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Trophic Transfer of Mercury in a Subtropical Coral Reef Food Web

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Trophic Transfer of Mercury in a Subtropical Coral Reef Food Web _______________________________________________________________________ A Thesis Presented to The Faculty of the College of Arts and Sciences Florida Gulf Coast University In Partial Fulfillment Of the Requirement for the Degree of Master of Science ________________________________________________________________________ By Christopher Tyler Lienhardt 2015 APPROVAL SHEET This thesis is submitted in partial fulfillment of the requirements for the degree of Master of Science ____________________________ Christopher Tyler Lienhardt Approved: July 2015 ____________________________ Darren G. Rumbold, Ph.D. Committee Chair / Advisor ____________________________ Michael L. Parsons, Ph.D. ____________________________ Ai Ning Loh, Ph. D. The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. i Acknowledgments This research would not have been possible without the support and encouragement of numerous friends and family. First and foremost I would like to thank my major advisor, Dr. Darren Rumbold, for giving me the opportunity to play a part in some of the great research he is conducting, and add another piece to the puzzle that is mercury biomagnification research. The knowledge, wisdom and skills imparted unto me over the past three years, I cannot thank him enough for. I would also like to thank Dr. Michael Parsons for giving me a shot to be a part of the field team and assist in the conducting of our research. I also owe him thanks for his guidance and the nature of his graduate courses, which helped prepare me to take on such a task. I would also like to thank Dr. Ai Ning Loh, for her comments and feedback on this thesis, and as someone who pushed me to pursue graduate school after working in her geochemistry laboratory as an undergraduate. I would also like to thank my friends and fellow graduate students Rheannon Ketover and Lacey Rains, your backing to apply to the program and support throughout it has been irreplaceable. Thanks are also owed to Alex Leynse, Amanda Ellsworth, Ashley Brandt, Nicole Fronczkowski, Megan Conkling, Adam Catasus, and Jeff Zingre (all from FGCU) for their assistance in collecting and processing samples. Finally, I am especially grateful to my mother Jamie Lienhardt and my best friend Lillie Simmons, for their moral support and constant encouragement throughout the many ups and downs that are the graduate school experience. ii Abstract Mercury is a widespread and damaging toxic metal that is trophically transferred through food webs. Coral reefs present an interesting dynamic in that they are comprised of complex food webs containing a high number of lateral or horizontal links that do not always end in top predators, thus possibly reducing trophic transfer. Therefore, the objective of the study was to assess the efficiency of trophic transfer in a coral reef food web, using mercury as the tracer. Concentrations of mercury and stable isotopes of nitrogen (δ15N) and carbon (δ13C) were measured in fish from two sites near the coastal waters of Long Key, Florida. The relationship between mercury and δ15N can be used to estimate biomagnification across the food web (i.e., trophic magnification slope, food web magnification factor). Using mercury and stable isotopes of nitrogen and carbon as tracers assisted in quantifying the efficiency at which coral reef ecosystems transfer these and other bioaccumulative toxins (e.g., ciguatoxins, etc.) through the food web, while also increasing our understanding of the associated flow of energy in the system. A total of 242 samples were collected from April 2012 through December 2013 using spear guns, hook and line, and hand collection techniques. Individual Hg concentrations ranged from 17.33 µg/kg in a gray angelfish (Pomacanthus arcuatus) to 3,317 µg/kg in a great barracuda (Sphyraena barracuda) at Long Key Hard Bottom, and 19.01 µg/kg in a rock beauty angelfish (Holacanthus tricolor) to 6,842 µg/kg in a porkfish (Anisotremus virginicus) at Tennessee Reef. Variability in both fish size and δ15N increased the variance in tissue Hg concentration both intra- and inter-specifically. As observed in other systems, the log transformed Hg concentrations in the food web, pooled across species, were significantly iii related to δ15N. The trophic magnification slope (i.e., slope of Log [Hg] regressed on δ15N), as an estimate of the biomagnification rate of Hg in the subtropical coral reef food web was 0.23 ± 0.03 (±95% confidence interval) at Tennessee Reef and 0.16 ± 0.04 for Long Key Hard Bottom. When δ15N was translated to trophic level, the food web magnification factor (calculated from slope of Log [Hg] regressed on trophic level) were 7.8 and 3.4 for Tennessee Reef and Long Key Hard Bottom, respectively. Although there was some evidence to support significant differences between the two sites (which could have been due to differences in water quality), this difference in slopes could also be a result of unbalanced sampling design. Nonetheless, these results clearly demonstrate that Hg is biomagnified through subtropical coral reef ecosystems and that the transfer efficiency (i.e., slopes) were consistent with previous reports for marine ecosystems. iv Table of contents Acknowledgments ............................................................................................................i Abstract .......................................................................................................................... ii Table of contents ............................................................................................................ iv List of Tables................................................................................................................... v List of Figures ................................................................................................................ vi Introduction ..................................................................................................................... 1 Objectives .................................................................................................................... 9 Significance ................................................................................................................. 9 Methods ........................................................................................................................ 10 Study Area ................................................................................................................. 10 Sample Collection and Processing ............................................................................. 11 Mercury Analysis ...................................................................................................... 12 Stable Isotope Analysis .............................................................................................. 13 Data Analysis ............................................................................................................ 14 Results ........................................................................................................................... 15 Variability in Mercury Concentration as a Function of Fish Size ................................ 16 Variability in Mercury Concentration as a Function of Location ................................ 17 Variability in Mercury Concentration in Relation to Stable Isotopes of Carbon and Nitrogen .................................................................................................................... 19 Trophic Magnification Slope (TMS) and Food Web Magnification Factor (FWMF) .. 21 Discussion ..................................................................................................................... 23 Conclusions ............................................................................................................... 38 Literature Cited ............................................................................................................. 39 v List of Tables Table 1. Summary of mercury concentration, total length, stable isotopes of δ13C and δ15N, and trophic level of invertebrate and finfish taxa collected from Tennessee Reef Light (TRL)………………………………………………………………………………56 Table 2. Summary of mercury concentration, total length, stable isotopes of δ13C and δ15N, and trophic level of invertebrate and finfish taxa collected from Long Key Hard Bottom (LKH)…………………………………………………………………………....57 Table 3. Coefficient of determination (r2) values for regression models for various relationships involving fish species at Tennessee Reef…………………………………...58 Table 4. Coefficient of determination (r2) values for regression models for various relationships involving fish species at Long Key Hard Bottom………………………….58 Table 5. Values for trophic magnification slopes of Total Hg biomagnification through food webs of different ecosystems as reported in the published literature……………….59 Table 6. Water quality conditions based on quarterly sampling from May 2010 – September 2013 at the two nearby sites (adapted from the FIU SERC Florida Keys National Marine Sanctuary). ………………………………………………………………………………60 vi List of Figures Figure 1. Map of study area where samples were collected off Long Key, FL………….61 Figure 2. (a) Relationship between log Hg (µg/kg) and total length (cm) in individuals at Tennessee
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