Wasting Disease and Environmental Variables Drive Sea Star Assemblages in the Northern Gulf of Alaska
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Wasting disease and environmental variables drive sea star assemblages in the northern Gulf of Alaska Item Type Thesis Authors Mitchell, Timothy James Download date 26/09/2021 12:37:49 Link to Item http://hdl.handle.net/11122/10521 WASTING DISEASE AND ENVIRONMENTAL VARIABLES DRIVE SEA STAR ASSEMBLAGES IN THE NORTHERN GULF OF ALASKA By Timothy James Mitchell, B.S. A Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science In Marine Biology University of Alaska Fairbanks May 2019 APPROVED: Brenda Konar, Committee Chair Katrin Iken, Committee Member Amanda Kelley, Committee Member Matthew Wooller, Chair Department of Marine Biology Trent Sutton, Dean College of Fisheries and Ocean Sciences Michael Castellini, Dean of the Graduate School Abstract Sea stars are ecologically important in rocky intertidal habitats. The recent (starting 2013) sea star die off attributed to sea star wasting disease throughout the eastern Pacific, presumably triggered by unusually warm waters in recent years, has caused an increased interest in spatial and temporal patterns of sea star assemblages and the environmental drivers that structure these assemblages. This study assessed the role of seven potential static environmental variables (distance to freshwater, tidewater glacial presence, wave exposure, fetch, beach slope, substrate composition, and tidal range) influencing northern Gulf of Alaska sea star assemblages before and after regional sea star declines. For this, intertidal surveys were conducted annually from 2005 to 2018 at five sites in each of four regions that were between 100 and 420 km apart. In the years leading up to the regional mortality events, assemblages were different among regions and were structured mainly by tidewater glacier presence, wave fetch, and tidal range. The assemblages after wasting disease were different from those before the event, and there was a partial change in the environmental variables that correlated with sea star structure. In these recent years, the environmental variables most highly correlated with sea star assemblages were slope, wave fetch, and tidal range, all of which relate to desiccation, attachment, and wave action. This indicates that the change in sea star density and structure by wasting disease left an assemblage that is responding to different environmental variables. Understanding the delicate interplay of some of the environmental variables that influence sea star assemblages could expand knowledge of the habitat preferences and tolerance ranges of important and relatively unstudied species within the northern Gulf of Alaska. iii iv Table of Contents Page Abstract........................................................................................................................................................ iii Table of Contents.......................................................................................................................................... v List of Figures.............................................................................................................................................. vii List of Tables...............................................................................................................................................vii Acknowledgements..................................................................................................................................... ix Introduction.................................................................................................................................................. 1 Materials and Methods.................................................................................................................................3 Study Area........................................................................................................................................3 Intertidal Surveys............................................................................................................................. 5 Static Environmental Variables........................................................................................................5 Data Analyses.................................................................................................................................. 6 Results.......................................................................................................................................................... 7 Regional Sea Star Assemblage Changes.......................................................................................... 7 Static Environmental Variable Correlations...................................................................................13 Discussion................................................................................................................................................... 16 References...................................................................................................................................................22 v vi List of Figures Page Fig. 1. Map derived from Konar et al. (2016) showing study sites........................................................... 4 Fig. 2. nMDS ordination plot of sea star assemblages.............................................................................. 8 Fig. 3. Changes in sea star density........................................................................................................... 10 Fig. 4. Sea star distribution across regions...............................................................................................12 Fig. 5. nMDS ordination plots of sites with years averaged.....................................................................14 Fig. 6. Yearly densities averaged over the western regions and eastern regions....................................15 Fig. 7. Monthly tidal water temperature anomalies................................................................................20 List of Tables Page Table 1. Site locations organized by zone.................................................................................................5 Table 2. ANOSIM R-values and p-values comparing differences in sea star assemblages.................... 11 Table 3. Average and standard error values.......................................................................................... 15 vii viii Acknowledgements I would first like to thank my advisor, Brenda Konar. She always made time for me and did everything in her power to keep me on the path to success. I would also like to thank my graduate committee members, Katrin Iken and Amanda Kelley. Their individual expertise and contributions were invaluable. I would also like to thank Heather Coletti from the National Park Service for her scrupulous data collection. Funding for this research was provided by the Exxon Valdez Oil Spill Trustee Council (EVOS), which enabled data collection by the Gulf Watch Alaska Monitoring Program. The Thesis Completion Fellowship from the University of Alaska Fairbanks Graduate School as well as EVOS helped to fund me as a graduate student. I must express my profound gratitude to all of the loved ones who offered limitless support. My parents, Carley and Sandy Mitchell, who provide me with continuous encouragement to reach my full potential. Lastly, my brilliant and loving wife, Amy, for believing in me and being there for me every step of the way. This accomplishment would not have been possible without them. Thank you. ix INTRODUCTION Intertidal zones represent some of the most extreme environments on earth (Helmuth and Hofmann, 2001; Sanford, 2002). Organisms living in the rocky intertidal zone must be able to manage extreme conditions, such as wave action, desiccation, and cold and warm air temperatures (Sanford, 2002). Nevertheless, this habitat supports diverse communities, which include ecologically important sea stars (Chenelot et al., 2007). Intertidal sea stars can differ in ecological roles, with some species having large- scale impacts on the structure and function of their community (Burt et al., 2018; Menge et al., 1994). The disproportionately high community impact that sea stars can have is perhaps best demonstrated by Pisaster ochraceus in the northeast Pacific, from which the keystone species concept was founded (Paine, 1966). The heavy predation by this species on mussels regulates intertidal community diversity by moderating the density of their prey and opening space for more species (Paine, 1966). Because sea stars can play such a pivotal role in structuring intertidal communities, changes to their distribution and abundance can have system-wide impacts. The structure of sea star assemblages can be influenced by environmental factors (Menge and Sutherland, 1987; Petes et al., 2008; Seabra et al., 2011). For example, salinity has been shown to correlate strongly with echinoderm distribution in rocky nearshore environments globally (Iken et al., 2010; Aguera et al., 2015). The sea star, P. ochraceus, can adapt to low salinity environments, although this will slow their feeding and mobility rates (Held and Harley, 2009). While some environmental parameters are dynamic in nature (e.g., temperature and salinity), many are static and do not vary widely over time. Static drivers, including wave exposure and wave fetch can impose strong physical forces in the intertidal environment, and wave splash can allow invertebrates to exist higher in the intertidal zone by