Oak Leaves
Hartley Nature Center
Natural History. Stewardship. Sustainability.
Home > Hartley Ecology > Pond and Stream Life > Water Transparency

Transparency of Hartley's Ponds and Streams

 Transparency is a measure of how far light can penetrate down into lakes, ponds and rivers and how much light is in shallow ponds and streams. Plants and algae are limited in the amount of their growth by the amount of light available for their photosynthesis. They produce sugars and other organic material for their growth by the energy they get from light.

 In deeper waters, transparency is measured by how far down one can see a secchi disk in the water from a boat. Below are the results from several years of measuring transparency in Hartley Pond by Citizen Scientist volunteers.

The color of unpolluted water is due to natural organic materials in the water, from the partial decay of plants and algae. The more of that is present, the browner the water and the less is the transparency of the water.  You’ve probably seen the root beer color of Northshore streams that flow out of boggy waters, high in dissolved organic matter.The brown color in the above chart is like the observable color seen on the secchi disk as it sinks deeper into the water.

Brown is also the color of Tischer Creek water that flows out of the pond over the dam. Some of the differences in the secchi depths above are due to differences in the amounts of dissolved brown organic materials in the water. Two of the greatest causes of those differences above is how much fresh water from snow melt and rainfall enters the pond and stream. This additional colorless water dilutes the dissolved brown organic material in
the pond and stream.

How much suspended material in the water, such as clay, silt and organic particles also is a major reason for reduction of transparency, (increasing turbidity).  Generally, the more rainfall, the more erosion there is of the land in the watershed of the pond and stream, and the more turbid the water of the pond and stream there is.  The interaction of these and other factors is too complex to be shown on the chart above and so some of the results above are not fully explainable by a few factors alone.  Temperature-evaporation, the speed of rainfall, how much newly exposed and disturbed soil, and the amount of moisture already in the soil before the rainfall are some of the other factors involved.

 

The water in shallow ponds and streams usually isn’t deep enough to allow a secchi disk disappear from sight. A turbidity tube is used instead. Water is poured into a 100 centimeter long tube with a white disk at the bottom. Then one measures the length of the column of water in the tube at which the disk can’t be seen, looking down into the water in the tube from above. You can figure out how many feet the 100+ cm readings represent by referring to the secchi disk readings of Hartley Pond at about the same time. One foot equals 30.5 cms.

The Citizen Science Tischer Creek turbidity-tube readings and Hartley Pond secchi disk readings are sent
to the Minnesota Pollution Control Agency every year. The creek is found to be typical of most northern Minnesota streams as having high average transparency (low turbidity). This is in contrastto the streams further south where there is a larger input from glacial deposits and accelerated soil erosion from farmland activity.
The brown color of Tischer Creek water reduces the degree of transparency but most loss of transparency
in Tischer Creek is due to suspended silt, clay and organic particles.

 

This project was funded in part by the Coastal Zone Management Act, by NOAA's Office of Ocean and Coastal Resource Management, in conjunction with Minnesota's Lake Superior Coastal Program.

 

Hartley Nature Center

3001 Woodland Ave
Duluth, MN 55803
218-724-6735

info@hartleynature.org

home    site map

Nature Center Hours
Monday-Friday: 9-5
Saturday: 10-5

Location map

Hartley on Facebook  Visit us on Facebook


2016 Presenting Sponsors

Miller Hill Subaru, Duluth, MN Subaru