Mercury Contamination of Aquatic 


Ecosystems

U.S. Department of the Interior–U.S. Geological Survey

Introduction

Mercury has been well known as an environmental pollutant for several decades. 

As early as the 1950’s it was established 

that emissions of mercury to the environment could have serious effects on 

human health. These early studies demonstrated that fish and other wildlife from 

various ecosystems commonly attain mercury levels of toxicological concern when 

directly affected by mercury-containing 

emissions from human-related activities. 

Human health concerns arise when fish 

and wildlife from these ecosystems are 

consumed by humans.


During the past decade, a new trend has 

emerged with regard to mercury pollution. Investigations initiated in the late 

1980’s in the northern-tier states of the 

U.S., Canada, and Nordic countries found 

that fish, mainly from nutrient-poor lakes 

and often in very remote areas, commonly have high levels of mercury. More 

recent fish sampling surveys in other regions of the U.S. have shown widespread 

mercury contamination in streams, wetlands, reservoirs, and lakes. To date, 

33 states have issued fish consumption 

advisories because of mercury contamination (fig. 1). These continental to 

global scale occurrences of mercury 

contamination cannot be linked to individual emissions of mercury, but instead are due to widespread air pollution. When scientists measure mercury levels in air and surface water, 

however, the observed levels are extraordinarily low (fig. 2). In fact, scientists have to take extreme precautions to 

avoid direct contact with water samples or sample containers, to avert sample contamination (fig. 3). Herein lies 

an apparent discrepancy: Why do fish 

from some remote areas have elevated 

mercury concentrations, when contamination levels in the environment are so 

low?


How does mercury become a 


toxicological problem? 

Like many environmental contaminants, mercury undergoes bioaccumulation. Bioaccumulation is the process by 

which organisms (including humans) 

can take up contaminants more rapidly 

than their bodies can eliminate them, thus 

the amount of mercury in their body accumulates over time. If for a period of time 

an organism does not ingest mercury, its 

body burden of mercury will decline. If, 

however, an organism continually ingests 

mercury, its body burden can reach toxic 

levels. The rate of increase or decline in 

body burden is specific to each organism. 

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 bioaccumulation 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 

problem is the fact that mercury concentrates in the muscle tissue of fish. So, unlike organic contaminants (for example 

PCBs and dioxins) which concentrate in 

the skin and fat, mercury cannot be 


filleted or cooked out of consumable 

game fish.


EXPLANATION

FISH CONSUMPTION ADVISORIES:


MERCURY

One or more advisories


No mercury advisories

0 400 800 MILES 0 400 800 MILES

Figure 1. States with at least one fish consumption advisory for 

mercury. Source: USEPA Fish Consumption Data Base

Figure 2. The droplet of mercury shown in this 

slide is about 1 gram; the same amount that is 

in a standard mercury thermometer and the 

total amount that is deposited annually on a 

lake in northern Wisconsin with a surface area 

of 27 acres.

Fact Sheet FS-216-95
What are the human health 


effects of mercury toxicity? 


Humans generally uptake mercury in 

two ways: (1) as methylmercury 

(CH3Hg+) from fish consumption, or 

(2) by breathing vaporous mercury 

(Hg0) emitted from various sources 

such as metallic mercury, dental amalgams, and ambient air. Our bodies are 

much more adapted for reducing the 

potential toxicity effects from vaporous mercury, so health effects from this 

source are relatively rare. Methylmercury, on the other hand, affects the central nervous system, and in severe cases 

irreversibly damages areas of the brain 

(fig. 5). The most well documented 

cases of severe methylmercury poisoning are from Minamata Bay, Japan in 

1956 (industrial release of methylmercury) and in Iraq in 1971 (wheat 

treated with a methylmercury fungicide). In each case, hundreds of people died, and thousands were affected, 

many with permanent damage. In 

milder cases of mercury poisoning, 

adults complain of reductions in motor 

skills and dulled senses of touch, taste, 

and sight. These milder effects are generally reversible if exposure to mercury is 

halted. Unborn children are at greatest 

risk from low-level exposure to methylmercury. Recent research suggests that 

prenatal effects occur at intake levels 5-


10 times lower than that of adults. If these 

results are confirmed, a substantial fraction of unborn children would be at risk.


Mercury Cycling in the 


Environment

Mercury can take a myriad of pathways 

through the environment. Figure 6 shows 

a schematic drawing of mercury cycling in 

an aquatic ecosystem. With the exception 

of isolated cases of known point sources, 

the ultimate source of mercury to most 

aquatic ecosystems is deposition from the 

atmosphere, primarily associated with 

rainfall. As depicted in this figure, 


atmospheric deposition contains the three 

principal forms of mercury, although the 

majority is as inorganic mercury (Hg2+, 

ionic mercury). Once in surface water, 

mercury enters a complex cycle in which 

one form can be converted to another. It 

can be brought to the sediments by partiFigure 3. Because there is actually very little 

mercury in most natural waters, scientists have 

to use extreme measures when sampling for 

mercury to avoid sample contamination from 

their hands and clothing. This entails the use of 

lint-free suits, plastic gloves, hoods, and 

stringently cleaned sampling equipment.

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 

remote from point sources, biomagnification 

effects can result in mercury levels of 

toxicological concern.

Figure 5. All forms of mercury are toxic to humans, but 

methylmercury is especially of concern because our bodies have 

a less well developed defense mechanism against this toxin. 

Effects on the nervous system are the most prevalent in humans.

MERCURY


HEALTH EFFECTS

Deteriorates nervous system


Impairs hearing, speech,


vision and gait


Causes involuntary muscle


movements


Corrodes skin and mucous


membranes


Causes chewing and 


swallowing to become


difficult

Hg

Hg

Hg

Hg

Hg

Hg

Hg

Hg

Hg

Hg

Hg

Hg

Hg

Hg

Hg

Hg

Hg

Hg

Hg

Hg

Hg

Hg

Hg

Hg

Hg

Hg

Hg

Hg

Hg

Hg

Hg

Hg

Hg Hg

Hg

Hg Hg

Hg

Hg
Figure 6. Mercury cycling pathways in aquatic environments are very complex. The various forms of 

mercury can be converted from one to the next; most important is the conversion to methylmercury 

(CH3Hg+), the most toxic form. Ultimately, mercury ends up in the sediments, fish and wildlife, or evades 

back to the atmosphere by volatilization. Reprinted with permission from Mercury Pollution: Integration 

and Synthesis. Copyright Lewis Publishers, an imprint of CRC Press.

AQUATIC MERCURY CYCLE

DEPOSITION

VOLATILIZATION


AND DEPOSITION

SEDIMENTATION

OUTFLOW

VOLATILIZATION


AND DEPOSITION

SEDIMENTATION

OUTFLOW

DIFFUSION/


SEDIMENT


RESUSPENSION

METHYLATION

BIOMAGNIFICATION

OUTFLOW

REDUCTION

DEPOSITION

Hg(II) DEPOSITION


AND RUNOFF

CH3HG DEPOSITION


AND RUNOFF

DEMETHYLATION

Hg(II)

Hg(II)

CH3Hg

CH3Hg

Hg(II)

cle settling and then later released by diffusion or resuspension. It can enter the 

food chain, or it can be released back to 

the atmosphere by volatilization. The 

concentration of dissolved organic carbon (DOC) and pH have a strong effect 

on the ultimate fate of mercury in an ecosystem. Studies have shown that for the 

same species of fish taken from the same 

region, increasing the acidity of the water 

(decreasing pH) and/or the DOC content 

generally results in higher body burdens 

in fish. Many scientists currently think 

that higher acidity and DOC levels 


enhance the mobility of mercury in the 

environment, thus making it more likely 

to enter the food chain. Many of the 


details of the aquatic mercury cycle are 

still unknown, however, and remain areas 

of active research. 


How does mercury enter the 


food chain? 


The exact mechanism(s) by which mercury enters the food chain remain largely 

unknown, and probably vary among ecosystems. We do know, however, that certain bacteria play an important early 

role. Studies have shown that bacteria 

that process sulfate (SO4

=) in the environment take up mercury in its inorganic form, and through metabolic 


processes convert it to methylmercury. 

The conversion of inorganic mercury to 

methylmercury is important for two reasons: (1) methylmercury is much more 

toxic than inorganic mercury, and 


(2) organisms require considerably longer to eliminate methylmercury. At this 

point, the methylmercury-containing 

bacteria may be consumed by the next 

higher level in the food chain, or the 

bacteria may release the methylmercury to the water where it can quickly 

adsorb to plankton, which are also consumed by the next level in the food 

chain. 


Where does atmospheric 


mercury come from? 


There are many sources of mercury to 

the environment, both natural and man 

related. Natural sources include volcanoes, natural mercury deposits, and 

volatilization from the ocean. The primary human-related sources include: coal 

combustion, chlorine alkali processing, 

waste incineration, and metal processing. 

Best estimates to date suggest that human 

activities have about doubled or tripled 

the amount of mercury in the atmosphere, 

and the atmospheric burden is increasing 

by about 1.5 percent per year. 


Has there always been mercury 

contamination, or is this a recent 

problem? 


This is a difficult question to answer, in 

part because of a lack of adequately preserved fish specimens of preindustrial age 

to compare against contemporary samples. However, several lines of evidence 

from recent studies on Wisconsin lakes 

suggest that increased emissions to the 


atmosphere, and subsequent higher deposition rates to lakes, likely translate into 

higher mercury levels in fish. Although 

the total amount of mercury delivered to 

one of these lakes annually is very small 

(fig. 2), it is strongly absorbed by organic material floating in the water such as 
plankton or bacteria. These microorganisms are consumed by organisms higher in 

the food chain, or after dying, settle to the 

bottom of the lake and are incorporated 

into bottom sediments. Studies of sediment cores show that younger sediments 

deposited since industrialization have mercury concentrations that are about 3-5 

times that of historical sediments. Thus, 

the fact that these sediments are primarily 

composed of dead microorganisms that 

were once the bottom of the food chain 

would suggest that modern levels of mercury in the food chain are elevated over 

preindustrial times. 


If human-related emissions could 

be eliminated or reduced, how long 

would it take for ecosystems to 


recover?

The only way to attempt to answer this 

question is to incorporate all the best 


information currently available on how 

mercury behaves in the environment into a 

computer model. Such a model was constructed as part of the research effort on 

northern Wisconsin lakes. Modeled 

scenarios predict that if emissions could be 

reduced by 5 percent, it would take 8 years 

before any change in fish concentrations 

would be observed, and the decrease 

would be small.


The Role of the USGS in 


Mercury Studies

As a national agency with a mission to 


describe the nation’s water resources, the 

USGS is uniquely positioned to provide a 

leadership role in aquatic mercury investigations. The USGS was a prominent participant in studies conducted in 

northern Wisconsin, which largely form 

the basis of current knowledge about 

mercury in aquatic ecosystems. With 

offices in every state, staffed with scientists trained in the collection of water 

samples, the USGS can conduct studies 

on mercury contamination throughout 

the country. The ongoing National 

Water-Quality Assessment (NAWQA) 

program provides additional infrastructure and expertise to gain a national perspective on mercury contamination. A 

recently established mercury research 

laboratory in Madison, Wisconsin gives 

USGS scientists the necessary analytical capability to conduct state-of-theart contamination studies. Thus, the 

USGS is well situated to advance the 

understanding of mercury cycling in 

aquatic ecosystems and to assist 


resource management agencies in 


developing strategies for reducing the 

effects of mercury contamination.


D.P. Krabbenhoft and D.A. Rickert


Suggested Reading


Mercury as a Global Pollutant (D.B. 


 Porcella, J.W. Huckabee, and B. 


 Wheatley editors), Water, Air and 


 Soil Pollution, 80 (1-4), 1995.

THE U.S. GEOLOGICAL SURVEY HAS 

PARTICIPATED IN MERCURY 

CYCLING STUDIES IN NORTHERN 

WISCONSIN LAKES.

CURRENTLY, THE U.S. GEOLOGICAL SURVEY 

IS CONDUCTING MERCURY CYCLING 


STUDIES IN THE FLORIDA EVERGLADES.

CURRENTLY, THE U.S. GEOLOGICAL SURVEY 

IS CONDUCTING MERCURY CYCLING 


STUDIES IN THE FLORIDA EVERGLADES.

CLEAN SAMPLING TECHNIQUES CAN BE 

ADAPTED FOR USE IN ALMOST ANY 

ENVIRONMENT, INCLUDING WETLANDS, 

LAKES, AND STREAMS.

For more information contact:


District Chief


U.S. Geological Survey


Water Resources Division


6417 Normandy Lane


Madison, WI 53719

U.S. Department of the Interior


U.S. Geological Survey


Fact Sheet FS-216-95