26th October, 2011 - Tony Groves, Pam Tyning, Paul Hausler - Progressive AE
A report of the first-ever assessment of the nation’s lakes was recently published by the U.S. Environmental Protection Agency (EPA).1 In this assessment, data from 1,028 randomly selected lakes from the lower 48 states was collected to gauge the general ecological condition of lakes across the United States. Data from 50 Michigan lakes was included in the nation-wide assessment. Standardized sampling methods were used that would provide unbiased estimates of the condition of natural and man-made lakes and ponds greater than 10 acres and at least three feet in depth. The assessment evaluated the biological health of lakes, key stressors, recreational conditions, trends in water quality, and trophic status. EPA and the US Geological Survey are also assessing streams and rivers, wetlands, coastal waters, and fish contaminants. Altogether, federal agencies are assembling a comprehensive view of the nation’s water resources.
The biological health of lakes was evaluated by examining plankton, i.e., microscopic
plant and animal life. The plankton in each of the lakes was compared to reference lakes, i.e., lakes that come as close as practical to those expected in a natural state. Lakes in “good” condition would contain nearly all the same plankton species as the reference lakes; lakes in “poor” condition would be missing more than 40% of the plankton compared to the reference lakes. Based on the plankton analyses, 56% of the lakes across the nation contained healthy biological communities, 21% of lakes were in fair condition, and 22% were in poor biological condition. However, results were quite variable from region to region. In the Midwest, 91% of lakes were in good biological condition while 90% of lakes in the Northern Plains were in poor biological condition.
The National Lakes Assessment also examined some, but not all, factors that can have a negative impact on lakes, including phosphorus, turbidity, dissolved oxygen, shoreline habitat, and others. Many negative factors were not included in the assessment, such as toxic contamination. Of the factors examined, the biggest problem facing the nation’s lakes was loss of lakeshore habitat, i.e., removal of tree, shrub, and herbaceous plant growth along the shoreline. In general, lakes with reduced shoreline vegetation were three times more likely to exhibit poor plankton growth compared to lakes with abundant vegetation.
The National Lakes Assessment also addressed the presence of algal toxins in lakes. Certain types of algae called cyanobacteria, also known as blue-green algae, can produce toxins called cyanotoxins that are potentially harmful to humans and wildlife. One variety of cyanotoxin called microcystin is believed to be the most common in lakes. While microcystin was found in about one-third of the lakes surveyed, toxins were at a level of concern in only about 1% of lakes.
Nationally, about 13% of lakes were classified as oligotrophic, 37% were mesotrophic, and 50% were eutrophic. In general, man-made lakes tended to exhibit more eutrophic characteristics than natural lakes. In the Upper Midwest, a majority of lakes (54%) were mesotrophic.
To assess general trends in trophic state, National Lakes Assessment data was compared to data collected in the 1970’s as part of the National Eutrophication Survey. In this comparison, about 51% of lakes showed no change in trophic state, 26% showed an improved trophic state and 23% had a degraded trophic state. With respect to phosphorus levels, a comparison of National Lakes Assessment and National Eutrophication Survey data indicate that phosphorus levels decreased in 50% of lakes, remained the same in 24% of lakes and increased in 26%. EPA noted that these results are encouraging and indicate wastewater treatment and other pollution control programs may be working despite increases in development pressure.
The National Lakes Assessment will provide a baseline of data to track future changes in water quality. Major stressors associated with excessive lakeshore development will necessitate stronger management of lakeshore development if this problem is to be addressed. Low Impact Development (LID) strategies need to be developed and implemented to help minimize the impact of shoreline development, and strategies need to be implemented to minimize both point and nonpoint source pollution loadings to the nation’s lakes. Several states, including Michigan, have implemented periodic, state-wide statistical sampling surveys. These data will be helpful in tracking trends in water quality and the effectiveness of lake restoration and pollution control programs.
- U.S. Environmental Protection Agency. 2010. National lakes assessment: A collaborative survey of the nation’s lakes. EPA 841-R-09-001. U.S. Environmental Protection Agency, Office of Water and Office of Research and Development, Washington D.C.