U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet FS 2004-3046

December 2002

Upper Midwest Environmental Sciences Center, La Crosse, Wisconsin

Effects of Ammonia on Freshwater Mussels in the St. Croix River

St. Croix River

Pocketbook mussels Photo by Kevin Cummings

(Lampsilis cardium) Used by permission

Freshwater mussels, animals sensitive to changes in habitat 

quality, are the largest group of endangered animals in North 

America. Freshwater mussels are a renewable resource that 

provides significant economic and environmental benefits to 

the Nation. Their shells are used in the cultured pearl industry; 

they are a food resource for many animals; and they filter 

contaminants, sediment, and nutrients from water, thereby 

improving water quality. Scientists see the mussels’ demise as 

a serious warning for lakes, rivers, and streams. When mussels 

begin to disappear, it is a sign that other species, and even 

whole ecosystems, may also be in jeopardy.

...un-ionized ammonia 

(NH3

) is particularly toxic to 

aquatic animals...

The St. Croix River basin (30 km east of Minneapolis-St. Paul, 

Minnesota) is experiencing rapid land-use changes—from 

forest and agriculture to suburbanization—as the metropolis of 

Minneapolis-St. Paul sprawls into the surrounding countryside. 

The National Park Service, which administers 

most of the St. Croix, is concerned that 

urbanization will also increase the output 

of nutrient-rich effluent from wastewater 

treatment plants into the river. This may result 

in an elevation of ammonia in river sediments. 

Ammonia is a common pollutant that enters 

rivers primarily from sewage discharge, 

fertilizers, and natural processes mediated by 

river bacteria. However, the un-ionized form of 

ammonia (NH3

) is particularly toxic to aquatic 

animals.

The St. Croix River contains a diverse and 

abundant group of freshwater mussels. The St. 

Croix is one of the few rivers in the Midwest 

not substantially affected by the invasion of 

the exotic zebra mussel, which encrusts and 

kills native freshwater mussels. Increased 

concentrations of ammonia in river sediments, 

however, poses a significant threat to mussels.

The Upper Midwest Environmental Sciences 

Center has completed a 3-year study that 

examined the effects of ammonia on native mussels. We used 

native mussels because they are considered one of the most 

important natural resources in the St. Croix River and they are 

very sensitive to ammonia.

In a series of laboratory studies, juvenile pocketbook mussels 

(Lampsilis cardium) were exposed to a range of ammonia 
Laboratory setup

0 100 200 300

Percent mortality

0

20

40

60

80

100 10-d exposure

Ammonia concentration (μg NH3/L)

Laboratory mortality

Mussel chamber

Elevated concentrations 

of ammonia in river 

sediments pose a signifi cant 

threat to mussels.

concentrations delivered into river sediments. We exposed 

mussels to ammonia for 4 and 10 days and then measured the 

concentration of ammonia that killed or reduced the growth of 

mussels. Concentrations as low as 127 μg NH3/L were lethal to 

50% of the mussels after 4 days; concentrations as low as 93 μg 

NH3/L killed 50% of the mussels after 10 days. The growth rate 

of mussels was substantially reduced at ammonia concentrations 

as low as 31 μg NH3/L. These concentrations are lower than the 

existing national water quality criteria, suggesting that these 

criteria may not be protecting juvenile mussels.

In a companion field study, we placed juvenile pocketbook 

mussels into small chambers that were placed into river sediment 

at eight locations throughout the river. After 4, 10, and 28 

days, we retrieved the chambers and measured the survival 

Ammonia concentration (μg NH3/L)

0 20 40 60 80 100 120 140

Percent mortality

0

20

40

60

80

100

4 day

10 day

28 day

and growth of the 

mussels. Ammonia 

concentrations in 

river sediments 

ranged from 0.1 to 

107 μg NH3/L and 

were highly variable 

within a location. 

Survival and growth 

of mussels were 

also highly variable 

and were generally 

unrelated to ammonia 

concentrations. The 

large variability in the 

field measures made 

it difficult to observe 

a relation between 

ammonia and 

survival or growth of 

mussels.

Although freshwater mussels are frequently used as indicators 

of contaminant exposure, the large variance associated with 

measuring survival and growth of juveniles in the field suggests 

that more sensitive effects-level endpoints need to be found in 

this imperiled faunal group. Laboratory experiments, conducted 

under highly controlled conditions, suggest that NH3 is quite 

toxic to juveniles at concentrations that are occasionally 

exceeded in the St. Croix River. In the river, however, sediments 

may not be consistently toxic; rather, periods of episodic toxicity 

may occur when certain conditions (high temperature and low 

flow) are present.

For more information, contact

Teresa Newton

USGS Upper Midwest Environmental Sciences Center

2630 Fanta Reed Road

La Crosse, Wisconsin 54603

Phone: 608.781.6217

Email: tnewton@usgs.gov

http://www.umesc.usgs.gov

Field mortality