Interactions between Thalassia testudinum Banks ex Koenig and Halimeda incrassata (Ellis) Lamouroux and Their Effects on...

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Interactions between Thalassia testudinum Banks ex Koenig and Halimeda incrassata (Ellis) Lamouroux and Their Effects on Carbon Dynamics in a Shallow, Tropical Lagoon
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english
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Barry, Savanna C
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University of Florida
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Degree:
Master's ( M.S.)
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University of Florida
Degree Disciplines:
Fisheries and Aquatic Sciences, Forest Resources and Conservation
Committee Chair:
Frazer, Tom K
Committee Members:
Jacoby, Charles A
Phlips, Edward J

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Subjects / Keywords:
acidification -- calcareous -- calcification -- caribbean -- macroalgae -- ocean -- production -- seagrass
Forest Resources and Conservation -- Dissertations, Academic -- UF
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Fisheries and Aquatic Sciences thesis, M.S.
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Abstract:
Ocean acidification poses a serious threat to a broad suite of calcifying organisms. Scleractinian corals and calcareous algae that occupy shallow, tropical waters around the globe are vulnerable to global changes in ocean chemistry because they already are subject to stressful and variable carbonate dynamics at the local scale. For example, net heterotrophy increases carbon dioxide concentrations, and pH varies with diurnal fluctuations in photosynthesis and respiration. Few researchers, however, have investigated the possibility that CO2 consumption during photosynthesis by non-calcifying photoautotrophs, such as seagrasses, can ameliorate deleterious effects of ocean acidification on sympatric calcareous algae. Naturally occurring variations in the density of seagrasses and associated calcareous algae provide an ecologically relevant test of the hypothesis that diel fluctuations in water chemistry driven by cycles of photosynthesis and respiration within seagrass beds create microenvironments that enhance macroalgal calcification. In Grape Tree Bay off Little Cayman Island BWI, we quantified net production and characterized calcification for thalli of the calcareous green alga Halimeda incrassata growing in beds of Thalassia testudinum with varying shoot densities. Results indicated that individual H. incrassata thalli were ~6% more calcified in dense seagrass beds. On an areal basis, however, far more CaCO3 was produced by H. incrassata in areas where seagrasses were less dense due to higher rates of production. In addition, diel pH regimes in vegetated and unvegetated areas were not significantly different, suggesting a high degree of water exchange and mixing throughout the lagoon. These results suggest that, especially in well-mixed lagoons, biotic interactions, especially competition, are likely to control carbon dynamics of calcareous algae.
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Statement of Responsibility:
by Savanna C Barry.
Thesis:
Thesis (M.S.)--University of Florida, 2012.
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Adviser: Frazer, Tom K.
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RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2013-02-28

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1 INTERACTIONS BETWEEN THALASSIA TESTUDINUM BANKS EX KOENIG AND HALIMEDA INCRASSATA (ELLIS) LAMOUROUX AND THEIR EFFECTS ON CARBON DYNAMICS IN A SHALLOW, TROPICAL LAGOON By SAVANNA C. BARRY A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DE GREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2012

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2 2012 Savanna C. Barry

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3 To my mom and dad

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4 ACKNOWLEDGMENTS I thank my parents for always being supportive and helping me believe in my abilities and I thank my grandmother for inspiring me to be a stronger woman. I also thank all of the University of Virginia graduate students that inspired me to pursue graduate e ducation. I appreciate logistical support provided by Morgan Edwards, Neil van Niekerk, Sky Notestein, Darlene Saindon, and Jessica Frost. I thank the staff of the Little Cayman Research Center for additional logistical support du ring the course of the study. I am grateful to the Cayman Islands Department of Environment and Marine Conservation Board for making this work possible. I also thank my advisory committee for guiding me through this experience and fostering my growth as a scientist.

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5 TABLE OF C ONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 6 LIST OF FIGURES ................................ ................................ ................................ .......... 7 LIST OF ABBREVIATIONS ................................ ................................ ............................. 8 ABSTRACT ................................ ................................ ................................ ..................... 9 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 11 2 METHODS AND MATERIALS ................................ ................................ ................ 15 Study Site ................................ ................................ ................................ ............... 15 Densities of Macrophytes and Site Selection ................................ .......................... 15 Water Quality Measurements ................................ ................................ ................. 16 Field Procedures and Laboratory Processing ................................ ......................... 16 Metrics of Production and Calcification ................................ ................................ ... 18 Statistical Analyses ................................ ................................ ................................ 19 3 RESULTS ................................ ................................ ................................ ............... 24 Densities of Macrophytes and Site Selection ................................ .......................... 24 Water Chemistry ................................ ................................ ................................ ..... 24 Rates of Production ................................ ................................ ................................ 25 Calcification ................................ ................................ ................................ ............ 27 4 DISCUSSION ................................ ................................ ................................ ......... 38 LIST OF REFERENCES ................................ ................................ ............................... 42 BIOGRAPHICAL SKETCH ................................ ................................ ............................ 47

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6 LIST OF TABLES Table page 3 1 Results of PERMANOVA analyses based on environmental data. ..................... 30 3 2 Results of ANOVAs based on rates of production for Thalassia testudinum an d Halimeda incrassata ................................ ................................ ................... 32 3 3 Back transformed mean rates of production for Thalassia testudinum shoots and Halimeda incrassata thal li ................................ ................................ ............ 32 3 4 Results of ANOVA based on rates of production for Halimeda incrassata standardized to initial sizes of thalli ................................ ................................ .... 34 3 5 Mean rates of production for Halimeda incrassata standardized to initial sizes of thalli ................................ ................................ ................................ ................ 34 3 6 Results of ANOVAs based on rates of areal production for Thalassia testudinum and Halimeda incrassata ................................ ................................ 34 3 7 Mean areal rates of production for Thalassia testudinum and Halimeda incrassata standardized to initial sizes of thalli ................................ ................... 35 3 8 Results of ANOVAs based ratios of calcium carbonate content to organic matter in thalli of Halimeda incrassata ................................ ................................ 37 3 9 Mean and back transformed mean ratios of calcium carbonate content to organic matter in thalli of Halimeda incrassata ................................ .................. 37 3 10 Results of an ANOVA based proportions of calcium carbonate in thalli of Halimeda incrassata ................................ ................................ ........................... 37 3 11 Back transformed mean proportions of calcium carbonate in thalli of Halimeda incrassata ................................ ................................ ........................... 37

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7 LIST OF FIGURES Figure page 2 1 Location of Little Cayman Island and Grape Tree Bay. ................................ ...... 21 2 2 Halimeda incrassata morphological characteristics. ................................ ........... 22 2 3 Regressions for inorganic and organic content versus total dry weight of Halimeda incrassata as determined by acidification and ashing. ........................ 23 3 1 Plot of Halimeda incrassata thalli density against Thalassia testudinum shoot density for the sites chosen to represent treatments ................................ .......... 29 3 2 Mean dissolved oxygen concentrations (mg L 1 ) among treatments on a 24 hour basis. ................................ ................................ ................................ .......... 30 3 3 Mean pH levels by treatment ................................ ................................ .............. 31 3 4 Back transformed mean rates of production ................................ ...................... 33 3 5 Mean areal rates of production. ................................ ................................ .......... 36

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8 LIST OF ABBREVIATION S A D Anderson Darling test for normality B F Brown Forsythe test for homoscedacity CaCO 3 Calcium c arbonate CL Confidence l imit CO 2 Carbon d ioxide CO 3 2 Carbonate i on DO Dissolved o xygen DW Dry w eight H + Hydrogen i on HHLT High Halimeda Low Thalassia LHHT Low Halimeda High Thalassia MHMT Medium Halimeda Medium Thalassia OA Ocean a cidification OM Organic m atter SD Standard d eviation

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9 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science INTERACTIONS BETWEEN THALASSIA TESTUDINUM BANKS EX KOENIG AND HALIMEDA INCRASSATA (ELLIS) LAMOUROUX AND THEIR EFFECTS ON CARBON DYNAMICS IN A SHALLOW, TROPICAL LAGOON By Savanna C. Barry August 2012 Chair: Thomas K. Frazer Major: Fisheries and Aquatic Science s Ocean acidification poses a serious threat to a broad suite of calcifying organisms. Scleractinian corals and calcareous algae that occupy shallow, tropical waters around the globe are vulnerable to global changes in ocean chemistry because they already ar e subject to stressful and variable carbonate dynamics at the local scale. For example, net heterotrophy increases carbon dioxide concentrations, and pH varies with diurnal fluctuations in photosynthesis and respiration. Few researchers, however, have inve stigated the possibility that carbon dioxide consumption during photosynthesis by non calcifying photoautotrophs, such as seagrasses, can ameliorate deleterious effects of ocean acidification on sympatric calcareous algae. Naturally occurring variations in the density of seagrasses and associated calcareous algae provide an ecologically relevant test of the hypothesis that diel fluctuations in water chemistry driven by cycles of photosynthesis and respiration within seagrass beds create microenvironments th at enhance macroalgal calcification. In Grape Tree Bay off Little Cayman Island BWI, we quantified net production and characterized calcification for thalli of the calcareous green alga Halimeda incrassata growing in beds of Thalassia testudinum with varying

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10 shoot densities. Results indicated that individual H incrassata thalli were ~6% more calcified in dense seagrass beds. On an areal basis, however, far more calcium carbonate was produced by H incrassata in areas where seagrasses were less d ense due to higher rates of production. In addition, diel pH regimes in vegetated and unvegetated areas were not significantly different, suggesting a high degree of water exchange and mixing throughout the lagoon. These results suggest that, especially in well mixed lagoons, carbonate production by calcareous algae may be more related to biotic interactions between seagrasses and calcareous algae than to seagrass mediated changes in local water chemistry.

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11 CHAPTER 1 INTRODUCTION oceans generated by a global rise in carbon dioxide ( CO 2 ) emissions represents a major ecological concern (Skirrow and Whitfield, 1975; Kleypas et al., 1999) As atmospheric CO 2 is absorbed by seawater, it alters the carbonate cycle primarily leading to l ower concentrations of carbonate ions (CO 3 2 ) and higher concentrations of hydrogen ions (H + ), which translates into lower pH values (McClendon, 1917, 1918; Guinotte and Fabry, 2008; Doney et al., 2012). These changes in concentrations of CO 3 2 and H + pote ntially interfere with sequestration of calcium carbonate (CaCO 3 ) by a broad suite of marine organisms that use this compound to form skeletons, shells, otoliths, statoliths and other key structures (Kleypas et al., 1999; Hoegh Guldburg et al., 2007; Kroek er et al., 2009). In shallow tropical waters, c orals and calcareous algae two key ecosystem components that sequester CaCO 3 are particularly vulnerable to an altered equilibrium because they already are subjected to other stresses mediated by a varying carbonate cycle. For example, diurnal fluctuations in seawater chemistry are driven by the relative intensities of photosynthesis and respiration and heterotrophic conditions dominate, which means release of CO 2 through remineraliza tion of organic matter exceeds consumption of CO 2 during photosynthe sis (Andersson and Mackenzie, 2012). Other shallow water organisms, however, are expected to be less affected and some may, in fact, benefit from changes in ocean chemistry driven by incre ased atmospheric CO 2 Seagrasses, for example, are likely to experience an increase in production especially in those areas where dissolved inorganic carbon is presently limiting (e.g., Palacios and Zimmerman, 2007). Moreover, seagrasses which consume CO 2 during photosynthesis,

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12 may serve to mediate and possibly ameliorate the deleterious effects of ocean acidification on a large number of sympatric species (Semesi et al., 2009a, 2009b; Kleypas et al., 2011) In shallow water, including tropical seas seag rass beds represent a predominant source of production and serve also as a key structural habitat ( Duarte et al., 2010 ). In fact, s eagrass beds are among the most prod uctive habitats on the planet, with g lobal estimates of seagrass production on the order of 21 101 Tg C y 1 depending on estimates of seagrass areal coverage ( Duarte et al., 2010 ). Thus, seagrasses are more productive than North American wetlands and undisturbed Amazonian rainforest ( Duarte et al., 2010 ). Seagrasses can exist as extensive beds or a complex mosaic of patches and they provide both refuge and a habitat for forag ing used by myriad species including a large number of commercially and recreationally important finfishes (Orth et al., 1984; Thayer et al., 1984; Virnstein and Howard, 1987). In addition seagrasses sequester carbon, stabilize bottom sediments, dampen wave action reduce turbulence, increas e water clarity and reduc e shoreline erosion ( Duarte 1995 ; Fourqurean et al. 2012 ). In the tropics, seagrasses often coexist with other ecologically important photoautotrophs (Littler and Littler, 1988, 1994; Dahlgren and Marr, 2004; Fong and Paul, 2011). A mong the co occurring species representatives of the phylum Chloro phyta, i.e. green algae are common (Littler, 1976; Littler and Littler, 1988, 1994) Calcareous green algae such as Halimeda spp. and Penicillus spp. are among the most cosmopolitan and well studied species because they perform a number of important ecol ogical functions. For example, photosynthetic production by Halimeda incrassata in

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13 Florida Bay was 20% of that attributed to the dominant seagrass, Thalassia testudinum (Davis and Fourqurean, 2001). In addition, several species of Halimeda serve as a key f ood source and are, in fact, a preferred food of several coral reef fishes (Overholtzer and Motta 1999; Mantyka & Bellwood 2007). More importantly, however, Halimeda spp. and other calcareous algae play critical roles in the formation of CaCO 3 (Vroom et al. 2003; Nelson, 2009). In the Bahamas, for example, rates of CaCO 3 production by Halimeda spp. are nearly equal to rates estimated for coral reefs (Milliman & Droxler, 1996). In fact, green calcifying algae can account for 35 40% of the carbonates generated in shallow, tropical marine waters, with corals and red, coralline algae account ing for an additional 50 55% (Lee and Carpenter, 2001). Furthermore, Halimeda spp. are known to be important producers of coarse grained sedimen ts in the Bahamas and elsewhere (Freile et al., 1995) and spalling of calcified plates is a primary mechanism by which sand is formed in tropical seas (Littler, 1976; Littler and Littler, 1988, 1994). Seagrasses and calcified algae are known to compete fo r nutrients in oligotrophic waters, with c ompetition reported to favor seagrasses (see Davis and Fourqurean 2001) F ew investigators however, have studied potential positive interactions where by seagrasses might promote the existence and co occurrence of key calcareous algae by raising pH, which alters the stoichiometry of calcification by enhancing release of hydrogen ions formed as byproducts (Semesi et al. 2009a, 2009b). In this scenario, diel fluctuations in water chemistry within seagrass beds drive n by cycles of photosynthesis and respiration are hypothesized to create microenvironments that are conducive to the calcification of algae ( Semesi et al., 2009a, 2009b ). Sites with varying densities of both

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14 seagras ses and rhizophyt ic calcareous algae pro vide opportu nities to test this hypothesis in situ In this study, sites with varying densities of T testudinum and H incrassata were identified in Grape Tree Bay off Little Cayman Island. At these sites, production for T testudinum and production and calcification for H incrassata were measured to determine if significant interactions existed. Measures of key water quality parameters provided data to assess potential causes of variations in production or calcification. Thus, the ca lcification rate of H. incrassata in dense seagrass was hypothesized to be increased relative to sites with less seagrass due to a favorable pH regime.

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15 CHAPTER 2 METHODS AND MATERIAL S Study S ite Grape Tree Bay is a shallow lagoon on the n orth coast of Lit tle Cayman Island, BWI (Figure 2 1) A mixed seagrass and calcareous alga l assemblage extends offshore for approximately 60 100 m where it is bounded by a fringing reef. The fringing reef delineates the seaward edge of Grape Tree Bay, which spans approxima tely 1.6 km of shoreline. Based on data from a National Oceanic and Atmospheric Administration Integrated Coral Reef Observation Network (ICON) station located just outside the fringing reef, ocean temperature ranged from 28.0 C to 30.6 C and salinity av eraged Densities of M acrophytes and S ite S election To select sites with differing densities of T testudinum and H incrassata benthic vegetation in Grape Tree Bay was surveyed within a systematic grid. Forty points along the shoreline were marked with GPS waypoints, and these points, which were separated by 10 m, served as the origins for transects that ran offshore to the fringing reef Along each transect a 0.25 m 2 quadrat was plac ed at the 10 m mark and also at every successive 10 m mark. Thus, 6 10 quadrats were sampled along each transect depending on the distance between the shoreline and the fringing reef. Within each quadrat, t hall i of all algal taxa were counted Subsequently a 0.0625 m 2 sub quadrat was thrown within each 0.25 m 2 quadrat. Within each subquadrat shoot s of all seagrass species were counted separately S ites for measurements of production for T testudinum and production and calcification for H incrassata were selected by comparing densities of shoots and thalli

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16 The goal was to identify sets of three replicate experimental sites that spanned the natural gradient in Grape Tree Bay Three levels, termed treatments, were targeted, i.e., i) l ow density Halimeda com bined with h igh density Thalassia ( LHHT ), ii) m edium density Halimeda and Thalassia ( MHMT ), and iii) h igh density Halimeda combined with l ow density Thalassia ( HHLT). At each experimental site shoot and thalli counts were repeated to verify densit ies were appropriate. Water Q uality M easurements During the course of the field experiment, a YSI 600R data sonde with a YSI 650 MDS data logger was deployed at each of the nine treatment sites and at an additional three unvegetated sites for at least 24 h. Temper ature, salinity, dissolved oxygen, and pH were recorded at 30 min intervals throughout each 24 h period. Measurements were taken at a height of approximately 5 cm above the sediment to document conditions within the seagrass can opy, when seagrass was present. Field P rocedures and L aboratory P rocessing Two methods were used to measure production for T testudinum and H incrassata Thalassia testudinum production was measured by the leaf marking technique (Zieman, 1974) with a needle forced through all blades in a shoot just above their basal meristems and marked shoots allowed to grow in situ for 7 d Halimeda incrassata production was measured using incorporation of Alizarin S dye ( Figure 2 2; Wefer, 1980; Multer, 1988; Davis & Fourqurean, 2001; Vroom et al., 2003) The dye stained existing tissue red which allowed new (unstained) algal biomass to be distinguished from tissue present at the start of the 7 d in situ growth period. When possible, 40 shoots and 25 thalli were marked within each experime ntal site; however, some sites did not contain 25 thalli so a ll available thalli were marked.

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17 After 7 d, marked macrophytes were harvested. Halimeda incrassata thalli were harvested by removing th eir basal holdfast s from the sediment. Thalassia testudinum shoots were harvested at the node where the short shoot meets the rhizome so that the entire sheath was retained. Individual algal thalli and seagrass shoots were placed in separate, labeled bags and frozen until processing. Individual T testudinum shoot s were rinsed in freshwater, scraped with a razor blade to remove epiphytic material and briefly rinsed in fresh water again For each blade with a hole, new and old growth w ere separated by cutting through the hole with a razor blade Unmarked blades were considered new growth. O ld and new material s were placed in separate borosilicate glass vial s and dried at 50 C to a constant weight. Individual Halimeda incrassata thalli were rinsed with freshwater using the focused stream from a wash bottle to remove sand, debris, and epiphytic material. New or unstained plates and old stained plates were separated, counted, placed into tared borosilicate glass vials and dried at 50 C to a constant weight. In addition to production, calcific ation was quantified for H incrassata to evaluate the influence of biogeochemical effects mediated by T testudinum Two methods were available to differentiate organic and inorganic content of H incrassata : acidification and ashing. Three consecutive exposures of dried algal tissue to 5% hydrochloric acid remove d the inorganic (CaCO 3 ) fraction of plates ( Vroom et al., 2003 ). This method relied on acid penetrat ing all of the tissue which could be problematic f or large samples (S. Barr y, pers. obs ). The alternative method subjected algal tissue to 500 C for 3 h in a muffle furnace to remove the organic fraction of plates ( Davis and Fourqurean, 2001 ) To test the methods a known number of H incrassata plates with a known dry weight

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18 w ere ash ed and a similar set of plates were acidifi ed Dry weights of the resulting material yielded estimates of both organic and inorganic carbon for each set of samples. Analyses of covariance (ANCOVAs) tested for significant relationships between weight s of organic matter and CaCO 3 fractions and total dry weights, with method being a covariate. The ANCOVAs indicated a significant difference between the methods for both organic matter and CaCO 3 fractions (organic: F 1, 19 = 9.84, p = 0.005; CaCO 3 : F 1, 19 = 9.84, p = 0.005). L inear regressions fitted to each set of measurements showed that acidification yielded higher estimates of CaCO 3 content and lower estimates of organic matter content (Figure 3 3 ), potentially due to loss of organic content, whic h has been reported previously (Roberts et al., 1973; Byers et al., 1978). T herefore, ashing was chosen as the method for this study After consistent dry weights were obtained, s amples of H incrassata plates were transferred into pre weighed aluminum dishes a shed for 3 h at 500 C, and allowed to cool before being weighed to determine quantities of inorganic carbon i.e., CaCO 3 For the purposes of t his study, contributions from silicon and other trace elements that might have remained after ashing were consid ered negligible O rganic content was estimated by subtracting the inorganic fraction from the total pre ashing dry weight of a sample. Metrics of P roduction and C alcification Dry weights (DW) were used to calculate metrics that would elucidate interaction s between H incrassata and T testudinum Metrics characterized production for T testudinum and H incrassata as well as calcification for H incrassata Dry weights yielded measures of production for shoots of T testudinum (mg DW shoot 1 d 1 ) and thalli of H incrassata (mg DW thallus 1 d 1 ) directly, and multiplying these individual growth rates by the appropriate mean density generated estimates of

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19 net areal production (mg DW m 2 d 1 ) For H incrassata dry weights of thalli, organic tissue and CaCO 3 were measured separately; therefore, relative rates of production were calculated by standardizing increases in these weights to their initial values. Relative rates of production provided insights into growth performance. C alcification for H in crassata thalli was characterized by the ratio of CaCO 3 to organic matter because the photosynthetic activity of living organic tissue is responsible for calcification of the thallus (Borowitzka and Larkum 1976a, 1977; de Beer and Larkum 2001). Estimate s were calculated separately for entire thalli and new growth. Statistical A nalyses Statistically significant differences in water chemistry at experimental and unvegetated sites were assessed with m ultivariate p ermutation analyses of variance (PERMANOVAs, Anderson et al. 2008). Analyses were based on range standardized mean water temperatures, salinities, dissolved oxygen concentrations and hydrogen ion concentrations calculated over 30 min intervals throughout the 24 h periods Due to anomalies caused by two days of bad weather, data from one MHMT site and one LHHT site were excluded from the analysis. In total, t hree PERMANOVAs were performed, with t he first examining differences among treatments (LHHT, MHHT, HHLT and unvegetated sand ) for all environmen tal data. T wo other analyses examined differences among treatments for i ) hydrogen ion concentrations across full 24 h periods and ii ) hydrogen ion concentrations during daytime periods in order to assess changes driven by photosynthesis. Univariate analys es of variance (ANOVAs) were used to evaluate growth rates and data characterizing calcification. Normality was evaluated with Anderson Darling tests, and homoscedasticity was evaluated with Brown Forsythe tests. If necessary,

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20 data were transformed to meet the assumptions. Areal production was analyzed with a one way ANOVA with treatment considered a fixed effect. All other growth rates and calcification data were analyzed using nested ANOVA s with treatment as a fixed effect and sites nested within treatmen ts. Ryan Einot Gabriel Welsch Q multiple comparison s were employed to discern differences among treatments For unbalanced ANOVAs, the Tukey Kramer adjustment was applied to generate degrees of freedom for the post hoc tests.

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21 Figure 2 1 Location of Little Cayman Island and Grape Tree Bay.

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22 Figure 2 2 Halimeda incrassata morphological characteristics.

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23 Figure 2 3 Regressions for inorganic and organic content versus total dry weight of Halimeda incrassata as determined by acidification and a shing.

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24 CHAPTER 3 RESULTS Densities of M acrophytes and S ite S election The maximum shoot density of T testudinum in Grape Tree Bay was greater than that of H incrassata thalli by approximately an order of magnitude. The shoot density of T testudinum range d from 0 to 1900 shoots m 2 whereas the H incrassata thallus density range d from 0 to 246 thalli m 2 S ites for measurements of productivity for T testudinum and H incrassata were selected to span the density gradient in Grape Tree Bay A total of nine sites were chosen with three replicate sites in each of three treatment s (Low Halimeda High Thalassia = LHHT; Medium Halimeda Medium Thalassia = MHMT; and High Halimeda Low Thalassia = HHLT ; Figure 3 1 ). Water C hemistry PERMANOVAs indicated that the time series for all environmental variables and hydrogen ion concentrations did not differ significantly among treatments (Table 3 1). Thus, macrophytes at all sites were subjected to similar temporal variation in environmental conditions. Although not significantly different, patterns in DO and pH were of interest because they could potentially affect interactions between T testudinum and H incrassata The highest mean DO concentrations were recorded during the day fo r the LHHT treatment sites (Figure 3 2 ), which would be expected given photosynthesis by T testudinum Also, as expected, sites where seagrass was less dense (including unvegetated sites) exhibited less pronounced diel varia tions in DO concentrations (Figure 3 2 ).

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25 Diel patterns for pH paralleled those of DO (Fig ure 3 3 ); increasing throughout the daylight hours and decreasing at night. T he dense seagrass treatment (LHHT) exhibited a higher mean pH during the day and a slightly lower mean pH at night (Fig ure 3 3 a) As a consequence, H incrassata thalli within th e LHHT treatment experienced on average slightly more variation in pH within a 24 h period than did thalli in other treatments. The mean pH values in the dense seagrass (LHHT) areas ranged from 7.68 at night to 8.15 during t he day time The se pH values correspond ed to a mean [H + ] of 21.17 nmol L 1 seawater at night and a mean of 7.22 nmol L 1 seawater during the day a 98.3% difference over the course of approximately 12 h. In addition, pH for the LHHT treatment increased fast er during the morning, which resulted in a slightly higher value than that in the other treatments by about 10:00 AM (Fig ure 3 3 b ). These patterns in pH were driven by metabolic activity. Rates of P roduction Rates of T testudinum production (mg DW shoot 1 d 1 ) were homoscedastic and normal when log transformed ( Table 3 2 ). P roduction rates differed significantly among treatments and also among sites ( Table 3 2 ). Variation among treatments was of greater biological interest, and post hoc pairwise compariso ns, with a Tukey Kramer adjustment, showed that T testudinum production increased with increasing shoot density (Table 3 3 ). S hoots in the treatment with the highest density (LHHT) were ~2.4 times more productive than shoots in treatment with the lowest d ensity ( Fig ure 3 4 a). For H incrassata rates of production (mg DW thallus 1 d 1 ) in terms of both organic material and CaCO 3 were homoscedastic and normal ( Table 3 2 ) after log transformation. Rates of production for organic and inorganic material differed among treatments ( Table 3 2) The rate at which organic matter was produced also varied

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26 significantly among s ites within treatments ( Table 3 2 ) but th ese result s were not explored further. P ost hoc pair wise comparisons showed that thalli produc ed organic matter and CaCO 3 at statistically equal rates in treatments with low and intermediate T testudinum density (HHLT and MHMT), and thalli produced significantly less of both types of carbon d 1 in the treatment with the densest T testudinum (LHHT ) In fact thalli in the LHHT treatment were roughly 3.5 times less productive than thalli in other treatments ( Table 3 3; Fig ure 3 4 b, c). Growth rates for H incrassata whether standardized by total dry weight, organic matter or CaCO 3 were homoscedastic without transformation ( Table 3 4 ). However, after repeated attempts at transforming data, normality could not be achieved The untransformed data were analyzed, and results were interpreted with caution Standardized growth rates based on to tal weights and CaCO 3 were significantly different among treatments, but rates based on organic matter were not statistically different (Table 3 4) P ost hoc comparisons showed that mean standardized growth rate s of thalli in dense seagrass (LHHT) were low er than rates recorded for other treatments in the case of total weight and CaCO 3 ( Table 3 5 ). T he mean standardized growth rate of thalli in dense seagrass treatments was 50% or 44% of that recorded for thalli in other treatments when expressed versus tot al weight or CaCO 3 respectively (Table 3 5 ). When rates of production for individual shoots and thalli were scaled to 1 m 2 using the mean number of individuals m 2 in a given treatment, the resulting rates of areal production were homoscedastic and normal ( Table 3 6 ). A real production was significantly different among treatments for T testudinum shoots, H incrassata organic

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27 matter and H incrassata CaCO 3 content ( Table 3 6 ). For T testudinum p ost hoc multiple comparisons showed sites with dense seagrass yielded significantly greater areal production and production decreased significantly with decreasing density ( Figure 3 5 a). Rates of areal production of organic matter and CaCO 3 f or H incrassata exhibited similar trends, with significant i ncreases in production as density of T testudinum decreased ( Fig ure 3 5 b, c). Mean rates of areal production for T testudinum and H incrassata spanned an order of magnitude (Table 3 7). Calcification R atios of CaCO 3 to organic material (CaCO 3 :OM) in the new growth of H incrassata were normal and homoscedastic (A D and B F test p > 0.05) without transformation. These ratio s were not significantly different among treatments, al though there was significant variation among sites within treatments ( Table 3 8). Thus, the CaCO 3 content of new plates varied among thalli but did not differ consistently among treatments. Ratios of CaCO 3 :OM for whole thalli were normal after log transformation, but the data remained h eter oscedastic so the results of the ANOVA were interpreted cautiously (Table 3 8) Ratios of CaCO 3 :OM in whole thalli differed significant ly a mong treatment s and among sites within treatments ( Table 3 8 ). Post hoc multiple comparisons indicated that thalli in the sites with the densest seagr ass (LHHT) had higher CaCO 3 :OM ratios than thalli in other treatments (Table 3 9 ), which indicated that relatively more calcification had occurred The proportion s of whole thalli comprising CaCO 3 were normal and homoscedastic after arcsine transformation ( Table 3 10 ). Thalli from different treatments exhibited significant differences in their CaCO 3 content, and thalli from different sites within

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28 treatments also differed significantly ( Table 3 10 ). P ost hoc multiple comparison tests, with Tukey Kramer adjus tment s showed that thalli in the dense seagrass treatment contained proportionately more CaCO 3 than thalli in other treatments. T halli growing in dense seagrass were 5 6% more calcified than thalli growing in areas with sparse seagrass (Table 3 11).

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29 Fi gure 3 1 Plot of Halimeda incrassata thalli density against Thalassia testudinum shoot density for the sites chosen to represent treatments. HHLT = high density of Halimeda and low density of Thalassia ; MHMT = medium density of Halimeda and medium densit y of Thalassia ; LHHT = low density of Halimeda and high density of Thalassia

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30 Table 3 1 Results of PERMANOVA analyses based on environmental data. [H+] = hydrogen ion concentrations Parameter Factor df SS MS Pseudo F p Unique permutations All environmental data Treatment 3 528.3 176.1 1.66 0.068 918 Error 6 637.9 106.3 24 h [H + ] Treatment 3 55.0 18.3 0.47 0.908 921 Error 6 31.4 0.2 Daytime [H + ] Treatment 3 53.8 17.9 0.89 0.562 922 Error 6 120.2 20.0 Figure 3 2 Mean dissolved oxygen concentrations (mg L 1 ) among treatments on a 24 hour basis. HHLT = high density of Halimeda and low density of Thalassia ; MHMT = medium density of Halimeda and medium density of Thalassia ; LHHT = low density of Halimeda and high density of Thalassia ; SAND = unvegetated sediment

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31 (a) (b) Figure 3 3 Mean pH levels by treatment during (a) a 24 h cycle and (b) daytime. HHLT = high density of Halimeda and low density of Thalassia ; MHMT = medium density of Halimeda and medium density of Thalassia ; LHHT = low density of Halimeda and high density of Thalassia ; SAND = unvegetated sediment

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32 Table 3 2 Results of ANOVAs based on rates of production for Thalassia testudinum and Halimeda incrassata A D p = p value for Anderson Darling test for normality; B F p = p value for Brown Forsythe test for homoscedasticity; OM = organic matter Trt = treatment Species Metric A D p B F p Factor df SS MS F p T halassia testudinum mg DW shoot 1 d 1 (OM) >0.25 0.67 Treatment 2 2.95 1.47 8.16 0.019 Site(Trt ) 6 1.08 0.18 8.96 < 0.001 Error 331 6.68 0.02 H alimeda incrassata mg DW thallus 1 d 1 (OM) 0.22 0.31 Treatment 2 2.73 1.36 7.15 0.026 Site(Trt ) 6 1.15 0.19 2.78 0.013 Error 179 12.29 0.07 mg DW thallus 1 d 1 ( CaCO 3 ) >0.25 0.06 Treatment 2 3.78 1.89 9.17 0.015 Site(Trt ) 6 1.24 0.21 1.76 0.110 Error 179 21.00 0.12 Table 3 3 Back transformed mean rates of production for Thalassia testudinum shoots and Halimeda incrassata thalli. 95% CL = lower and upper 95% confidence limits; HHLT = high density of Halimeda and low density of Thalassia ; MHMT = medium density of Halimeda and medium density of Thalassia ; LHHT = low density of Halimeda and high density of Thalassia ; OM = organic matter Species HHLT MHMT LHHT Metric Mean 95% CL Mean 95% CL Mean 95% CL T halassia testudinum 0.86 0.74, 0.99 1.40 1.25, 1.55 2.09 1.89, 2.31 mg DW shoot 1 d 1 (OM) H alimeda incrassata 2.20 1.72, 2.71 2.46 2.01, 2.97 0.69 0.42, 1.02 mg DW thallus 1 d 1 (OM) mg DW thallus 1 d 1 (CaCO3) 3.19 2.41, 4.16 3.45 2.74, 4.29 0.91 0.53, 1.38

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33 a b c Figure 3 4 Back transformed mean rates of production for a) Thalassia testudinum shoots and b) organic matter and c) CaCO 3 in thalli of Halimeda incrassata Error bars indicate 95% confidence limits. Different capital letters above columns indicate statistically significant differences as determined by pairwise follow up tests. Note t hat scales of y axes differ among panels. HHLT = high density of Halimeda and low density of Thalassia ; MHMT = medium density of Halimeda and medium density of Thalassia ; LHHT = low density of Halimeda and high density of Thalassia A A B

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34 Table 3 4 Results of A NOVA based on rates of production for Halimeda incrassata standardized to initial sizes of thalli. A D p = p value for Anderson Darling test for normality; B F p = p value for Brown Forsythe test for homoscedasticity; OM = organic matter Trt = treatment M etric A D p B F p Factor df SS MS F p mg DW new mg DW old 1 d 1 (thallus) < 0.005 0.283 Treatment 2 3.79 x 10 4 1.89 x 10 4 6.64 0.030 Site(Trt) 6 1.71 x 10 4 2.85 x 10 5 1.40 0.217 Error 173 3.52 x 10 3 2.04 x 10 5 mg DW new mg DW old 1 d 1 (thallus) < 0.005 0.113 Treatment 2 8.28 x 10 4 4.14 x 10 4 2.41 0.171 Site(Trt) 6 1.03 x 10 3 1.72 x 10 4 2.05 0.061 Error 173 1.50 x 10 2 8.40 x 10 5 mg DW new mg DW old 1 d 1 (CaCO3) < 0.005 0.066 Treatment 2 2.62 x 10 4 1.30 x 10 4 9.52 0.014 Site(Trt) 6 8.25 x 10 5 1.37 x 10 5 1.01 0.418 Error 173 2.35 x 10 3 1.36 x 10 5 Table 3 5 Mean rates of production for Halimeda incrassata standardized to initial sizes of thalli. SD = standard deviation; HHLT = high density of Halimeda and low density of Thalassia ; MHMT = medium density of Halimeda and medium density of Thalassia ; LHHT = low density of Halimeda and high density of Thalassia ; OM = organic matter; SD = standard deviation Metric HHLT MHMT LHHT Mean SD Mean SD Mean SD mg DW new mg DW old 1 d 1 (thallus) 0.00710 0.00477 0.00701 0.00450 0.00359 0.00417 mg DW new mg DW old 1 d 1 (OM) 0.01414 0.00909 0.01437 0.00848 0.00997 0.01113 mg DW new mg DW old 1 d 1 ( CaCO 3 ) 0.00541 0.00391 0.00532 0.00376 0.00236 0.00131 Table 3 6 Results of ANOVAs based on rates of areal production for Thalassia testudinum and Halimeda incrassata A D p = p value for Anderson Darling test for normality; B F p = p value for Brown Forsythe test for homoscedasticity; OM = organic matter T rt=treatment Species and Metric A D p B F p Factor df SS MS F p Thalassia testudinum mg DW m 2 d 1 (OM) >0.25 0.65 Trt 2 2.33 x 10 7 1.17 x 10 7 158.52 >0.25 Error 6 4.42 x 10 5 7.36 x 10 4 Halimeda incrassata mg DW m 2 d 1 (OM) 0.14 0.74 Trt 2 4.01 x 10 5 2.00 x 10 5 311.31 0.14 Error 6 3.86 x 10 3 6.43 x 10 2 Halimeda incrassata mg DW m 2 d 1 (CaCO3) 0.19 0.71 Trt 2 1.19 x 10 6 5.94 x 10 5 335.63 < 0.001 Error 6 1.06 x 10 4 1.77 x 10 3

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35 Table 3 7 Mean areal rates of production for Thalassia testudinum and Halimeda incrassata standardized to initial sizes of thalli. SD = standard deviation; HHLT = high density of Halimeda and low density of Thalassia ; MHMT = medium density of Halimeda and medium density of Thalassia ; LHHT = low density of Halimeda and high density of Thalassia ; OM = organic matter Species and Metric HHLT MHMT LHHT Mean SD Mean SD Mean SD T halassia testudinum mg DW m 2 d 1 (OM) 214.4 152.3 1379.1 343.0 4060.4 282.8 H alimeda incrassata mg DW m 2 d 1 (OM) 531.2 34.6 325.2 25.7 17.8 8.5 Halimeda incrassata mg DW m 2 d 1 (CaCO3) 915.0 59.6 505.7 40.0 26.2 12.6

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36 a b c Figure 3 5 Mean areal rates of production for a) Thalassia testudinum shoots and b) organic matter and c) CaCO 3 in thalli of Halimeda incrassata Error bars indicate 1 standard deviation. Different capital letters above columns indicate statistically significant differences as determined by pairwise follow up tests. Note that scales of y axes differ among panels. HHLT = high density of Halimeda and low density of Thalassia ; MHMT = medium density of Halimeda and medium density of Thalassia ; LHHT = low density of Halimeda and high density of Thalassia

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37 Table 3 8 Results of ANOVAs based ratios of calcium carbonate content to organic matter in thall i of Halimeda incrassata A D p = p value for Anderson Darling test for normality; B F p = p value for Brown Forsythe test for homoscedasticity; OM = organic matter Trt = treatment Metric A D p B F p Factor df SS MS F p CaCO 3 :OM for new growth > 0.25 0.41 Treatment 2 0.46 0.23 0.34 0.723 Site(Trt ) 6 4.01 0.67 3.35 0.004 Error 157 31.38 0.20 CaCO 3 :OM for thalli 0.21 < 0.01 Treatment 2 0.56 0.280 14.71 0.005 Site(Trt ) 6 0.11 0.020 3.79 0.001 Error 180 0.90 0.005 Table 3 9 Mean and back transformed mean ratios of calcium carbonate content to organic matter in thalli of Halimeda incrassata SD = standard deviation; 95% CL = upper and lower 95% confidence limits; HHLT = high density of Halimeda and low density of Thalassia ; MH MT = medium density of Halimeda and medium density of Thalassia ; LHHT = low density of Halimeda and high density of Thalassia ; OM = organic matter Metric HHLT MHMT LHHT Mean SD or 95% CL Mean SD or 95% CL Mean SD or 95% CL CaCO 3 :OM for new growth 1.48 0.49 1.49 0.42 1.24 0.50 CaCO 3 :OM for thalli 3.76 3.62, 3.91 4.00 3.82, 4.18 5.64 5.16, 6.16 Table 3 10 Results of an ANOVA based proportions of calcium carbonate in thalli of Halimeda incrassata A D p = p value for Anderson Darling test for normality; B F p = p value for Brown Forsythe test for homoscedasticity; OM = organic matter A D p B F p Factor df SS MS F P > 0.25 0.06 Treatment 2 0.15 0.070 12.18 0.008 Site(Treatment) 6 0.04 0.010 4.16 < 0.001 Error 180 0.27 0.001 Table 3 11 Back transformed mean proportions of calcium carbonate in thalli of Halimeda incrassata 95% CL = upper and lower 95% confidence limits; HHLT = high density of Halimeda and low density of Thalassia ; MHMT = medium density of Halimeda and medium density of Thalassia ; LHHT = low density of Halimeda and high density of Thalassia ; OM = organic matter HHLT MHMT LHHT Mean 95% CL Mean 95% CL Mean 95% CL 0.79 0.78, 0.80 0.80 0.79, 0.81 0.85 0.84, 0.86

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38 CHAPTER 4 DISCUSSION Predicting the potential impact of ocean acidification on organisms that rely on CaCO 3 requires an understanding of their responses to local variations in factors affecting the carbonate cycle and physiological carbon dynamics. For example, seawater chemistry in shallow, tropical lagoons tends to vary across 24 h periods because rates of photosynthesis and respiration vary. Furthermore, seagrasses and sympatric macroalgae interact ecologically in positive and negative ways. By measuring in situ rates of production and calcification at sites with different densities of T testudinum and H incrassata this study contributes insights into the relative importance of biotic and abiotic factors at the local scale. Biotic interactions between T. testudinum and H. incrassata include facili tation and competition. Halimeda incrassata and other r hizophytic algae can successfully colonize and persist in unstable, nutrient poor sediments due to their ability to anchor themselves and more efficiently garner scarce nutrients ( Hillis Colinvaux, 198 0; Williams, 1981 ; Demes et al., 2010 ) Thus, rhizophytic algae can facilitat e colonization and growth of T testudinum and other seagrasses by stabilizing sediments increasing the accumulation of nutrients in the sediment as thalli decompose, and reducin g transfer of nutrients in to the water column by protecting sediments from turbulence (McRoy and McMillan 1977; Orth, 1977; Williams, 1984a, 1990). After colonization and establishment by seagrass calcifying macroalgae often decrease in abundance because seagrasses tend to be superior competitors for space, light or nutrients. In fact, T testudinum represents a particularly strong competitor, and it dominates many tropical seagrass beds via exploitative competition (Zieman and Wetze l, 1980; Williams, 1987, 1990). Furthermore,

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39 T testudinum has been shown to compete for nitrogen with H incrassat a when present at densities of 400 800 shoots m 2 (Davis and Fourqurean, 2001) and T testudinum densities as low as 200 shoots m 2 were associated with a decl ine in the abundance of rhizophytic algae (Williams, 1990). In the present study, no significant negative effect on H incrassata producti on was observed at seagrass densities up to 1312 shoots m 2 (i.e. HHLT and MHMT treatments). However, rates of produc ti on were reduced at seagrass densities at or above 1650 shoots m 2 (i.e., LHHT treatment), suggesting that some level of competition ( sensu Davis and Fourqurean, 2001) occurs at high seagrass densities in Grape Tree Bay. Given light regimes in clear, shal low, tropical waters, such competition is likely to be for nutrients rather than light (Davis and Fourqurean, 2001). Therefore, biotic interactions, i.e., competition, could affect the dynamics of CaCO 3 production by H incrassata at scales similar to Grape Tree Bay. As thalli grow, Halimeda spp. produce both organic matter and CaCO 3 where production of CaCO 3 is driven by photosynthesis in the thallus. Jensen et al. (1985) reported that 77% of the variation in calcification of H. copiosa, H, cryptica, H. discoidea, and H. lacrimosa could be explained by variation in the rate of photosynthesis. In addition, net carbonate accretion by Halimeda spp. does not occur in the d ark (Borowitzka and Larkum, 1976a, 1976b, 1976c, 1977; de Beer and Larkum, 2001), which further demonstrates the link between calcification and photosynthesis. Thus ratios of CaCO 3 to organic matter (OM) yield insights into the dynamics of calcification. When only new growth of H incrassata was considered, CaCO 3 :OM ratios were not significantly different regardless of the density of T testudinum surrounding the

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40 algae, with the range of ratios across densities of seagrass being 0.25 In contrast, the maximum difference among CaCO 3 :OM ratios for whole thalli, i.e., ratios including older plates, was 1.88, which is 7 times the range observed for new growth, suggesting that continued CaCO 3 accumulation varied significantly among sites with different densities of seagrass. In fact, the highest CaCO 3 :OM ratio was recorde d for H incrassata growing among the highest density of T testudinum In combination, these results indicate that new H incrassata plates, i.e., those less than 7 d old, are produced with a relatively constant CaCO 3 :OM ratio and CaCO 3 content continues to increase in older plates, especially for thalli in dense seagrass (LHHT) Previously, van Tussenbroek and van Dijk (2007) found that mature, basal plates of H incrassata were heavier than newly produced plates and other studies have documented an incr ease in CaCO 3 content as plates age (Borowitzka and Larkum, 1976a, 1977; Multer, 1988). This is the first study to report that accumulation of CaCO 3 may depend on the density of surrounding seagrasses. Competition for nutrients represents a potential influ ence on calcification if thalli amid dense seagrass continue to photosynthesize and produce CaCO 3 in existing tissues without the nutrients required to synthesize new living tissue. S ome investigators have reported abiotic influences on calcification, with seagrasses or fleshy macroalgae creating seawater chemistry favorable to algal and coral calcification (Semesi et al., 2009a, b; Anthony et al., 2011; Kleypas et al., 2011). These results were obtained under naturally (Kleypas et al., 2011) or artificiall y (Semesi et al., 2009a, b; Anthony et al., 2009) low mixing conditions (high water residence time). Findings here suggest that abiotic effects may not be very significant in well mixed conditions, because (1) the pH regime in the seagrass canopy did not d iffer significantly

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41 from unvegetated areas in Grape Tree Bay and (2) growth rates for H. incrassata in dense T testudinum bed s were significantly lower, which further indicates that biotic interactions, e.g., competition between the se macrophytes could r epresent a key factor. Collectively, the results presented here suggest that enhancement of calcification will depend on water residence time (Anthony et al., 2011; Kleypas et al., 2011) and ecological processes, such as competition (Davis and Fourqurean, 2001) and succession (Williams, 1990) that affect both the abundances of macrophytes and their ability to garner resources (van Tussenbroek and van Dijk, 2007). Further research combining manipulative and mensurative experiments is needed to elucidate the outcomes of positive and negative interactions between seagrasses, like T testudinum and rhizophytic, calcareous algae, like H incrassat a in oceans that are becoming increasingly acidic. Data on local hydrodynamics and diel variations in seawater chem istry also will be important in furthering our understanding of biotic and abiotic influences on calcium carbonate production

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42 LIST OF REFERENCES Anderson, R., Gorley, R.N., Clarke, K.R., 2008. PERMANOVA+ for PRIMER: guide to software and statistical methods. PRIMER E, Plymouth, UK. Andersson, A.J., Mackenzie, F.T., 2012. Revisiting four scientific debates in ocean acidification research. Biogeosciences 9, 893 905. Anthony, K.R.N., Kleypas, J.A., Gattuso, J., 2011. Coral reefs modify th eir seawater carbon chemistry implications for impacts of ocean acidification. Glob Change Biol. 17, 3655 3666. Borowitzka, M.A., Larkum, A.W.D., 1976 a Calcification in the green alga Halimeda II. Exchange of Ca 2+ and occurrence of age gradients in ca lcification and photosynthesis. J. Exp. Bot. 27, 864 878. Borowitzka, M.A., Larkum, A.W.D., 1976 b Calcification in the green alga Halimeda III. Sources of inorganic carbon for photosynthesis and calcification and a model of mechanism of calcification. J. Exp. Bot. 27, 879 893. Borowitzka, M.A., Larkum, A.W.D., 1976 c Calcification in the green alga Halimeda IV. Action of metabolic inhibitors on photosynthesis and calcification. J. Exp. Bot. 27, 894 907. Borowitzka, M.A., Larkum, A.W.D., 1977. Calcificati on in the green alga Halimeda I. Ultrastructure study of thallus development. J. Phycol. 13, 6 16. Byers, S.C., Mills, E.L., Stewart, P.L., 1978. Comparison of methods of determining organic carbon in marine sediments, with suggestions for a standard meth od. Hydrobiologia 58, 43 47. Dahlgren, C., Marr, J., 2004. Back reef systems: Important but overlooked components of tropical marine ecosystems. Bull. Mar. Sci. 75, 145 152. Davis, B.C., Fourqurean, J.W., 2001. Competition between the tropical alga, Halime da incrassata and the seagrass, Thalassia testudinum Aquat. Bot. 71, 217 232. d e Beer, D., Larkum, A.W.D., 2001. Photosynthesis and calcification in the calcifying algae Halimeda discoidea studied with microsensors. Plant Cell Environ. 24, 1209 1217. Demes, K.W., Littler, M.M., Littler, D.S., 2010. Comparative phosphate acquisition in giant celled rhizophytic algae (Bryopsidales, Chlorophyta): Fleshy vs. ca lcified forms. Aquat. Bot. 92 157 160.

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43 Doney, S.C., Ruckelshaus, M., Duffy, J.E., Barry, J.P., C han, F., English, C.A., Galindo, H.M., Grebmeier, J.M., Hollowed, A.B., Knowlton, N., Polovina, J., Rabalais, N.N., Sydeman, W.J., Talley, L.D., 2012. Climate change impacts on marine ecosystems. Annu. Rev. Mar. Sci. 4, 11 37. Duarte, C.M., 1995. Submerged aquatic vegetation in relation to different nutrient regimes. Ophelia 41, 87 112. Duarte, C.M., Marba, N., Gacia, E., Fourqurean, J.W., Beggins, J., Barron, C., Apostolaki, E.T., 2010. Seagrass community metabolism: Assessing the carbon sink capacity of s eagrass meadows. Glob al Biogeochem. Cy 24, GB4032. Fong, P., Paul, V.J., 2011. Coral reef algae, in: Dubinsky, Z., Stambler, N. (Eds.), Coral reefs: An Ecosystem in Transition. Springer, Dordrecht, Netherlands, pp. 241 272. Fourqurean, J.W., Duarte, C.M., Kennedy, H., Marba, N., Holmer, M., Mateo, M.A., Apostolaki, E.T., Kendrick, G.A., Krause Jensen, D., McGlathery, K.J., Serrano, O., 2012. Seagrass ecosystems as a globally significant carbon stock. Nature Geosci advance online publication, 1 5. Freile, D., Milliman, J.D., Hillis, L., 1995. Leeward bank margin Halimeda meadows and draperies and their sedimentary importance on the western Great Bahama Bank slope. Coral Reefs 14, 27 33. Guinotte, J.M., Fabry, V.J., 2008. Ocean acidification and its potentia l effects on marine eco systems. Ann. NY Acad. Sci. 1134 320 342. Hillis Colinvaux, L., 1980. Ecology and taxonomy of Halimeda : primary producer of coral reefs. Adv. Mar. Biol. 17, 1 327. Hoegh Guldberg, O., Mumby, P.J., Hooten, A.J., Steneck, R.S., Greenf ield, P., Gomez, E., Harvell, C.D., Sale, P.F., Edwards, A.J., Caldeira, K., Knowlton, N., Eakin, C.M., Iglesias Prieto, R., Muthiga, N., Bradbury, R.H., Dubi, A., Hatziolos, M.E., 2007. Coral reefs under rapid climate change and ocean acidification. Scien ce 318, 1737 1742. Jensen, P.R., Gibson, R.A., Littler, M.M., Littler, D.S., 1985. Photosynthesis and calcification in 4 deep water Halimeda species (Chlorophyceae, Caulerpales). Deep Sea Res. 32, 451 464. Kleypas, J.A., Anthony, K.R.N., Gattuso, J., 2011. Coral reefs modify their seawater carbon chemistry case study from a barrier reef (Moorea, French Polynesia). Glob Change Biol. 17, 3667 3678. Kleypas, J.A., Buddemeie r, R.W., Archer, D., Gattuso, J P., Langdon, C., Opdyke, B.N., 1999. Geochemical conseq uences of increased atmospheric CO 2 on coral reefs. Science 284, 118 120.

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47 BIOGRAPHICAL SKETCH Savanna Barry grew up in central Virginia and attended Patrick Henry High School, where she gradu ated as Valedictorian in 2006. She studied b iology at the University of Virginia and held multiple internships at the Anheuser Busch Coastal Research Center in Oyster, VA. She graduated from UVA with honors in 2010 entered the University of Florida Fisheri es and Aquatic Sciences Master of Science program in the fall of the same year. Her graduate work was completed in Little Cayman, BWI and she plans to enroll in a Doctor of Philosophy program at the University of Florida in the fall of 2012.