What WCOLA’s Secchi Disc Readings Tell Us About Our Lakes

For more than five decades, volunteers around the Wabana Chain have helped build a record of lake-water clarity. One of the simplest tools in that work is also one of the most useful: the Secchi disc.

What is a Secchi disc?

A Secchi disc is a circular black-and-white disk lowered into the water until it can no longer be seen. The depth at which it disappears provides a repeatable measure of water transparency, or clarity. Minnesota’s volunteer lake-monitoring programs use Secchi measurements because they are inexpensive, consistent and valuable when collected over many years.

What does water clarity tell us?

Clarity is influenced by several factors. Algae can reduce transparency when nutrients such as phosphorus encourage growth. Suspended sediment, tannins and other natural conditions can also affect how far a Secchi disc can be seen. That means one reading should not be treated as a complete diagnosis of a lake.

The real value comes from looking at measurements over time. Long-term records can help identify patterns, seasonal variation and meaningful changes that deserve closer attention.

WCOLA has a long monitoring history

WCOLA’s historical records indicate that Secchi readings on Wabana began in 1975. Chemical sampling began later, in 1991. Historical trend reports prepared by RMB Environmental Laboratories combined WCOLA information with Minnesota Pollution Control Agency data to examine conditions in several lakes in the chain.

Not every lake has the same amount of historical data, so comparisons need to be made carefully. The older reports are valuable as a record of past monitoring, but they should not be interpreted as a statement of today’s lake condition without current measurements.

Why consistent monitoring matters

A single unusually clear or cloudy reading may reflect weather, wind, algae, runoff or the time of year. Repeated measurements taken using consistent methods are much more informative. When volunteers collect data year after year, they create a record that can help WCOLA and state resource professionals recognize longer-term trends.

What lake residents can do

Water clarity is connected to the larger health of a lake and its watershed. Property owners can help by limiting erosion and runoff, maintaining native shoreline vegetation, keeping leaves and fertilizer out of the water, maintaining septic systems and preventing the spread of aquatic invasive species.

Explore WCOLA’s historical lake records

WCOLA has preserved its earlier lake trend reports and stewardship materials in the Historical & Reference Documents section. For current stewardship recommendations, visit Lake Education & AIS.

Historical dates and monitoring information in this article are drawn from WCOLA’s archived lake-health materials. Current lake conditions should be evaluated using current monitoring data.