WQ – Lakes – Clarity – Trends

This data layer indicates Trends in Lake Clarity based on Secchi disk measurements. It was developed by the Natural Resources Research Institute using data from the Minnesota Pollution Control Agency. A Secchi disk measures water transparency or clarity and indicates the depth of light penetration into a lake. Secchi depth also provides an indirect measure of the amount of suspended material, algae, or sediment in the water. The trend analysis, which shows both the presence or absence and direction of a significant trend, was performed using a Seasonal Mann Kendall statistical test. Lake water clarity must change by more than half a foot (0.15 meters) per decade to be considered a detectable change, or trend. The analysis was based on 47 years of data (1972-2019), although not all lakes had data covering this entire length. Long-term transparency monitoring provides a cost-effective, valuable basis for detecting trends in water quality. Generally, the sooner water-quality problems are detected, the easier and less expensive it is to restore the lake to its previous state.