Temporal changes in metal and arsenic concentrations in blood and feathers of tropical seabirds after one of the largest environmental disasters associated with mining.

Monitoring of contaminant levels in wildlife over time is a tool for assessing the presence and persistence of environmental impacts at ecosystem, community and population levels. Tropical seabirds breeding in the Abrolhos Archipelago, 70 km off the Brazilian coast, forage in areas under the influence of the Doce River discharge. In 2015, the Fundão Dam collapsed and released ca 60 million tons of iron ore tailings into the ocean. In the present study, red-billed tropicbirds Phaethon aethereus and brown boobies Sula leucogaster breeding in Abrolhos were monitored over four years (2019-2022) for metal (Fe, Mn, Zn, Cu, Cr, Hg, Pb, Cd) and metalloid (As) concentrations in blood and feathers. Over six sampling events, metal (loid) concentrations showed strong temporal variation in both tissues. Overall, feathers showed greater element concentrations than blood, with stronger correlations between elements, especially Mn and the nonessential As, Cd, Hg and Pb. Mn is one of the major chemical markers of the Fundão Dam tailings. Metal (loid) concentrations in the tropical seabirds evaluated were above suggested threshold levels for most nonessential elements (As, Cd and Pb), especially in February 2021, when metal (loid) concentrations peaked in feathers. In this case, values were orders of magnitude higher than those observed in other sampling events. This occurred one year after a major rainy season in the Doce River basin, which increased river discharge of contaminated mud into the ocean, where contaminants are further remobilized by winds and currents, resulting in transference through the marine food web. This finding is consistent to what has been observed for other ecosystem compartments monitored in the region under the influence of the Doce River. Our findings highlight the utility of using tropical seabirds as sentinels of marine pollution, revealing strong temporal patterns in metal (loid) concentrations associated to bottom-up climatic processes.

Seabirds subsidize terrestrial food webs and coral reefs in a tropical rat-invaded archipelago.

Allochthonous resource fluxes mediated by organisms crossing ecosystem boundaries may be essential for supporting the structure and function of resource-limited environments, such as tropical islands and surrounding coral reefs. However, invasive species, such as black rats, thrive on tropical islands and disrupt the natural pathways of nutrient subsidies by reducing seabird colonies. Here, we used stable isotopes of nitrogen and carbon to examine the role of seabirds in subsidizing the terrestrial food webs and adjacent coral reefs in the Abrolhos Archipelago, Southwest Atlantic Ocean. By sampling invasive rats and multiple ecosystem compartments (soil, plants, grasshoppers, tarantulas, and lizards) within and outside seabird colonies, we showed that seabird subsidies led to an overall enrichment in 15 N across the food web on islands. However, contrary to other studies, δ15 N values were consistently lower within the seabird colonies, suggesting that a higher seabird presence might produce a localized depletion in 15 N in small islands influenced by seabirds. In contrast, the nitrogen content (%N) in plants and soils was higher inside the colonies, corresponding to a higher effect of seabirds at the base of the trophic web. Among consumers, lizards and invasive rats seemed to obtain allochthonous resources from subsidized terrestrial organisms outside the colony. Inside the colony, however, they showed a more direct consumption of marine matter, suggesting that subsidies benefit these native and invasive animals both directly and indirectly. Nonetheless, in coral reefs, scleractinian corals assimilated seabird-derived nitrogen only around the two smaller and lower-elevation islands, as demonstrated by the substantially higher δ15 N values in relation to the reference areas. This provides evidence that island morphology may influence the incorporation of seabird nutrients in coral reefs around rat-invaded islands, likely because guano lixiviation toward seawater is facilitated in small and low-elevation terrains. Overall, these results showed that seabirds affected small islands across all trophic levels within and outside colonies and that these effects spread outward to coral reefs, evidencing resiliency of seabird subsidies even within a rat-invaded archipelago. Because rats are consumers of seabird chicks and eggs, however, rat eradication could potentially benefit the terrestrial and nearshore ecosystems through increased subsides carried by seabirds.

Resource partitioning influences levels of toxic trace elements in sympatric tropical seabirds.

Morphologically similar species that occupy resource-limited environments tend to differ in their ecological traits in order to coexist, which may result in differential exposure to environmental threats. For instance, partitioning of feeding resources may influence contaminant exposure and bioaccumulation in marine predators through different diets or foraging habitats. Here, we sampled three tropical seabird species breeding in sympatry in the southwestern Atlantic Ocean for blood and feather trace element concentrations (As, Hg, Cd, Pb), and assessed their foraging ecology with bio-logging (GPS tracks and time-depth recorders), analysis of regurgitated prey, and carbon and nitrogen stable isotope mixing models. Red-billed tropicbirds (Phaethon aethereus), brown (Sula leucogaster) and masked (S. dactylatra) boobies differed in their preferred foraging locations, the range of foraging trips, diving parameters and diets. In addition, interspecific differences were detected in blood or feathers for all trace elements analyzed, suggesting influence of the differences observed in diet and space use. Red-billed tropicbirds had the largest foraging range over the continental shelf and over the slope, suggesting lower exposure to continental sources of metals. Brown and masked boobies had higher Hg concentrations than tropicbirds, higher δ15N values (a proxy for trophic level), and δ15N correlated with Hg levels, suggesting biomagnification of Hg along the food chain. Nonetheless, red-billed tropicbirds showed comparable levels of As and Cd in blood or feathers, and higher levels of Pb in both tissues in comparison to boobies, which may suggest overall exposure of seabirds in the region, through their diets and foraging areas. Resource partitioning is critical for allowing coexistence of different seabird species in shared breeding sites where they act as central-place foragers. Nonetheless, in scenarios of environmental pollution, these species-specific strategies lead to differential bioaccumulation, and thus distinct effects of pollution on populations are expected.