This article was researched and written by a student at the University of Massachusetts, Amherst participating in the Encyclopedia of Earth's (EoE) Student Science Communication Project. The project encourages students in undergraduate and graduate programs to write about timely scientific issues under close faculty guidance. All articles have been reviewed by internal EoE editors, and by independent experts on each topic.
Mercury is a naturally occurring element often found at low concentrations and with low toxicity in various natural environments. Mercury enters terrestrial and aquatic systems through atmospheric deposition: processes such as refining and burning fossil fuels and incinerating municipal waste concentrate mercury while volcanic eruptions can naturally release mercury into the atmosphere. Many forms of mercury can affect the health of humans and wildlife resulting in neurological, digestive, endocrine and skin disorders. Unfortunately, high levels of mercury are present in many ecosystems, connected by air, water, and the organisms that inhabit them.
Despite its harmful effects, mercury is still legally emitted in many countries including the United States, where power plants are currently the leading source. When the Clean Air Act Amendments of 1990 were passed, municipal waste combustors, utility coal boilers, and medical waste incinerators accounted for two-thirds of all mercury emissions. By 1999, emissions from burning municipal and medical waste had decreased by over 90% but utility coal boilers still released 48 tons of mercury into the air annually, accounting for approximately 40% of U.S. mercury emissions. This is partly because many coal fired power plants were protected from having to reduce emissions under the original Clean Air Act of 1970. When the Clean Air Mercury Rule of 2005 is fully implemented, the U.S. Environmental Protection Agency (USEPA) predicts a reduction of utility coal boiler mercury emissions to 15 tons of a predicted 68 tons of total U.S. mercury emissions.
Mercury can be found in small quantities in a variety of common household products, including fluorescent light bulbs, thermometers, batteries, thermostats, switches, and dental fillings. Some states have made it illegal to sell products containing mercury in the state, while awareness and education have led to discontinued use, removal and disposal of contaminated products. Proper disposal as hazardous waste is key to protecting the environment from these products.
All geographic regions do not face equal risks from mercury. In North America one of the regions identified as having high levels of mercury is the Gulf of Maine watershed. The Gulf’s watershed spans 165,185 square kilometers in the Canadian provinces of Nova Scotia, New Brunswick and Quebec, the U.S. states of Maine, New Hampshire and Massachusetts. It is home to over 6 million humans, and has shown some of the highest levels of mercury in wildlife anywhere in North America. The 60 rivers of the watershed empty into the 90,700 square kilometers Gulf of Maine. Its geographic location, which places it at the tail end of prevailing winds, make the Gulf of Maine watershed particularly susceptible to atmospheric deposition of contaminants including mercury.
Mercury in the atmosphere is inorganic (elemental Hg and Hg2+); however, once combined with carbon in the sediments through methylation by sulfur and iron reducing bacteria it becomes organic and can enter the food chain as a biologically active compound. The most common form of organic mercury is monomethyl mercury, which can be found in the tissues of plankton, invertebrates, fish, birds, and [mammal]]s. This process by which organisms take up Hg from food and water is called bioaccumulation. Mercury in these consumers likely originates from estuaries, where conditions are conducive for bacteria to methylate mercury, or from the open ocean, where regions called the oxygen minimum zone appear to have water chemistry that is favorable for methylation. Methyl mercury is produced and released into the water where it is taken up by phytoplankton, zooplankton and later consumed by fish that use the estuaries or open ocean during a portion of their life cycle. This toxic form of mercury is transferred to predators high in the food chain that feed directly on [invertebrate]s and [fish].
In the Gulf of Maine, blue mussels (Mytilus edulis), common loons (Gavia immer), common eider (Somateria mollissima), Leach’s storm-petrel (Oceanodroma leucorhoa), double-crested cormorant (Phalocrocorax auritus) and black guillemot (Cepphus grylle) have all been shown to be effective bioindicators of mercury in the environment. Bioindicators are organisms which are useful in environmental monitoring. Each species has a different foraging strategy and participates in a different food web, making it of particular interest for monitoring programs. The four seabird species characterize four different food webs within the Gulf of Maine: inshore, site specific (common eider); offshore pelagic and mesopelagic (leach’s storm petrel); broad pelagic (double-crested cormorant); and local benthic (black guillemot).
In contrast to the previously mentioned species, there are currently no monitoring programs investigating mercury levels of marine fish. Over 90% of the global fish harvest is from marine fisheries. In the United States, tuna, swordfish, pollock, shrimp, and cod are all significant sources of mercury uptake for humans. Currently, mercury loads are not monitored in marine fish consumed by humans and wildlife in marine ecosystems including the Gulf of Maine and the pathways of mercury throughout food webs are not known. A large gap exists in data on mercury levels in the food web linking lower trophic levels and seabirds. Data on mercury levels in fish would enhance our understanding of the toxin’s pathways and illuminate ecological relationships. A design has been proposed but not yet implemented for such a monitoring program, including methods and focal species.
Compounding the problem of insufficient data is the lack of understanding determining variation in feeding and reproductive behaviors of many marine fish. Characteristics such as age, habitat, migration, abundance, and climate may play important roles in transfer of methyl mercury throughout marine food webs. Understanding these factors is crucial to determining the uptake of methyl mercury in marine fish and what the uptake means for humans eating those species.
Although efforts to reduce atmospheric, terrestrial, and aquatic disposal of mercury are ongoing, the amount of mercury currently available to marine organisms is extremely high. Cutting mercury emissions from coal-fired power plants will likely play a key role in reducing levels of mercury in the Gulf of Maine. More research is needed on the transfer of mercury through food webs, at-risk species, combined with monitoring programs for those species showing us how mercury levels change over time. Understanding threats to humans and wildlife of the Gulf of Maine will help determine where education for humans and remediation for wildlife should be focused.
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