Water: Lakes
National Lakes Assessment Frequently Asked Questions
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What is the major news of this study? What is the major news of this study?The National Lakes Assessment (NLA) finds that, compared to least disturbed sites, 56% of the nation’s lakes support healthy biological communities, another 21% are in fair condition and 24% are in poor biological condition. The leading stressor assessed in the NLA is poor lakeshore habitat, based on information on both the extent of stressors and their relationship to lake biological health. The nutrients nitrogen and phosphorus rank as the next most significant stressors.
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 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. Why did you survey only lakes?Lakes are the third in a series of several major water resource categories that EPA, states and other partners are surveying under the National Aquatic Resource Survey program. The goal of this program is to generate scientifically-valid information on the status of all our nation’s waters. Coastal waters were the subject of the first national survey of this type, which resulted in the National Coastal Condition Reports. The second survey focused on wadeable streams of the United States. The Wadeable Streams Assessment was published in 2006. EPA and states have also conducted a national survey of fish tissue in lakes (a summary is presented in this report). A national survey of rivers and streams is currently underway and a survey of wetland condition will follow in future years. What’s so new about this study? Who conducted it and why?This report describes the condition of the nation’s lakes, ponds and reservoirs and establishes a national baseline we can use to compare results from future studies. This information will help us evaluate the success and shortcomings of national efforts to protect and restore water quality. The National Lakes Assessment, along with studies of other water resource types, responds to criticisms by the Government Accountability Office and others that the nation’s monitoring programs are not providing answers to key questions about water quality. This is the first time EPA and the states can report on the condition of all U.S. lakes. It applies statistical survey techniques as a cost-effective alternative to sampling every lake across the country. Results are presented both for the nation as a whole and for major ecological regions within the lower 48 states. EPA, states and tribes worked cooperatively together to conduct the study. How did you define lakes for this study?Lakes are defined for purposes of this study as natural or man-made freshwater lakes, ponds, or reservoirs greater than 10 acres (4 hectares), at least 3.3 feet (1 meter) deep and with a minimum of a quarter acre (0.1 hectares) of open water. The Great Lakes and the Great Salt Lake were not included in the survey, nor were commercial treatment and/or disposal ponds, brackish lakes, or ephemeral (temporary) lakes. Only lakes in the lower 48 states were surveyed. Hawaii has no lake resources and a separate study of a subset of Alaska’s enormous lake resource is currently underway. What types of natural lakes were sampled? What types of reservoirs were sampled?The NLA sampled all types of natural and man-made lakes as long as they met the definition of lakes developed for this study (see above). This included lakes formed by glacial, volcanic or tectonic action and oxbow lakes formed from meandering river channels. It also included large and small man-made reservoirs created for energy production, drinking water supply and other purposes. What does the report say are the most important stressors to lakes?Using information on both the extent of stressors and their relationship to lake biological health, the leading stressor assessed in the NLA is poor lakeshore habitat. Lakeshore habitat is rated poor in 36% of lakes. Poor biological health is three times more common in lakes with poor lakeshore habitat relative to lakes with good lakeshore habitat. About 20% of lakes have high levels of nutrients (i.e., nitrogen or phosphorus). Poor biological health is 2.5 times more common in lakes with high nutrient levels. What does the report say about the risk posed by microcystin?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. Using cyanbacteria cell counts as an indicator, approximately 27% of lakes nationwide pose a high or moderate risk for potential exposure to algal toxins. This could potentially have wide ranging impacts on human health and the swimmability of many lakes.
The NLA provides an important baseline for understanding the distribution and general levels of microcystin in lakes, but much more remains to be learned about algal toxins. What does the report say about different pollution sources, like urban runoff or agriculture?This report addresses key chemical and physical stressors to lake condition, but it does not examine their sources. Are lakes getting better or worse?We will not be able to determine comprehensive trends in lake water quality until we have conducted subsequent national lake assessments. However, for the NLA, a representative subset of wastewater-impacted lakes that were sampled for the 1972 National Eutrophication Study (NES) were re-sampled in order to look for trends in trophic status since that time. (Trophic status refers to how much living material is in a lake, as measured by different parameters such as nutrient or chlorophyll levels, water clarity, or amount of plant life. Lakes that are excessively enriched are classified as eutrophic or hypereutrophic and may also be characterized by algae blooms and clouded water. Oligotrophic and mesotrophic lakes, on the other hand, are often characterized by lower nutrient and algae levels and higher water clarity.) 800 lakes of the National Eutrophication Study were originally selected because they were likely influenced by nutrients from wastewater treatment facilities. The survey found that, over the past 35 years, trophic status has improved or remained the same in more than 75% of those lakes This study also shows that nearly 75% of the NES lakes showed either improvements or no change in phosphorus levels despite increased population pressures across the United States. This suggests that the nation’s investment in wastewater treatment and other pollution control activities are working for those lakes. What does the NLA say about my lake?There is no lake-specific information in the NLA. NLA results are not applicable to any particular lake but rather describe the target population of lakes as whole. However, we will be making the NLA data available via the Web. It will be possible to search this data to look for information by geographic area. What does the NLA say about my state’s lakes?NLA results are not applicable to any given state’s lakes but rather describe the target population of the nation’s lakes as whole. However, many states opted to increase sampling coverage to at least 50 sites within their boundaries to allow state-level findings to be made. These states include: MI, WI, IN, MN, TX and OK. Individual states will make the results of their state-level lake surveys available to the public. We should also be seeing the results of their surveys in future state water quality assessment reports under Section 305(b) of the Clean Water Act. It is one of EPA’s goals to encourage and support states in conducting these kinds of state-scale monitoring programs. How do the findings compare to what the EPA has reported previously on lake condition in 305(b)/303(d) reports? Why are the findings different?The 2004 National Water Quality Inventory Report (305(b) report) found that 64% of the nation’s assessed lake acres were impaired for one or more of their designated uses (such as fishing or swimming). Leading causes of impairment in lakes were mercury, PCBs, nutrients, metals and organic enrichment/oxygen depletion. Top reported sources of these problems included atmospheric deposition, unknown or unspecified sources, agriculture, natural/wildlife sources and hydrologic modifications. However, the results of the NLA and the 305(b) report should not be compared for the following reasons:
How were the NLA sites selected?To select lakes for sampling, EPA used a statistical sampling approach incorporating state-of-the-art survey design techniques developed by EPA’s research program. These techniques, based on the same statistical principles as election polls, make it possible to sample a relatively small number of sites and make unbiased estimates about conditions across the entire population under study with statistical confidence. The greater the number of sites sampled, the more confidence in the results and the smaller the margin of error. The number of sites included in the survey allows EPA to determine the percentage of lakes nationwide and within predetermined ecoregions that exceed a threshold of concern with 95% confidence. EPA used the National Hydrography Dataset (NHD) to compile a list of target lakes and their locations. (The NHD is a multi-layered series of digital maps that reveal key attributes of the nation’s waters, such as topography, area, flow and location.). A total of 1,208 lakes were chosen to be part of the survey. What are reference sites?References sites are those locations that display the best available (or least-disturbed) chemical, physical and biological habitat condition given the current state of the landscape. These sites were identified by evaluating data from proposed sites according to a set of explicit screening levels that define what is least disturbed by human activities. To reflect the natural variability of the U.S. and historical patterns of land use, these levels varied across the country. For most indicators, findings were compared to conditions at reference sites in order to come up with determinations of good/fair/poor. What are ecoregions?Ecoregions are areas that are similar in climate, vegetation, soil type and geology. Water resources within an ecoregion have similar natural characteristics and similar responses to stressors. Lakes were evaluated compared to reference lakes in their ecoregion. How are good, fair and poor defined in the report?Thresholds for good, fair and poor (or high, moderate, low) were defined in two ways for the NLA report, depending on the specific indicator. The first type of threshold used in the NLA is the fixed threshold. The values for indicators such as dissolved oxygen are based on longstanding, accepted values from the literature. They are well-established and frequently used. For example, all lakes were compared to the following dissolved oxygen thresholds: ≥5 mg/L (high), >3 to <5 mg/L (moderate) and ≤3 mg/L (low). The second type of threshold is based on reference condition. In order to assess the condition of the nation’s lakes, findings for a number of indicators including biological condition, nutrients, habitat condition, etc were compared to conditions in a suite of reference lakes. For these indicators, a lake was classified as either “good”, “fair” or “poor” condition relative to conditions found in reference lakes. Good denotes an indicator value similar to reference condition while poor denotes conditions definitely different from reference. Specifically, the NLA used the following percentages of the reference condition to establish the good, fair, poor thresholds. Lake results above the 25% of the reference values are considered good; those between the 5% and 25% are considered fair; and those below the 5% of the reference values are considered poor. How do streams and lakes compare (WSA to NLA)?Stream and lake condition as described in the Wadeable Streams Assessment (WSA) and the National Lakes Assessment are not entirely comparable because different indicators of biological condition were used. In the NLA, EPA used a taxa loss model based on planktonic communities as the indicator of overall condition. In the WSA, EPA assessed the overall condition of streams using an index of biological integrity based on benthic macroinvertebrates. Nationally, the WSA showed that 42% of our nation’s stream length is in poor biological condition relative to the least disturbed reference condition. This contrasts with NLA findings of only 24% of lakes in poor biological condition. However, it can be noted that both assessments found that nutrients and poor habitat conditions were leading stressors and posed the highest risk to good biological health. |