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MIT 7 014 - Solutions to 7.014 Quiz III

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Solutions to 7.014 Quiz III4/14/04Class Average = 71Median = 73Grade Range %A 84-100 15B 70-83 42C 58-69 25D 43-57 17F 0-42 12Question 1A colleague gives you a sample of marsh mud and water that she would like you to analyze.You create three identical tanks containing water, NO3-, H2S, and all essential trace elements.These tanks can be sealed such that only you can control the gas allowed in. You then addidentical samples of the marsh mud and water to each tank.For questions a and b, choose from:Oxygenic photosynthesis Anoxygenic photosynthesis ChemosynthesisAerobic respiration Anaerobic respirationa) You place Tank 1 in the dark and supply CO2, but no oxygen. After two days you examinethe atmosphere and the organisms found in Tank 1. You find CO2 and some N2, but no O2.What types of metabolisms must organisms found in Tank 1 use?Chemosynthesis and Anaerobic respirationb) You place Tanks 2 and 3 in the light and supply CO2, but no oxygen. After two days youexamine the atmosphere and the organisms found in Tank 2, but you leave Tank 3 sealed forthe time being.• After two days you examine the atmosphere and the organisms found in Tank 2. Youfind CO2, SO42-, some N2, but no O2. What types of metabolisms must organisms foundin Tank 2 use?Anoxygenic photosynthesis (chemosynthesis O.K.) Anaerobic respiration• After two months you examine the atmosphere and the organisms found in Tank 3.You find CO2 and O2, but no N2. What types of metabolisms must organisms found inTank 3 use?Oxygenic photosynthesis Aerobic respirationc) You know that Tanks 2 and 3 were identical so if you had examined Tank 3 at two days youwould have seen CO2, SO42-, some N2, but no O2. However, after two months you find CO2and O2, but no N2.• Why is oxygen not present at two days but found at two months?At two days the predominant organisms are carrying out Anoxygenic photosynthesis which usesH2S and releases SO42-. However, the organisms capable of oxygenic photosynthesis which uses H2Oand releases O2 out-compete the anoxygenic photosynthesizers.• What happened to the N2 that was present at two days?The atmospheric N2 is fixed into NH3which is incorporated into nitrogenous bases and aminoacids in the cells. Without N2 to fix, the organisms cannot survive.3Question 2As part of a field trip for 7.014 you travel to the Amazon River Basin. There is much concernabout the deforestation occurring throughout the Basin.a) Deforestation is occurring in both temperate and tropical rain forests. In both cases, trees areremoved to clear land for agricultural uses.• How does this affect the global carbon cycle?It increases the CO2 in the atmosphere. It contributes to global warming.• Why do you see this effect?By removing the trees, you decrease photosynthesis and thus remove less CO2 from theatmosphere. Also, agricultural practices burn fossil fuel as gas for farm equipment and energy tosupply the factories that make farm equipment and fertilizers. This burning of fossil fuelincreases the CO2 in the atmosphere.• Why is creating agricultural land by deforestation of tropical rain forests less productivethan creating agricultural land by deforestation of temperate forests?The nutrients in the tropical rain forests are stored primarily in the biomass of the trees. When atropical forest is cleared, the trees are removed and the soil will only support a few growingseasons. In temperate forests, the soil stores more nutrients because there is exposed rock thatcontinues to provide nutrients to the soil. More growing seasons can be supported.b) While in South America you feast on seafood supplied by the thriving fishing industryfound along the West coast. Explain the relationship between coastal upwelling and thefishery industry in this area.Coastal upwelling brings limiting nutrients that have sunk into the deep waters back to the surface.These nutrients support a bloom of primary producers (phytoplankton) that support the zooplanktonthat feed the fish that are caught for consumption.Use only space above4Question 2, continuedYou decide to create a small model of the Amazon River in your backyard fishpond. Youstock it with some algae, the algae-eating fish H. plecostomus (pleco) and the carnivorous fish A.ocellatus (oscar). Carbon and energy are transferred in the following manner:sun --> algae --> pleco --> oscarAssume that total solar input for a system is 1,000,000 kcal/yr. This system has the followingefficiencies:photosynthetic efficiency of algae = 4%algae production efficiency = 80%exploitation efficiency of plecos = 60%production efficiency of plecos = 30%ecological efficiency of plecos = 12%ecological efficiency of oscars = 20%ecological efficiency of turtles = 15%c) What is the annual Net Primary Production (NPP) of this ecosystem? Show your work.1e6 * .04 * .8 = 32,000 kcal/yrd) What is the assimilation efficiency of the plecos? Show your work..12 = .6 * .3 * x ; x = .66 ; 66%e) Suppose that another predator (turtles) requires a minimum of 750kcal/yr of availableenergy from the oscars to maintain itself. Assume these turtles only eat oscar fish. Could apopulation of turtles be supported in this pond ecosystem without any additional energyinput? Explain or show why or why not.32,0004 * .12 * .2 * = 768kcal/yr. There is enough energy to support the turtles.5Question 2, continuedYour neighbor decides to copy you. He stocks his pond with some algae, the algae-eating fishH. plecostomus (pleco) and the carnivorous fish A. ocellatus (oscar). Although his pond is thesame size, it is in a sunnier location, and is able to maintain a turtle population. The ecosystemof your neighbor’s pond quickly reaches and maintains an equilibrium. However, after oneyear he discovers that his oscar population is decreasing.Your neighbor believes that something else has invaded his pond and is eating the algae, butthis invader is not prey for the oscars.You believe that his turtles are now eating the plecos as well as the oscars.You estimate the biomass of the turtles and find that it is increasing.f) Is the increase in turtle biomass consistent with your neighbor’s hypothesis, yourhypothesis, or neither? Explain.Yours. Your neighbor’s hypothesis predicts that the oscar population would decrease, but it would alsopredict a decrease in the turtle population (biomass). The decrease in the oscar population and increasein turtle biomass is only consistent with your hypothesis.Use only space


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MIT 7 014 - Solutions to 7.014 Quiz III

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