Mercury concentrations in Seaside Sparrows and Marsh Rice Rats differ across the Mississippi River Estuary.

Mercury (Hg) concentrations and their associated toxicological effects in terrestrial ecosystems of the Gulf of Mexico are largely unknown. Compounding this uncertainty, a large input of organic matter from the 2010 Deepwater Horizon oil spill may have altered Hg cycling and bioaccumulation dynamics. To test this idea, we quantified blood concentrations of total mercury (THg) in Seaside Sparrows (Ammospiza maritima) and Marsh Rice Rats (Oryzomys palustris) in marshes west and east of the Mississippi River in 2015 and 2016. We also tested for a difference in THg concentrations between oiled and non-oiled sites. To address the potential confounding effect of diet variation on Hg transfer, we used stable nitrogen (δ15N) and carbon (δ13C) isotope values as proxies of trophic position and the source of primary production, respectively. Our results revealed that five to six years after the spill, THg concentrations were not higher in sites oiled by the spill compared to non-oiled sites. In both species, THg was higher at sites east of the Mississippi River compared to control and oiled sites, located west. In Seaside Sparrows but not in Marsh Rice Rats, THg increased with δ15N values, suggesting Hg trophic biomagnification. Overall, even in sites with the most elevated THg, concentrations were generally low. In Seaside Sparrows, THg concentrations were also lower than previously reported in this and other closely related passerines, with only 7% of tested birds exceeding the lowest observed effect concentration associated with toxic effects across bird species (0.2 µg/g ww). The factors associated with geographic heterogeneity in Hg exposure remain uncertain. Clarification could inform risk assessment and future restoration and management actions in a region facing vast anthropogenic changes.

Consistencies in the dietary and isotopic niche of spotted seatrout, Cynoscion nebulosus, across a salinity gradient within a coastal Louisiana estuary.

Estuaries are essential habitats for recreational and commercial fish that are shaped by both natural and anthropogenic processes. In Louisiana a combination of climate change and planned coastal restoration actions is predicted to increase freshwater introduction to coastal estuaries. As such there is a need to quantify the relationships between estuarine fish ecology and salinity to aid in predicting how species will respond to shifts in salinity. We investigated the relative abundance and dietary niches of adult (24.5 ± 5.4 cm standard length) spotted seatrout Cynoscion nebulosus across varying salinity regimes (oligohaline, mesohaline, and polyhaline) within Barataria Bay, Louisiana, using a combination of net sampling and gut content and stable isotopes analysis. We found that the relative abundance of C. nebulosus was lowest at the oligohaline site, translating to approximately five fewer fish captured for every single psu decrease in a site's average annual salinity. In contrast, we found that diets and, to a lesser extent, isotopic niches had a high degree of overlap across sites with differing salinity regimes. Fish and penaeid shrimp were the most common and important prey taxa recovered from guts at all sites. The small isotopic differences found among sites were likely due to spatial variation in hydrogeochemical baselines, and the observed isotopic overlap provides support for the idea that C. nebulosus move between adjacent salinity regimes and forage throughout Barataria Bay. Our results contribute to a greater understanding of the salinity preference and trophic ecology of C. nebulosus that can aid in predicting their responses to future salinity and habitat changes within Barataria Bay associated with predicted climate change and planned coastal restoration actions.

Intercontinental analysis of temperate steppe stream food webs reveals consistent autochthonous support of fishes.

Quantifying the trophic basis of production for freshwater metazoa at broad spatial scales is key to understanding ecosystem function and has been a research priority for decades. However, previous lotic food web studies have been limited by geographic coverage or methodological constraints. We used compound-specific stable carbon isotope analysis of amino acids (AAs) to estimate basal resource contributions to fish consumers in streams spanning grassland, montane and semi-arid ecoregions of the temperate steppe biome on two continents. Across a range of stream sizes and light regimes, we found consistent trophic importance of aquatic resources. Essential AAs of heterotrophic microbial origin generally provided secondary support for fishes, while terrestrial carbon did not seem to provide significant, direct support. These findings provide strong evidence for the dominant contribution of carbon to higher-order consumers by aquatic autochthonous resources (primarily) and heterotrophic microbial communities (secondarily) in temperate steppe streams.

Divergent trophic responses of sympatric penguin species to historic anthropogenic exploitation and recent climate change.

The Southern Ocean is in an era of significant change. Historic overharvesting of marine mammals and recent climatic warming have cascading impacts on resource availability and, in turn, ecosystem structure and function. We examined trophic responses of sympatric chinstrap (Pygoscelis antarctica) and gentoo (Pygoscelis papua) penguins to nearly 100 y of shared environmental change in the Antarctic Peninsula region using compound-specific stable isotope analyses of museum specimens. A century ago, gentoo penguins fed almost exclusively on low-trophic level prey, such as krill, during the peak of historic overexploitation of marine mammals, which was hypothesized to have resulted in a krill surplus. In the last 40 y, gentoo penguin trophic position has increased a full level as krill declined in response to recent climate change, increased competition from recovering marine mammal populations, and the development of a commercial krill fishery. A shifting isotopic baseline supporting gentoo penguins suggests a concurrent increase in coastal productivity over this time. In contrast, chinstrap penguins exhibited no change in trophic position, despite variation in krill availability over the past century. The specialized foraging niche of chinstrap penguins likely renders them more sensitive to changes in krill availability, relative to gentoo penguins, as evinced by their declining population trends in the Antarctic Peninsula over the past 40 y. Over the next century, similarly divergent trophic and population responses are likely to occur among Antarctic krill predators if climate change and other anthropogenic impacts continue to favor generalist over specialist species.

Receding ice drove parallel expansions in Southern Ocean penguins.

Climate shifts are key drivers of ecosystem change. Despite the critical importance of Antarctica and the Southern Ocean for global climate, the extent of climate-driven ecological change in this region remains controversial. In particular, the biological effects of changing sea ice conditions are poorly understood. We hypothesize that rapid postglacial reductions in sea ice drove biological shifts across multiple widespread Southern Ocean species. We test for demographic shifts driven by climate events over recent millennia by analyzing population genomic datasets spanning 3 penguin genera (Eudyptes, Pygoscelis, and Aptenodytes). Demographic analyses for multiple species (macaroni/royal, eastern rockhopper, Adélie, gentoo, king, and emperor) currently inhabiting southern coastlines affected by heavy sea ice conditions during the Last Glacial Maximum (LGM) yielded genetic signatures of near-simultaneous population expansions associated with postglacial warming. Populations of the ice-adapted emperor penguin are inferred to have expanded slightly earlier than those of species requiring ice-free terrain. These concerted high-latitude expansion events contrast with relatively stable or declining demographic histories inferred for 4 penguin species (northern rockhopper, western rockhopper, Fiordland crested, and Snares crested) that apparently persisted throughout the LGM in ice-free habitats. Limited genetic structure detected in all ice-affected species across the vast Southern Ocean may reflect both rapid postglacial colonization of subantarctic and Antarctic shores, in addition to recent genetic exchange among populations. Together, these analyses highlight dramatic, ecosystem-wide responses to past Southern Ocean climate change and suggest potential for further shifts as warming continues.

Evidence of Pathogen-Induced Immunogenetic Selection across the Large Geographic Range of a Wild Seabird.

Over evolutionary time, pathogen challenge shapes the immune phenotype of the host to better respond to an incipient threat. The extent and direction of this selection pressure depend on the local pathogen composition, which is in turn determined by biotic and abiotic features of the environment. However, little is known about adaptation to local pathogen threats in wild animals. The Gentoo penguin (Pygoscelis papua) is a species complex that lends itself to the study of immune adaptation because of its circumpolar distribution over a large latitudinal range, with little or no admixture between different clades. In this study, we examine the diversity in a key family of innate immune genes-the Toll-like receptors (TLRs)-across the range of the Gentoo penguin. The three TLRs that we investigated present varying levels of diversity, with TLR4 and TLR5 greatly exceeding the diversity of TLR7. We present evidence of positive selection in TLR4 and TLR5, which points to pathogen-driven adaptation to the local pathogen milieu. Finally, we demonstrate that two positively selected cosegregating sites in TLR5 are sufficient to alter the responsiveness of the receptor to its bacterial ligand, flagellin. Taken together, these results suggest that Gentoo penguins have experienced distinct pathogen-driven selection pressures in different environments, which may be important given the role of the Gentoo penguin as a sentinel species in some of the world's most rapidly changing environments.

A multi-biomarker approach supports the use of compound-specific stable isotope analysis of amino acids to quantify basal carbon source use in a salt marsh consumer.

RATIONALE: Determining the flow of energy from primary producers to higher trophic levels in complex systems remains an important task for ecologists. Biomarkers can be used to trace carbon or energy sources contributing to an organism's tissues. However, different biomarkers vary in their ability to trace carbon sources based on how faithfully they transfer between trophic levels. Comparing emerging biomarker techniques with more commonly used techniques can demonstrate the relative efficacy of each in specific systems. METHODS: Two common biomarker techniques, fatty acid analysis (FAA) and bulk stable isotope analysis (SIA), and one emerging biomarker technique, compound-specific stable isotope analysis of amino acids (CSIA-AA), were compared to assess their ability to characterize and quantify basal carbon sources supporting the seaside sparrow (Ammodramus maritimus), a common salt marsh species. Herbivorous insect and deposit-feeding fiddler crab biomarker values were analyzed as proxies of major terrestrial and aquatic basal carbon sources, respectively. RESULTS: All three biomarker techniques indicated that both terrestrial and aquatic carbon sources were important to seaside sparrows. However, FAA could only be evaluated qualitatively, due to a currently limited understanding of trophic modification of fatty acids between primary producer and this consumer's tissues. Quantitative stable isotope (SIA or CSIA-AA) mixing models predicted nearly equal contributions of terrestrial and aquatic carbon sources supporting seaside sparrows, yet estimates based on CSIA-AA had greater precision. CONCLUSIONS: These findings support the use of CSIA-AA as an emerging tool to quantify the relative importance of basal carbon sources in salt marsh consumers. Integrating multiple biomarker techniques, with their differing benefits and limitations, will help to constrain models of carbon and energy flow in future ecosystem studies.

Comparative population genomics reveals key barriers to dispersal in Southern Ocean penguins.

The mechanisms that determine patterns of species dispersal are important factors in the production and maintenance of biodiversity. Understanding these mechanisms helps to forecast the responses of species to environmental change. Here, we used a comparative framework and genomewide data obtained through RAD-Seq to compare the patterns of connectivity among breeding colonies for five penguin species with shared ancestry, overlapping distributions and differing ecological niches, allowing an examination of the intrinsic and extrinsic barriers governing dispersal patterns. Our findings show that at-sea range and oceanography underlie patterns of dispersal in these penguins. The pelagic niche of emperor (Aptenodytes forsteri), king (A. patagonicus), Adélie (Pygoscelis adeliae) and chinstrap (P. antarctica) penguins facilitates gene flow over thousands of kilometres. In contrast, the coastal niche of gentoo penguins (P. papua) limits dispersal, resulting in population divergences. Oceanographic fronts also act as dispersal barriers to some extent. We recommend that forecasts of extinction risk incorporate dispersal and that management units are defined by at-sea range and oceanography in species lacking genetic data.

Google Haul Out: Earth Observation Imagery and Digital Aerial Surveys in Coastal Wildlife Management and Abundance Estimation.

As the sampling frequency and resolution of Earth observation imagery increase, there are growing opportunities for novel applications in population monitoring. New methods are required to apply established analytical approaches to data collected from new observation platforms (e.g., satellites and unmanned aerial vehicles). Here, we present a method that estimates regional seasonal abundances for an understudied and growing population of gray seals (Halichoerus grypus) in southeastern Massachusetts, using opportunistic observations in Google Earth imagery. Abundance estimates are derived from digital aerial survey counts by adapting established correction-based analyses with telemetry behavioral observation to quantify survey biases. The result is a first regional understanding of gray seal abundance in the northeast US through opportunistic Earth observation imagery and repurposed animal telemetry data. As species observation data from Earth observation imagery become more ubiquitous, such methods provide a robust, adaptable, and cost-effective solution to monitoring animal colonies and understanding species abundances.

Stable isotope analyses of feather amino acids identify penguin migration strategies at ocean basin scales.

Identifying the at-sea distribution of wide-ranging marine predators is critical to understanding their ecology. Advances in electronic tracking devices and intrinsic biogeochemical markers have greatly improved our ability to track animal movements on ocean-wide scales. Here, we show that, in combination with direct tracking, stable carbon isotope analysis of essential amino acids in tail feathers provides the ability to track the movement patterns of two, wide-ranging penguin species over ocean basin scales. In addition, we use this isotopic approach across multiple breeding colonies in the Scotia Arc to evaluate migration trends at a regional scale that would be logistically challenging using direct tracking alone.

Relationships between isotopic values and oxidative status: insights from populations of gentoo penguins.

Feeding strategies can affect the balance between the production of reactive oxygen species and antioxidant defences (i.e. oxidative status). This is ecologically relevant, as variation in oxidative status can in turn strongly affect fitness. However, how animals regulate their oxidative status through their feeding behaviour under natural conditions remains poorly understood. Thus, relating the isotopic values of free-ranging animals to their oxidative status may prove useful. Here, we considered three colonies of gentoo penguins (Pygoscelis papua) in which we measured (1) δ(13)C and δ(15)N values, and (2) antioxidant defences and oxidative damage. We found that colonies with the highest δ(13)C and δ(15)N values also had the highest levels of antioxidant defences and oxidative damage, resulting in positive relationships between isotopic values and markers of oxidative status. As a result, colony segregation in terms of isotopic values was reflected by segregation in terms of oxidative markers (although more markedly for oxidative damage than for antioxidant defences). Interestingly, variation in the estimated contribution of krill in the diet of penguins followed an opposite pattern to that observed for markers of oxidative status, providing evidence that inter-population differences in terms of foraging strategies can result in inter-population differences in terms of oxidative status. More studies examining simultaneously oxidative status, isotopic signature, foraging behaviour and food allocation between parents and young are, however, needed to understand better the interplay between the foraging strategies adopted by animals in their natural habitat and their oxidative status.

Responses of high-elevation herbaceous plant assemblages to low glacial CO2 concentrations revealed by fossil marmot (Marmota) teeth.

Atmospheric CO2 cycles of the Quaternary likely imposed major constraints on the physiology and growth of C3 plants worldwide. However, the measured record of this remains both geographically and taxonomically sparse. We present the first reconstruction of physiological responses in a late Quaternary high-elevation herbaceous plant community from the Southern Rocky Mountains, USA. We used a novel proxy-fossilized tooth enamel of yellow-bellied marmots (Marmota flaviventris)-which we developed using detailed isotopic analysis of modern individuals. Calculated C isotopic discrimination (Δ) of alpine plants was nearly 2 ‰ lower prior to the Last Glacial Maximum than at present, a response almost identical to that of nonherbaceous taxa from lower elevations. However, initial shifts in Δ aligned most closely with the onset of the late Pleistocene bipolar temperature "seesaw" rather than CO2 increase, indicating unique limitations on glacial-age high-elevation plants may have existed due to both low temperatures and low CO2. Further development of system-specific faunal proxies can help to clarify this and other plant- and ecosystem-level responses to past environmental change.

Multi-tissue analyses reveal limited inter-annual and seasonal variation in mercury exposure in an Antarctic penguin community.

Inter-annual variation in tissue mercury concentrations in birds can result from annual changes in the bioavailability of mercury or shifts in dietary composition and/or trophic level. We investigated potential annual variability in mercury dynamics in the Antarctic marine food web using Pygoscelis penguins as biomonitors. Eggshell membrane, chick down, and adult feathers were collected from three species of sympatrically breeding Pygoscelis penguins during the austral summers of 2006/2007-2010/2011. To evaluate the hypothesis that mercury concentrations in penguins exhibit significant inter-annual variation and to determine the potential source of such variation (dietary or environmental), we compared tissue mercury concentrations with trophic levels as indicated by δ(15)N values from all species and tissues. Overall, no inter-annual variation in mercury was observed in adult feathers suggesting that mercury exposure, on an annual scale, was consistent for Pygoscelis penguins. However, when examining tissues that reflected more discrete time periods (chick down and eggshell membrane) relative to adult feathers, we found some evidence of inter-annual variation in mercury exposure during penguins' pre-breeding and chick rearing periods. Evidence of inter-annual variation in penguin trophic level was also limited suggesting that foraging ecology and environmental factors related to the bioavailability of mercury may provide more explanatory power for mercury exposure compared to trophic level alone. Even so, the variable strength of relationships observed between trophic level and tissue mercury concentrations across and within Pygoscelis penguin species suggest that caution is required when selecting appropriate species and tissue combinations for environmental biomonitoring studies in Antarctica.

Seasonal variability in the feeding ecology of an oceanic predator.

Complementary approaches (stomach contents, DNA barcoding, and stable isotopes) were used to examine seasonal shifts in the feeding ecology of an oceanic predator, yellowfin tuna (Thunnus albacares, n = 577), in the northern Gulf of Mexico. DNA barcoding greatly enhanced dietary resolution and seasonally distinct prey assemblages were observed for both sub-adults and adults. In general, diet was characterized by ommastrephid squids and exocoetids in spring, juvenile fishes (i.e., carangids and scombrids) in summer, migratory coastal fishes during fall, and an increased consumption of planktonic prey (e.g., amphipods) in winter. Seasonal variability in bulk stable isotope values (δ13C, δ15N, and δ34S) was also observed, with low δ15N values and high δ34S values during late summer/early fall and high δ15N values (low δ34S) during late winter/early spring. Bayesian stable isotope mixing models corroborated seasonal diet shifts, highlighting the importance of oceanic nekton in spring/summer, coastal nekton during fall, and oceanic plankton during winter. Seasonal shifts in diet appeared to be influenced by prey reproductive cycles, habitat associations, and environmental conditions. Findings highlight the complex food web dynamics supporting an opportunistic oceanic predator and the importance of seasonal cycles in prey availability to predator resource utilization in open-ocean ecosystems.

Tree canopy cover affects basal resources and nutrient profiles of Aedes and Culex larvae in cemetery vases in New Orleans, Louisiana, United States.

Cemetery vases are important habitat for vector mosquito production, yet there is limited understanding on their food web dynamics and how they vary across environmental gradients. Tree cover is one factor that varies widely across cemeteries, and influence food webs by means of detrital inputs, temperature mediation, and light availability. Such information can be important for determining mosquito adult body size, fecundity, and competition outcomes, all of which may influence mosquito population and disease risk. This study evaluates the relationship between tree canopy cover and indicators of basal resources for Aede aegypti (L.), Aedes albopictuss (Skuse), and Culex quinquefasciatus (Say) larvae, such as stable isotopes (δ13C and δ15N) and nutrient stoichiometry in cemeteries of New Orleans, Louisiana (USA). Stable isotope values suggest that larvae feed directly on the Particulate Organic Matter (POM) suspended in the vase's water, and that POM composition influence the nutrient profiles of mosquito larvae. The POM of open canopy vases had higher δ13C values, than that of closed canopy vases indicating differences in relative proportion of basal carbon sources, with open canopy POM having a lower proportion of allochthonous carbon, and a higher proportion of authoctonous carbon. Accordingly, mosquito larvae collected from open canopy vases had higher δ13C values, and higher C:N than larvae from closed canopy vases. The results of this study show a shift in food web dynamics driven by canopy cover in cemetery vases that directly influence the nutrient profiles of mosquito larvae. The implications for mosquito ecology, and vector management are discussed.

Wanted: dead or alive? Isotopic analysis (δ13C and δ15N) of Pygoscelis penguin chick tissues supports opportunistic sampling.

RATIONALE: Physiological stress and starvation have been shown to affect δ(13)C and δ(15)N isotope values and, given that animals often die from starvation, the cause of death may be an important factor to consider in stable isotope analyses of opportunistically collected samples. METHODS: We addressed this issue by comparing tissue stable isotope values of living and deceased Adélie (Pygoscelis adeliae) and Chinstrap Penguin (P. antarctica) chicks collected from the same respective populations. RESULTS: No significant difference was found between living and deceased penguin chick feather, down, and toenail isotope values and both groups displayed similar isotopic trends between tissue types. In addition, similar relationships were observed between both species and across several seasons. Furthermore, sub-dermal adiposity and cause of death (starvation and/or predation) had no significant effect on the δ(13)C and δ(15)N values. CONCLUSIONS: Our findings suggest that tissues from deceased penguins can be isotopically representative of tissues obtained from the living population, despite the cause of death, and support the use of opportunistic sampling in stable isotope analyses.

Pattern of mercury allocation into egg components is independent of dietary exposure in Gentoo penguins.

Avian eggs have become one of the most common means of evaluating mercury contamination in aquatic and marine environments and can serve as reliable indicators of dietary mercury exposure. We investigated patterns of mercury deposition into the major components of penguin eggs (shell, membrane, albumen, and yolk) using the Gentoo penguin (Pygoscelis papua) as a model species. Eggs were collected from both wild and captive populations of Gentoo penguins to compare the allocation of mercury into individual egg components of birds feeding at disparate trophic positions as inferred by stable isotope analysis. Mercury concentrations in captive penguins were an order of magnitude higher than in wild birds, presumably because the former were fed only fish at a higher trophic position relative to wild penguins that fed on a diet of 72-93% krill (Euphausia spp.). Similar to previous studies, we found the majority of total egg mercury sequestered in the albumen (92%) followed by the yolk (6.7%) with the lowest amounts in the shell (0.9%) and membrane (0.4%). Regardless of dietary exposure, mercury concentrations in yolk and membrane, and to a lesser degree shell, increased with increasing albumen mercury (used as a proxy for whole-egg mercury), indicating that any component, in the absence of others, may be suitable for monitoring changes in dietary mercury. Because accessibility of egg tissues in the wild varies, the establishment of consistent relationships among egg components will facilitate comparisons with any other study using eggs to assess dietary exposure to mercury.

Population dynamics and resource availability drive seasonal shifts in the consumptive and competitive impacts of introduced house mice (Mus musculus) on an island ecosystem.

Background: House mice (Mus musculus) are widespread and invasive on many islands where they can have both direct and indirect impacts on native ecological communities. Given their opportunistic, omnivorous nature the consumptive and competitive impacts of house mice on islands have the potential to vary over time in concert with resource availability and mouse population dynamics. Methods: We examined the ecological niche of invasive house mice on Southeast Farallon Island, California, USA using a combination of mouse trapping, food resource surveys, and stable isotope analysis to better understand their trophic interactions with native flora and fauna. Specifically, we coupled the analysis of seasonal variation in resource availability over a 17-year period (2001-2017), carbon (δ 13C) and nitrogen (δ 15N) stable isotope values of mouse tissue and prey resources in a single year (2013), and isotopic niche and mixing models to quantify seasonal variation in mouse diets and the potential for resource overlap with native species. Results: We found that plants were the most important resource for house mice during the spring months when vegetation is abundant and mouse populations are low following heavy precipitation and declines in mouse abundance during the winter. While still consumed, plants declined in dietary importance throughout the summer and fall as mouse populations increased, and seabird and arthropod resources became relatively more available and consumed by house mice. Mouse abundance peaks and other resource availability are low on the island in the fall months when the isotopic niches of house mice and salamanders overlap significantly indicating the potential for competition, most likely for arthropod prey. Discussion: Our results indicate how seasonal shifts in both mouse abundance and resource availability are key factors that mediate the consumptive and competitive impacts of introduced house mice on this island ecosystem. As mice consume and/or compete with a wide range of native taxa, eradication has the potential to provide wide-reaching restoration benefits on Southeast Farallon Island. Post-eradication monitoring focused on plant, terrestrial invertebrate, salamander, and seabird populations will be crucial to confirm these predictions.

Mercury exposure driven by geographic and trophic factors in Magellanic penguins from Tierra del Fuego.

Penguins accumulate mercury due to their long-life span together with their high trophic position. We sampled adult and juveniles' feathers from three colonies of Spheniscus magellanicus from Tierra del Fuego along an inshore-offshore corridor. We integrated toxicological information (mercury concentrations) and foraging biomarkers (δ13C, δ15N) into a common data analysis framework (isotopic niche analysis) to evaluate the influence of age, location, and foraging behaviors on mercury concentrations. Adults had higher feather mercury concentrations, δ13C, and δ15N values compared to juveniles. Also, adult and juvenile feather mercury concentrations differed between colonies, with lower mercury concentrations at the nearest inshore colony relative to the farther offshore colonies. Trophic position and the isotopic niche analyses suggest that this geographic gradient in mercury concentrations is due to differences in colonies' foraging areas. Understanding penguins' exposure to mercury derived from local food webs is a crucial first step in evaluating the impacts of this heavy metal on their conservation status.

Mercury and stable isotopes portray colony-specific foraging grounds in southern rockhopper penguins over the Patagonian Shelf.

Mercury pollution is a serious global environmental issue and the characterization of its distribution and its driving forces should be urgently included in research agendas. We report unusually high mercury (Hg) concentrations (>5 µg/g) along with stable isotopes values in feathers of southern rockhopper penguins (Eudyptes chrysocome) from colonies in the Southwest Atlantic Ocean. We found a highly heterogenous prevalence of Hg throughout the study area and over a three-fold higher mean Hg concentration in southernmost colonies. Variation in Hg concentrations among colonies is primarily explained by site, rather than by trophic position. We provide further support to the existence of a Hg hotspot in the food web of the Patagonian Shelf and spatially restrict it to the southern tip of South America. Our findings highlight the need for regional and colony-based seabird conservation management when high local variability and plasticity in foraging habits is evident.