The Importance of Lakes
About 71% of the earth’s surface is covered in water, most of which (97%!) is in the oceans. That means that only 3% of earth’s water is fresh water, and most of that in in the glaciers of Greenland and Antarctica. In fact, only about 0.6% of the earth’s water is fresh water that is readily usable.
Lakes serve as traps for materials generated within their watersheds. The trapped material generally impairs water quality and may severely impact beneficial uses and significantly shorten the life of the lake. Suspended and deposited sediments can affect certain lake uses. Excessive aquatic plant growth and/or algal blooms often result from the addition of nutrients such as nitrogen and phosphorus. An overabundance of plant life may tend to limit recreational and public water supply usage. Lakes may also collect heavy metal and organic contamination from urban, industrial, and agricultural sources. Dissolved oxygen deficiencies may limit biological habitat or result in taste and odor problems for public water supplies. Thus, knowing about the quality of water that we use is of great importance.
Lake Decatur
There are 3,256 lakes with a surface area of six acres or more in Illinois, and more than 87,000 ponds. In addition to being valuable recreational and ecological resources, these waterbodies serve as potable, industrial and agricultural water supplies; as cooling water sources; and as flood control structures. Lake Decatur, covering 2800 acres, serves all of these functions.
Lake Decatur was formed in 1920-22 at the urging of the A.E. Staley Company to provide its growing demand for water for corn and soybean processing. At the time, it was the largest man-made lake in Illinois. It is 11 miles long, has an average width of 0.4 miles, and an average depth of only 8 feet. Its capacity is 6.5 billion gallons. Water that drains into Lake Decatur comes from a watershed that spans 925 square miles, or 592,000 acres, over seven counties of prime Illinois farmland.
The site of this EarthCache is on the Fairies Park Basin. From the time of its creation until 1983, it is estimated that the lake lost 35% of its capacity due to sediment filling from soil erosion of the watershed’s farm fields. This basin was most affected, since it is the uppermost part of the reservoir. Thus, in 1993-94, the Fairies Park Basin was dredged. Ongoing projects will restore about 16% of the lake’s capacity. In addition, directly across the lake from this EarthCache site is the Bois du Sangamon Nature Preserve. The state of this area is therefore critical to the overall health of Lake Decatur.
The Trophic State Index
Limnology is the study of inland waters, and includes the study of lakes, ponds, rivers, streams, and wetlands. Limnologists classify lakes (or other bodies of water) according to the trophic state index, or TSI. The most common measure, and that used by the EPA, is the Carlson index. The trophic state is defined as the total weight of biomass in a given water body at the time of measurement. The quantities of nitrogen, phosphorus, and other biologically useful nutrients are the primary determinants of a body of water's TSI. Nutrients such as nitrogen and phosphorus tend to be limiting resources in standing water bodies, so increased concentrations tend to result in increased plant growth, followed by an increase in their trophic levels. As a result, a body of water's trophic index may sometimes be used to make a rough estimate of its biological condition.
Three independent variables can be used to calculate the Trophic State Index: chlorophyll pigments, total phosphorus and Secchi depth. Of these three, chlorophyll will probably yield the most accurate measures, because it is the most accurate predictor of biomass. Phosphorus may be a more accurate estimation of a water body's summer trophic status than chlorophyll if the measurements are made during the winter. Finally, the Secchi depth is probably the least accurate measure, but also the most affordable and expedient one. Consequently, citizen monitoring programs and other volunteer or large-scale surveys will often use the Secchi depth.
Lake Classification
A lake is usually classified as being in one of three possible classes: oligotrophic, mesotrophic or eutrophic. Lakes with extreme trophic indices may also be considered hyperoligotrophic or hypereutrophic.
An oligotrophic lake is a lake with low nutrient content. These lakes have low algal production, and consequently, often have very clear waters, with high drinking-water quality. The bottom waters of such lakes typically have ample oxygen; thus, such lakes often support many fish species, like lake trout, which require cold, well-oxygenated waters. Oligotrophic lakes are most common in cold regions. An oligotrophic lake will have a TSI up to 40, and its Secchi depth will be 4 to more than 8 meters (at least 13 feet).
Mesotrophic lakes are lakes with an intermediate level of productivity. These lakes are commonly clear water lakes and ponds with beds of submerged aquatic plants and medium levels of nutrients. A mesotrophic lake will have a TSI of 40-50, and a Secchi depth of 2-4 meters (6.5 to 13 feet).
Eutrophic lakes or ponds have high biological productivity. Due to excessive nutrients, especially nitrogen and phosphorus, these water bodies are able to support an abundance of aquatic plants. When aquatic plants dominate the water tends to be clear; when algae dominates, the water tends to be darker. The algae engages in photosynthesis which supplies oxygen to the fish and biota which inhabit these waters. Occasionally an excessive algae bloom will occur and can ultimately result in fish kills due to respiration by algae and bottom living bacteria. The process of eutrophication can occur naturally and by human impact on the environment. Eutrophic lakes have a TSI of 50 to 70, and a Secchi depth of 0.5 to 2 meters (1.5 to 6.5 feet).
Hypereutrophic lakes are very nutrient-rich lakes characterized by frequent and severe nuisance algal blooms and low transparency. Hypereutrophic lakes are the most biologically productive lakes, and support large amounts of plants, fish and other animals. The excessive algal blooms can also significantly reduce oxygen levels and prevent life from functioning at lower depths creating dead zones beneath the surface. Hypereutrophic lakes have a TSI of 70 to 100+, and a Secchi depth of less than 0.5 meters (18 inches or less).
Secchi Depth
An 8” disc with a standard pattern is used to measure the Secchi depth. (The pattern is black and white, and looks like the BMW logo!) The disc is mounted on a pole or line, and lowered slowly down in the water. The depth at which the pattern on the disk is no longer visible is taken as a measure of the transparency of the water and is known as the Secchi depth.
Secchi disk readings do not provide an exact measure of transparency, as there can be errors due to the sun's glare on the water, or variations between the vision of the observers. However a Secchi disk is an inexpensive and straightforward method of measuring water clarity. Because of the potential for variation between practitioners, methods should be standardized as much as possible. Typically, a Secchi disk measurement should be taken off the shady side of a boat or dock between 9 a.m. and 3 p.m. One can approach the measurement by lowering the disk beyond a point of disappearance, then raising it and lowering it slightly to set the Secchi depth. Another method is to record the depth at which the disk disappears, lower another few feet, then record the depth at which the disk reappears as it is slowly brought up. The Secchi depth is taken as the average of the two values.
Secchi disk measurements have been an integral part of water quality monitoring in several states, most notably Minnesota, Indiana, and Illinois. The Illinois EPA established the Volunteer Lake Monitoring Program (VLMP) in 1981, and utilizes the time and talents of citizen volunteers to serve as a cost-effective method for gathering fundamental information on inland lakes. Further information on this program can be found at http://www.epa.state.il.us/water/conservation/vlmp/index.html.
Logging this EarthCache
The above coordinates will take you to a public boat launch on Lake Decatur, at which are a couple of short piers from which you can work. The area may be quite busy at times, so plan your activities accordingly so as to not interfere with the primary purpose of this site! To log this cache, you’ll need to take your own Secchi depth measurement. Don’t worry about the specific time of day, or about having a standard disc. Improvise! Use a weighted disc on a string, or a lure at the end of a fishing line. Measure the depth at which the object is no longer visible. The point is to get an idea of the classification of Lake Decatur, and what implications this may have for its current and future uses.
To log this cache, send me an e-mail with the answers to the following questions. If I do not get an e-mail with your answers, using your best judgment, I will reluctantly have to delete your log.
- What is your estimate of the Secchi Depth at this point of the lake?
- Based on that number, would you classify Lake Decatur as oligotrophic, mesotrophic, eutrophic, or hypereutrophic?
- From your knowledge of the area and the information above, what factors contribute to Lake Decatur being in this classification?
- Add any other observations or comments, such as temperature, presence of algae or plants, etc., that may be relevant to your visit.
I hope you enjoyed this EarthCache and your visit to Lake Decatur!
Information used in developing this EarthCache was sourced from Wikipedia.org, Decatur Magazine, and the Illinois Environmental Protection Agency website.