WHAT DOES IT TELL US?

Water transparency can tell us a lot about a lake. When water is not stained by DOC, transparency can indicate how much algae is present in the water and thus how productive the lake is. More algae mean lower transparency and higher productivity. Transparency can also influence how deep in the lake rooted aquatic plants are able to grow – the clearer the water, the deeper these plants may be found.

Transparency of lake water is most often determined by using a Secchi disk. The disk is lowered down vertically into the water, and the depth at which the disk can no longer be seen from the surface is measured. (learn more about how Secchi disks work on the How is it Measured? page)

Measuring transparency with a Secchi disk is one of the most useful ways of showing whether or not a lake is changing through time. Changes in transparency are frequently associated with changes in the amount of algal growth, or the amount of dissolved or particulate materials in a lake. These changes could be the result of human disturbance or other impacts to the land surrounding the lake. These changes are of particular concern for lakes that are in danger of becoming too productive in their algal communities. Many volunteer lake monitors conduct Secchi depth tests to keep track of the algal growth in their lake.

An example of changes in transparency due to human impact might be if a camp road introduces sediment and nutrients to the lake causing an increase in algal growth. The way that this can happen, is when it rains, the impact of raindrops falling on bare soil can dislodge soil particles and cause erosion by carrying this soil away, downstream. One heavy rain storm can produce large amounts of erosion and wash soil, sediments and nutrients into a stream or lake. Road and culvert construction can also result in harmful erosion. Sediment washed into a lake can cloud the water, and settle to the lake bottom. This layer can smother organisms such as algae growing on the rocks and soil, or damage fish and amphibian eggs.

Transparency values in Maine vary from aproximately a half meter (1.5 feet) to 20 meters (67 ft) with the average being 4 m (16.2 ft). Unless a lake is highly colored, a transparency of 2m (6.5 ft) or less indicates that there may be a water quality problem stemming from elevated productivity, for example from an algal bloom. In Maine, the mean (average) Secchi disk readings are related to algal productivity using the following guidelines:

• 4m (13 ft) or less = Productive (“eutrophic”)
• 4-7m (13-23 ft) = Moderately Productive (“mesotrophic”)
• 7m (23 ft) or greater = Unproductive (“oligotrophic”)

The graph below summarizes transparency in Maine lakes.

Distribution of Secchi Disk Transparency in Maine Lakes (2005)
Figure courtesy of Maine Volunteer Lake Monitoring Program..