2000), and the brown alga Undaria pinnatifida increased the diversity of subcanopy native macroalga

(Battershill et al. 1998, Wear and Gardner 1999, Forrest and Taylor 2002). Thus, exotic species can create novel ecosystems that benefit native species; however, continued observation of these species and their effects on native flora and fauna is necessary to maintain healthy, functional, and productive ecosystems.

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80 TABLES

Table 10. Recovered tagged crabs relative to crabs placed initially.

2012 2014 Crabs Crabs recovered/crabs recovered/crabs Combined placed placed recovered/placed Seagrass 3/10 8/10 11/20 Downriver Gracilaria 5/10 6/10 11/20 Unvegetated 3/10 2/10 5/20

Gracilaria 6/10 10/10 16/20 Midriver Unvegetated 7/9 7/9 14/18

81 Table 11. Percent sediment grain size composition by component a) inside and outside of cages and b) downriver and midriver. 95% confidence intervals are in parentheses. a. % Gravel % Sand % Silt % Clay 0.077 66.073 22.685 11.166 Inside (-0.074, 0.228) (55.690, 76.455) (16.247, 29.122) (6.968, 15.364) 0.027 67.12 21.871 10.982 Outside (-0.015, 0.069) (58.023, 76.216) (16.258, 27.484) (7.279, 14.685) b. 0.111 81.59 13.93 4.369 Downriver (-0.052, 0.273) (75.715, 87.465) (9.184, 18.676) (3.132, 5.606) 0.000 53.268 29.698 17.034 Midriver (0.000, 0.000) (45.710, 60.826) (25.136, 34.260) (13.935, 20.132)

82 Table 12. Pair-wise comparisons of mean weekly growth rate using Tukey’s HSD test (confidence level = 0.95). Values are p-values.

Unvegetated Gracilaria Gracilaria 0.385 - Seagrass 0.726 0.950

83 -1 Table 13. Mean proportional growth rate (mm CW week /mm CWI) in each habitat with standard errors and 95% confidence intervals.

Mean SE 95% CI Gracilaria 0.083 0.012 0.025 Seagrass 0.111 0.012 0.027 Unvegetated 0.101 0.012 0.026

84 FIGURES

Figure 11. Locations of (i) the York River, a tributary of Chesapeake Bay (inset); (ii) study areas midriver (a) and downriver (b, c); and (iii) all caging locations in 2012 (black) and 2014 (gray).

Figure 12. Final carapace width versus initial carapace width in 2012 (filled) and 2014 (shaded) with linear regressions from an additive model with year as a covariate (2012: y = 10.616 + 0.837x; 2014: y = 13.526 + 0.837x; R2 = 0.85).

Figure 13. Proportional weekly growth rate relative to initial carapace width for all crabs with linear regression (y = 0.224 - 0.004x; R2 = 0.48).

87 APPENDIX I

Detailed results and discussion for conspecific abundance and size.

Untagged conspecifics were present in 80.0% of cages in 2012 and in 30.3% in 2014. Conspecifics were found in higher abundance in cages in Gracilaria vermiculophylla than in seagrass (Supplementary Figure 1A), although this difference was only marginally significant (p = 0.072). More conspecifics were found in cages in G. vermiculophylla midriver than in seagrass downriver (p = 0.015), but there were no significant differences between other habitat types (Supplementary Figure 1B). Conspecific size (mm CW) did not differ between habitat type (p = 0.692; Supplementary Table 1). Unintentionally, the cages attracted untagged juvenile blue crabs while deployed, which allowed us to gain some insight on the attractiveness of additional structure to juvenile crabs in different habitats and the impact of resource competitors on growth of the tagged experimental crabs. While untagged conspecifics were found in similar numbers in cages in unvegetated habitat both downriver and midriver, patterns of abundance were different in structured habitats. Interestingly, more conspecifics were found in cages in G. vermiculophylla than in seagrass, and conspecific abundance was highest in the alga midriver. Because seagrass is the native structural nursery habitat in the York River, the addition of the cage as structure may not have provided any additional perceived benefit to juvenile crabs, whereas G. vermiculophylla may be used more opportunistically so the additional structure provided by the cages may have been attractive. Midriver, where seagrass has been lost, G. vermiculophylla is the only subtidal vegetative habitat available and its distribution is patchy (Chapter 2), which is likely why juvenile crabs were more attracted to cages there. Furthermore, while juvenile blue crab densities tend to be highest in seagrass beds near the mouth of the York River (Orth and van Montfrans 1987), megalopae disperse to areas midriver and upriver (Stockhausen and Lipcius 2003), indicating that G. vermiculophylla may represent an emerging subtidal nursery habitat that is disproportionately used by juvenile crabs midriver. The presence of conspecifics had no noticeable effect on growth of experimental crabs, although we cannot know if conspecifics present in cages at the end of the experiment were there for hours, days, or weeks. Of the 142 conspecifics recovered from 30 cages with experimental crabs present, 48 could not escape because they were larger than 15 mm CW. Conspecifics likely used cages as shelter during molting.

89 Supplementary Table 1. Mean size (mm CW) of untagged conspecifics found in cages with tagged crabs.

Mean SE 95% CI Gracilaria 18.994 2.229 4.369 Seagrass 12.350 4.850 9.506 Unvegetated 19.229 3.968 7.777

CHAPTER 5

Simulated effects of benthic habitat on juvenile blue crab recruitment in the York River

91 ABSTRACT

The degradation and loss of nursery habitat can severely impact the recruitment and population dynamics of marine species. In Chesapeake Bay, loss of seagrass, the primary nursery habitat for juvenile blue crabs, may be mediated by the exotic macroalga

Gracilaria vermiculophylla. Anthropogenic factors such as climate change and eutrophication are likely to contribute to the continued decline of seagrass in Chesapeake

Bay while improving conditions for macroalgae. In this study, we modeled density- dependent migration of early juvenile crabs between subtidal habitats by using a piecewise dispersal function to simulate dispersal of crabs in excess of the carrying capacity of a habitat. Model simulations of both three- and two-dimensional models were used to examine changes in juvenile blue crab recruitment caused by habitat availability based on current, historic, and predicted future habitat area. In three-dimensions, juvenile crab migration was modeled among seagrass, G. vermiculophylla, and unvegetated habitats; in two-dimensions, seagrass and G. vermiculophylla area was summed as vegetated bottom and an analytical solution was derived. These simulations suggest that the loss of subtidal nursery habitat area from historic to current levels caused total juvenile crab abundance to decline by 22 - 37%. Continued loss of seagrass was not fully ameliorated by increases in algal area, but these results suggest that the alga may stabilize the juvenile crab population in the future if it continues to spread in Chesapeake Bay.

92 INTRODUCTION

Many benthic invertebrates have complex life histories that often involve changes in habitat use throughout ontogeny (sensu Thorson 1950), typically requiring a transition from the pelagic zone to the benthos. Decapod crustaceans, for example, develop in the plankton of the pelagic zone, migrate to nursery habitats before metamorphosing into the benthic stage, and leave nurseries for adult foraging grounds as they mature. Blue crab

(Callinectes sapidus) larvae (zoea) develop in the plankton along the continental shelf

(Epifanio et al. 1989, Epifanio 2007). The postlarval (megalopal) stage reinvades coastal and estuarine areas (Epifanio and Garvine 2001) and move up-estuary to nursery habitats

(DeVries et al. 1994, Forward et al. 1997, 2003). When the megalopae reach nursery habitats, they metamorphose into the first benthic instar (J1) (Etherington and Eggleston

2000). Juveniles typically remain in these habitats until they reach about 20-30 mm carapace width (CW; J7 – J9), after which they emigrate to less-structured habitats like shallow unvegetated areas (Pile et al. 1996, Lipcius et al. 2007).

Crustaceans including lobsters and crabs often exhibit preferences for particular nursery habitats or grounds during their juvenile stages (Marx and Herrnkind 1985,

Wahle et al. 1991, Welch et al. 1997). Small juvenile American lobsters (Homarus americanus) provide one example; they are restricted to structured habitats in the Gulf of

Maine and avoid unvegetated habitats where adults are common (Wahle et al. 1991).

Juvenile spiny lobsters (Panulirus argus) in Florida were found to use alternative shelters only after sponges, their primary nursery habitat, were locally depleted (Herrnkind et al.

1997). Blue crabs in the first benthic instar (J1) phase settle at accelerated rates when

93 seagrasses, macroalgae, and salt marshes are present (McConaugha 1988, Forward et al.

1997). These preferred nursery habitats increase juvenile crab growth and survival by providing abundant food resources and physical protection from foraging predators (Heck et al. 2003, Lipcius et al. 2005), which is likely why they are favored as settlement sites.

Habitat degradation or loss may have severe negative impacts on the recruitment and population demographics of species that prefer few nursery habitats (Caddy 1986, Wahle et al. 1991).

Seagrasses provide important nursery areas in Chesapeake Bay for many species and are the preferred nursery habitat for the blue crab, Callinectes sapidus (Orth and van

Montfrans 1987). Chesapeake Bay seagrasses (Zostera marina and Ruppia maritima) have been in decline since the 1960s due to a combination of anthropogenic and natural causes, such as decreased water quality (Kemp et al. 2004, 2005), increased nutrient inputs (Moore and Wetzel 2000, Kemp et al. 2004), physical disturbances (Pulich and

White 1991), disease (Muehlstein et al. 1988, Muehlstein 1989, Muehlstein et al. 1991,

Muehlstein 1992), climate change (Thayer et al. 1975, Evans et al. 1986, Knutson et al.

1998, Knutson and Tuleya 1999, Scavia et al. 2002, Bintz et al. 2003, Moore and Jarvis

2008), and sea level rise (Najjar et al. 2010). The continued decline of seagrasses in the

Chesapeake Bay may lead to decreased blue crab larval settlement and increased juvenile mortality. Seagrass cover has been correlated with juvenile blue crab abundance, and simulations of juvenile crab settlement indicate that historic losses of seagrasses have caused a moderate (10 – 25%) reduction in megalopal settlement and a substantial (40 –

45%) reduction in juvenile recruitment due to a reduction in seagrass cover in the York

River, a tributary of Chesapeake Bay (Stockhausen and Lipcius 2003). While juvenile

94 blue crab growth is similar among subtidal habitats (Chapter 4), juvenile crab survival is much higher in vegetated habitats like seagrass beds and macroalgae compared to unvegetated habitat (Johnston and Lipcius 2012). Therefore, the number of juvenile crabs that reach the adult stage likely relies on the quality of nursery habitats, the behavior of juveniles as they move through nurseries, and their ability to survive in different habitats.

Although juvenile crabs typically remain in nursery habitats until they reach about 20-30 mm CW (J7 – J9), they may emigrate earlier due to density-dependent interactions with conspecifics (Pile et al. 1996, Reyns and Eggleston 2004). Emigration by larger juveniles to less-structured habitats like shallow unvegetated areas may be triggered by a lack of suitable refuges in nursery habitats as they grow (Pile et al. 1996, Lipcius et al. 2005,

Seitz et al. 2005, Lipcius et al. 2007, Johnston and Lipcius 2012).

The presence of some macroalgal species, such as Gracilaria vermiculophylla

(hereafter, Gracilaria) in the York River, may ameliorate the loss of seagrass as nursery habitat in some areas (Chapter 2). Gracilaria, an exotic, coarsely branching, red macroalga originating from the Western Pacific (Ohmi 1956), has colonized shallow coastal systems of North America and Europe (Bellorin et al. 2004, Thomsen et al. 2005,

Freshwater et al. 2006, Thomsen et al. 2006, 2007, Gulbransen et al. 2012, Miller 2012).

While it is unknown when Gracilaria was introduced to Chesapeake Bay, this macroalga has become ubiquitous in shallow shoals and coves of tributaries of Chesapeake Bay and seaside lagoons of the Eastern Shore (Thomsen et al. 2006). Although it is exotic, since biomass levels in the York River are currently low to moderate, there is no indication that it is negatively impacting seagrasses and their associated fauna in Chesapeake Bay

(Chapter 2, Chapter 3).

95 Eutrophication, one of the primary causes of seagrass decline, has led to frequent macroalgal blooms around the world (Duarte 1995, Burkholder et al. 2007), and these nonvascular plants may outcompete and replace seagrasses in eutrophied areas. This occurs when unvegetated substrate left by retreating seagrass beds is colonized by macroalgae (Valiela et al. 1997). Gracilaria may fill some of the ecological roles of seagrasses where they have been lost (Rodriguez 2006); for example, it provides structured nursery habitat and refuge for juvenile blue crabs and other structure-reliant species (Beck et al. 2001) (Lipcius et al. 2007, Thomsen 2010). Johnston and Lipcius

(2012) found that juvenile blue crab survival in Gracilaria was similar to that in seagrass and higher than that in unvegetated bottom.

While past investigations into habitat impacts on juvenile blue crab settlement and recruitment have focused primarily on seagrasses (Stockhausen and Lipcius 2003), this study addresses alternative subtidal habitats while also allowing for movement between habitat types. Migratory behavior based on changing habitat conditions must be better investigated to further understand population dynamics in species that rely on sensitive habitats. This study focuses on young juvenile blue crabs (J1 - J5) that undergo early secondary dispersal because of density-dependent interactions with conspecifics. We examine the change in blue crab recruitment (here considered surviving to the end of the simulation) due to variations in subtidal habitat availability based on historic, recent, and predicted habitat area using a habitat-specific population model that incorporates density- dependent behavior.

96 MATERIALS AND METHODS

Blue crab nursery habitat model

The basic model analyzes the daily change in numbers of blue crabs in seagrass,

Gracilaria, and unvegetated shallow-water nursery habitats with respect to time (Figure

14):

Ni,t 1 si Ni,t f Ni,t ,ki sd pi f Ni,t ,ki (1) i

where Ni,t is abundance of juvenile blue crabs on day t ; i s for seagrass, g for Gracilaria, or u for unvegetated bottom; si is daily survival of juveniles that remained in habitat i; ki is habitat-specific carrying capacity; sd is daily survival of emigrating juveniles as they migrate between habitats; and pi is the fraction of emigrating juveniles

that migrate to a particular habitat. The piecewise function f Ni,t ,ki is used to simulate dispersal by the excess of

crabs Ni,t ki over the carrying capacity ki in each habitat (Figure 15): 0, if N k f N ,k i,t i (2) i,t i , if Ni,t ki Ni,t ki

Thus, the number of crabs in a specific habitat i at time t 1, Ni,t 1 equals the number of crabs in that habitat at time t that did not emigrate Ni,t f Ni,t,ki and survived at rate si, plus the sum of crabs immigrating from the other two habitats and those returning to the same type of habitat f Ni,t ,ki , where immigrating crabs survive i

at rate sd and enter a particular habitat at rate pi . Substituting actual habitat values for i 97 in Equations (1) and (2) yields a set of three coupled equations, one for each habitat type.

The equation for seagrass habitat, for instance, is:

N s N f N ,k s p f N ,k f N ,k f N ,k (3) s,t 1 s s,t s,t s d s s,t s g,t g u,t u

which is depicted conceptually in Figure 14.

To derive an analytical solution, we simplified the model (Equation (1)) to a set of

two equations by summing the area of seagrass and Gracilaria habitat as vegetated

habitat, and by assuming that the parameters for seagrass and Gracilaria are equal.

Vegetated area was set at 40 m2 and unvegetated bottom area was set to 682 m2. Now

i v for vegetated or u for unvegetated bottom. For instance, the equation for vegetated

habitat is: Nv,t 1 sv Nv,t f Nv,t ,kv sd pv f Nv,t,kv f Nu,t ,ku. (4)

Numerical simulations

Numerical simulations were conducted using parameter values from the literature

(Table 14) over a time span of t 30 days to better understand early secondary dispersal

by young (J1 – J5) crabs (Matlab v. 2016a). Initial population (n = 1,000,000,000 crabs)

was allocated proportionally by carrying capacity to each vegetated habitat, where there

was more than one, and no individuals were allocated to unvegetated habitat because it is

not a preferred settlement site (Penry 1982). The total shallow (1.5 m depth MLLW)

bottom area of the York River was estimated to be approximately 72,345 km2. Seagrass

area was estimated for the York River in 2013 (“current”) (Orth et al. 2014, Chesapeake

Bay and Coastal Bays SAV Annual Monitoring Data) and 1971 (“historic”) (Moore et al.

98 2001, Chesapeake Bay and Coastal Bays SAV Annual Monitoring Data), Gracilaria area was estimated for the York River in 2013 only (Chapter 2), and unvegetated habitat was allocated the remaining shallow water area.

For the three-habitat model, simulations assessed the impacts on juvenile crab recruitment of: 1) current seagrass and Gracilaria area; 2) a 50% decline in seagrass from current levels with Gracilaria remaining stable; 3) a concurrent 50% decrease in seagrass area and 50% increase in Gracilaria area; 4) historic seagrass area (215% increase in current area) with Gracilaria absent; 5) current seagrass area with Gracilaria absent; 6) current seagrass area with a 50% increase in Gracilaria area; and 7) a 50% decline in current seagrass area with a concurrent increase in Gracilaria area so that total vegetated area remains the same (Table 15). Similarly, for the two-habitat model, simulations evaluated the impacts on juvenile crab recruitment of: 1) current vegetation area (sum of current seagrass and Gracilaria area); 2) a 50% decrease in vegetation area; 3) a 50% increase in vegetation area; and 4) historic vegetation area (only seagrass) (Table 16).

Sensitivity analysis

Sensitivity analyses were conducted for the current simulation (simulation 1) for both the two- and three-habitat models (Table 19, 20). Each parameter was increased and decreased by 5 and 7%, and sensitivity model runs were compared to the standard model run.

99 RESULTS

Three-habitat model

Juvenile blue crab population sizes in seagrass, Gracilaria, and unvegetated habitat predicted by all modified simulations (2 – 7) for the three-habitat model were compared to model Simulation 1, the simulation assuming current seagrass and

Gracilaria areas. Simulation 1 predicted a total juvenile blue crab abundance of 26 million crabs, with about 58% of crabs residing in seagrass, 16% in Gracilaria, and 26% in unvegetated habitat after 30 days (Table 17). Compared to Simulation 1, Simulation 2 indicated that a 50% decline seagrass from current levels with no decline in Gracilaria would cause a 22% decrease in juvenile crab abundance, with the largest decline in seagrass (-42%); however, Gracilaria reduced the impact of seagrass decline by hosting

12% more juvenile crabs. Adding a concurrent 50% increase in Gracilaria area to the

50% decline in seagrass area, shown by Simulation 3, reduced the loss of total juvenile blue crab abundance to 14% relative to Simulation 1, because a substantial increase

(66%) in crabs residing in Gracilaria offset declines in seagrass (-43%). Simulation 4 showed historic conditions to increase juvenile crab abundance by 23% overall, with a

69% increase in seagrass and a 4% increase in unvegetated habitat. When Gracilaria was removed leaving only current seagrass area in Simulation 5, there was a 13% decline in the crab population, although there were slight gains in abundance in seagrass (4%).

Simulation 6, which expanded Gracilaria area by 50% and maintained current seagrass area, showed a juvenile blue crab abundance increase of 6% with substantial increases in

100 Gracilaria (48%). A 50% decline in seagrass area with a concurrent 265% increase in

Gracilaria area, such that total vegetated area remained constant (Simulation 7), had a

positive, but small, impact on total blue crab abundance (3%) due to a 182% increase in

crab abundance in Gracilaria. In all applicable simulations, emigration from seagrass and

Gracilaria caused an initial increase in abundance in unvegetated habitat (Figure 16).

Crab abundance in Simulation 1 also temporarily increased by about 26.7 million crabs in

seagrass due to immigration of crabs from Gracilaria.

Two-habitat model

Because there is neither direct interaction between the number of individuals in

vegetated habitat and the number of individuals in unvegetated habitat, nor constant

influx of individuals to either habitat, there is only one steady stat at 0,0. This makes

intuitive sense, because the populations of juvenile blue crabs in these habitats should go

extinct if there is no further influx of individuals. To determine the eigenvalues of the

system and their stability, the Jacobian matrix for the system was calculated and

evaluated at 0,0:

kv ku s svkv sd pvkv e s p sd pvku e s 1 v d v v kv 2 ku 2 1 e 1 ekv 1 e 1 eku J0,0 . (5) kv ku s p sd pukv e s suku sd puku e d u s 1 u kv 2 u ku 2 1 e kv 1 e ku 1 e 1 e

A solution to this two-dimensional system is: t 1 t 0 N c 0.08 c 0.19 . (6) t 1 0 2 1

101 Two-habitat simulations compared juvenile crab abundance in vegetated habitat and unvegetated habitat and total population between Simulation 1 and each modified simulation (Simulations 2 – 4; Table 18). Total crab abundance predicted by Simulation 1 was approximately 21 million crabs, of which 98% resided in vegetated habitat and 2% in unvegetated habitat after 30 days. Reducing vegetated habitat by 50% in Simulation 2 resulted in a 36% reduction in total crab abundance with a 38% reduction in vegetated habitat. In Simulation 3, increasing vegetated habitat by 50% increased total juvenile blue crab abundance by 29% with a 30% increase in vegetated habitat. Vegetation at historic seagrass levels, modeled in Simulation 4, increased total abundance by 37% and abundance in vegetated habitat by 38%. In each scenario, abundance in unvegetated habitat increased initially as crabs emigrated from vegetated habitat (Figure 17), and this pattern was most pronounced when vegetated habitat was reduced (Simulation 2).

Sensitivity analysis

For both the two- and three-habitat models, total abundance was most sensitive to

changes in survival in vegetated habitat ( sv ) and seagrass ( ss), respectively, and

emigration survival rate ( sd ) (Table 19, 20). Neither model was sensitive to changes in survival rate in unvegetated habitat ( su ).

102 DISCUSSION

This study is unique in modeling density-dependent migration of juvenile crabs to different habitats. Matrix models incorporating density-dependence have been developed for bivalves like sea scallops (Placopecten gellanicus) (Barbeau and Caswell 1999) and the clams Spisula ovalis (David et al. 1997) and Mesodesma mactroides (Lima et al.

2000). Past models of juvenile blue crab population dynamics and behavior have focused primarily on stage-based growth (Miller 2001, 2003) and megalopal settlement patterns

(Stockhausen and Lipcius 2003). Post-settlement behavior has not been modeled explicitly and habitat preference has only rarely been incorporated prior to this study

(Stockhausen and Lipcius 2003). While this model assumes no change in survival over time, there is considerable variability in juvenile survival rates depending on habitat type, crab instar stage, and environmental factors. Although this model is limited by the exclusion of instar stage-based differences in survival due to time constraints, it provides a starting point, which future studies may build upon to include stage-specific survival rates; this would require the addition of growth information.

Model simplification

For simplicity, several factors that would have complicated the model were excluded, including: 1) multiple inputs of juvenile blue crabs; 2) instar stage-based survival rates; 3) juvenile crab growth rates; and 4) explicit spatial information, such as configuration of the habitats, habitat quality variability, and physical factors that

103 influence initial settlement locations. Inputing only one large pulse of crabs into the model allowed for better visualization of habitat use patterns that would be obscured with additional pulses and reduced the need to use multiple survival rates for different cohorts at various developmental stages. Had instar stage-based survival rates been included, a more complex model incorporating growth would have been required. Since growth rates are similar among the three habitats investigated in this model (Chapter 4) and the simulations were run over a relatively short time period, incorporating juvenile growth was determine to be unnecessary. Explicit spatial information, including the arrangement of habitats, variability in habitat quality, and physical factors like currents that impact initial settlement location, was excluded because it was outside of the scope of the study, which focused on density-dependent early secondary dispersal rather than mechanisms of megalopal settlement.

Habitat changes and implications for juvenile blue crabs

These simulations suggest that habitat loss from historic levels has caused a 22 –

37% decline in total juvenile blue crab abundance in the York River, depending on whether vegetated habitats were considered separately or together. This decline in abundance is comparable to the estimated reduction in blue crab megalopal settlement and recruitment to the first juvenile stage (10 – 25%) in seagrass due to habitat loss from historic levels in the York River (Stockhausen and Lipcius 2003). Increases in Gracilaria area were shown to have positive impacts on crab population size but could not fully

104 mediate the continued loss of seagrass. If this alga continues to spread and seagrass remains stable, juvenile blue crabs will benefit.

Seagrasses in Chesapeake Bay may not remain stable if anthropogenic effects continue to impact them. Eelgrass, Zostera marina, is more susceptible to stress from disturbance and eutrophication in Chesapeake Bay, because it is at the southernmost edge of its range in the Bay (Green and Short 2003). It seems likely that seagrass declines in

Chesapeake Bay will continue due to increased water temperatures which can cause mass mortality events, specifically in eelgrass (Najjar et al. 2009, 2010). Gracilaria will likely continue to spread, because increased temperatures and nutrients increase algal growth

(Pedersen and Borum 1996, Valiela et al. 1997, McGlathery 2001). In the York River, algal biomass increased 41% from 2013 to 2014 (Chapter 2), indicating that substantial fluctuations in algal cover are possible over a short time period. Additionally, the range of Gracilaria expanded approximately 150 km in Sweden over two years (Nyberg et al.

2009), and Gracilaria biomass tripled over two years in the Baltic Sea (Weinberger et al.

2008), giving further evidence that this alga is an efficient colonizer and giving rise to the possibility that it could expand its extent in Chesapeake Bay. However, Gracilaria fragments and mats often tangle in seagrass beds, and algal cover increases with seagrass presence (Chapter 2). Therefore, any additional loss of seagrass will likely have a negative impact on the spread of Gracilaria in Chesapeake Bay, which limits the ability of the alga to replace seagrass as a nursery habitat for blue crabs.

Although the model presented here assumes no changes in survival over time, the particular nursery habitat a juvenile crab uses can have a significant impact on the probability that it will survive to recruit to the fishery. Future work may expand this

105 model to include survival rates for each developmental stage, growth information, a density-independent emigration behavior function for larger juvenile crabs (J7 – J9), and observed megalopal settlement estimates that would allow for more realistic and biologically meaningful results. Better understanding the behavior of juvenile blue crabs and how they interact with ecologically sensitive habitats will ultimately lead to a better understanding of the blue crab population in Chesapeake Bay as a whole and lead to better, ecosystem-based management of this commercially important species.

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111 TABLES

Table 14. Table of parameters used in the simulations with Equations (1), (2), and (4).

Parameter Definition Units Value Reference†

-1 ss daily survival rate in seagrass day 0.91 1 -1 sg daily survival rate in Gracilaria day 0.93 1, 2

-1 su daily survival rate in unvegetated day 0.81 1

-1 sv daily survival rate in vegetated day 0.92

ks seagrass carrying capacity ind. 19.4 As 3

kg Gracilaria carrying capacity ind. 12.9 Ag 3

ku unvegetated carrying capacity ind. 0.1 Au 4

kv vegetated carrying capacity ind. 16.2 Av

-1 sd daily survival rate of emigrants day 0.81 †References: 1(Schulman 1996), 2 (Johnston and Lipcius 2012), 3 (Chapter 3), 4 (Penry 1982)

112 1 Table 15. Model scenarios and parameter values used with the three-habitat model (Equations (1) and (2)). 2

Simulation Scenario Ns,1 Ng,1 As Ag Au ps pg pu ind. ind. m2 m2 m2 1 Current seagrass & Gracilaria 832,676,982 167,323,018 3,122,178 943,515 68,279,448 0.761 0.153 0.086 2 Seagrass decline 713,321,822 286,678,178 1,561,089 943,515 69,840,537 0.613 0.246 0.141 Seagrass decline & Gracilaria 3 623,893,496 376,106,504 1,561,089 1,415,272 69,368,780 0.546 0.329 0.125 spread 4 Historic seagrass 1,000,000,000 0 6,715,000 0 65,630,141 0.952 0 0.048 Current seagrass & Gracilaria 5 1,000,000,000 0 3,122,178 0 69,222,936 0.897 0 0.103 absent Current seagrass & Gracilaria 6 768,392,136 231,607,864 3,122,178 1,415,272 67,807,691 0.708 0.213 0.079 spread Seagrass decline & Gracilaria 7 483,830,333 516,169,667 1,561,089 2,504,604 68,279,448 0.436 0.465 0.098 replaces 3

113 1 Table 16. Table of model scenarios and parameter values used with the two-habitat model 2 (Equation (4)). 3

Simulation Scenario Nv,1 Av Au pv pu ind. m2 m2 1 Current vegetation 1,000,000,000 4,065,693 68,279,448 0.906 0.094 2 Vegetation decline 1,000,000,000 2,032,847 70,312,294 0.824 0.176 3 Vegetation spread 1,000,000,000 6,098,540 66,246,601 937 0.063 4 Historic vegetation 1,000,000,000 6,715,000 65,630,141 0.943 0.057 4 5

114 1 Table 17. Simulation results for the three-habitat model, where values are the number of 2 individuals (millions) remaining at time t = 31 with percent change from simulation 1 in 3 parentheses. 4 Simulation Seagrass Gracilaria Unvegetated Total 1 15.039 4.185 6.824 26.049 2 8.616 4.687 6.959 20.262 (-42.7) (12.0) (2.0) (-22.2) 3 8.502 6.941 6.919 22.362 (-43.5) (65.8) (1.4) (-14.1) 25.371 0 6.551 31.922 4 (68.7) (-100) (-4.0) (22.5) 15.644 0 6.902 22.546 5 (4.0) (-100) (1.1) (-13.4) 14.732 6.19 6.769 27.691 6 (-2.0) (47.9) (-0.8) (6.3) 8.081 11.803 6.822 26.706 7 (-46.3) (182.0) (0.0) (2.5) 5

115 1 Table 18. Simulation results for the two-habitat model, where values are the number of 2 individuals (millions) remaining at time t = 31 with percent change from simulation 1 in 3 parentheses. 4 Simulation Vegetated Unvegetated Total 1 20.298 0.311 20.608 2 12.542 0.55 13.092 (-38.2) (77.0) (-36.5) 3 26.298 0.21 26.608 (30.1) (-32.3) (29.1) 28.048 0.19 28.238 4 (38.1) (-38.8) (37.0) 5 6

116 1 Table 19. Table of model sensitivity analysis for the three-habitat model (Equation (4)), 2 where values are the number of individuals (millions) remaining at time t = 31 with 3 percent change from the standard run in parentheses. 4 Parameter Sensitivity Seagrass Gracilaria Unvegetated Total 93% 3.658 (-75.7) 3.643 (-13.0) 6.824 (0.0) 14.125 (-45.8) 95% 5.593 (-62.8) 3.831 (-8.5) 6.809 (-0.2) 16.234 (-37.7) ss 105% 34.766 (131.2) 4.920 (17.6) 6.817 (-0.1) 46.503 (78.5) 107% 45.896 (205.2) 5.439 (30.0) 6.823 (0.0) 58.158 (123.3) 93% 14.420 (-4.1) 1.290 (-69.2) 6.806 (-0.3) 22.516 (-13.6) 95% 14.594 (-3.0) 1.833 (-56.2) 6.801 (-0.3) 23.228 (-10.8) sg 105% 15.575 (3.6) 8.772 (109.6) 6.828 (0.0) 31.174 (19.7) 107% 15.850 (5.4) 11.428 (173.1) 6.828 (0.1) 34.106 (30.9) 93% 15.039 (0.0) 4.185 (0.0) 6.824 (0.0) 26.049 (0.0) 95% 15.039 (0.0) 4.185 (0.0) 6.824 (0.0) 26.049 (0.0) su 105% 15.039 (0.0) 4.185 (0.0) 6.824 (0.0) 26.049 (0.0) 107% 15.039 (0.0) 4.185 (0.0) 6.824 (0.0) 26.049 (0.0) 93% 11.393 (-24.2) 3.384 (-19.1) 6.823 (-0.0) 21.599 (-17.1) 95% 12.220 (-18.7) 3.568 (-14.7) 6.811 (-0.2) 22.599 (-13.2) sd 105% 20.023 (33.1) 5.220 (24.7) 6.823 (0.0) 32.066 (23.1) 107% 23.183 (54.2) 5.907 (41.1) 6.803 (-0.3) 35.893 (37.8) 5 6

117 1 Table 20. Table of model sensitivity analysis for the two-habitat model (Equation (4)), 2 where values are the number of individuals (millions) remaining at time t = 31 with 3 percent change from the standard run in parentheses 4 Parameter Sensitivity Vegetated Unvegetated Total 93% 5.054 (-75.1) 0.207 (-33.2) 5.261 (-74.5)

95% 7.733 (-61.9) 0.226 (-27.1) 7.969 (-61.4) sv 105% 45.222 (122.8) 0.504 (62.4) 45.726 (121.9) 107% 58.008 (185.8) 0.701 (125.9) 58.709 (184.9)

93% 19.978 (-1.6) 0.096 (-69.2) 0.074 (-2.6) 95% 20.070 (-1.1) 0.135 (-56.5) 20.205 (-2.0) su 105% 20.572 (1.4) 0.664 (114.0) 21.236 (3.0) 107% 20.672 (1.8) 0.891 (186.8) 21.563 (4.6) 93% 15.578 (-23.3) 0.891 (186.8) 15.738 (-23.6)

95% 16.618 (-18.1) 0.188 (-39.4) 16.806 (-18.4) sd 105% 26.816 (32.1) 0.628 (102.2) 27.444 (33.2)

107% 31.087 (53.2) 0.905 (191.4) 31.992 (55.2) 5 6

118 1 FIGURES

2 3

4 5 6 Figure 14. Schematic of the blue crab habitat model, where N is the initial abundance of 7 juveniles placed in a particular habitat at time t = 0. 8

119 1 2 3 Figure 15. Values of the dispersal function.

120 1 2 Figure 16. Results of simulations of the three-habitat model with the number of juvenile 3 blue crabs (in millions) remaining in seagrass (solid), Gracilaria (dashed), and 4 unvegetated (dotted) habitat after t = 30 days.

121 1 2 3 Figure 17. Results of simulations of the two-habitat model with the number of juvenile 4 blue crabs (in millions) remaining in vegetated (solid) and unvegetated (dotted) habitat 5 after t = 30 days.

122

CHAPTER 6

Summary and Conclusions

123 Exotic species are often exclusively evaluated for negative impacts on native species, communities, and ecosystems. Few studies focus on potential benefits that exotic species can provide. Habitat modifiers like macroalgae can have broad ecological impacts, especially in degraded systems, such as Chesapeake Bay. My research provides evidence that Gracilaria vermiculophylla is an emerging ecosystem in Chesapeake Bay and acts as valuable nursery habitat for the ecologically and economically important blue crab Callinectes sapidus (Chapter 2, 3, 4). This dissertation also is the first attempt to quantify the shallow bottom habitat area of an exotic species to use as a predictor of density-dependent juvenile blue crab migration (Chapter 5).

Gracilaria vermiculophylla may ameliorate the loss of seagrass as a nursery habitat for juvenile blue crabs and other organisms in Chesapeake Bay. A number of studies on G. vermiculophylla distribution outside of its native range have concluded that salinity has no impact on the ability of the alga to invade (Thomsen et al. 2007,

Weinberger et al. 2008, Sfriso et al. 2012). My research indicated that G. vermiculophylla presence in the York River was correlated with river region, which was linked to salinity as well as other characteristics like sediment type and turbidity. The alga was 53% more likely to be present downriver than midriver. Because of seagrass loss in lower salinity areas in Chesapeake Bay, seagrass is also most abundant in high salinity areas and interacts with G. vermiculophylla. Seagrass presence doubled the likelihood that G. vermiculophylla would also be present. The alga is the dominant subtidal vegetation present midriver, where seagrasses have been extirpated, providing some structural refuge in an area where megalopal settlement rates can be high (Stockhausen and Lipcius

2003). I examined prey community composition and found that epifaunal communities

124 were dissimilar between G. vermiculophylla and seagrass, but infaunal communities in the two habitats were relatively similar. Gracilaria vermiculophylla supported fewer epifaunal prey animals than seagrass, indicating that blue crabs may face greater food resource competition in the alga than in seagrass. Juvenile blue crab densities were similar between native seagrass and the alga; however, smaller (J1 and J2) crabs preferred seagrass. Juvenile blue crab growth was similar in G. vermiculophylla, seagrass, and unvegetated habitat (p = 0.746), indicating that survival, not growth, drives ontogenetic shifts in habitat use by juvenile crabs when they reach 25 – 30 mm CW.

Similar growth rates in the alga and native seagrass illustrate that G. vermiculophylla has the capacity to act as a nursery habitat for juvenile blue crabs by hosting suitable prey abundance. Therefore, the alga likely acts as a secondary dispersal site for juvenile blue crabs in downriver areas that are experiencing density-dependent effects in seagrass and as a primary nursery habitat midriver where G. vermiculophylla is the only subtidal vegetation present.

Nursery habitat availability is important for the distribution and abundance of juvenile blue crabs. Previous studies modeling juvenile crab settlement and recruitment focus solely on changes to seagrass habitat (Stockhausen and Lipcius 2003). My study followed crab population dynamics shaped by density-dependent early secondary dispersal and changes in seagrass, G. vermiculophylla, and unvegetated habitat in a number of scenarios, including historic, current, and potential future conditions. Increases in G. vermiculophylla area had positive impacts on the crab population but did not fully mediate any continued loss of seagrass habitat.

125 The exotic macroalga Gracilaria vermiculophylla is providing valuable nursery habitat to juvenile blue crabs in the York River and Chesapeake Bay. It acts as a superior secondary nursery to unvegetated habitat when young juvenile blue crabs emigrate from seagrass beds due to high densities by increasing survival of juveniles (Johnston and

Lipcius 2012) and maintaining growth. Future work should track G. vermiculophylla cover across Chesapeake Bay to better determine the alga’s impact on juvenile blue crabs

Baywide and in other physiographic settings to better understand how salinity and other factors impact G. vermiculophylla abundance. Additionally, the model should be linked to a hydrodynamic model to make it spatially explicit to better understand megalopal settlement patterns in all habitats, including macroalgae. This, along with better estimates of megalopal influx into the Bay and juvenile population size, will contribute greatly to the management of the blue crab in Chesapeake Bay.

LITERATURE CITED

Johnston, C. A., and R. N. Lipcius. 2012. Exotic macroalga Gracilaria vermiculophylla provides superior nursery habitat for native blue crab in Chesapeake Bay. Marine Ecology Progress Series 467: 137–146. Sfriso, A., M. A. Wolf, C. Andreoli, K. Sciuto, and I. Moro. 2012. Spreading and autoecology of the invasive species Gracilaria vermiculophylla (Gracilariales, Rhodophyta) in the lagoons of the north-western Adriatic Sea (Mediterranean Sea, Italy). Estuarine, Coastal and Shelf Science 114: 192–198. Stockhausen, W. T., and R. N. Lipcius. 2003. Simulated effects of seagrass loss and restoration on settlement and recruitment of blue crab postlarvae and juveniles in the York River, Chesapeake Bay. Bulletin of Marine Science 72: 409–422. Thomsen, M. S., P. A. Stæhr, C. D. Nyberg, S. Schwærter, D. Krause-Jensen, and B. R. Silliman. 2007. Gracilaria vermiculophylla (Ohmi) Papenfuss, 1967 (Rhodophyta, Gracilariaceae) in northern Europe, with emphasis on Danish conditions, and what to expect in the future. Aquatic Invasions 2: 83–94. Weinberger, F., B. Buchholz, R. Karez, and M. Wahl. 2008. The invasive red alga Gracilaria vermiculophylla in the Baltic Sea: adaptation to brackish water may compensate for light limitation. Aquatic Biology 3: 251–264.

126 VITA

Megan A. Wood

Born in Hampton, Virginia, March 30, 1987. Graduated from Poquoson High School in Poquoson, Virginia. Earned a B.S. in Biology with a concentration in Environmental and Biological Conservation from the University of Virginia in 2009. Entered the M.S. program at the College of William and Mary, School of Marine Science in 2010. Successfully by-passed the M.S. degree, entering the doctoral program in August 2012.

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