Sampling - Strategy and Methods

Lake water-quality assessment (LWQA) monitoring is coordinated with current Michigan Department of Environmental Quality (MDEQ) water-quality activities to help support, (1) statewide water quality assessments, (2) the National Pollutant Discharge Elimination System (NPDES) permitting process, and (3) resource management decisions (MDEQ, 1997). The LWQA monitoring program incorporated the watershed management units and five-year rotational cycle currently being used by MDEQ’s Ambient Surface Water Chemistry Monitoring Program (MDEQ, 2000) to assess Michigan’s rivers, Great Lakes connecting channels, and bays.

Lakes targeted for monitoring under the LWQA monitoring program were restricted to those of 25 acres or larger in size and have public boat access. Lakes that are sampled in a given year are randomly selected from 7 to 10 major watersheds. Each year, lakes will be selected from another set of watershed units within the state. Watersheds will be visited on a five year rotation until all lakes with public boat access and 25 acres or larger in size are sampled. After five years all 45 major watersheds in Michigan will have some portion of its lakes sampled.

All lakes selected for monitoring within a given year will be sampled once during spring turnover (usually April) and again in late summer (August and September) when the lake may be stratified thermally. Samples collected during spring turnover represent baseline water-quality characteristics of the lake because the water is well mixed. Samples collected in late summer, when the water is the warmest and plant growth is at its peak, will give insight into the biological productivity of the lake. As the lake water warms throughout the summer, a lake can stratify and water quality can vary appreciably with depth. Thermal stratification occurs during the summer in many Michigan lakes where a layer of less dense, warmer water is isolated from mixing with a deeper, more dense, colder layer. This condition is affected by the lake morphology, any inflow or outflow of water, and prevail¬ing winds. Prior to each sampling, profiles of dissolved-oxygen concentration, water temperature, specific conductance, and pH are obtained at the deepest part of the lake basin to measure the variation with depth to determine if the lake is stratified.

The sample site in the major basins of each lake was located as close as possible to the known historical sample location. The exact location of each sample site was verified with a hand-held Global Positioning System (GPS). Sample sites are located at the deepest portion of each basin. Once the sample site is located by GPS, the site is then verified by comparing the depth measured with a electronic depth finder to the known historical depth from previous samplings.

Standard USGS field methods were used to collect preserve samples from the water-column. During spring sampling prior to thermal stratification three discrete samples were collected. One sample was collected 3.0 ft below the lake surface, another 3.0 ft above the lake bottom and the third at mid-depth. Summer sampling used the same methodology as used in the spring except in lakes that stratify; the mid-depth sample was collected from the center of metalimnion. Only the mid-depth sample was collected from those lakes whose shallow depths prohibited the collection of three samples.

All water samples except for the chlorophyll-a were collected with a Van Dorn style sampler. Water samples for chlorophyll-a analysis were a depth-integrated composite sample collected by lowering a bottle sampler through the photic zone (twice the depth of the secchi disc measurement).

For the purpose of data storage in the USGS National Water Information System (NWIS) and access through the Internet, sampled basins are referenced with a Site Identification number. The Site identification number is the latitude-longitude (Lat-long) of the approximate location that the sample was collected at. A sequence number indicating the depth at which a sample was collected was then incorporated in the Site ID as the last two digits. The lake station identification numbering system is summarized below.

• Lat-long-01, Vertical profile data.
• Lat-long-02, Sample collected 3ft. above lake bottom.
• Lat-long-03, Depth integrated sample through the photic zone.
• Lat-long-05, Sample collected 3ft. below lake surface.
• Lat-long-06, Sampled collected at mid-depth or metalimnion

All LWQA monitoring data will be archived in the U.S. Environmental Protection Agency’s data management system STORET as well as the USGS National Water Information System (NWIS). These data are also available to the public through the Internet at URL http://mi.waterdata.usgs.gov/nwis/qw.