Secchi Transparency Trend Lists

One of the primary uses for Secchi transparency data is to determine lake water quality trends over time. The document below provides trend analysis results for all Minnesota lakes using data collected through September, 2010. Only lakes with at least eight years of data were included in the trend analysis.

Lakes in the document below are organized alphabetically by county. A description of the analysis method used to calculate the trends follows:

• 2010 Secchi Transparency Trend Summary (wq-s2-01)

Secchi Trend Analysis Description

To determine Secchi transparency trend results, all available Secchi data were extracted from EQuIS, the Minnesota Pollution Control Agency’s water quality database. The statistical software program "R" was used to perform a Seasonal Kendall test to determine whether the data for each lake exhibit increasing or decreasing trends.

The statistical results include p-value, slope and 90% confidence interval. These results were used to determine a "significance factor" which resulted in the trend description language found in the report above. A total of nine possible trend descriptions were used (the number of lakes in each category is provided in parentheses). Of 3,955 lakes with Secchi disk data, 1,330 had sufficient data for trend analysis :

For increasing trends (324)

• Weak evidence for possible trend (30)
• Evidence for possible trend (45)
• Evidence for trend (96)
• Strong evidence for trend (153)

For decreasing trends (124)

• Weak evidence for possible trend (25)
• Evidence for possible trend (28)
• Evidence for trend (44)
• Strong evidence for trend (27)

Other

• There is no evidence yet of a long term trend in either direction (882)

In addition to the probability categories, the slope and the confidence interval of the trend are expressed in feet per decade. An example of a complete description is provided below:

“The median transparency at this lake from 1988 to 2010 increased by 0.24 feet per decade. Given the variability over these years, this is weak evidence for a possible long-term trend. A plausible range for the long-term trend is between no trend and an increase of 0.94 feet per decade.”

If you have questions or comments about the Secchi transparency trends or the methods used to calculate them, please contact the CLMP coordinator at clmp.pca@state.mn.us or call 1-800-657-3864 (Greater Minnesota) or 651-296-6300.