The impact of mercury on North American songbirds: effects, trends, and predictive factors

Global mercury concentrations in biota: their use as a basis for a global biomonitoring framework

An important provision of the Minamata Convention on Mercury is to monitor and evaluate the effectiveness of the adopted measures and its implementation. Here, we describe for the first time currently available biotic mercury (Hg) data on a global scale to improve the understanding of global efforts to reduce the impact of Hg pollution on people and the environment. Data from the peer-reviewed literature were compiled in the Global Biotic Mercury Synthesis (GBMS) database (>550,000 data points). These data provide a foundation for establishing a biomonitoring framework needed to track Hg concentrations in biota globally. We describe Hg exposure in the taxa identified by the Minamata Convention: fish, sea turtles, birds, and marine mammals. Based on the GBMS database, Hg concentrations are presented at relevant geographic scales for continents and oceanic basins. We identify some effective regional templates for monitoring methylmercury (MeHg) availability in the environment, but overall illustrate that there is a general lack of regional biomonitoring initiatives around the world, especially in Africa, Australia, Indo-Pacific, Middle East, and South Atlantic and Pacific Oceans. Temporal trend data for Hg in biota are generally limited. Ecologically sensitive sites (where biota have above average MeHg tissue concentrations) have been identified throughout the world. Efforts to model and quantify ecosystem sensitivity locally, regionally, and globally could help establish effective and efficient biomonitoring programs. We present a framework for a global Hg biomonitoring network that includes a three-step continental and oceanic approach to integrate existing biomonitoring efforts and prioritize filling regional data gaps linked with key Hg sources. We describe a standardized approach that builds on an evidence-based evaluation to assess the Minamata Convention's progress to reduce the impact of global Hg pollution on people and the environment.

Wildfire burn severity and stream chemistry influence aquatic invertebrate and riparian avian mercury exposure in forested ecosystems

Terrestrial soils in forested landscapes represent some of the largest mercury (Hg) reserves globally. Wildfire can alter the storage and distribution of terrestrial-bound Hg via reemission to the atmosphere or mobilization in watersheds where it may become available for methylation and uptake into food webs. Using data associated with the 2007 Moonlight and Antelope Fires in California, we examined the long-term direct effects of wildfire burn severity on the distribution and magnitude of Hg concentrations in riparian food webs. Additionally, we quantified the cross-ecosystem transfer of Hg from aquatic invertebrate to riparian bird communities; and assessed the influence of biogeochemical, landscape variables, and ecological factors on Hg concentrations in aquatic and terrestrial food webs. Benthic macroinvertebrate methylmercury (MeHg) and riparian bird blood total mercury (THg) concentrations varied by 710- and 760-fold, respectively, and Hg concentrations were highest in predators. We found inconsistent relationships between Hg concentrations across and within taxa and guilds in response to stream chemical parameters and burn severity. Macroinvertebrate scraper MeHg concentrations were influenced by dissolved organic carbon (DOC); however, that relationship was moderated by burn severity (as burn severity increased the effect of DOC declined). Omnivorous bird Hg concentrations declined with increasing burn severity. Overall, taxa more linked to in situ energetic pathways may be more responsive to the biogeochemical processes that influence MeHg cycling. Remarkably, 8 years post-fire, we still observed evidence of burn severity influencing Hg concentrations within riparian food webs, illustrating its overarching role in altering the storage and redistribution of Hg and influencing biogeochemical processes.

Mercury in Neotropical birds: a synthesis and prospectus on 13 years of exposure data

Environmental mercury (Hg) contamination of the global tropics outpaces our understanding of its consequences for biodiversity. Knowledge gaps of pollution exposure could obscure conservation threats in the Neotropics: a region that supports over half of the world's species, but faces ongoing land-use change and Hg emission via artisanal and small-scale gold mining (ASGM). Due to their global distribution and sensitivity to pollution, birds provide a valuable opportunity as bioindicators to assess how accelerating Hg emissions impact an ecosystem's ability to support biodiversity, and ultimately, global health. We present the largest database on Neotropical bird Hg concentrations (n = 2316) and establish exposure baselines for 322 bird species spanning nine countries across Central America, South America, and the West Indies. Patterns of avian Hg exposure in the Neotropics broadly align with those in temperate regions: consistent bioaccumulation across functional groups and high spatiotemporal variation. Bird species occupying higher trophic positions and aquatic habitats exhibited elevated Hg concentrations that have been previously associated with reductions in reproductive success. Notably, bird Hg concentrations were over four times higher at sites impacted by ASGM activities and differed by season for certain trophic niches. We developed this synthesis via a collaborative research network, the Tropical Research for Avian Conservation and Ecotoxicology (TRACE) Initiative, which exemplifies inclusive, equitable, and international data-sharing. While our findings signal an urgent need to assess sampling biases, mechanisms, and consequences of Hg exposure to tropical avian communities, the TRACE Initiative provides a meaningful framework to achieve such goals. Ultimately, our collective efforts support and inform local, scientific, and government entities, including Parties of the United Nations Minamata Convention on Mercury, as we continue working together to understand how Hg pollution impacts biodiversity conservation, ecosystem function, and public health in the tropics.

Mercury contamination and potential health risks to Arctic seabirds and shorebirds

Since the last Arctic Monitoring and Assessment Programme (AMAP) effort to review biological effects of mercury (Hg) on Arctic biota in 2011 and 2018, there has been a considerable number of new Arctic bird studies. This review article provides contemporary Hg exposure and potential health risk for 36 Arctic seabird and shorebird species, representing a larger portion of the Arctic than during previous AMAP assessments now also including parts of the Russian Arctic. To assess risk to birds, we used Hg toxicity benchmarks established for blood and converted to egg, liver, and feather tissues. Several Arctic seabird populations showed Hg concentrations that exceeded toxicity benchmarks, with 50 % of individual birds exceeding the "no adverse health effect" level. In particular, 5 % of all studied birds were considered to be at moderate or higher risk to Hg toxicity. However, most seabirds (95 %) were generally at lower risk to Hg toxicity. The highest Hg contamination was observed in seabirds breeding in the western Atlantic and Pacific Oceans. Most Arctic shorebirds exhibited low Hg concentrations, with approximately 45 % of individuals categorized at no risk, 2.5 % at high risk category, and no individual at severe risk. Although the majority Arctic-breeding seabirds and shorebirds appeared at lower risk to Hg toxicity, recent studies have reported deleterious effects of Hg on some pituitary hormones, genotoxicity, and reproductive performance. Adult survival appeared unaffected by Hg exposure, although long-term banding studies incorporating Hg are still limited. Although Hg contamination across the Arctic is considered low for most bird species, Hg in combination with other stressors, including other contaminants, diseases, parasites, and climate change, may still cause adverse effects. Future investigations on the global impact of Hg on Arctic birds should be conducted within a multi-stressor framework. This information helps to address Article 22 (Effectiveness Evaluation) of the Minamata Convention on Mercury as a global pollutant.

Feather mercury concentrations in omnivorous and granivorous terrestrial songbirds in Southeast Michigan

Sublethal exposure to methylmercury (MeHg) can have consequences for the reproductive, neurological, and physiological health of birds. Songbirds, regardless of trophic position, are often exposed to mercury (Hg) and may be at risk for health effects - especially if they inhabit a place that is subject to high Hg atmospheric deposition and/or have local conditions that are prone to methylation. This study investigates Hg concentrations in terrestrial songbirds of Southeast Michigan, where historical and present-day anthropogenic emissions of heavy metals are elevated. We collected tail feather samples from 223 songbirds across four different species during summer and fall of 2018 and 2019. The mean (±SE) Hg concentration across all samples was 103 ± 3.43 ng/g of dry feather weight. Mercury concentration varied significantly among species, and by age and site in some species, but not by sex. Mean concentrations were nearly seven times higher in two omnivore species, American robin (Turdus migratorius) and European starling (Sturnus vulgaris), than in the two granivore species, American goldfinch (Spinus tristus) and house sparrow (Passer domesticus). Juveniles had higher feather Hg concentrations than adults in all species except American goldfinches - which feed their young primarily seeds, further supporting a role of diet in exposure. We also found a negative correlation between Hg concentration and body condition in American robins, but further research is needed to verify this relationship. While our sample concentrations do not exceed the threshold for sublethal effects, our findings provide insight into the patterns of Hg concentrations in terrestrial songbirds, which may help in understanding Hg exposure pathways, bioaccumulation and risks in terrestrial species.

Mercury exposure of tidal marsh songbirds in the northeastern United States and its association with nest survival

The biogeochemistry of tidal marsh sediments facilitates the transformation of mercury (Hg) into the biologically available form methylmercury (MeHg), resulting in elevated Hg exposures to tidal marsh wildlife. Saltmarsh and Acadian Nelson's sparrows (Ammospiza caudacutua and A. nelsoni subvirgatus, respectively) exclusively inhabit tidal marshes, potentially experiencing elevated risk to Hg exposure, and have experienced range-wide population declines. To characterize spatial and temporal variation of Hg exposure in these species, we sampled total mercury (THg) in blood collected from 9 populations spanning 560 km of coastline, including individuals resampled within and among years. Using concurrent nesting studies, we tested whether THg was correlated with nest survival probabilities, an index of fecundity. Blood THg ranged from 0.074-3.373 µg/g ww across 170 samples from 127 individuals. We detected high spatial variability in Hg exposure, observing differences of more than 45-fold across all individuals and 8-fold in mean blood THg among all study plots, including 4-fold between study plots within 4 km. Intraindividual changes in blood Hg exposure did not vary systematically in time but were considerable, varying by up to 2-fold within and among years. Controlling for both species differences and maximum water level, the dominant driver of fecundity in this system, nest survival probability decreased by 10% across the full range of female blood THg concentrations observed. We conclude that Hg has the potential to impair songbird reproduction, potentially exacerbating known climate-change driven population declines from sea-level rise in saltmarsh and Acadian Nelson's sparrows.

Geographic variation of mercury in breeding tidal marsh sparrows of the northeastern United States

Saltmarsh sparrows (Ammospiza caudacuta) and seaside sparrows (A. maritima) are species of conservation concern primarily due to global sea-level rise and habitat degradation. Environmental mercury (Hg) contamination may present additional threats to their reproductive success and survival. To assess site-specific total mercury (THg) exposure and identify environmental correlates of THg detection, we sampled blood from adult male saltmarsh and seaside sparrows at 27 sites between Maine and Virginia, USA. The mean THg concentration (±1 SD) throughout the entire sampling range was 0.531 ± 0.287 µg/g wet weight (ww) for saltmarsh sparrows and 0.442 ± 0.316 µg/g ww for seaside sparrows. Individual THg concentrations ranged from 0.135-1.420 µg/g ww for saltmarsh sparrows and 0.153-1.530 µg/g ww for seaside sparrows. Model averaging from a suite of linear mixed models showed that saltmarsh sparrows averaged 20.1% higher blood THg concentrations than seaside sparrows, potentially due to differences in diet or foraging behavior. We found no evidence for a relationship between sparrow THg concentrations and land cover surrounding sampled marshes or average precipitation-based Hg deposition. Overall, our results suggest considerable, unexplained variation in tidal marsh sparrow blood THg concentrations over their co-occurring breeding ranges.

Differential reliance on aquatic prey subsidies influences mercury exposure in riparian arachnids and songbirds

Cross-ecosystem subsidies move substantial amounts of nutrients between ecosystems. Emergent aquatic insects are a particularly important prey source for riparian songbirds but may also move aquatic contaminants, such as mercury (Hg), to riparian food webs. While many studies focus on species that eat primarily emergent aquatic insects, we instead study riparian songbirds with flexible foraging strategies, exploiting both aquatic and terrestrial prey sources. The goal in this study is to trace reliance on aquatic prey sources and correlate it to Hg concentrations in common riparian arachnids (Families Tetragnathidae, Opiliones, and Salticidae) and songbirds (Common Yellowthroat Geothlypis trichas, Spotted Towhee Pipilo maculatus, Swainson's Thrush Catharus ustulatus, Song Sparrow Melospiza melodia, and Yellow Warbler Setophaga petechia). We used stable isotopes of δ13C and δ15N and Bayesian mixing models in MixSIAR to determine the reliance of riparian predators on aquatic prey sources. Using mixed effects models, we found that arachnid families varied in their reliance on aquatic prey sources. While songbird species varied in their reliance on aquatic prey sources, songbirds sampled earlier in the season consistently relied more on aquatic prey sources than those sampled later in the season. For both arachnids and songbirds, we found a positive correlation between the amount of the aquatic prey source in their diet and their Hg concentrations. While the seasonal pulse of aquatic prey to terrestrial ecosystems is an important source of nutrients to riparian species, our results show that aquatic prey sources are linked with higher Hg exposure. For songbirds, reliance on aquatic prey sources early in the breeding season (and subsequent higher Hg exposure) coincides with timing of egg laying and development, both of which may be impacted by Hg exposure.

A synthesis of patterns of environmental mercury inputs, exposure and effects in New York State

Mercury (Hg) pollution is an environmental problem that adversely affects human and ecosystem health at local, regional, and global scales-including within New York State. More than two-thirds of the Hg currently released to the environment originates, either directly or indirectly, from human activities. Since the early 1800s, global atmospheric Hg concentrations have increased by three- to eight-fold over natural levels. In the U.S., atmospheric emissions and point-source releases to waterways increased following industrialization into the mid-1980s. Since then, water discharges have largely been curtailed. As a result, Hg emissions, atmospheric concentrations, and deposition over the past few decades have declined across the eastern U.S. Despite these decreases, Hg pollution persists. To inform policy efforts and to advance public understanding, the New York State Energy Research and Development Authority (NYSERDA) sponsored a scientific synthesis of information on Hg in New York State. This effort includes 23 papers focused on Hg in atmospheric deposition, water, fish, and wildlife published in Ecotoxicology. New York State experiences Hg contamination largely due to atmospheric deposition. Some landscapes are inherently sensitive to Hg inputs driven by the transport of inorganic Hg to zones of methylation, the conversion of inorganic Hg to methylmercury, and the bioaccumulation and biomagnification along food webs. Mercury concentrations exceed human and ecological risk thresholds in many areas of New York State, particularly the Adirondacks, Catskills, and parts of Long Island. Mercury concentrations in some biota have declined in the Eastern Great Lakes Lowlands and the Northeastern Highlands over the last four decades, concurrent with decreases in water releases and air emissions from regional and U.S. sources. However, widespread changes have not occurred in other ecoregions of New York State. While the timing and magnitude of the response of Hg levels in biota varies, policies expected to further diminish Hg emissions should continue to decrease Hg concentrations in food webs, yielding benefits to the fish, wildlife, and people of New York State. Anticipated improvements in the Hg status of aquatic ecosystems are likely to be greatest for inland surface waters and should be roughly proportional to declines in atmospheric Hg deposition. Efforts that advance recovery from Hg pollution in recent years have yielded significant progress, but Hg remains a pollutant of concern. Indeed, due to this extensive compilation of Hg observations in biota, it appears that the extent and intensity of the contamination on the New York landscape and waterscape is greater than previously recognized. Understanding the extent of Hg contamination and recovery following decreases in atmospheric Hg deposition will require further study, underscoring the need to continue existing monitoring efforts.