A Secchi disk is named after Fr. Pietro Angelo Secchi, an astrophysicist and scientific advisor to the Pope in the mid-1800s. He was requested to measure transparency in the
I have been asked at some lake monitoring training sessions why in this day and age we don't use something fancier like a laser for this measurement. The reason we still use a Secchi disk is because of its ease, low cost and accessibility. Also, since we have so many years of data from Secchi disk readings, we can continue to compare measurements using the same technique. The Pollution Control Agency gives Secchi disks out for free when you sign up for their Citizens
A Secchi disk measures water transparency. Transparency is how easily light can pass through a substance. In lakes this means how deep sunlight penetrates through the water. Plants and algae need sunlight to grow, so they are only able to grow in areas of lakes where the sun penetrates. The top section of the lake that sunlight penetrates is called the euphotic zone.
Water
transparency depends on the amount of particles in the water. These particles
can be algae or sediment from erosion, the more particles - the less water
transparency. In other words, when the water is murky or cloudy and contains a
lot of particles, the light cannot penetrate as deeply into the water column.
Taking weekly Secchi readings will tell you a lot about the dynamics going on in a lake. Some lakes vary greatly over the ice-out season, while other lakes stay relatively the same. Most lakes have high transparency in the early spring, decreased transparency in late spring, a rebound in transparency in mid-summer, and another decrease in late summer. These dynamics are mostly based on algae growth cycles.
Most lakes will experience increased boat activity on week ends and holidays. Taking Secchi readings on Mondays and the day following a holiday, and comparing these readings with other readings at other times may reveal the affect of boating activity on transparency depths. Boats can churn up the bottom of the lake, re-suspending sediment that contains nutrients. These nutrients feed algae and plants and can cause an algae bloom.
Significant storm events within the watershed with the resultant stormwater runoff can also cause lower Secchi disk readings. Comparing Secchi disc readings immediately after a storm with readings between storms may suggest that runoff is decreasing water clarity. Runoff carries phosphorus and eroded particles into the water which can feed an algae bloom and cause the transparency to decrease.
Good water transparency in a lake is beneficial to the fish and other animals living in the lake. Plants and algae produce oxygen as a by product of photosynthesis, and fish, and other aquatic organisms need oxygen to survive. When the euphotic zone is deep, oxygen is being produced in a large area of the vertical water column. When the euphotic zone is shallow, there is no new oxygen being produced deeper in the water column. This has implications for animals living there and other dynamics occurring at the bottom of the lake.
In 2006, 1,095 lakes in
Until next week, enjoy the lakes!