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Biological-Physical Interactions in Pacific Coral Reef Ecosystems

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Biological-Physical Interactions in Pacific Coral Reef Ecosystems BIOLOGICAL-PHYSICAL INTERACTIONS IN PACIFIC CORAL REEF ECOSYSTEMS A DISSERTATION SUBMITTED TO THE GRADUATE DIVISION OF THE UNIVERSITY OF HAWAI‘I AT MĀNOA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN OCEANOGRAPHY DECEMBER 2013 By Jamison M. Gove Dissertation Committee: Margaret A. McManus, Co-chairperson Jeffrey C. Drazen, Co-chairperson Craig R. Smith Mark A. Merrifield Alan M. Friedlander ACKNOWLEDGEMENTS I would like to express the utmost gratitude to Margaret McManus for the tremendous amount of advice, supervision, and assistance she gave me throughout this research. Her wisdom will stay with me for the rest of my life. I would like to thank my dissertation committee – Jeff Drazen, Mark Merrifield, Craig Smith and Alan Friedlander – for their invaluable scientific input and guidance. I would also like to thank Rusty Brainard for 12 years of support and for providing me with the amazing opportunity to dive throughout the Pacific researching some of the world’s most incredible coral reef ecosystems. In addition, I would like to thank the many individuals who contributed greatly to this research. Thank you to Gareth Williams who served as an important mentor, collaborator, and friend. Thanks to Oliver Vetter, Chip Young, Danny Merritt, and Ron Hoeke for not only being my closest friends in life, but for putting in ridiculously long hours in the field to help with data collection . Finally, thanks to Dave Foley, my original mentor who taught me so much about satellite oceanography. I am especially thankful and indebted to my family who has given me a substantial amount of support and love. Particularly I would like to thank my mother, father, sister, and grandmother for being such a solid foundation throughout my life and for their invaluable advice and constant encouragement. Lastly, I would like to thank my friends, who have played a crucial role in my happiness throughout this research and my life here in Hawai‘i. ii ABSTRACT Coral reefs are some of the most diverse and productive marine ecosystems on earth. They are also among the most threatened by human disturbance. On a local scale, many of these systems are subject to over-fishing and land-based pollution, and on a global scale, these systems are impacted by climate change and ocean acidification: human activities clearly influence the structure of coral reef communities. However, before considering anthropogenic influence, it is necessary to consider the influence of local and regional environmental forcings on these ecosystems. In this research, I investigate natural environmental and anthropogenic drivers of benthic community organization in 41 coral reef ecosystems across the Pacific (14.2°S – 28.4°N, 144.8°E – 155.4°W). These systems have been the focus of a long-term, multi-disciplinary NOAA-led monitoring effort. I present a new methodological approach to spatially constrain environmental forcings at the scale of individual islands and atolls. The results indicate considerable spatial heterogeneity in environmental forcings, namely sea surface temperature, waves, chlorophyll-a concentration (a proxy for phytoplankton biomass) and irradiance. Further examination of long-term (10-year) chlorophyll-a concentrations revealed sustained increased phytoplankton biomass just offshore of reefs compared to surrounding oceanic waters around a majority (91%) of islands and atolls, providing widespread evidence of the occurrence and scale of the “island mass effect”. Additionally, significant differences in horizontal gradients in chlorophyll-a between island and atoll systems were observed. Variations in reef area, bathymetric slope, geomorphic type (e.g. atoll versus island), and human population were identified as important drivers of increased phytoplankton biomass, together explaining 77% of the variability observed. In order to investigate biological-physical relationships at a smaller iii spatial scale, I focused on one oceanic atoll with a history of minimal human influence, Palmyra Atoll (5.8°N, 162.1°W). At intra-island scales, wave forcing and reef geomorphology were important drivers of benthic community organization. Model performance improved when hard coral cover was modeled in distinct morphological groups (encrusting, plating, branching), highlighting the response of coral reef communities to extrinsic physical forcings. Superimposed on natural variations in coral reef benthic community organization are the effects of anthropogenic disturbance. iv TABLE OF CONTENTS Acknowledgements……………………………………………………………………………….ii Abstract…………………………………………………………………………………………..iii List of Tables……………..………………………………………………………...…….…........vi List of Figures…………………………………………………………………………….….......vii Chapter I: Introduction………………………...…………………………………………………..1 Chapter II: Quantifying Climatological Ranges and Anomalies for Pacific Coral Reef Ecosystems…..…………………………………………………………………..…….5 Chapter III: The Island Mass Effect: Explaining Enhanced Phytoplankton Biomass for Island Ecosystems Across the Pacific…………………………………….………………..42 Chapter IV: Intra-island Gradients in Physical Forcings Drive Spatial Patterning in Coral Reef Benthic Communities……………………………………………………….………72 Chapter V: Conclusions……………………………………………………………………..….115 References…………….…………………………….……………………………………….….121 v LIST OF TABLES 2.1 General information for the 41 Pacific island and atoll coral reef ecosystems………….30 3.1 General information on the islands and atolls used to study the island mass effect……..60 3.2 List of biogeophysical drivers of increased phytoplankton biomass…………………….61 3.3 Best-fit generalized linear model results for the island mass effect………………….….61 4.1 Information on the physical environmental data collected from Palmyra Atoll………..100 4.2 Summary of cold-pulses by mooring location around Palmyra Atoll………….…...….101 4.3 Significant drivers of benthic community structure around Palmyra Atoll…………….101 vi LIST OF FIGURES 2.1 Map highlighting the coral reef ecosystems of the U.S. Pacific……….……………..…32 2.2 Spatially constrained chlorophyll-a at Pearl and Hermes Reef………………………….33 2.3 Long-term mean of chlorophyll-a by data inclusion zone at Pearl and Hermes Reef…...34 2.4 Map of long-term means in environmental forcings across the U.S. Pacific…………....35 2.5 Climatological ranges and anomalies for U.S. Pacific coral reef ecosystems…………...36 2.6 Principle component analysis of environmental forcings………………………………..37 3.1 Map of long term mean chlorophyll-a across the U.S Pacific…………………………...63 3.2 Spatially constrained long-term mean chlorophyll-a from Midway Atoll…………….....63 3.3 Long-term mean of chlorophyll-a by data inclusion zone at Midway Atoll……………..64 3.4 Gradients in chlorophyll-a and biogeophysical predictors of the island mass effect…....65 3.5 Long-term mean and in situ chlorophyll-a in the Hawaiian Archipelago……………….66 4.1 Map Palmyra Atoll in the northern Line Islands……………………………………….103 4.2 Climatological and time series information for Palmyra Atoll…………………………104 4.3 Current ellipses and dominant tidal frequencies at Palmyra Atoll……………………..105 4.4 In situ temperature time series obtained from six locations at Palmyra Atoll………….106 4.5 In situ temperature and current data highlighting an observed internal wave………….107 4.6 Numerical wave model results representing maximum annual wave forcing………….108 4.7 Spatial variability in benthic structure and environmental forcings around Palmyra…..109 4.8 Partial dependency plots from ecological modeling of benthic functional groups……..110 vii CHAPTER I INTRODUCTION Coral reefs are among the most diverse and productive marine ecosystems on earth. They provide economic benefits to millions of people as sources of food, employment, natural products, coastal protection and recreation (Knowlton 2001). Coral reef ecosystems are also among the most threatened by human disturbance, both at local scales due to over-fishing and land-based pollution, and on local to global scales owing to climate change and ocean acidification (Hughes et al. 2003; Knowlton and Jackson 2008). Despite the societal importance and susceptibility of reef systems to human impacts, until these contributions the intrinsic natural biological-physical relationships in coral reef ecosystems have not been well explored. Now, more than any other time in our history, effective ecosystem-based management and successful strategies to mitigate anthropogenic impacts to coral reef ecosystems require a fundamental understanding of the underlying abiotic-biotic interactions determining coral reef ecosystem function and health. Coral reef ecosystems are influenced by a suite of physical, chemical and biological environmental forcings that are highly variable across time and space (Brown 1997; Done 1999). Over time, coral reefs have adapted to exist within an envelope of environmental forcings that is region-specific and governed in part by a reef’s geographic location (Done 1999; Kleypas et al. 1999). Regional variation (hundreds to thousands of kilometers) in long-term environmental conditions drive spatial differences in coral reef communities (Brown 1997; Kleypas et al. 1999). Therefore, long-term gradients in environmental forcings should lead to clear spatial patterning in the benthic community as it organizes to adapt to local conditions (Hughes et al. 2012). 1 Human activity can also profoundly influence coral reef ecosystems (Knowlton 2001) . Urbanization, coastal development and land-use can result in increased sedimentation and nutrient enrichment, resulting
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