Fact Sheet FS-216-95 Mercury Contamination of Aquatic Ecosystems U.S. Department of the Interior–U.S. Geological Survey Introduction 33 states have issued fish consumption Mercury has been well known as an envi­ advisories because of mercury contami­ ronmental pollutant for several decades. nation (fig. 1). These continental to As early as the 1950’s it was established global scale occurrences of mercury that emissions of mercury to the environ­ contamination cannot be linked to indi­ ment could have serious effects on vidual emissions of mercury, but human health. These early studies dem­ instead are due to widespread air pollu­ onstrated that fish and other wildlife from tion. When scientists measure mercury various ecosystems commonly attain mer­ levels in air and surface water, howev­ cury levels of toxicological concern when er, the observed levels are extraordinar­ directly affected by mercury-containing ily low (fig. 2). In fact, scientists have emissions from human-related activities. to take extreme precautions to avoid Human health concerns arise when fish direct contact with water samples or Figure 2. The droplet of mercury shown in this slide is about 1 gram; the same amount that is and wildlife from these ecosystems are sample containers, to avert sample con­ in a standard mercury thermometer and the consumed by humans. tamination (fig. 3). Herein lies an appa­ total amount that is deposited annually on a lake in northern Wisconsin with a surface area rent discrepancy: Why do fish from of 27 acres. During the past decade, a new trend has some remote areas have elevated mer­ emerged with regard to mercury pollution. cury concentrations, when contamina­ Investigations initiated in the late 1980’s tion levels in the environment are so than their bodies can eliminate them, thus in the northern-tier states of the U.S., low? the amount of mercury in their body accu­ Canada, and Nordic countries found that mulates over time. If for a period of time fish, mainly from nutrient-poor lakes and How does mercury become a an organism does not ingest mercury, its often in very remote areas, commonly toxicological problem? body burden of mercury will decline. If, have high levels of mercury. More Like many environmental contami­ however, an organism continually ingests recent fish sampling surveys in other nants, mercury undergoes bioaccumula­ mercury, its body burden can reach toxic regions of the U.S. have shown wide­ tion. Bioaccumulation is the process by levels. The rate of increase or decline in spread mercury contamination in streams, which organisms (including humans) body burden is specific to each organism. wet-lands, reservoirs, and lakes. To date, can take up contaminants more rapidly For humans, about half the body burden of mercury can be eliminated in 70 days if no mercury is ingested during that time. Biomagnification is the incremental increase in concentration of a contaminant at each level of a food chain (fig. 4). This phenomenon occurs because the food source for organisms higher on the food chain is progressively more concentrated in mercury and other contaminants, thus magnifying bioaccumulation rates at the top of the food chain. The bioaccumula­ tion effect is generally compounded the longer an organism lives, so that larger predatory game fish will likely have the highest mercury levels. Adding to this EXPLANATION problem is the fact that mercury concen­ FISH CONSUMPTION ADVISORIES: MERCURY trates in the muscle tissue of fish. So, One or more advisories unlike organic contaminants (for example No mercury advisories PCBs and dioxins) which concentrate in 0 400 800 MILES 0 400 800 MILES the skin and fat, mercury cannot be filleted or cooked out of consumable Figure 1. States with at least one fish consumption advisory for game fish. mercury. Source: USEPA Fish Consumption Data Base What are the human health milder effects are generally reversible if effects of mercury toxicity? exposure to mercury is halted. Unborn Humans generally uptake mercury in children are at greatest risk from low-level two ways: (1) as methylmercury exposure to methylmercury. Recent + research suggests that prenatal effects (CH3Hg ) from fish consumption, or (2) by breathing vaporous mercury occur at intake levels 5­ (Hg0) emitted from various sources 10 times lower than that of adults. If these such as metallic mercury, dental amal­ results are confirmed, a substantial frac­ gams, and ambient air. Our bodies are tion of unborn children would be at risk. much more adapted for reducing the potential toxicity effects from vaporous Mercury Cycling in the mercury, so health effects from this Environment source are relatively rare. Methylmer­ Mercury can take a myriad of pathways cury, on the other hand, affects the cen­ through the environment. Figure 6 shows Figure 3. Because there is actually very little tral nervous system, and in severe cases a schematic drawing of mercury cycling in mercury in most natural waters, scientists have irreversibly damages areas of the brain to use extreme measures when sampling for an aquatic ecosystem. With the exception mercury to avoid sample contamination from (fig. 5). The most well documented of isolated cases of known point sources, their hands and clothing. This entails the use of cases of severe methylmercury poison­ lint-free suits, plastic gloves, hoods, and the ultimate source of mercury to most stringently cleaned sampling equipment. ing are from Minamata Bay, Japan in aquatic ecosystems is deposition from the 1956 (industrial release of methyl-mer­ atmosphere, primarily associated with cury) and in Iraq in 1971 (wheat treated rainfall. As depicted in this figure, with a methylmercury fungicide). In atmospheric deposition contains the three Hg each case, hundreds of people died, and principal forms of mercury, although the thousands were affected, many with 2+ Hg majority is as inorganic mercury (Hg , permanent damage. In milder cases of ionic mercury). Once in surface water, Hg mercury poisoning, adults complain of Hg mercury enters a complex cycle in which Hg Hg reductions in motor skills and dulled one form can be converted to another. It Hg senses of touch, taste, and sight. These can be brought to the sediments by particle Hg Hg Hg Hg Hg Hg Hg MERCURY Hg Hg HEALTH EFFECTS Hg Hg Hg Hg Deteriorates nervous system Hg Hg Impairs hearing, speech, vision and gait Hg Hg Hg Hg Hg Hg Hg Hg Hg Causes involuntary muscle movements Corrodes skin and mucous Hg Hg Hg Hg Hg Hg membranes Causes chewing and Hg Hg swallowing to become difficult Hg Figure 4. Mercury (Hg) biomagnifies from the bottom to the top of the food chain. Even at very low input rates to aquatic ecosystems that are Figure 5. All forms of mercury are toxic to humans, but remote from point sources, biomagnification methylmercury is especially of concern because our bodies have effects can result in mercury levels of a less well developed defense mechanism against this toxin. toxicological concern. Effects on the nervous system are the most prevalent in humans. settling and then later released by diffu­ tain bacteria play an important early human-related sources include: coal com­ sion or resuspension. It can enter the food role. Studies have shown that bacteria bustion, chlorine alkali processing, waste = chain, or it can be released back to the that process sulfate (SO4 ) in the envi­ incineration, and metal processing. Best atmosphere by volatilization. The con­ ronment take up mercury in its inorgan­ estimates to date suggest that human centration of dissolved organic carbon ic form, and through metabolic activities have about doubled or tripled (DOC) and pH have a strong effect on the processes convert it to methylmercury. the amount of mercury in the atmosphere, ultimate fate of mercury in an ecosystem. The conversion of inorganic mercury to and the atmospheric burden is increasing Studies have shown that for the same spe­ methylmercury is important for two rea­ by about 1.5 percent per year. cies of fish taken from the same region, sons: (1) methylmercury is much more increasing the acidity of the water (de­ toxic than inorganic mercury, and Has there always been mercury creasing pH) and/or the DOC content gen­ (2) organisms require considerably lon­ contamination, or is this a recent erally results in higher body burdens in ger to eliminate methylmercury. At this problem? fish. Many scientists currently think that point, the methylmercury-containing This is a difficult question to answer, in higher acidity and DOC levels bacteria may be consumed by the next part because of a lack of adequately pre­ enhance the mobility of mercury in the higher level in the food chain, or the served fish specimens of preindustrial age environment, thus making it more likely bacteria may release the methylmercury to compare against contemporary samples. to enter the food chain. Many of the to the water where it can quickly adsorb However, several lines of evidence from details of the aquatic mercury cycle are to plankton, which are also consumed recent studies on Wisconsin lakes suggest still unknown, however, and remain areas by the next level in the food chain. that increased emissions to the of active research. atmosphere, and subsequent higher depo­ Where does atmospheric sition rates to lakes, likely translate into How does mercury enter the mercury come from? higher mercury levels in fish. Although food chain? There are many sources of mercury to the total amount of mercury delivered to The exact mechanism(s) by which mercu­ the environment, both natural and man one of these lakes annually is very small ry enters the food chain remain largely related. Natural sources include volca­ (fig. 2), it is strongly absorbed by organic unknown, and probably vary among eco­ noes, natural mercury deposits, and vol­ material floating in the water such as systems.