Indicator 54: Lake water quality


Rationale: Water quality affects the desirability of a lake for fishing, swimming and other recreation. Extreme cases of poor water quality can produce toxic algae blooms.
About this indicator: Volunteers in the Minnesota Pollution Control Agency's Citizen Lake Monitoring Program (CLMP) measure the transparency (clarity) of lakes statewide using a Secchi Disk. Transparency is one simple measure of water quality. The results of a 2009 trend analysis performed on lakes with sufficient data for analysis show substantially more lakes with an improving water clarity trend, 514, than with a decreasing water clarity trend, 210. The remaining 538 lakes showed no clear trend. A Seasonal Kendall statistical test was used to determine trends for each lake. Only lakes with 8 or more years of data are included in the trend analysis.

Changes in Minnesota lake water quality measured by Citizen Lake Monitoring Program | ||
2008 | 2009 | |
Total improving | 455 | 514 |
Possibly improving | 67 | 78 |
Likely improving | 47 | 49 |
Very likely improving | 113 | 122 |
Almost certainly improving | 228 | 265 |
Total declining | 231 | 210 |
Possibly declining | 50 | 40 |
Likely declining | 38 | 29 |
Very likely declining | 53 | 65 |
Almost certainly declining | 90 | 76 |
No clear trend | 515 | 538 |
For comparison: Typical Secchi disk measures vary by ecoregion. They are highest in the Northern Lakes and Forests ecoregion in northeastern Minnesota. The Northern Glaciated Plains and Western Cornbelt Plains ecoregions in southern Minnesota have the lowest Secchi measures.
Technical notes: In 2009, 1263 Minnesota CLMP volunteers monitored the quality of 1237 lakes in the state. Participants take weekly transparency measurements on their lakes using a Secchi disk. A Secchi disk is a circular disk with a distinct pattern. The disc is lowered into the water, and the depth at which the pattern is no longer visible is used as a measure of transparency. At least eight to ten readings per season are needed to give reliable results.
Secchi transparency (clarity) is a measure of the depth of light penetration in the water column. Factors affecting water clarity in lakes include algae, dissolved organic compounds, and suspended sediments.
Sources:http://www.pca.state.mn.us/clmp
- Minnesota Pollution Control Agency, http://www.pca.state.mn.us/water/clmpfactsheets.html
Local data: