National Lakes Assessment 

Fact Sheet April 2010

What is the National Lakes Assessment? 

The National Lakes Assessment (NLA) is the first-ever baseline study of the condition of the 

nation’s lakes. It is the latest in a series of surveys of the nation’s aquatic resources being 

conducted by EPA and its state and tribal partners. 

The NLA provides unbiased estimates of the condition of natural and man-made freshwater 

lakes, ponds, and reservoirs greater than 10 acres and at least one meter deep. Using a 

statistical survey design, lakes were selected at random to represent the condition of the 

larger population of lakes across the lower 48 states. A total of 1,028 lakes were sampled for 

the NLA during summer 2007, representing the condition of about 50,000 lakes nationwide. 

The Great Lakes and Great Salt Lake were not included in the survey. 

Field crews collected samples using the same methods at all lakes to ensure that results can 

be compared across the country. Researchers processed and analyzed 680,000 

measurements, including indicators of water quality such as nutrients, dissolved oxygen and 

algal density; biological indicators such as phytoplankton and zooplankton (algae and 

microscopic animals); recreational indicators such as algal toxins and pathogens; and 

physical habitat indicators such as lakeshore and shallow water habitat cover. 

What are the Key Findings of the NLA? 

Biological Health 

The NLA finds that 56% of 

the nation’s lakes support 

healthy biological 

communities when compared to least disturbed sites 

(Fig.1). Another 21% of lakes are in fair condition, and 

22% are in poor biological condition. This rating is 

based on an index of phytoplankton and zooplankton 

taxa loss – the percentage of taxa observed compared to 

those that are expected, based on conditions at leastdisturbed lakes. 

The survey measured a set of key stressors to lake 

condition to determine their extent across the nation. 

Analysts also examined the relationship between these 

stressors and lake biological health, much as medical 

researchers examine the relationship between 

cholesterol and heart health. 

 Purpose ________________

Report on the 

condition of the 

Nation’s lakes, 

ponds, and 

reservoirs 

Help states and 

tribes implement 

lake monitoring 

and assessment 

programs 

Establish a 

baseline for lake 

condition that 

can be used for 

future trend 

assessments 

Figure 1. Biological Condition of the Nation’s Lakes 
Poor habitat conditions along the lakeshore and high levels of the nutrients nitrogen and phosphorus are the most 

significant stressors of those assessed in the survey (Fig. 2). These stressors increase the likelihood (i.e., relative risk) 

of degraded biological condition. 

• Lakeshore habitat is rated poor in 36% of 

lakes. Poor biological health is three times 

more likely in lakes with poor lakeshore 

habitat relative to lakes with good habitat. 

• The nutrients nitrogen and phosphorus are at 

high levels in about twenty percent of lakes. 

Poor biological health is 2.5 times more likely 

in lakes with high nutrient levels. 

Recreational Condition 

Lakes are widely used for swimming, boating, and 

many other recreational activities. However, a 

number of microbial organisms, algal toxins, and 

other contaminants in lakes can potentially affect 

human health. The NLA conducted the first-ever 

national study of algal toxins in lakes. Microcystin – 

a toxin that can harm humans, pets, and wildlife – 

was found to be present in about one third of lakes 

and at levels of concern in 1% of lakes. 

A parallel study of fish tissue contaminants in the nation’s lakes shows that mercury concentrations in game fish 

exceed health-based limits in 49% of lakes, and polychlorinated biphenyls (PCBs) are found at potential levels of 

concern in 17% of lakes. 

Trends and Trophic Status 

A comparison of NLA data to a subset of wastewater-impacted lakes sampled 35 years ago suggests that the nation’s 

investments in wastewater treatment and other pollution control activities are working despite increased population 

pressures across the United States. The report finds that nearly 75% of the 800 lakes sampled in the 1970s showed 

either improvements or no change in phosphorus levels. Trophic status, a measure of the biological productivity of 

lakes which changes very slowly under natural conditions, also improved or remained the same in about 75% of those 

lakes. The NLA provides a national baseline against which lake managers can track the rate of change in trophic 

status at national and regional scales, and compare that to the rate of change in specific lakes they manage. 

 

Figure 2. Relative Extent of Stressors and Increased Likelihood of 

Degraded Biology when Stressor Rated Poor

This is the first time a national monitoring study of the overall condition of lakes has been conducted using a statisticallyvalid approach. EPA has produced reports using a similar approach for streams (The Wadeable Streams Assessment) and 

estuaries and coastal waters (The National Coastal Condition Reports). National studies of rivers and streams and wetlands 

are underway. To learn about these National Aquatic Resource Surveys, visit www.epa.gov/aquaticsurveys. 

For more information on the National Lakes Assessment, visit http://www.epa.gov/lakessurvey. 

For more information on the National Lake Fish Tissue Study, visit http://www.epa.gov/waterscience/fish/study/. 

 

April 2010, EPA 841-F-09-007. USEPA (4503T), 1200 Pennsylvania Ave NW, Washington, DC 20460