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Indicator of the Month: January 2002 Indicator of the Month - How low can you go? Water transparency in Minnesota's lakes

Think about water transparency in Minnesota's lakes during the cold temperatures and chilling winds of January? Are we kidding - our lakes are now covered by ice and snow (or are supposed to be)! What better way to chase away cabin fever than to think about visiting a Minnesota lake for all types of recreation. January's indicator looks at one aspect that gives lakes their special appeal - the transparency (or clarity) of the water.

Water transparency is an easy measurement that quickly tells scientists a lot about a lake's water quality. First it indicates the amount of light penetration into a lake. Second, water transparency provides an indirect measure of the amount of suspended material in the water, which in many cases is an indication of the amount of algae in the water. Long-term monitoring helps scientists detect signs of degradation in a lake. Generally, the sooner water-quality problems are detected, the easier and less expensive it is to restore the lake to its previous state.

Water transparency is measured with a Secchi disk. The depth of light penetration in water (transparency) is tracked by observing how far down into the lake (in feet or meters) you can see a Secchi disk. The measured water depth is called Secchi transparency. Figure 1 shows a Secchi disk in use in a clear lake with a deeper Secchi transparency and a cloudy lake (due to high turbidity) with a more shallow Secchi transparency. The Secchi disk is probably the most inexpensive and easy-to-use tool in water quality monitoring.

Figure 1 - A Secchi Disk in Use

Most of the Secchi transparency data is collected by volunteer lake monitors in the MPCA's Citizen Lake-Monitoring Program (CLMP). Any person or group willing to devote a small amount of time and energy to conduct simple water-quality checks regularly can become a volunteer monitor. This includes anyone who lives on or near a lake or is a frequent user of a particular lake. Through this process, the volunteers can learn about the water quality of lakes in their area and gain a greater awareness of the causes and effects of lake degradation. Currently, the MPCA has Secchi transparency data on over 800 lakes. The CLMP program continues to be a cost-effective mechanism for obtaining good, basic water quality data on many of our State's lakes. To learn more, visit the MPCA's Citizen Lake Monitoring Program.

Plotting the summer-mean Secchi transparency readings of a lake over time is one means for identifying patterns or trends. Summer-mean transparency of a lake often varies from year to year in response to changes in the amount of algae. Variation in the amount of algae may be caused by changes in amounts of nutrients reaching a lake, fisheries composition, and/or climatic changes. It is important to consider these factors when trying to determine if significant long-term changes have occurred or if changes are merely natural variation in a living system

Based on an analysis of several lakes with long-term Secchi transparency data, yearly mean Secchi transparencies tend to vary within one to two feet (or about 20 percent) of the long-term mean (Heiskary and Lindbloom, 1993). Consistent increases or decreases in Secchi transparency summer-means, or consistent variation by more than 20 percent of the long-term mean, may be indicative of a trend.

Case Study: Turtle Lake (Lake ID No.: 31-0725)

To illustrate the use of Secchi transparency as an indicator, we will look at a case study of Turtle Lake. Turtle Lake is located in Itasca County at Marcell, Minnesota, and has a good data set available for assessing trends in Secchi transparency. Volunteer lake monitors collected the majority of the data; county and MPCA staff supplied the remaining data.

Figure 2 displays Secchi transparency data for Turtle Lake from 1986 to 2000. Based on 15 years of record, the long-term mean Secchi transparency depth is 4.6 m (15 feet). As Figure 2 shows, a slight improvement in Secchi transparency over time has occurred. Summer-mean Secchi transparency measured 11.2 feet (3.4 m) in 1986 and continued to improve to a maximum of 17.7 feet (5.4 m) in 1998. These changes in Secchi transparency may be due to changes in the amount or type of algae growing in the lake, runoff from the lake's watershed, changes in land uses near the lake, and the amount of precipitation in the lake's watershed.

Figure 2. Turtle Lake Summer-Mean Secchi Transparency

Typical year-to-year variability for Minnesota lakes is on the order of 1 to 2 feet (or 20 percent of the long-term mean). For Turtle Lake, the Secchi transparency depth varies within a range of 0 - 3.8 feet (or 1 - 26 percent of the long-term mean). Additional supporting chemistry data for Turtle Lake consists of only one year of sampling conducted in 1992. This chemistry data is too limited to perform trend analysis.

To search for Secchi transparency data for a lake near you, visit the MPCA's CLMP Search Web page.

More than just clear water

Secchi transparency measurements also provide a basis for assessing water quality, estimating a lake's trophic status, and documenting trends in water quality over time. Secchi transparency can vary greatly among Minnesota's lakes and throughout different regions of the state, due to differences in geology, land use, and natural vegetation. Figure 3 provides a map of Minnesota's ecoregions, illustrating these variations.

In most Minnesota lakes, Secchi transparency provides an indirect measure of the amount of algae in the water. Suspended sediments or color (such as root-beer or tea colored water and deep turquoise colored water) may also limit transparency. Secchi transparency data can be used to convey information on the quality of lakes and allow for estimation of the amount of algae (chlorophyll-a) and nutrient (phosphorus) status of a lake.

Carlson's Trophic State Index (TSI) is a common means for characterizing a lake's trophic state and associating Secchi transparency, chlorophyll-a, and phosphorus measurements. Figure 4 shows the associations that scientists make with the three mentioned parameters. Although total phosphorus and chlorophyll-a concentrations are not measured in the CLMP, the summer-mean Secchi transparency generally provides a good indication of trophic status for Minnesota's lakes and can be used to estimate likely ranges of total phosphorus and chlorophyll-a for the lake.

Figure 3. Typical Summer-Mean Secchi Transparency for Selected Ecoregions in MN
(Based on Interquartile Ranges for Reference Lakes in MN)

Figure 4. Carlson's Trophic State Index Scale

Detecting trends in a lake's trophic status requires taking numerous measurements over several years. Lakes typically require a minimum of 8 - 10 years of data to detect trends in trophic status. As a result, Secchi transparency is the preferred parameter for developing lake water quality trends because the method is:

• low cost
• easily understood and incorporated into volunteer lake monitoring programs, and
• allows for the collection of a large number of samples in a given sampling period on many different lakes.

Glossary

Secchi Disk: A Secchi disk is a circular metal plate attached to a calibrated rope. It is probably the most inexpensive and easy to use tool in water quality monitoring. There have been many revisions to the first disks used in terms of size and color. The two most common color variations in use today are the all-white disk and black and white quadrant version disk (Figure 5). In Minnesota, we use an all-white, 8-inch diameter metal disk with notched sides for rope storage when the disk is not in use (Figure 5).

Figure 5. Secchi Disk Styles and Color Variations

Citizen Lake Monitoring Program (CLMP): The CLMP began in 1973 at the University of Minnesota, and was developed by Dr. Joe Shapiro. During that first year, volunteers monitored 74 lakes. Administration of the CLMP was transferred to the Minnesota Pollution Control Agency (MPCA) in 1978. Currently, we have Secchi transparency data on over 800 lakes. Any person or group willing to devote a small amount of time and energy to conduct simple water-quality checks on a regular basis can become a volunteer monitor. This includes anyone who lives on or near a lake or is a frequent user of a particular lake. The only restriction is that volunteers need to have access to a boat or canoe in order to take the transparency readings.

Trophic Status: The level of growth or productivity of a lake as measured by phosphorus content, algae abundance, and depth of light penetration.

Chlorophyll-a: A pigment produced by algae (and other plants). Chlorophyll-a is measured in a water sample and is used as an estimate of the amount (biomass) of algae in water.

Phosphorus: An essential plant nutrient. Excess quantities promote excessive growth of algae and plants in lakes and streams. Total phosphorus refers to the most common form measured in water and includes both dissolved and particulate phosphorus.

Turbidity: Particles in solution (e.g. soil or algae) which scatter light and reduce transparency.

Ecoregion: EPA ecoregions have been defined for Minnesota based on areas of relative homogeneity for land use, soils, landform, and potential natural vegetation.

Citations

Heiskary, S.A. and Lindbloom, J.L. 1993. Lake Water Quality Trends in Minnesota. Minnesota Pollution Control Agency. Saint Paul, MN.