Note that the data published in the 2002 State of the Nation’s Ecosystems Report as well as the 2003 and 2005 Web-Only Updates have been superseded by the 2008 Report and thus should be used with caution. For the most recent data, purchase the 2008 Report from Island Press.

Two approaches for measuring water clarity are measurements of Secchi depth and satellite-based estimates. Since 1994, the U.S. Environmental Protection Agency (EPA) has supported an impressive program that aggregates Secchi disk measurements made by volunteers during July across parts of the United States and Canada (The Great North American Secchi Dip-In; see http://dipin.kent.edu). In 2000, lakes in 43 states were sampled, but the coverage varied considerably from state to state—in Minnesota, Michigan, and Maine, large numbers of lakes were tested, while in West Virginia and Wyoming, no lakes were sampled, and in states such as Pennsylvania and the Dakotas, relatively few lakes were sampled. In order to make the data nationally representative, this program should be expanded to include more lakes in more states. Because clarity is greatly affected by algal blooms, measurements of clarity should be carried out at the height of the growing season (mid-July to mid-September) in each ecoregion, which may or may not fit with the July observations of the Dip-In program. In addition, scientists are developing ways to measure water clarity from satellite data, which could greatly improve our understanding of how water clarity varies across the country and over time.

The Indicator

This discussion assumes that water clarity will be measured in lakes and reservoirs by the Secchi-disk method, although a satellite-based method may become the preferred approach. Secchi depth measurements of water clarity (or transparency) will be reported in three ranges: low (<3 ft), medium (3–10 ft), and high (>10 ft). The Secchi disk is a white plate with a diameter of 8 inches with black lines radiating from the center. The disk is lowered into the water until it can no longer be seen. The depth at which this occurs is called the Secchi disk transparency or Secchi depth (SD). It is a simple but effective way to measure water clarity.

Water clarity values for lakes and reservoirs will be reported in two ways: by lake area falling into the low, medium, and high categories, and as averages for freshwater ecoregions. (Ecoregions are areas that are similar in climate, geography, and ecological conditions and are defined in Ricketts et al. 1997). Measurements should be made annually during an “index” period near the height of the algal growing season, which generally corresponds with the height of the recreational use season. In lakes of the Upper Midwest, for example, the index period is mid-July to mid- September, when Secchi-disk transparency is relatively constant and at annual minimum values. The appropriate length of the index period in other parts of the country needs to be determined, but the mid-July to mid-September period should be suitable for all lakes in temperate climate zones. One measurement during this period should be adequate to define ecoregional growing-season minimum values, although one measurement is not sufficient to define the minimum transparency for an individual lake.

Humic-colored lakes and reservoirs are found in many areas of the country (e.g., in northern forests of Minnesota, Wisconsin, Michigan, New York, and New England and in wetland forests throughout the Southeast, from Virginia to Florida). Clay turbidity is a dominant factor in water clarity in lakes and reservoirs of the central plains and the Piedmont region of the Southeast. Humic color and clay turbidity tend not to have a strong seasonal pattern in lakes, so a mid- to late-summer sampling period designed to capture the peak influence of algal growth on transparency should also be appropriate for these lakes and reservoirs.

Ponds have been excluded from this indicator, mostly because the hydraulic properties of ponds are quite different from those of lakes. Because of their shallow nature (typically less than 2 meters, or 6.5 feet), ponds can readily be completely mixed by strong winds. Such mixing can suspend sediments in the water column, which would decrease clarity. Lakes (and reservoirs) typically have a warm layer of water at the surface (epilimnion) that does not easily mix with deeper, colder waters (hypolimnion). Full wind-driven mixing of lakes typically occurs only during the fall and spring when temperatures are fairly uniform across all depths.

The Data Gap

The Great North American Secchi Dip-In program has been evolving since 1994. Supported by the EPA in cooperation with the North American Lake Management Society, the Dip-In is the largest-scale program for collecting SD data in the United States. The program relies upon volunteers who measure the Secchi depth of lakes in their area over a 2-week period in the beginning of July. Data are collected and maintained at http://dipin.kent.edu. While the data do not cover the whole country, they are substantial. In 2000, lakes in 43 states were sampled, but coverage varies considerably from state to state. Several states (Minnesota and Wisconsin in particular) have extensive volunteer monitoring programs coordinated by state agencies, and some state agencies have extensive collections of historical data.

Using satellite imagery is promising as a way of obtaining essentially complete coverage of lake water clarity. This approach is being tested by a NASA-funded consortium involving the Universities of Minnesota, Wisconsin, and Michigan. The consortium is applying a recently developed protocol using Landsat satellite images from the early 1990s and from 1999 to all lakes over 50 acres in the three-state region (see http://resac.gis.umn.edu/water/regional_water_clarity/regional_water_clarity.htm and Kloiber et al. 2000).

References

Kloiber, S.M., T. Anderle, P.L. Brezonik, L. Olmanson, M.E. Bauer, and D.A. Brown. 2000. Trophic state assessment of lakes in the Twin Cities (Minnesota, USA) region by satellite imagery. Arch. Hydrobiol. Ergebn. Limnol. 85:1–15

Ricketts, T.H., et al. 1997. A conservation assessment of the terrestrial ecoregions of North America. Volume 1: The United States and Canada. Washington, DC: Island Press.