University Lake is a reservoir with a surface area of 8 acres and an average depth of 10 feet. The volume of the lakes is 222.2 acre-ft. University Lake receives water from stream inflow at a rate of 302 acre-ft/year (373 million L/year) with phosphorus (P) concentration of 0.045 mg P/L. Direct precipitation to the lake totals 80 cm/year and has a phosphorus (P) concentration of 0.011 mg/L. Estimated lake evaporation is 1.89 million L/year. The stream outflow from the lake is 297 acre-ft/year (366 million L/year). You can assume that the evaporation is pure water and carries no other chemicals with it. (There are 1233.5 m3 in 1 acre-ft) Show all of your work! (Hand write calculations and submit assignment in class.)
1. Draw a diagram showing all the inputs and outputs into University Lake (show flows and contaminants in a box model).
2. Determine the average residence time of phosphorus in the lake in days.
3. What is the surface area of the lake in meters squared? What is the Q (flow) of water to the lake from direct precipitation in meters cubed per year?
4. What is the concentration of phosphorus in the lake? (report in µg/L, assume that the concentration associated with the out flow equals the concentration in the lake).
Discussion Questions: address all questions in short essay format (use your text and Carlson’s paper):
1. What is eutrophication, and why is it an ecosystem effect?
2. What is the “twofold effect” of sediment pollution?
3. Based on your calculations in question 4, what is the trophic status of the lake using Carlson’s Trophic State Index? What does this trophic status imply about the lake? What would the associated Secchi depth transparency be, and what would that tell us?
4. If you could manipulate the average residence time of the lake to help reduce the amount of phosphorus in the system, what would you need to change?
5. The water from the lake will eventually drain into the Gulf of Mexico contributing to the dead zone. Explain the impact of this flow into the Gulf of Mexico. What is meant by “cultural eutrophication” (see “A Closer Look,” 19.2).
6. What are some ways that we can reduce phosphorus and nitrogen loading to lakes and streams? Consider specific best management practices (BMPs) within a watershed that would help reduce nutrient loading.