Zarn et al.: Isotope and mercury analysis of Galápagos seabirds 71 STABLE ISOTOPE AND MERCURY ANALYSES OF THE GALAPAGOS ISLANDS SEABIRD COMMUNITY ANNA M. ZARN1*, CARLOS A. VALLE2,3, REBECKA BRASSO4, WILLIAM D. FETZNER5 & STEVEN D. EMSLIE1 1University of North Carolina, Department of Biology and Marine Biology, 601 S. College Rd., Wilmington, NC 28403, USA *([email protected]) 2Universidad San Francisco de Quito USFQ, Colegio de Ciencias Biológicas y Ambientales COCIBA, Campus Cumbavá Av. Diego de Robles S/N e Interoceánica, Quito, Ecuador 3Galápagos Science Center GSC-USFQ, Av. Alsacio Northia, Isla San Cristóbal, Galápagos, Ecuador 4Weber State University, Department of Zoology, 1415 Edvalson St., Ogden, UT 84408, USA 5Texas Tech University, Department of Biological Sciences, Lubbock, TX 79409, USA Received 08 July 2019, accepted 10 December 2019 ABSTRACT ZARN, A.M., VALLE, C.A., BRASSO, R., FETZNER, W. & EMSLIE, S.D. 2020. Stable isotope and mercury analyses of the Galapagos Islands seabird community. Marine Ornithology 48: 71–80. The Galapagos Islands seabird community is directly impacted by El Niño Southern Oscillation (ENSO) cycles, which makes understanding seabird foraging behavior in response to these events important for future conservation plans. In this study, we used stable isotope analysis (δ15N and δ13C) to investigate trophic status and foraging location in the seabird community before, during, and after the 2015–2016 El Niño event. Mercury (Hg) analysis was also performed to provide a more thorough understanding of the relationship between contaminant exposure and foraging behavior. We analyzed breast feathers collected across five years (2011, 2014–2017) from eight nesting seabird species (Sula sula, S. granti, S. nebouxii excisa, Fregata minor, F. magnificens, Oceanodroma tethys tethys, Creagrus furcatus, and Phaethon aethereus) for δ15N and δ13C isotopes and total Hg (ppm). These sampling periods occurred at different points in the ENSO cycle, which allowed shifts in foraging behavior to be monitored as environmental conditions changed. Our findings indicate that higher Hg contamination is positively correlated with La Niña. Additionally, as prey abundance decreased with the onset of El Niño in 2015, most species showed more negative δ13C values, which indicates a shift to more pelagic foraging. Furthermore, isotopic nitrogen values revealed that while foraging by most species decreased in trophic level during the 2015–2016 El Niño, some populations, mainly Sula species, increased in trophic level. Both responses indicate a change in diet, suggestive of flexible foraging behavior. Key words: foraging, δ15N, δ13C, mercury, ENSO events, tropical seabirds, dietary shifts INTRODUCTION concern. Organisms at high trophic levels, such as seabirds, have a high risk of encountering health problems due to Hg exposure The Galapagos Islands support a diverse seabird community that because of the bioaccumulation of Hg through trophic transfer includes six endemic species that occur within the Galapagos (Elliott 2005, Szumilo-Pilarska et al. 2016). This is especially Marine Reserve (Sibley & Monroe 1990). Although the ecology problematic in coastal upwelling zones where increases in Hg of many of these species has been extensively investigated, an concentrations in surface waters has been documented (Gworek understanding of how they respond to variations in prey availability et al. 2016). Impacts of Hg toxicity include reduced reproductive remains unknown. success (Finkelstein et al. 2006, Winder et al. 2012), behavioral changes, physiological impairments, and increased susceptibility to The most recent El Niño began in 2015 and quickly became a very disease (Grajewska et al. 2015, Evers 2018). strong El Niño (VSE), reaching an Oceanic Niño Index (ONI) of 2.6, similar to previous VSEs in 1982 and 1997 (Stramma et al. Understanding the foraging behavior of seabirds can be challenging 2016). These strong events reduce primary production and, in turn, because they spend much of their lives at sea, making continuous the availability of prey (Forero & Hobson 2003, Cherel et al. 2008, observation by humans difficult. Fortunately, stable isotope analysis, Young et al. 2010b, Sigman & Hain 2012, Mendez et al. 2016). specifically δ15N and δ13C, has become a valuable technique for As a result, the foraging habits of individual birds are affected, providing a proxy in the investigation of the diets of seabirds including expansion of their range or foraging at a different trophic (Jiménez-Uzcátegui et al. 2019). This technique provides indirect level (England 2000). Past VSEs have resulted in a decrease in indicators of foraging location and trophic level, using carbon as an reproductive success of seabird species, most notably in 1982–1983 indicator of foraging location and nitrogen as an indicator of trophic when many nests were deserted (Valle et al. 1987, Schreiber & level (Finkelstein et al. 2006, Fry 2006, Bond & Jones 2009). Burger 2001). The objective of this study was to use δ15N and δ13C analyses to In addition, mercury (Hg) emission and pollution has steadily enhance our understanding of the foraging behavior of Galapagos increased with urbanization (Wiener 2013), becoming a public seabirds, and, more specifically, to determine whether foraging Marine Ornithology 48: 71–80 (2020) 72 Zarn et al.: Isotope and mercury analysis of Galápagos seabirds behavior changes in response to El Niño Southern Oscillation At each sampling location, at least 20 adult individuals of a given (ENSO) events. In addition, we sought to monitor the Hg exposure species were captured when possible, and five to six breast feathers of this community over time to identify populations that are at risk of were plucked from each individual. Fewer Magnificent Frigatebirds being adversely affected by Hg toxicity. We addressed the following were captured because of their small population sizes, and we questions: (1) are there differences in diet among colonies of the same captured only adults without chicks because of a high chance of nest species; (2) do δ15N and δ13C values, and thus foraging behavior, shift abandonment when disturbed. Because feathers are metabolically in relation to ENSO phases; and (3) does Hg exposure correlate with inert once fully grown, a feather will contain stable isotopes and ENSO phases and their effects on upwelling? mercury assimilated at the time of its growth (Robertson 2004). This makes feathers a good source of information on foraging STUDY AREA AND METHODS behavior during the molting period or non-breeding season (Forero & Hobson 2003, Young et al. 2010a). Breast feathers were used Breeding colonies of Great Fregata minor and Magnificent (rather than other feather types) because variation in stable isotope Frigatebirds F. magnificens; Nazca Sula granti, Blue-footed S. and Hg values between contemporaneous breast feathers is low nebouxii excisa, and Red-footed Boobies S. sula; Wedge-rumped (Kojadinovic et al. 2007). Low variation was confirmed by testing Storm Petrels Oceanodroma tethys tethys; Red-billed Tropicbirds a subset of samples for each species in triplicate. Phaethon aethereus; and Swallow-tailed Gulls Creagrus furcatus—located in the Galapagos Islands (0°46ʹ38.1324″S, Unfortunately, very little is known about the body molting process 91°8ʹ33.2808″W)—were sampled across eight locations (specific among these species. The onset of molt is generally related to sampling locations given in parentheses, if applicable): Genovesa breeding phenology, either starting directly after, or ending prior, to (Darwin Bay and Prince Philip’s Steps), Seymour Norte, Daphne breeding (Howell 2003). Additionally, a prebasic molt, in which all Major, Plaza Sur, San Cristóbal (Punta Pitt), Floreana (The body feathers are replaced, occurs once per year (Howell 2003). We Caves), and Española (Punta Suárez and Punta Cevallos; Fig. 1). assumed that the feathers we collected represented the ENSO phase We chose these eight species to address our questions because we that occurred immediately before sampling. anticipated that, collectively, they would encompass the full range of foraging strategies (high to low trophic foraging) and locations The feathers were stored in paper bags and shipped to the (pelagic versus inshore or local foraging) used within the seabird University of North Carolina Wilmington (UNCW) for analysis. community. These species also breed in small-to-large colonies, All sample collection was conducted under an active IACUC Permit which facilitated our sampling efforts and increased our chances (A1617-012) to SDE. Before stable isotope analysis, the feathers of obtaining adequate sample sizes. Sampling was conducted in were washed in a 2:1 chloroform:methanol solution. The feather August 2011 (La Niña), October 2014 (Neutral), October 2015 barbs were then cut, avoiding the rachis, and run through a Thermo (El Niño), June 2016 (El Niño), and June 2017 (La Niña). In 2014 V Delta Plus Isotope Ratio Mass Spectrometer at the UNCW and 2015, we sampled at more than one location to address inter- Center for Marine Science. Raw δ values were reported and then island variation. normalized on a two-point scale using certified reference material Fig. 1. Map of sampling locations in the Galapagos Islands: Genovesa (GEN), Daphne Major (DAP), Seymour Norte (SEY), Plaza Sur (PLA), Punta Pitt (PITT), Punta Suárez (SUAR), Punta Cevallos (CEV), Floreana (FLO). Created using ArcGIS® software by Esri. Source layer credits: Esri, HERE,