The Secchi Disk

The Kings County Volunteer Lake Monitoring Program uses Secchi disks to help determine variations in the water transparency of our lakes. These instruments (originally with diameters exceeding two meters) were first used by Angelo Secchi, an Italian scientist and advisor to the Pope. The Secchi disks used today are only 20 cm wide and have black and white quadrants.They are important tools of modern limnology that are as old as they are controversial.

Secchi readings are taken on the shady side of a boat preferably between 10 a.m. and 2 p.m. The Secchi disk is lowered into the lake water with a calibrated line until it can be no longer seen by the volunteer. This depth is recorded. The disk is then lifted until it reappears and the second depth is recorded. The average of these two depths is called the Secchi depth and is a measure of the lake water transparency.

The Secchi depth will depend partly on the eyesight of the viewer, the reflectivity of the disk, the contrast between the disk and the surrounding water and the disk's diameter. The water transparency itself can be affected by the lake's color, the amount of algae present, and suspended sediments. In general, transparency diminishes as these other factors increase. In many Nova Scotian lakes the Secchi depths are quite shallow because the water is stained brown by decaying plant matter and humic acids. In fact, some say that our lakes are the color of tea.

Algae are small, green aquatic plants whose numbers are related to the quantity of plant nutrients, especially phosphorus. As a result, transparency can be affected by the amount of plant nutrients coming into the lake from sources such as cottage septic systems and lawn and agricultural fertilizer.

Suspended sediments enter the water column from sources such as construction sites, agricultural fields, and re-suspension of the lake bottom.

Transparency is an indicator of the impact of human activity on the lakeshore. If transparency is measured annually or monthly, trends become apparent. Transparency can serve as an early-warning signal that activities on the land are having a negative effect on a lake. The Kings County Volunteer Lake Monitoring Program records Secchi depths at ten Gaspereau watershed lakes on a monthly basis between May and October, so that small changes in lake water quality won't go unnoticed.