Transparency trends

Secchi Transparency

One of the primary uses for transparency data is to determine water quality trends over time. The following results are for Minnesota lakes and stream sites using data collected by volunteer water monitors using Secchi disks and tubes through September 2015.

Trend analysis

To determine Secchi transparency trends, all available Secchi data were extracted from EQuIS, the Minnesota Pollution Control Agency’s water quality database.

  • Lakes: 1,624 had sufficient data for trend analysis (out of 4,520 with Secchi disk data)
  • Streams: 968 had sufficient data for trend analysis (out of 4,991 sites with Secchi tube data)

The Seasonal Kendall statistical test was used to determine whether the data for each lake or stream exhibit increasing or decreasing trends. A minimum of 8 years of data is required to run the test.

Secchi transparency trends: Lakes

Based on 44 years of collected data (1972-2015): File 2015 Lake trend results by county (wq-s2-06)

Map showing transparency trends for monitored lakes and streams in Minnesota

For increasing trends

Lakes

Streams 

Weak evidence for possible trend

37

1

Evidence for possible trend

58 1

Evidence for trend

109 2

Strong evidence for trend

196 0

Total

400 4

 

For decreasing trends

Lakes

Streams

Weak evidence for possible trend

32

0

Evidence for possible trend

38 1

Evidence for trend 

46 7

Strong evidence for trend

39

0

Total

155 8

 

Other

Lakes

Streams

There is no evidence yet of a long term trend in either direction

1,069 956