Lake Water Quality Index
Humans as sources of food, water, and recreation have long cherished lakes. There are not many lakes in the world that do not have evidence of man's tampering. This has changed the character of the lake ecosystems. Many organisms no longer exist in the ecosystems due to the effects of human pressure. We are creating smaller and smaller pockets where these organisms exist in greater densities, putting them in greater danger of catastrophic change or extinction. It has also changed the perspective of how we evaluate lakes. More and more emphasis is placed on creating clear lakes that can be used for many recreational uses and will support the aquatic life that humans value the most, e.g. walleye, trout. Lakes are classified as either oligotrophic, mesotrophic, eutrophic, or hypereutrophic. Oligotrophic and mesotrophic are the most desirable for humans.
We are just realizing that lakes are simply collection points in larger watersheds and when we devise a management plan for lakes we need to look at the whole watershed. Eutrophic and hypereutrophic lakes often are the result of some contamination of human source. Once identified, we need to look at all the issues surrounding the source and devise a plan to deal with it. Unlike streams, lake water quality is indexed through four major parameters: total phosphorus, total nitrogen, and Secchi transparency depth. Any one of these can give us an indication as to the trophic state of the lake. Other tests such as dissolved oxygen help to confirm diagnosis.
Total Phosphate:
Phosphorus, usually found in aquatic systems as phosphate, is the limiting factor for growth in the ecosystem. This is due to the fact that plants need it to grow and it is found in such small quantities. When phosphorus is in excess in a system, algal blooms occur. The excess algae die and take up oxygen in the decomposition process. The algal bloom will also cut down on the depth that light can penetrate the water, affecting the other plant life that lives deeper in the water. Excess phosphate usually occurs from human sources, laundry detergent, fertilizers, sewage treatment plants, or cabin septic tanks. Levels of 0.013 mg/l are considered the problem threshold for streams or lakes. 0.13 mg/l indicates that there is a pollution problem and levels of 1.3 mg/l indicate severe problems for the ecosystem.
Follow the instructions from your water test kits to test your section of the lake for total phosphate.
1. What is your phosphate level for the lake? ______
2. Is it in excess to normal levels? ______
3. Are there any potential phosphate sources in your area? If so identify them._____
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Nitrates:
Nitrogen is needed by all life to produce protein, the building block of tissue. The air contains nitrogen in the form of N2. This form is not of use to most plants except legumes. Blue-green algae are also able to use this form of nitrogen from the air. Most plants need to take up a form called nitrate, NO3. It too, like phosphate, can cause over-growth in an aquatic ecosystem. Sources of nitrates is the decomposition of any organic substance and fertilizers. Nitrate are of particular concern for humans in that if they are present in high levels in drinking water, they will pose a risk to infants if used to make formula. The problem threshold for nitrogen is 0.092 mg/l with 0.92 mg/l indicating a likely problem. 10 mg/l is the limit set by the U.S. Public Health Service for allowable concentration of nitrates in public drinking water. The baby's blood loses the ability to carry oxygen leading to "blue baby" syndrome.
Again, follow the procedure from your test kits to measure nitrate levels..
4. Record your nitrate level. ______
5. Would this pose a risk to humans? ______
6. What is the most likely source of nitrates in your area ? ______
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Turbidity:
Turbidity is the measure of the clarity of the water. High turbidity can cause lower photosynthetic levels, warmer water, and possible clogging of fish gills. Turbidity is measured with a Secchi disk, or with shallow waters, a colorimetric test. Use whichever one that is available to you.
7. What is the turbidity value for your lake? ______
8. What might be some sources for increased turbidity in a lake? ______
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Chlorophyll A:
Chlorophyll A is often used to estimate the amount of algae in a lake. The method works best in lakes due to the interference of suspended solids in streams. Once the amount of chlorophyll A is found the following calculations can be done:
1 mg dry plankton = 0.5 mg carbon
1 mg dry plankton = 4.45 mg plankton wet biomass
1 mg dry plankton = 0.93 mg oxygen
1 mg dry plankton = 0.92 dry plankton biomass
1 mg dry plankton = 0.03 mg chlorophyll a
To determine chlorophyll A, filter a liter of water using a vacuum pump. A hand-held brake bleeder works great. Take the filter and place it in 50 or 100 ml 90% acetone solution and refrigerate for 24 hours. This will remove most of the chlorophyll from the filter. Centrifuge at 3000 to 4000 rpm for ten minutes. Carefully decant or pipet the acetone; do not disturb the bottom sediments. Zero the spectrophotometer at wavelength 630 nm using a tube containing only 90% acetone. Place the chlorophyll-containing acetone tube in the spectrophotometer and record the reading. Repeat the zero and reading procedure at 647, 664, and 750 nm. Also record the light path of the spectometer cell in centimeters.
Chlorophyll a, in micrograms per milliliter
= 11.85e664 - 1.54e647 - 0.08e630;
where
e664 = (664 nm reading - 750 nm reading) / light path distance
e647 = (647 nm reading - 750 nm reading) / light path distance
e630 = (630 nm reading - 750 nm reading) / light path distance
Carlson Trophic State Index
The Carlson Index was developed by Robert E. Carlson, Limnological Research Center, University of Minnesota. This index rates lakes on a scale of 0 to 100. Each major division (10, 20 30, etc.) represents a doubling in algal biomass. The index number can be calculated from any of several parameters: Secchi disk, chlorophyll, total nitrogen, and total phosphorus. Attached is a diagram that can be used to determine the trophic index of the lake. Using your phosphorus reading, Secchi disk reading, and chlorophyll A results find the point on the appropriate scale and read straight up on the trophic scale to determine the classification of the lake.
9. What is the trophic state for the lake? ______
10. Describe what other chemical conditions you would expect in this lake and what types of life we should find in it.
Carlson Trophic State Index
Dissolved Oxygen:
Dissolved Oxygen (DO) is the most important parameter of water for aquatic organisms. All organisms need oxygen to fuel the chemical reactions involved in respiration. The absence of oxygen is often a sign of severe pollution within the lake. Different species of organisms have different DO requirements. Only a few are able to live in low concentrations, e.g. carp, catfish, and bloodworms. The higher the level of DO, the more variety of life the stream can support. For this reason, levels of 9 or higher are considered healthy.
11. Using the test procedures you have learned with the test kits, what is the DO content in your sample?
12. How much variety of life would you expect to find in your stream? Explain____
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It is also useful to determine % saturation, or actual amount of DO in the water vs. total amount that can be held by the water. This can be determined using an oxygen saturation chart. Line the DO reading with the temperature of the sample on the scale and obtain the % saturation. Consistent readings in the 90% level indicates a very healthy stream. Readings over 100% indicate eutrophication problems due to over production of plants.
13. What is your % saturation? ______
pH:
pH is a measurement of the H+ concentration in liquids and substances. This concentration is reported on a scale of 0 to 14 with 7 being neutral and lower numbers representing a higher concentration of H+ and higher numbers indicating higher concentrations of OH- . This is a logarithmic scale, each unit is ten times stronger than the previous. This is why a slight change in pH can have such a great effect on organisms.
Aquatic organisms have evolved within ecosystems that have narrow ranges of pH. Therefore, their tolerance to pH changes is low. Even hardy fish like carp and suckers can only live in a pH range of 6 to 10. pH changes often are greatest during the spring melt when all of the acidic snow is dumped into the streams over a short period of time. pH readings under 5 cause developmental problems with young fish and immature stages of insects; this disrupts the food chain of the ecosystem.
Measure pH of your study stream following the instructions on your test kit. Do the test at different regions of the stream
14. What is the pH of the lake? ______
15. Does the pH change drastically anywhere on the lake? ______
16. Is this lake susceptible to acid rain? Explain. ______
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Biochemical Oxygen Demand (BOD):
When organisms die, their tissues will decompose through the process of aerobic respiration, which requires oxygen. This process removes oxygen from the aquatic ecosystem. Therefore, a large influx of organic matter into a stream can greatly decrease the amount of oxygen that is available to organisms, possibly causing periods of die-off. The sources of this organic matter include wastes from paper mills, meat-packing plants, food processing industries, cabin/home septic tanks, farms, urban runoff, and water treatment plants. Healthy water should have a low BOD after five days. The five day bottle should have at least one third the oxygen content of the original.
Procedure for BOD
A. Fill three BOD bottles and wrap two in tin foil. Set this one out of the sun.
B. Using a small hose, siphon water out of the first bottle into a DO bottle; make sure the hose is on the bottom of the bottle and let the water overflow for a few seconds and test for DO. Record
C. Place the covered BOD bottles in a drawer at a temperature of 68° F (20°C) for five days.
D. Using the same siphoning technique, test one of the covered bottles for DO on day two.
E. After five days, test the second bottle.
BOD = mg/l DO original bottle - mg/l DO five day bottle
If the five day bottle is at 0, repeat sample and dilute samples with half distilled water.
17. What is the BOD of the lake? ______
18. What was the clarity of the lake? ______
19. Is the BOD a healthy value? Justify your answer. ______
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20. Did your results for the above tests agree with your expectations based on your trophic determination? Explain.
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21. Describe what uses the lake might support.
22. Describe a management plan for this lake. Justify each of your suggestions.