Mercury concentrations in Alaska Pacific halibut muscle relative to stable isotopes of C and N and other biological variables

Biogeochemical patterns in prey species reveal complex mercury exposure pathways from the environment to Aleutian Steller sea lions

Several wildlife species exhibit marked spatial variation in toxicologically relevant tissue concentrations of mercury across the Aleutian Islands of Alaska, most notably the endangered Steller sea lion (Eumetopias jubatus). To unravel potential environmental and trophic pathways driving mercury variation in this species of concern, we investigated spatiotemporal and ecological patterns in total mercury concentrations and stable isotope ratios of carbon and nitrogen from muscle tissues of twelve mid-trophic level prey species of the region (n = 1461). Dividing samples into island groups explained biogeochemical variation better than larger spatial resolutions, with Amchitka Pass and Buldir Pass acting as strong geographic break points. Species-specific relationships between muscle biogeochemistry and length were common, and benthic feeding and increased trophic position were associated with higher total mercury concentrations. Considered together, tissue biogeochemical markers in this region are dependent on location and the species upon which they forage in remarkably nuanced manners.

Regional variations and drivers of essential and non-essential elements in Steller sea lion pups from the Aleutian Islands, Alaska

Exposure and resulting tissue concentrations of various elements from natural and anthropogenic sources are influenced by multiple factors, such as geographic location, age, diet, and metabolic rate, that can influence wildlife health. Essential and non-essential elements were assessed in lanugo and whole blood collected in 2019 from 102 Steller sea lion (Eumetopias jubatus) pups from two rookeries from the western and central Aleutian Islands: Agattu (WAI, n = 54) and Ulak Islands (CAI, n = 48). Rookery, sex, dorsal standard length, and trophic ecology (ẟ15N, ẟ13C values) effects on element concentration were evaluated. Significant differences in element concentrations of lanugo were exhibited across rookeries (p < 0.05), except for zinc (Zn). For example, higher mercury (Hg) and selenium (Se) concentrations were observed in WAI than CAI, while other elements were lower in WAI. Whole blood showed higher sulfur (S) and Se concentrations in CAI compared to WAI, while WAI had elevated strontium (Sr) and Hg concentrations relative to CAI. Trophic ecology significantly influenced most element concentrations, possibly due to regional variations in adult female feeding and food web dynamics. Interactions between elements were found in lanugo across both rookeries, with varying strengths. Whole blood displayed less pronounced yet consistent associations, with variable intensities. Essential elements sodium (Na), potassium (K), and calcium (Ca) formed a distinct group whose interaction is crucial for nervous system function and muscle contraction. Another group comprised zinc (Zn), iron (Fe), manganese (Mn), magnesium (Mg), phosphorous (P), S, and Se, which are known for indirectly interacting with enzyme function and metabolic pathways. Hg and Se formed a distinct group probably due to their known chemical interactions and physiological protective interactions.

Spatial and cumulative organochlorine and mercury exposure assessments in Steller Sea lions of Alaska: Emphasizing pups

Steller sea lions (SSL) are sentinels for monitoring environmental contaminants in remote areas of the Aleutian Islands, Alaska. Therefore, concentrations of several organochlorines (OCs) were measured in blood from 123 SSL pups sampled from 3 regions; the western Aleutian Islands (WAI), central Aleutian Islands (CAI), and the central Gulf of Alaska. Blood, blubber, and milk from 12 adult female SSL from WAI, CAI and southeast Alaska also were analyzed. Findings included the following. SSL pups had higher concentrations of some OCs and mercury (Hg) on rookeries in the WAI than those more easterly. Pups had significantly higher blood concentrations of many OC classes than adult females sampled within the same region; some pups had PCB concentrations exceeding thresholds of concern (∑PCBs >8600 ng/g lw). ∑PCB concentration in pup whole blood was positively correlated with the trophic marker, δ15N within the regions sampled, along with two PCB congeners (PCB138 and PCB153). This suggests that the dams of pups with higher ∑PCBs, PCB138, and PCB153 concentrations were feeding on more predatory prey. Adult female blubber ∑DDT and hexachlorocyclohexane concentrations were also positively correlated with δ15N values. Several pups (mostly from WAI) had blood Hg concentrations and/or blood PCB concentrations (surrogate for overall OC exposures) of concern. The finding that WAI SSL pups have been exposed to multiple contaminants calls for future investigation of their cumulative exposure to a mixture of contaminants especially their transplacental and then transmammary exposure routes.

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.

Compound-specific stable isotopes of amino acids reveal influences of trophic level and primary production sources on mercury concentrations in fishes from the Aleutian Islands, Alaska

Total mercury concentrations ([THg]) exceed thresholds of concern in some Steller sea lion (Eumetopias jubatus) tissues from certain portions of the Aleutian Islands, Alaska. We applied compound-specific stable isotope analyses of both carbon and nitrogen in amino acids from fish muscle tissue to quantify the proportional contributions of primary production sources and trophic positions of eight prey species (n = 474 total) that are part of Steller sea lion diets. Previous THg analyses of fish muscle, coupled with monomethylmercury analyses of a subset of samples, substantiated previous findings that fishes from the west of Amchitka Pass, a discrete oceanographic boundary of the Aleutian Archipelago, have higher muscle Hg concentrations relative to fishes from the east. The δ13C values of essential amino acids (EAAs) in fish muscle demonstrated that although most fishes obtained their EAAs primarily from algae, some species varied in the extent to which they relied on this EAA source. The δ15N values of phenylalanine (0.9 to 7.8 ‰), an indicator of the isotopic baseline of a food web, varied widely within and among fish species. Trophic position estimates, accounting for this baseline variation, were higher from the west relative to the east of the pass for some fish species. Trophic magnification slopes using baseline-corrected trophic position estimates indicated similar rates of Hg biomagnification to the east and west of Amchitka Pass. Multiple linear regression models revealed that trophic position was the most important driver of fish muscle [THg] with less variation explained by other parameters. Thus, higher trophic positions but not the rate of Hg biomagnification to the west of Amchitka Pass may play a role in the regional differences in both fish and Steller sea lion [THg]. Although, differences in Hg contamination and uptake at the base of the east and west food webs could not be excluded.

Ecotoxicology of mercury in burbot (Lota lota) from interior Alaska and insights towards human health

Fish consumption has many health benefits, but exposure to contaminants, such as mercury (Hg), in fish tissue can be detrimental to human health. The Tanana River drainage, Alaska, USA supports the largest recreational harvest of burbot (Lota lota) in the state, yet information to evaluate the potential risks of consumption by humans is lacking. To narrow this knowledge gap, we sought to (i) quantify the concentrations of total Hg ([THg]) in burbot muscle and liver tissue and the ratio between the two tissues, (ii) assess the effect of age, length, and sex on [THg] in muscle and liver tissue, (iii) evaluate if [THg] in muscle tissue varied based on trophic information, and (iv) compare observed [THg] to consumption guidelines and statewide baseline data. The mean [THg] was 268.2 ng/g ww for muscle tissue and 62.3 ng/g ww for liver tissue. Both muscle [THg] and liver [THg] values were positively associated with fish length. Trophic information (δ¹⁵N and δ¹³C) was not significantly related to measured [THg] in burbot muscle, which is inconsistent with typical patterns of biomagnification observed in other fishes. All burbot sampled were within the established categories for consumption recommendations determined by the State of Alaska for women of childbearing age and children. Our results provide the necessary first step towards informed risk assessment of burbot consumption in the Tanana drainage and offer parallels to fisheries and consumers throughout the subarctic and Arctic region.

Regional variations and drivers of mercury and selenium concentrations in Steller sea lions

Mercury (Hg) can be neurotoxic to mammals and impact reproduction, whereas selenium (Se) is an important antioxidant known to ameliorate some adverse effects of Hg. Total Hg concentrations ([THg]) were measured in lanugo (pelage grown in utero) of 812 Steller sea lion (Eumetopias jubatus) pups across Alaska and Russia to assess fetal exposure during late gestation. The molar ratio of total Se to THg (TSe:THg) was determined in whole blood collected from 291 pups. Stable isotope ratios of carbon and nitrogen were measured in sections of vibrissae (whiskers, n = 498) and in lanugo (n = 480) of pups grown during late gestation to track diet variations among adult females that can drive Hg and Se exposure during this critical fetal development period. Lanugo [THg] ranged from 1.4 to 73.7 μg/g dry weight with the lowest median [THg] in Southeast Alaska. Pups from the Western Aleutian Islands had higher median lanugo [THg] than pups from other metapopulations in Alaska. Over 25% of pups in the Western Aleutian Islands had [THg] above published risk thresholds (20 μg/g) for other mammals. Whole blood molar TSe:THg was significantly lower in the Western Aleutian Islands and in some parts of the Central Aleutian Islands with higher molar ratios found in the Eastern Aleutian Islands and Central Gulf of Alaska. This suggests a limitation on potential protective functions of Se in the western regions with the highest relative [THg]. The Central Aleutian Island pups with [THg] over 20 μg/g had higher δ¹⁵N ratios than pups with lower [THg] suggesting dams consuming higher trophic level prey is a key driver for Hg exposure. However, regional differences likely reflect variability in diet of the dam during gestation and in Hg food web dynamics between oceanic regimes east and west of key passes in the Aleutian Islands.

Mercury in Juvenile Solea senegalensis: Linking Bioaccumulation, Seafood Safety, and Neuro-Oxidative Responses under Climate Change-Related Stressors

Mercury (Hg) is globally recognized as a persistent chemical contaminant that accumulates in marine biota, thus constituting an ecological hazard, as well as a health risk to seafood consumers. Climate change-related stressors may influence the bioaccumulation, detoxification, and toxicity of chemical contaminants, such as Hg. Yet, the potential interactions between environmental stressors and contaminants, as well as their impacts on marine organisms and seafood safety, are still unclear. Hence, the aim of this work was to assess the bioaccumulation of Hg and neuro-oxidative responses on the commercial flat fish species Solea senegalensis (muscle, liver, and brain) co-exposed to dietary Hg in its most toxic form (i.e., MeHg), seawater warming (ΔT°C = +4 °C), and acidification (pCO2 = +1000 µatm, equivalent to ΔpH = −0.4 units). In general, fish liver exhibited the highest Hg concentration, followed by brain and muscle. Warming enhanced Hg bioaccumulation, whereas acidification decreased this element’s levels. Neuro-oxidative responses to stressors were affected by both climate change-related stressors and Hg dietary exposure. Hazard quotient (HQ) estimations evidenced that human exposure to Hg through the consumption of fish species may be aggravated in tomorrow’s ocean, thus raising concerns from the seafood safety perspective.

Mercury concentrations in marine species from the Aleutian Islands: Spatial and biological determinants

Several species found in the Bering Sea show significant spatial variation in total mercury concentrations ([THg]) longitudinally along the Aleutian Island chain. We assessed [THg] in other members of the Bering Sea food web to better understand the factors shaping regional differences. [THg] and stable carbon and nitrogen isotope ratios (δ¹⁵N and δ¹³C values) were measured in muscle tissue from 1052 fishes and cephalopods from parts of the Bering Sea and North Pacific Ocean adjacent to the Aleutian Islands. The spatial distribution of the samples enabled regional comparisons for 8 species of fish and one species of cephalopod. Four species showed higher mean length-standardized [THg] in the western Aleutian Islands management area. [THg] in yellow Irish lord were very different relative to those observed in other species and when included in multi-species analyses drove the overall regional trends in mean [THg]. Multi-species analyses excluding measurements for yellow Irish lord showed mean length-standardized [THg] was greater in the western Aleutian Islands than in the central Aleutian Islands management area. Linear regression of [THg] and δ¹⁵N values showed a significant and positive relationship across all species, varying between regions and across species. Isotopic space of all species was significantly different between the western Aleutian Islands and central Aleutian Islands, driven largely by δ¹³C values. Stable isotope values observed follow the same regional trend of lower trophic taxa reported in the literature, with significantly lower δ¹³C values in the western Aleutian Islands. We conclude that there are regional differences in carbon and nitrogen stable isotope ecology, as well as species-specific feeding ecology that influence [THg] dynamics in part of the marine food web along the Aleutian Island chain. These regional differences are likely contributors to the observed regional variations of [THg] in some high-level predators found in these regions.

Nitrogen and sulfur isotopes predict variation in mercury levels in Arctic seabird prey

Mercury (Hg) biotransformation and biomagnification are processes that affect Hg burdens in wildlife. To interpret variation in Hg in seabird eggs, used as Hg bioindicators in the Arctic, it is important to understand how Hg biomagnifies through the food web. We evaluated the use of δ³⁴S, along with other commonly used stable isotope signatures (δ¹⁵N and δ¹³C), for the determination of possible sources of Hg in an Arctic food web (56 individuals of 15 species of fish and invertebrates). Hg correlated with δ³⁴S (R² = 0.72). When the combined effects of δ³⁴S and δ¹⁵N were considered in mixed-effects models, both δ³⁴S and δ¹⁵N together described Hg patterns in Arctic food webs better than either isotope alone. Our results demonstrate the usefulness of δ³⁴S to account for variation in Hg among marine animals and to study the possible underlying effects that MeHg production may have on Hg pathways in Arctic ecosystems.

Mercury concentrations in three ray species from the Pacific coast of Baja California Sur, Mexico: Variations by tissue type, sex and length

Total mercury concentrations ([THg]) were determined in muscle and liver of the bat ray (Myliobatis californica), shovelnose guitarfish (Pseudobatos productus) and banded guitarfish (Zapteryx exasperata). Generalized linear models (GLM) were used to determine the effects of size and sex in [THg] and showed that both are determinants of [THg] in these species. The [THg] in both tissues significantly increased with length especially in sexually mature organisms with a steeper slope for mature male than mature female. This may relate to elasmobranchs sexual dimorphism driven variation in growth rates. Median muscle [THg] was significantly greater than liver in each ray species but there were some individuals with higher liver [THg] than muscle. There were individuals with muscle [THg] higher than the advisory thresholds of 0.2 and 0.5mgkg^-1ww (2.4 and 11% of the bat ray; 2.1 and 10% of the shovelnose guitarfish; 12.6 and 45% of the banded guitarfish, respectively).