Lake Learning column: Monitoring water clarity
Did you know that the easiest, most inexpensive way to monitor water quality in a lake is with a simple white disk? This type of water monitoring is free. The measurement tool is called a Secchi disk. It is a metal disk, 8 inches in diameter that has a cord attached to the middle. This cord has 6 inch increments marked on it. Each foot is marked in black, while each half foot is marked in red. The way to take a Secchi disk reading is to lower the disk on the cord into the water until you can't see it anymore. Then, you slowly pull it back up until you see it again. You average these two depths from the surface of the water to the disk and that is called the Secchi depth.
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 Mediterranean Sea, and he did so using white disks. Since then, we have used the same concept to tell us about a lake's condition.
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 Lake Monitoring Program http://www.pca.state.mn.us/water/clmp.html.
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 Minnesota were monitored with a Secchi disk; 50 lakes in Becker County and 54 lakes in Ottertail County. The average range of Secchi depth for Ottertail County and western Becker County is 5-10.5 feet. To look up individual lake data, you can visit www.rmbel.info and click on the Lakes Monitoring Program link. If your lake is not there, you should consider starting to monitor water transparency this year. This type of water quality monitoring is free. You can receive a free Secchi disk when you sign up for the MPCA's Citizens Lake Monitoring Program: http://www.pca.state.mn.us/water/clmp.html.
Until next week, enjoy the lakes!
(Moriya Rufer is the Lakes Monitoring Program Coordinator for RMB Environmental Laboratories in Detroit Lakes, 218-846-1465, firstname.lastname@example.org.)