A caveat of industrial development is increased anthropogenic mercury (Hg) release, including aerial deposition and accumulation near industrial dischargers. Under certain conditions, microbes convert inorganic mercury to methyl mercury, which has potent neurotoxic effects on both humans and wildlife. The combination of bacteria and anoxia in marsh and wetland sediments creates the perfect environment for Hg conversion and methyl mercury accumulation. This has significant implications for the health of native species, including birds, as methyl mercury is biomagnified through the food web.
Dianne Kopec, PhD, is a research biologist at the Senator George J. Mitchell Center for Sustainability Solutions at the University of Maine, and her research documents toxic contaminants detected in various species of wildlife. Via long-term monitoring of fish, shellfish, mammals, and birds, she is able to document trends in contaminant concentrations to identify whether government policy is contributing to reduced contaminant levels in wildlife. Kopec was the staff biologist to the court-ordered Penobscot River Mercury Study, in which she and her colleagues measured mercury exposure in Penobscot River marsh birds by analyzing blood and feather samples. While feathers may seem like an unorthodox sample tissue, they can help to tell a more complete story about contaminant exposure.
The Importance of Studying Birds
The marshes bordering the lower Penobscot River in Maine are home to several species of marsh birds, including various sparrows, red-winged blackbirds, and the Virginia rail. The area is also home to the site of the former HoltraChem plant that operated from 1967 to 2000. The plant used the chlor-alkali electrolysis process to produce chlorine, sodium hydroxide, sodium hypochlorite, hydrochloric acid, and chloropicrin, resulting in mercury contamination along the river. Mercury is associated with a wide range of health effects in birds, including reproductive effects manifesting as reductions in the number of eggs laid, chicks hatched, and juveniles fledged.
“Several factors led us to study mercury concentrations in breeding songbirds inhabiting the lower Penobscot River,” says Kopec. “The river’s marshes are depositional areas for the fine-grained sediment that pulses up and down the river with the twice-daily tides. Particle-bound mercury accumulates in these depositional areas and the biogeochemistry of the marsh sediment enhances the bacterial methylation of inorganic mercury into methylmercury, the more toxic form that biomagnifies in the food web.” According to Kopec, it was important to study birds during their breeding season. “During the breeding season, many songbird species switch to an invertebrate diet. The birds in our study are obligate marsh foragers consuming benthic invertebrates, insects, and spiders. Mercury concentrations in the birds’ invertebrate diet would reflect the bioavailable methyl mercury in the marsh.”
Feathers Contain a Detailed History
To capture birds for sampling, the team used a combination of mist nets erected around the marsh, and a remotely activated woosh net for Virginia rails—as this species flies infrequently, spending more time walking through shallow waters.
In addition to blood samples, Kopec and her colleagues obtained feather samples by clipping a feather—specifically, the first primary feather, P1—at the base of the shaft next to the skin. “It is critical to sample the same exact feather from a given bird species,” explains Kopec. “Mercury is depleted from the blood during feather formation, even when dietary exposure remains constant. Mercury declines in feathers molted later in the molt cycle. We were interested in documenting exposure on the summer breeding grounds, so we sampled the P1, which in the species we sampled is the first feather to molt at the end of the summer breeding season.”
Feather sampling is minimally invasive, and analysis offers researchers valuable insight into the birds’ mercury exposure at the time the feather was formed. “Since feathers are not biologically active, the mercury incorporated into feathers is stable for years—even decades or longer,” explains Kopec. Their structural composition lends them to mercury accumulation, as Kopec explains: “Mercury partitions to proteins, especially to amino acids containing di-sulfide bonds, including cysteine. Keratin, which is a primary structural component of feathers, has a high cysteine content. The mercury in feathers reflects the concentration in circulating blood at the time of feather formation.”
This is an important point to consider when sampling. “In using feathers to document mercury exposures, it is critical to identify where the bird was living at the time of feather formation, and in the months immediately preceding feather growth. A feather sampled during the summer breeding season may have been grown on the bird’s wintering grounds thousands of miles away, and so reflect mercury exposures on the wintering grounds, not the summer breeding grounds where it was sampled,” says Kopec.
In order to ensure accurate interpretation of their results, Kopec and her team examined the birds’ molt cycle and migration timing. Nelson’s sparrows, one of the species sampled in the study, typically winter along the eastern United States, stretching from North Carolina to Florida, and arrive at the breeding grounds in Maine near the end of May and into early June. Their complete pre-basic molt then begins in early August, and during this time the P1 feather is replaced. Based on this molt pattern, the new primary feathers grown in late summer reflect the birds’ mercury exposure on their breeding grounds.
Kopec and colleagues used a modified version of EPA method 1631e, relying on cold vapor atomic fluorescence spectroscopy to determine total mercury in the samples. Feather samples were first washed in dilute soap with a low mercury concentration, rinsed, dried, and held at room temperature prior to analysis.
Taken together, results from blood and P1 feather analysis revealed that despite the HoltraChem plant closure in 2000, marsh birds in the surrounding area were still being exposed to residual mercury contamination in the environment.
The mid-summer mercury concentrations in the blood of adult birds were six to 10 times higher compared to birds sampled outside of the Penobscot watershed. Blood mercury has significant implications for reproductive function, as it may be transferred to developing eggs, and across all five species studied, the concentration exceeded the lower toxicity threshold.
Results also revealed a strong correlation between the mercury content of P1 feathers, and the exposure location at the time of feather formation. The P1 feathers of Nelson’s sparrows, which are indicative of mercury exposure during time spent in the breeding grounds along the Penobscot, were the highest of all species sampled. These findings highlight an important interplay between blood and feather mercury content in birds, and have implications for future study design. Others have used feathers to determine the concentration of various different heavy metals and contaminants, relying on different techniques including inductively coupled plasma mass spectrometry.
Looking Toward a Brighter Future
In our modern, industrial world, it is perhaps unsurprising to read that remnants of past industrial activity in the Penobscot River were still observed in bird species years later. However, studies such as the one carried out by Kopec and her colleagues help to identify the scope of the problem and can support decision-making and cleanup efforts.
In the case of the Penobscot, the Maine Department of Environmental Protection is working on cleaning up the plant site. As for the river and marshes, the Natural Resources Defense Council and Maine People’s Alliance filed suit in 2000, and subsequent court orders required both mercury and engineering studies. Several remediation options have been recommended and are currently being debated.
Environmental analysis techniques, such as cold vapor atomic fluorescence spectroscopy, can be used to monitor environmental contamination. Combined with some help from our feathered friends, these techniques enable scientists to determine the source of contamination, and monitor its progression. In this way, Kopec hopes “we can begin to document the externalities of industrial processes to take steps to ensure that future operations do not damage our environment.”