Foraging in marine habitats increases mercury concentrations in a generalist seabird

The role of seabird foraging strategies on the uptake of mercury: A case study using gulls and shearwaters from the Portuguese coast

Mercury (Hg) is a non-essential element that bioaccumulates and biomagnifies in food webs through site-specific biogeochemical processes. Seabirds are valuable bioindicators of Hg contamination, yet certain regions, like the Portuguese coast, remain underrepresented. This study measured Hg concentrations in the blood of yellow-legged gulls (Larus michahellis), Audouin's gulls (Ichthyaetus audouinii), and Cory's shearwaters (Calonectris borealis) breeding along the Portuguese coastline. The influence of foraging ecology on Hg contamination was investigated using stable isotopes (δ13C, δ15N, δ34S) along with GPS-loggers. Thus, 52 % of the adults were at low risk (0.95-4.8 μg g-1 dry weight, dw), while 37 % were at moderate risk of Hg toxicity (>4.8 μg g-1 dw). The highest trophic positions (indicated by δ15N values) were associated with higher Hg concentrations, particularly in gulls from Deserta Island. Yellow-legged gulls foraging in terrestrial habitats (inferred from lower δ13C and δ34S values and GPS data) generally exhibited lower Hg concentrations, although patterns varied between regions. Gulls from Deserta Island had elevated Hg concentrations, likely because they feed on discarded demersal fish. In contrast, those from Porto showed some of the lowest concentrations, likely because they fed on terrestrial food. However, other factors not evaluated here, like age, sex, or even phylogeny could have also influenced Hg uptake and bioaccumulation and should not be disregarded in future research. This study highlights the critical role of foraging strategies in Hg contamination and stresses the importance of estimate food web-specific baseline isotopic composition to better understand how these differences may impact Hg trophic transfer.

Interactions between contaminants and the trophic ecology of two seabirds in a coastal lagoon of the Gulf of California

Monitoring the dynamics of contaminants in ecosystems helps understand their potential effects. Seabirds have been used as biomonitors of marine ecosystems for this purpose. However, exposure and vulnerability to pollutants are understudied in tropical species, and the relationships between various pollutants and the trophic ecology of seabirds are poorly understood. In this study, we quantified mercury (Hg), lead (Pb), cadmium (Cd), and organochlorine pesticide (OC) concentrations in the blood of Laughing Gulls and Magnificent Frigatebirds breeding in Bahía Santa María, México. Using carbon and nitrogen isotopic ratios (δ13C and δ15N), we examined the interaction between contaminants and trophic ecology. Laughing Gulls exhibited higher concentrations of dichlorodiphenyltrichloroethane and its metabolites (ΣDDTs), endrins (ΣDrins), and chlordanes, while Magnificent Frigatebirds had elevated levels of Hg and hexachlorocyclohexane isomers (ΣHCHs). Both species displayed temporal and sex-related variations in isotopic signatures. Some blood pollutant concentrations in Laughing Gulls were explained by diet: ΣOCs in plasma were directly related to trophic levels, indicating biomagnification, whereas higher Hg levels were associated with changes in habitat use. In contrast, the differences in sex-related isotopic signatures in Magnificent Frigatebirds did not reflect pollutant accumulation patterns, possibly due to their opportunistic feeding habits.

Calonectris shearwaters reveal a gradient of mercury contamination along the Atlantic and Mediterranean waters of the Iberian Peninsula

This study examines blood mercury (Hg) concentrations in Calonectris spp. shearwaters from three colonies along the Atlantic and Mediterranean waters of the Iberian Peninsula (southwestern Europe), investigating their relationship with foraging ecology through GPS tracking and stable isotopes (δ15N and δ13C) data during the breeding season. Hg levels exhibited a spatial gradient, increasing from the Atlantic Ocean (1.8 ± 0.4 μg g-1 dw) towards the Mediterranean Sea, with shearwaters from the Columbretes Islands (NW Mediterranean) showing the highest Hg levels (6.5 ± 2.1 μg g-1 dw). Individuals breeding in the Alboran Sea, a transition area between both basins, had intermediate Hg concentrations (3.1 ± 1.5 μg g-1 dw). All individuals were above the Hg toxicity threshold associated with negative reproductive, body condition, and immune system outcomes. However, all shearwaters had a Se:Hg molar ratio above 4, indicating effective protection of Se against Hg toxicity. Positive significant relationships between Hg concentrations, δ15N values, and time spent foraging in deep sea waters were observed in Mediterranean colonies, highlighting the ecological context's role in Hg accumulation. Results suggest that feeding on higher trophic level prey, in deep-sea areas, and geographic location contribute to Hg accumulation in these populations. Given the potential health risks associated with elevated Hg levels, further research is warranted to explore the ecological factors driving Hg accumulation and the implications for the health status of these populations.

Coastal foraging increases mercury concentrations in a breeding seabird: Insights from isotopes, biologging, and prey

Mercury concentrations can vary substantially across spatial and temporal scales. As mobile marine predators, seabirds offer a unique opportunity to directly link foraging tactics with mercury burdens, because temporal variation in mercury can be related to spatial variation in foraging. Breeding razorbills (Alca torda) forage in various habitats around colonies, which can affect their mercury burdens. Here, we explore how mercury concentrations in red blood cells are influenced by foraging tactics (movement and trophic ecology) using GPS tracking and stable isotope dietary analysis (δ15N, δ13C, and δ34S), as well as by environmental signals, assessed through prey observations using nest-based cameras in two years (2021 and 2022) in breeding razorbills in the Gulf of St-Lawrence, Canada. Total mercury levels varied significantly between years, and all individuals exceeded low-risk toxicity thresholds. In the higher mercury year, razorbills foraged closer to the coast and had lower δ13C and δ34S values, suggesting a shift in foraging tactics. Although prey species composition did not change between years, individuals in the high mercury year brought back smaller prey and more items per load. These findings suggest that elevated mercury concentrations in razorbills may be linked to foraging in more coastal areas. Thus, small-scale changes, such as a shift to inshore coastal foraging, may expose seabirds to cumulative freshwater/terrestrial inputs and potentially higher mercury concentrations in prey. By investigating ecotoxicological risks associated with foraging-related contamination using multiple simultaneous approaches, our study provides insights into how feeding tactics can drive mercury contamination in sympatric seabirds foraging in coastal environments.

Legacy and emergent contaminants in glaucous-winged gull eggs from Canada's Pacific coast: Spatial distribution, temporal trends, and risks for human consumers

Using glaucous-winged gull (Larus glaucescens) eggs from Canada's Pacific coast, we investigated spatial and temporal trends (2008-2022) of a suite of legacy and emergent contaminants, including 16 perfluoroalkyl substances (PFAS), 15 polybrominated diphenyl ethers (PBDEs), 7 alternative halogenated flame retardants (AHFRs), total mercury (THg), as well as stable isotopes of carbon (δ13C) and nitrogen (δ15N) to control for diet. Legacy organochlorines (OCs) were also measured in eggs in 2020 for a preliminary human health risk assessment (HHRA). Between 2008 and 2022, glaucous-winged gull eggs from more urban-influenced colonies (Mandarte Island) were up to ∼2x more contaminated with PFAS, PBDEs, AHFRs, and THg than eggs from the offshore colony (Cleland Island), suggesting different source regions and dietary exposures. Concentrations of Σ15PBDEs declined linearly among colonies (p < 0.001), consistent with several North American phase-outs and regulatory restrictions dating back to the early/mid 2000s. Conversely, temporal trends for PFOS, Σ12PFCAs, Σ7AHFRs, and THg were characterized by a combination of second-order declines and non-linear increases in recent years. After correcting THg for dietary shifts using δ15N, THg concentrations followed a U-shaped trend at Mandarte and Cleland Islands, while those at Mitlenatch Island remained relatively constant over time. Increasing trends for some contaminants coincided with both an increase in δ13C and δ15N. For the HHRA, all gull eggs collected in 2020 had hazard quotients (HQs) < 0.2, indicating no foreseeable risk or harm for First Nations consumers for certain contaminants. Our findings indicate that spatio-temporal trends of persistent contaminants in Pacific glaucous-winged gull eggs are influenced by a combination of factors, including the impact of regulations on anthropogenic emissions, coupled with changes in foraging behaviour and food-web structure.

Assessing the impacts of trace element contamination on the physiology and health of seabirds breeding along the western and southern coasts of Portugal

Coastal seabirds serve as sentinels of ecosystem health due to their vulnerability to contamination from human activities. However, our understanding on how contaminant burdens affect the physiological and health condition of seabirds is still scarce, raising the uncertainty on the species' vulnerability vs tolerance to environmental contamination. Here, we quantified 15 Trace Elements (TE) in the blood of gull (yellow-legged gull Larus michahellis and Audouin's gull Ichthyaetus audouinii) and shearwater (Cory's shearwater Calonectris borealis) adults, breeding in five colonies along the Portuguese coastline. Additionally, stable isotopes of carbon (δ13C) and nitrogen (δ15N) were quantified to elucidate foraging habitat and trophic ecology of adults, to identify potential patterns of TE contamination among colonies. We used immuno-haematological parameters as response variables to assess the influence of TE concentrations, stable isotope values, and breeding colony on adults' physiological and health condition. Remarkably, we found blood mercury (Hg) and lead (Pb) concentrations to exceed reported toxicity thresholds in 25% and 13% of individuals, respectively, raising ecotoxicological concerns for these populations. The breeding colony was the primary factor explaining variation in five out of six models, underlining the influence of inherent species needs on immuno-haematological parameters. Model selection indicated a negative relationship between erythrocyte sedimentation rate and both Hg and selenium (Se) concentrations, but a positive relationship with δ13C. The number of immature erythrocyte counts was positively related to Hg and Se, particularly in yellow-legged gulls from one colony, highlighting the colony-site context's influence on haematological parameters. Further research is needed to determine whether essential TE concentrations, particularly copper (Cu) and Se, are falling outside the normal range for seabirds or meet species-specific requirements. Continuous monitoring of non-essential TE concentrations like aluminium (Al), Hg, and Pb, is crucial due to their potential hazardous concentrations, as observed in our study colonies.

Factors influencing mercury levels in Leach's storm-petrels at northwest Atlantic colonies

Mercury (Hg) is a globally distributed heavy metal, with negative effects on wildlife. Its most toxic form, methylmercury (MeHg), predominates in aquatic systems. Levels of MeHg in marine predators can vary widely among individuals and populations. Leach's storm-petrels (Hydrobates leucorhous) have elevated levels of Hg but the role of Hg in storm-petrel population declines is unknown. In this study, we used egg and blood samples to study variation in Hg exposure among several northwest Atlantic colonies during breeding seasons, thereby evaluating relative toxicity risk within and among colonies. Total mercury (THg) concentrations were higher with increasing colony latitude, and were more pronounced in blood than in eggs. THg concentrations in blood were mostly associated with low toxicity risk in birds from the southern colonies and moderate risks in birds from the northern colonies; however, those values did not affect hatching or fledging success. THg concentrations in both eggs and blood were positively correlated with δ³⁴S, emphasizing the role of sulfate-reducing bacteria in methylation of THg acquired through marine food webs, which is consistent with enriched δ³⁴S profiles. By associating tracking data from foraging trips with THg from blood, we determined that blood THg levels were higher when storm-petrel's intensive search locations were over deeper waters. We conclude that spatial variation in THg concentrations in Leach's storm-petrels is attributable to differences in ocean depth at foraging locations, both at individual and colony levels. Differences in diet among colonies observed previously are the most likely cause for observed blood THg differences. As one of the few pelagic seabird species breeding in Atlantic Canada, with limited overlap in core foraging areas among colonies, Leach's storm-petrels can be used as biomonitors for less sampled offshore pelagic regions. The global trend in Hg emissions combined with legacy levels warrant continued monitoring for toxicity effects in seabirds.

Active breeding seabirds prospect alternative breeding colonies

Compared to other animal movements, prospecting by adult individuals for a future breeding site is commonly overlooked. Prospecting influences the decision of where to breed and has consequences on fitness and lifetime reproductive success. By analysing movements of 31 satellite- and GPS-tracked gull and tern populations belonging to 14 species in Europe and North America, we examined the occurrence and factors explaining prospecting by actively breeding birds. Prospecting in active breeders occurred in 85.7% of studied species, across 61.3% of sampled populations. Prospecting was more common in populations with frequent inter-annual changes of breeding sites and among females. These results contradict theoretical models which predict that prospecting is expected to evolve in relatively predictable and stable environments. More long-term tracking studies are needed to identify factors affecting patterns of prospecting in different environments and understand the consequences of prospecting on fitness at the individual and population level.

Stable Mercury Trends Support a Long-Term Diet Shift Away from Marine Foraging in Salish Sea Glaucous-Winged Gulls over the Last Century

Marine predators are monitored as indicators of pollution, but such trends can be complicated by variation in diet. Glaucous-winged gulls (Larus glaucescens) have experienced a dietary shift over the past century, from mainly marine to including more terrestrial/freshwater inputs, with unknown impacts on mercury (Hg) trends. We examined 109-year trends in total mercury (THg) and methylmercury (MeHg) concentrations in glaucous-winged gull feathers (1887-1996) from the Salish Sea. Adult flank feathers had higher MeHg concentrations than immature feathers, and males head feathers had higher THg concentrations than females. Overall, we found no evidence of a trend in feather MeHg or THg concentrations over time from 1887 to 1996. In the same individuals, δ¹⁵N, δ¹³C, and δ³⁴S declined over time in gull feathers. In comparison, egg THg concentrations declined from 1970 to 2019 in two species of cormorants, likely reflecting decreases in local Hg sources. We conclude that diet shifts through time may have countered increased Hg deposition from long-range transport in glaucous-winged gulls. The lack of Hg trends over time in glaucous-winged gull feathers provides additional support that these gulls have decreased the amount of marine forage fish in their diet.

Blood mercury concentrations in four sympatric gull species from South Western France: Insights from stable isotopes and biologging

Mercury (Hg) is a toxic trace element widely distributed in the environment, which particularly accumulates in top predators, including seabirds. Among seabirds, large gulls (Larus sp) are generalist feeders, foraging in both terrestrial and marine habitats, making them relevant bioindicators of local coastal Hg contamination. In the present study, we reported blood Hg concentrations in adults and chicks of four different gull species breeding on the French Atlantic coast: the European herring gull (Larus argentatus), the Lesser black-backed gull (L. fuscus), the Great black-backed gull (L. marinus) and the Yellow-legged gull (L. michahellis). We also investigated the potential role of foraging ecology in shaping Hg contamination across species, using the unique combination of three dietary tracers (carbon, nitrogen and sulfur stable isotopes) and biologging (GPS tracking). A high concentration of Hg was associated with high trophic position and a marine diet in gulls, which was corroborated by birds' space use strategy during foraging trips. Adults of all four species reached Hg concentrations above reported toxicity thresholds. Specifically, adults of Great black-backed gulls had a high trophic marine specialized diet and significantly higher Hg concentrations than the three other species. Blood Hg was 4-7 times higher in adults than in chicks, although chicks of all species received mainly marine and high trophic position prey, which is expected to be the cause of blood Hg concentrations of toxic concern. By using both stable isotopes and GPS tracking, the present study provides compelling insights on the main feeding habits driving Hg contamination in a seabird assemblage feeding in complex coastal environments.