By examining data for animals such as crayfish and salamanders, researchers have identified new ways of comparing mercury levels both within and across watersheds. In addition, by carefully analyzing new data scientists have discovered high mercury levels in unexpected places. Not only does mercury pose a threat to fish and the people eating them, but animals living in habitats as diverse as mountain-tops and small headwater streams should now be considered at risk for mercury poisoning.

Crayfish as mercury yardsticks (Paper 12)

Crayfish are relatively long-lived invertebrates (organisms without backbones) that reside in many different habitats within a watershed. They live in small headwater streams, large lakes and all water types in bet ween. Crayfish also have small home ranges and remain wit hin the same area for most of their life. As such, they reflect mercury in their immediate surroundings and provide a useful yardstick for comparing mercury levels throughout a specific watershed (Figure 8). These same characteristics make crayfish useful locators of high mercuy levels that may originate from local point sources such as an old landfill.

Researchers collected and analyzed crayfish over a period of four years from sites in Vermont, New Hampshire and Maine. Mercury concentrations ranged from 0.04 to 0.50 ppm. Half of the crayfish examined had mercury levels above the expected background level of 0.10 ppm. Larger crayfish and crayfish living in rivers and streams showed higher mercury levels than other individuals. As is the case for fish and wildlife, nearly all of the tail mercury existed in t he toxic methyl form (88 percent). Animals that regularly eat crayfish include bass, loons and raccoons.

Salamanders detect mercury in headwaters (Paper 14)

Scientists analyzed mercury in northern two-lined salamanders that inhabit streams throughout eastern North America. Their study sites included Acadia National Park (ANP) and Bear Brook Watershed (BBW) in Maine, as well as Shenandoah National Park (SNP) in Virginia. Streams in each of these study sites represent differences in mercury deposition and land use history. This is the first study to analyze the effects of chronic acidification, fire history and forest cover on mercury levels in a stream-dwelling amphibian species.

The mercury in two-lined salamanders was elevated and ranged from 0.02 to 0.08 ppm. The mercury concentrations in these salamanders were higher than those found in brook trout, and most of it occurred as methylmercury.

Data from the acidic stream in BBW indicate that mercury bioaccumulation was higher in this acidic environment. This is presumably due to the greater presence of sulfate reducing bacteria and transfer of mercury through the food web in this acidic environment.

The highest mercury levels in two-lined salamanders were found in the conifer-dominated watershed in ANP where there was no history of fire. This result suggests that fire history and forest cover may also affect mercur y bioaccumulation, as has been suggested by previous research. This study illustrates the important role t hat acidification, land use and forest cover play in mercury cycling and underscores t he importance of assessing sensitive watersheds.

Scientists document widespread mercury in aquatic birds (Papers 15, 17 and 18)

The use of aquatic birds as indicators of mercury contamination has been a common practice for years. Recently, scientists have discovered the importance of using several bird species to compare pollution levels across different ecosystem types (e.g. lakes versus wetlands). A dataset of more than 4,700 records representing 38 different bird species was compiled to assess differences among bird species, geographic areas, habitat types, size, age and gender.

Elevated mercury levels were detected in most aquatic and even some terrestrial habitats (Figure 11). Selected indicator species that represent fish and insect food chains are useful for monitoring changing mercury levels and identif ying sensitive areas across the Northeast. In particular, the common loon serves this role well because of its position on the food chain, prey choice, habitat, and abundance (Box 2).

To understand how mercur y levels compare in different bird species living in the same environment, researchers evaluated mercur y data for five species on Aziscohos and Flagstaff lakes in Maine. The results show t hat large fish-eating birds had the highest mercury levels and plant-eating birds had the lowest. In general, mercur y levels ranged from low to high as follows: wood duck < tree swallow < belted kingfisher < common merganser < common loon. This information is useful when choosing indicators species and confirms the common loon serves that role well.

Insect-eating birds in aquatic environments generally had lower mercury than their fish-eating neighbors, but some did not follow this pattern. Specifically, a northern waterthrush from a river in Massachusetts had mercury levels of 1.6 ppm in its blood. This level was higher than the mercury found in all of the more than 100 juvenile bald eagles that were sampled. Scientists attribute these high mercury levels in a non-fish-eating bird because it is at the top of a food chain that has multiple links. The more linkages there are in a food chain, the greater the rate of biomagnification.

In another review of this extensive dataset, scientists analyzed samples from bald eagles and belted kingfishers and determined that mercury tends to increase across habitat types from marine areas, to estuaries and rivers, and is highest in lakes (Figure 9). Therefore, a bald eagle nesting near a lake would likely have higher mercury levels than one near the coast. This information can be used to help focus mercury reduction efforts on ecosystems with high mercury in wildlife.

Research reveals mercury in forest songbirds (Paper 16)

One of the most significant discoveries made in this comprehensive data analysis is the presence of mercury in non-aquatic songbirds. Scientists collected blood and feather samples from four species of mountain-dwelling songbirds at sites on Mt. Mansfield in Vermont: Bicknell’s thrush, blackpoll warbler, white-throated sparrow and yellow-rumped warbler. In addition, they sampled Bicknell’s thrush at 20 other sites from Vermont to Gaspe Peninsula in Quebec. The data on Bicknell’s thrush provide the most comprehensive information to date on mercury in a strictly terrestrial, insect-eating songbird.

The results from this new study show that songbirds in mountain forests are accumulating mercury. Among the four species sampled on Mt. Mansfield, mercury concentrations in blood were highest in the Bicknell’s thrush (0.08 to 0.38 ppm). Feather mercury levels were greatest in Bicknell’s thrush older than two years, suggesting that the mercury in these birds is building up over time. Nearly all of the mercury measured in these birds was in the methyl form, indicating that mercury is accumulating in food webs within high elevation forest environments.

The spatial pattern of mercury in the blood of Bicknell’s thrush shows that levels are highest in areas that are expected to receive high inputs of mercury in litterfall (Figure 10). The higher mercury blood concentrations of Bicknell’s thrushes in the southern versus northern Green Mountains of Vermont parallels deposition estimates for these sites. Overall, Bicknell’s thrush blood mercur y levels were highest in the western Maine mountains and lowest in the Gaspe Peninsula, Quebec. Known mercury sources, mercury deposition models, and new songbird and fish mercury data all suggest that the Catskill Mountains and nearby areas of the Appalachian Mountains are potentially at greater ecological risk for mercury accumulation.

Mercury found in mink and river otter (Paper 19)

Mink and river otter are mammals that feed on fish and crayfish and have the potential to accumulate toxic levels of mercury in their bodies. Scientists compiled mercury data for mink and otter across New York, New England and Nova Scotia. The average mercury concentrations in mink liver ranged from 1.01 to 3.01 ppm with the highest levels occurring in Massachusetts and Connecticut. Average mercury levels in river otter liver ranged from 0.85 to 2.10 ppm, with no clear regional pattern.

When evaluating ecological impacts, it is important to compare mercury concentrations to levels that are associated with adverse effects. For mink and otter, that level has been established at 20 ppm of mercury in fur. It can also be helpful to look beyond the average mercury level to the maximum level, since these high levels could have acute effects. Thresholds for acute mercury toxicity leading to the death of mink and otter have been defined from laboratory studies and field observations as approximately 47 ppm in fur. Figure 12 compares these thresholds to mercury levels found in mink and otter fur in the Northeast.

A long-term dataset from New York state allowed scientists to evaluate changes in mercury levels over time. They found a statistically significant decrease in both otter and mink mercur y levels between the periods 1982-1984 and 1998-2000. Mercury in the liver of otters decreased approximately 26 percent between these two periods and mink liver mercury declined roughly 37 percent. The declines were remarkably similar between adult and young otter as well as bet ween male and female mink. The uniform decline suggests decreases in mercury will produce improvements in mink and otter regardless of species, age and gender.