Barkley
Hannah C.
Barkley
Hannah C.
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ArticleObservations and a model of net calcification declines in Palau's largest coral reef lagoon between 1992 and 2015(American Geophysical Union, 2020-07-06) Lentz, Steven J. ; Cohen, Anne L. ; Shamberger, Kathryn E. F. ; Barkley, Hannah C.Net ecosystem calcification (NEC) rates of Palau's largest lagoon and barrier reef system between 1992 and 2015 are estimated from sparse total alkalinity (TA) and salinity measurements and a tidal exchange model in which surface lagoon water transported offshore on the ebb tide is replaced by saltier (denser) ocean water that sinks to the bottom after entering the lagoon on the flood tide. Observed lagoon salinities are accurately reproduced by the model with no adjustable parameters. To accurately reproduce observed lagoon TA, NEC for the lagoon‐barrier reef system was 70 mmols m−2 day−1 from 1992 to 1998, 35 mmols m−2 day−1 from 1999 to 2012, and 25 mmols m−2 day−1 from 2013 to 2015. This indicates that Palau's largest lagoon and barrier reef system has not recovered, as of 2015, from the 50% decline in NEC in 1998 caused by the loss of coral cover following a severe bleaching event. The cause of the further decline in NEC in 2012–2013 is unclear. Lagoon residence times vary from 8 days during spring tides to 14 days during neap tides and drive substantial spring‐neap variations in lagoon TA (~25% of the mean salinity‐normalized ocean‐lagoon TA difference). Sparse measurements that do not resolve these spring‐neap variations can exhibit apparent long‐term variations in alkalinity that are not due to changes in NEC.
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PreprintCoral macrobioerosion is accelerated by ocean acidification and nutrients( 2014-10) DeCarlo, Thomas M. ; Cohen, Anne L. ; Barkley, Hannah C. ; Cobban, Quinn ; Young, Charles W. ; Shamberger, Kathryn E. F. ; Brainard, Russell E. ; Golbuu, YimnangCoral reefs exist in a delicate balance between calcium carbonate (CaCO3) production and CaCO3 loss. Ocean acidification (OA), the CO2-driven decline in seawater pH and CaCO3 saturation state (Ω), threatens to tip this balance by decreasing calcification, and increasing erosion and dissolution. While multiple CO2 manipulation experiments show coral calcification declines under OA, the sensitivity of bioerosion to OA is less well understood. Previous work suggests that coral and coral reef bioerosion increase with decreasing seawater Ω. However, in the surface ocean, Ω and nutrient concentrations often covary, making their relative influence difficult to resolve. Here, we exploit unique natural gradients in Ω and nutrients across the Pacific basin to quantify the impact of these factors, together and independently, on macrobioerosion rates of coral skeletons. Using an automated program to quantify macrobioerosion in 3-D computerized tomography (CT) scans of coral cores, we show that macrobioerosion rates of live Porites colonies in both low-nutrient (oligotrophic) and high-nutrient (>1 µM nitrate) waters increase significantly as Ω decreases. However, the sensitivity of macrobioerosion to Ω is ten times greater under high-nutrient conditions. Our results demonstrate that OA (decreased Ω) alone can increase coral macrobioerosion rates, but the interaction of OA with local stressors exacerbates its impact, accelerating a shift toward net CaCO3 removal from coral reefs.
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PreprintReplanting reduces frog diversity in oil palm( 2015-06) Kurz, David J. ; Turner, Edgar C. ; Aryawan, Agung A. ; Barkley, Hannah C. ; Caliman, Jean-Pierre ; Konopik, Oliver ; Ps, Sudharto ; Foster, William A.A growing body of literature has demonstrated significant biodiversity losses for many taxa when forest is converted to oil palm. However, no studies have directly investigated changes to biodiversity throughout the oil palm life cycle, in which oil palm matures for 25-30 years before replanting. This process leads to major changes in the oil palm landscape that likely influence species assemblages and ecosystem function. We compare frog assemblages between mature (21-27 year old) and recently replanted (1-2 year old) oil palm in Sumatra, Indonesia. Across eighteen 2.25-ha oil palm plots, we found 719 frogs from 14 species. Frog richness was 31 percent lower in replanted oil palm (9 species) than mature oil palm (13 species). Total frog abundance was 47 percent lower in replanted oil palm, and frog assemblage composition differed significantly between the two ages of oil palm. The majority of frog species were disturbance- tolerant, although we encountered four forest-associated frog species within mature oil palm despite a distance of 28 km between our study sites and the nearest extensive tract of forest. Although it is clear that protection of forest is of paramount importance for the conservation of tropical fauna, our results indicate that management decisions within tropical agricultural landscapes also have a profound impact on biodiversity. Practices such as staggered replanting or variable retention of mature oil palm patches could help maintain frog diversity in the oil palm landscape.
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ArticleDiverse coral communities in naturally acidified waters of a Western Pacific reef(John Wiley & Sons, 2014-01-16) Shamberger, Kathryn E. F. ; Cohen, Anne L. ; Golbuu, Yimnang ; McCorkle, Daniel C. ; Lentz, Steven J. ; Barkley, Hannah C.Anthropogenic carbon dioxide emissions are acidifying the oceans, reducing the concentration of carbonate ions ([CO32−]) that calcifying organisms need to build and cement coral reefs. To date, studies of a handful of naturally acidified reef systems reveal depauperate communities, sometimes with reduced coral cover and calcification rates, consistent with results of laboratory-based studies. Here we report the existence of highly diverse, coral-dominated reef communities under chronically low pH and aragonite saturation state (Ωar). Biological and hydrographic processes change the chemistry of the seawater moving across the barrier reefs and into Palau's Rock Island bays, where levels of acidification approach those projected for the western tropical Pacific open ocean by 2100. Nevertheless, coral diversity, cover, and calcification rates are maintained across this natural acidification gradient. Identifying the combination of biological and environmental factors that enable these communities to persist could provide important insights into the future of coral reefs under anthropogenic acidification.
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ArticleRepeat bleaching of a central Pacific coral reef over the past six decades (1960–2016)(Nature Publishing Group, 2018-11-08) Barkley, Hannah C. ; Cohen, Anne L. ; Mollica, Nathaniel R. ; Brainard, Russell E. ; Rivera, Hanny E. ; DeCarlo, Thomas M. ; Lohmann, George P. ; Drenkard, Elizabeth J. ; Alpert, Alice ; Young, Charles W. ; Vargas-Ángel, Bernardo ; Lino, Kevin C. ; Oliver, Thomas A. ; Pietro, Kathryn R. ; Luu, VictoriaThe oceans are warming and coral reefs are bleaching with increased frequency and severity, fueling concerns for their survival through this century. Yet in the central equatorial Pacific, some of the world’s most productive reefs regularly experience extreme heat associated with El Niño. Here we use skeletal signatures preserved in long-lived corals on Jarvis Island to evaluate the coral community response to multiple successive heatwaves since 1960. By tracking skeletal stress band formation through the 2015-16 El Nino, which killed 95% of Jarvis corals, we validate their utility as proxies of bleaching severity and show that 2015-16 was not the first catastrophic bleaching event on Jarvis. Since 1960, eight severe (>30% bleaching) and two moderate (<30% bleaching) events occurred, each coinciding with El Niño. While the frequency and severity of bleaching on Jarvis did not increase over this time period, 2015–16 was unprecedented in magnitude. The trajectory of recovery of this historically resilient ecosystem will provide critical insights into the potential for coral reef resilience in a warming world.
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ThesisA scientific framework for evaluating coral reef resilience to climate change(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2016-09) Barkley, Hannah C.The 21𝑠𝑡 century warming and acidification of tropical oceans will impact the structure and function of coral reef ecosystems. Consequently, conservation efforts are increasingly focused on identifying and protecting reef communities that demonstrate resilience to these changes. In this thesis, I develop a scientific framework for identifying climate change resilience in coral communities and, using Palau’s coral reefs as a case study, demonstrate the application of this approach. First, I use coral skeletal records to evaluate the sensitivity of coral communities to episodes of severe thermal stress. This information reveals coral reef communities that consistently exhibit weak responses to multiple high temperature events. Second, I evaluate coral reef community structure across a strong, natural pH gradient using metrics informed by laboratory ocean acidification studies. The coral communities of Palau’s Rock Island reefs show a level of pH tolerance that is unique amongst reefs studied to date. Third, I conduct laboratory and field experiments to constrain the pH thresholds of these resilient corals and investigate potential mechanisms for pH tolerance. Finally, I combine archipelago-wide coral temperature and pH sensitivity data to construct climate change resilience indices. My study succeeds in identifying a small number of coral communities that have the potential to withstand 21𝑠𝑡 century climate change and highlights the spatial variability in community responses to ocean warming and acidification. Critically, I present a set of scientific tools and approaches for identifying resilient coral reef communities that has applicability to coral reefs worldwide.