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Lakes Health

National Lakes Assessment - Probabilistic Survey

National Lakes Assessment Survey

Monitoring Objectives

The objective of the probabilistic survey is to determine statewide lake condition across all lake types and sizes. By repeating the survey over time, changes in statewide lake condition over time will also be determined.

Monitoring Design

The probabilistic surveys will be completed in conjunction with the National Lakes Assessment (NLA), a monitoring effort led by the Environmental Protection Agency (EPA). The NLA is conducted once in a 5-year period. Fifty lakes will be sampled once within a single summer field season, which is a sufficient sample size for a statewide assessment. Lakes > 1 meter deep and > 2.5 acres area are randomly selected from a sample stratified by ecoregion and weighted by lake size. NLA surveys were conducted in 2007 and 2012; the next survey will be in 2017. If additional funding is secured, the WDNR's goal is to sample a total of 100 lakes within a 2-year period on the NLA cycle in order to characterize lake condition in northern and southern Wisconsin.

Water Quality Indicators

At the deepest point in the lake, samples are collected for a wide variety of parameters: depth profiles of temperature, pH, and dissolved oxygen (DO), Secchi depth, water chemistry (NH4, NO3, major anions and cations, alkalinity, dissolved organic carbon, total suspended solids, silica, conductivity), chlorophyll a, nutrients, phytoplankton assemblage, zooplankton assemblage, triazine pesticide screen, and algal toxins. In addition, a sediment core is taken, dated, and analyzed for diatoms and mercury. At ten littoral sites located equidistantly around the lake, benthic macroinvertebrates and shoreline habitat are sampled. Aquatic macrophytes are also surveyed at five of the littoral sites. At a single littoral site, chlorophyll a, algal toxins, and phytoplankton are collected.

Given funding to monitor an additional 150 lakes, WDNR will scale back the NLA protocol to do fewer metrics at more sites, omitting the triazine pesticide screen, benthic macroinvertebrate, zooplankton, and sediment mercury sampling from analysis. Instead of following the NLA macrophyte protocol, WDNR will do full aquatic macrophyte point-intercept surveys on all lakes. Monitoring will be conducted by a centralized crew based at the Science Operations Center in Madison

Data Management

To date, data has been collected on tablets and given directly to EPA. EPA screens the data and then sends back to WDNR after approximately two years. The data are then stored on personal computers of the WDNR research staff. In the future, the NLA data should also be stored in SWIMS. Data from the additional 50 lakes will be entered directly into SWIMS. The SWIMS database will need to be set up for new types of data (e.g., lakeshore habitat inventories).


EPA releases a nationwide report following each NLA survey. WDNR researchers present Wisconsin-specific results in the form of oral presentations and posters at statewide meetings and national conferences. NLA results are also included in the Integrated Report. In the future, NLA results shall also be reported on the WDNR website. These data not used for statewide assessments, but trigger further monitoring and assessment when SWIMS capture USEPA data.

Programmatic Evaluation

Apart from EPA evaluations, WDNR will assess probabilistic monitoring every five years.

Last revised:

Tuesday May 30 2017