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): 
2015 Lake trend results by county (wq-s2-06)
|
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 |