MERCURY BIOMAGNIFICATION IN AQUATIC FOOD WEBS ON A GLOBAL SCALE
Mercury (Hg) concentrations in aquatic organisms of a food web increase with trophic level (TL) through a process called biomagnification. The trophic magnification factor (TMF) metric using the slope of the relationship between Hg concentration and TL (measured using ratios of stable isotopes of nitrogen; d15N) is generally used to quantify biomagnification. TMFs can be compared among systems to understand drivers of biomagnification. TMF averages approximately 8 times increase of Hg concentrations per trophic level (times/TL) for methyl Hg, but ranges from 1 to 64 times/TL across diverse food webs on a global scale. Based on an analysis of 69 food web studies conducted between 1992 and 2013 in marine and freshwater systems across the globe, we previously showed that some of this variation is related to latitude, with Arctic food webs having higher TMFs (~10 times/TL) than those from the tropics (~4 times/TL). Higher biomagnification in colder food webs is believed to be the result of slower growth rate leading to accrued bioaccumulation. Also, areas with high Hg inputs (via atmospheric deposition) tend to have food webs that exhibit lower trophic magnification of this metal, thus lessening risk from exposure in these otherwise contaminated food webs. However, much of the variation in TMFs across aquatic systems remains unexplained. Here we revisit and add TMF and ancillary (e.g., pH) data to this global dataset to further explore patterns and drivers. We identified an additional 74 studies completed between 2013 and 2017 for potential inclusion in the existing database. These include studies on under-represented systems such as hydroelectric reservoirs that are known to result in high Hg concentrations in predatory fishes after dam construction. In addition, we assess whether variations in bioenergetics and food web dynamics could explain variations in TMF. Given ongoing Hg emissions, deposition and transformations, understanding Hg behavior in food webs at a global scale allows for critical links to be made between system characteristics and top predators in food webs that can be used to identify areas at greatest risk from legacy inputs or those that may respond more quickly to global reductions in Hg emissions.
RECOVERY OF AQUATIC INSECT-MEDIATED METHYLMERCURY FLUX FROM PONDS FOLLOWING DRYING DISTURBANCE
Small ponds exist across a permanence gradient and pond permanence is hypothesized to be a primary determinant of insect community structure and insect-mediated methylmercury (MeHg) flux from ponds to the surrounding terrestrial landscape. Here we present the first experiment examining the recovery of insect-mediated MeHg flux following a drying disturbance that converted permanent ponds with insectivorous fish to semi-permanent ponds without fish. We used floating emergence traps to collect emergent insects for 10 weeks in the spring and summer from five ponds with fish (permanent) and five ponds that were drained to remove fish, dried and refilled with water (semi-permanent). Methylmercury flux from dragonflies (Odonata: Anisoptera) and phantom midges (Diptera: Chaoboridae) was significantly greater from newly refilled semi-permanent ponds than permanent ponds but the other eight taxa did not differ between treatments. Beginning 11 days after semi-permanent ponds were refilled, total MeHg flux from semi-permanent ponds was never significantly less than permanent ponds, indicating that insect-mediated MeHg flux had rapidly recovered in semi-permanent ponds following the drying disturbance. The present study demonstrates the impact of an ecological factor like drying disturbance on the cross-system transport of contaminants from aquatic to terrestrial ecosystems.
MERCURY AND STABLE ISOTOPES OF NITROGEN REVEAL AN AQUATIC DIET FOR SEVEN TAXA OF SHORELINE SPIDERS
Mercury (Hg) is a hazardous aquatic contaminant that can be transferred from the aquatic to the terrestrial environment via emerging aquatic insects. Spiders transfer Hg to terrestrial consumers such as birds, but how spiders become contaminated with Hg is not well understood. In the present study, we used stable isotopes of nitrogen (δ15N) in combination with total Hg (THg) to determine the source of Hg to seven taxa of shoreline spiders. We collected orb weavers (Araneidae), wolf spiders (Lycosidae), long-jawed orb weavers (Tetragnathidae), lynx spiders (Oxyopidae), fishing spiders (Pisauridae), jumping spiders (Salticidae), and crab spiders (Thomisidae) along with a variety of aquatic and terrestrial plant, invertebrate, and vertebrate samples from 10 experimental ponds located in north Texas, USA. Orb weavers, long-jawed orb weavers and fishing spiders were the most strongly connected to the aquatic food web and had the highest concentrations of THg. However, δ15N and THg values suggest that all taxa of shoreline spiders examined in the present study consume aquatic organisms.
PATTERNS OF MERCURY AND ORGANIC CO-CONTAMINANTS IN MARINE AND FRESHWATER FISH
Fish consumption advisories have been established throughout the US for freshwater and coastal ecosystems. Most of the fish consumption advisories in the US are for mercury (81%), but significant numbers of others have been established for organic contaminants including PCBs and their congeners, chlordane, DDT, and others. Although the advisories are contaminant specific, humans are exposed to multiple contaminants through fish consumption. Past studies have focused only on individual contaminants, in particular mercury, without relating those exposures to others such as organic contaminants. Using publicly available datasets in which multiple contaminants were measured in individual fish and invertebrate samples, we examined the relationships between different contaminants (metals, pesticides, PCBs, PAHs, and PFCs) from the EPAs Environmental Monitoring and Assessment Program (EMAP) in lakes, National Coastal Condition Assessment (NCCA), National Rivers and Streams Assessment (NRSA), and the Mussel Watch Program. Preliminary results indicate species-specific relationships between contaminants and some general differences between freshwater species and marine species. Across datasets, Hg in fish and invertebrate tissue correlates both positively and negatively with PCBs, DDT formulations, and some pesticides (e.g., dieldrin, aldrin, chlordane). Correlations of Hg with organic contaminants appear to be mostly negative in rivers and lakes but positive in coastal waters. We examined the co-occurrence of mercury and organic contaminants in fish from a range of ecosystems in order to begin to evaluate the risk of human exposure to multiple contaminants posed by eating seafood and understand the processes related to their co-occurance in aquatic organisms.
SYSTEMATIC REVIEW OF TRENDS IN FISH TISSUE MERCURY CONCENTRATIONS
Since the 1970s there has been a major international effort to monitor mercury (Hg) concentrations in fish tissue to identify areas with fish Hg concentrations that are of concern to human health and wildlife. More recently, numerous data sets, based on these monitoring programs, have been compiled and used to evaluate trends in mercury in fish from both freshwater and marine systems. We reviewed the academic literature and published government reports that assessed temporal trends in fish or invertebrate tissue mercury concentrations. Our goals were to summarize the overall trend patterns in fish tissue Hg concentrations, and to review and evaluate the statistical methods used for trend assessment.
We identified more than 180 papers published between 2005 and 2015. Together, these papers reported on the analysis of over 1,050,000 tissue samples from at least 17,000 sampling sites. In many cases, these studies compared fish Hg data to Hg emissions and deposition data to assess the effects of local controls on Hg emissions as well as the influence of global atmospheric Hg emissions. We summarized and catalogued the results of the trend analyses by waterbody type, species, and geographic location, and by the direction (increase/decrease) and magnitude of trends. Generally, the results of the published studies are equivocal. The direction and magnitude of significant trends in fish tissue Hg concentrations vary by region and are not always consistent with trends in atmospheric emissions. In some analyses, where consistent trends have been identified in a geographic area, the results indicate that the fish Hg levels either declined or remained stable between 1970 and 1990 during the period of the largest reductions in mercury emissions. Numerous studies show no trend or an increasing trend in fish tissue concentrations between 1995 and 2012.
During the course of the review, information on the influence of other factors that affect the relationship between atmospheric Hg deposition and tissue Hg levels was recorded. These factors include temperature effects and food web structure. The statistical methods used in the reviewed trend analyses are summarized with the goal of providing information to optimize future sampling and analysis efforts.
EFFECTS OF POND PERMANENCE AND SEASON ON ODONATE-MEDIATED MERCURY FLUX AND ASSOCIATED RISK TO NESTLING RED-WINGED BLACKBIRDS
In the U.S. Great Plains, millions of human-made ponds have been contaminated with mercury (Hg) deposited from the atmosphere. Methylmercury (MeHg) can be transferred from these ponds to terrestrial predators by emergent aquatic insects. The effects of pond permanence and season on insect-mediated MeHg flux from ponds have not been well studied. Small human-made ponds exist across a permanence gradient from permanent, that always contain water and fish, to temporary, that dry periodically and are fishless. The presence of fish reduces the population sizes of odonate taxa (damselflies and dragonflies) and could therefore reduce odonate-mediated MeHg flux from permanent ponds. We examined the effects of pond permanence and season on odonate-mediated MeHg flux in experimental ponds and its potential risk to nestling red-winged blackbirds (Agelaius phoeniceus), an odonate predator. Exuviae of emergent odonates were collected from emergence platforms in permanent ponds with bluegill (Lepomis macrochirus) (n = 5) and temporary ponds without fish (n = 5) over an eight-month period (January August, 2015). Methylmercury flux was computed by multiplying the number of exuvia collected on emergence platforms by the biomass of adult odonates and the average MeHg concentration of adult odonates captured in floating-emergence traps. Fish predation reduced odonate-mediated MeHg flux by suppressing emergence of damseflies and aeshnid and libellulid dragonflies. Emergence of damselflies, aeshnid dragonflies and libellulid dragonflies peaked in April, May, and June, respectively, and overlapped with reproduction of red-winged blackbirds. We calculated odonate-based wildlife values (the minimum odonate MeHg concentrations causing physiologically significant doses in consumers) to assess exposure risks for three sizes (0.004, 0.02 and 0.04 kg) of nestling red-winged blackbirds with diets consisting of a low (40%) and high (90%) percentage of odonates. Methylmercury concentrations in odonates exceeded wildlife values for all three sizes of nestlings with both low- and high-odonate diets. The present study suggests that MeHg concentrations in odonates can pose a threat to small-bodied nestling birds that consume large amounts of odonates. This MeHg threat to songbirds may extend throughout the U.S. Great Plains.
SPANNING THE (ECOSYSTEM) GAP: AQUATIC INVERTEBRATES SUBSIDIZE NUTRIENTS AND MERCURY TO TERRESTRIAL RIPARIAN SONGBIRDS IN LENTIC AND LOTIC HABITATS OF THE WILLAMETTE RIVER, OREGON
Aquatic and terrestrial food webs are intrinsically linked through cross-ecosystem subsidies of nutrients. Despite the importance of cross-ecosystem nutrient subsidies for ecological function, bioaccumulative and harmful contaminants, such as mercury (Hg), also follow these subsidies through energy pathways. The flux of Hg out of aquatic systems may cause deleterious impacts to terrestrial species that rely on aquatic subsidies. Riparian songbirds, in particular, are sensitive to Hg effects and can be heavily dependent on the flux of emergent aquatic insects for prey sources during the breeding season. In this study, we elucidated the relationship between carbon and Hg movement from aquatic to terrestrial ecosystems along the Willamette River in western Oregon, a water body with a legacy of Hg pollution from gold and Hg mining in its headwaters. The Willamette River, though now largely channelized, was historically made up of a series of braided channels. The legacy of this braided system is found in backwater lentic habitats with varying degrees of connection to the main stem lotic habitats. These lentic habitats serve an integral conservation role for fish, flood control, and riparian habitat, but also may facilitate higher Hg methylation rates.
This study quantifies MeHg and carbon-13 isotope in terrestrial and aquatic invertebrate communities in both lentic and lotic systems of the Willamette River. In 2013, riparian songbird blood samples (N = 480) and baseline aquatic and terrestrial invertebrate (non-predators and predators) were collected at 12 sites (7 lotic and 5 lentic). We then compared MeHg and carbon signatures in the base of the food web with THg and carbon-13 in blood samples from riparian songbirds. Our preliminary findings indicated that terrestrial and aquatic non-predatory invertebrates differ in both Hg and carbon-13 signature, when accounting for site, date, and invertebrate family. Non-predatory aquatic insects in lentic habitats had higher Hg concentrations than in lotic habitats, but no there was no difference in Hg between terrestrial invertebrates living near lentic or lotic habitats. Predatory invertebrates in terrestrial habitats showed Hg bioaccumulation and carbon signatures that suggest a reliance on aquatic-based nutrients. Preliminary data suggest that birds that forage on more aquatic based carbon (based on carbon-13 isotope) also showed higher levels of Hg in their blood.
MERCURY EXPOSURE OF BREEDING LEACH’S STORM-PETRELS RELATED TO THEIR FORAGING HABITATS IN THE NORTHWEST ATLANTIC OCEAN
Leach’s storm-petrels (Oceanodroma leucorhoa = Hydrobates leucorhous) are small seabirds that breed on coastal islands and make long, multi-day foraging trips during incubation, often beyond the continental shelf. We are working to assess the role of mercury as a stressor on breeding populations of this bird, which are declining at many breeding colonies in Atlantic Canada. In 2013-15, we attached geolocator tags to adult storm-petrels at seven breeding colonies in Atlantic Canada, to monitor their foraging trips during incubation. When the birds were recaptured 2-4 weeks later, the geolocator data were downloaded and a small blood sample was collected for analysis of total mercury and stable carbon and nitrogen isotope ratios (δ13C and δ15N). Leach’s storm-petrels made offshore foraging trips that averaged 4.3 days in length, covered 1,481 km in total distance, and foraged over ocean depths ranging 44 – 5700 m. Mean mercury concentrations (± SE) in Leach’s storm-petrel blood (n=193) were significantly greater from four colonies around Newfoundland (Baccalieu 1.11 ± 0.05 μg/g, Gull 1.00 ± 0.06 μg/g, Middle Lawn 1.28 ± 0.09 μg/g, and Grand Colombier Islands 1.27 ± 0.05 μg/g) than from three colonies in the Gulf of Maine (Bon Portage 0.65 ± 0.08 μg/g, Kent 0.57 ± 0.04 μg/g, and Machias Seal Islands 0.37 ± 0.06 μg/g). Blood mercury concentrations were positively related to the depth of the ocean over which birds foraged and to the distance that they foraged away from their breeding colonies, and were negatively related to sea surface temperatures where they foraged. Blood mercury concentrations were also positively related to δ15N. Storm-petrels breeding at Newfoundland colonies consistently foraged over deep ocean beyond the edge of the continental shelf and had higher blood mercury levels. In contrast, some birds in the Gulf of Maine foraged over shallow ocean on the continental shelf and had lower blood mercury concentrations. Determining mercury concentrations in the blood of these seabirds, along with their foraging patterns, allows us learn about methylmercury dynamics in offshore ocean foodwebs.