One of the primary uses for water clarity data (also known as transparency data) is to determine water quality trends over time. The following results are for Minnesota lakes using data collected by volunteer water monitors using Secchi disks through 2017.
Trend analysis
To determine Secchi clarity trends, all available Secchi data were extracted from EQuIS, the Minnesota Pollution Control Agency’s water quality database.
- Lakes: 2,165 had sufficient data for trend analysis (out of 4,522 with Secchi disk data)
The trend analysis was performed with a Seasonal Mann Kendall test. This statistical test detects changes in water clarity over time by comparing months across years (example – Mays are compared to Mays, Junes to Junes, etc…).
Secchi transparency trends: Lakes
Based on 45 years of data collected (1972-2017): 
2017 Lake trend results by county (wq-s2-06)
We are currently making improvements to the way that we determine trends in water clarity on streams. Stream Secchi clarity trend results will be available in spring 2019. Thanks for your patience!