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Quiz & test questions Grade 11 Science

Ripplebrook Estuary Unit Test: Salinity, Oxygen, and Nutrients

A 50-point grade 11 unit test on estuary stratification, summer oxygen loss, and nitrogen loading, mixing multiple choice, computation, and an extended evaluation item.

Objectives

  1. Students will interpret a depth profile of temperature, salinity, and dissolved oxygen.
  2. Students will calculate a nutrient load in pounds per day from a stream flow and concentration.
  3. Students will evaluate two nutrient-reduction proposals against oxygen and habitat criteria.

Vocabulary

Estuary
An estuary is a place where a river meets the sea and fresh water mixes with salt water. Estuaries such as bays and coastal marshes are nurseries for fish, crabs, and birds.

Also: stratification, halocline, hypoxia, nutrient load, internal loading

Teacher overview

This 50-point unit test covers estuary stratification, summer oxygen loss, and the nitrogen load a watershed delivers, using an August depth profile from the fictional Ripplebrook Estuary. It mixes four multiple-choice items worth 2 points each, five computation items, two short-answer explanations, and one extended evaluation item worth 10 points. Students need a calculator and the conversion that 1 million gallons per day equals 1.547 cubic feet per second. A misconception this test targets directly is the belief that low bottom-water oxygen means the estuary has stopped producing oxygen; item 10 asks students to explain that the surface stays well oxygenated while stratification cuts the bottom layer off from resupply, and that decomposition is what draws the bottom water down.

Ripplebrook Estuary Unit Test: Salinity, Oxygen, and Nutrients — Grade 11 Science
Name: ____________________________ Date: ______________

Assessment

All items describe the Ripplebrook Estuary and the town of Beaumaris, both fictional. Station R-3 was profiled on an August afternoon.

Depth (meters) Temperature (degrees Celsius) Salinity (parts per thousand) Dissolved oxygen (mg/L)
0 24.0 4.0 8.1
2 23.6 5.2 7.6
4 21.0 12.0 5.4
6 18.5 20.0 2.1
8 18.0 22.0 0.8
  1. (2 points) In which depth interval does salinity change the most? A. 0 to 2 meters. B. 2 to 4 meters. C. 4 to 6 meters. D. 6 to 8 meters.
  2. (2 points) Why does the saltier water lie beneath the fresher water at Station R-3? A. Salt water holds more dissolved oxygen. B. Salt water is denser than fresh water at the same temperature. C. Fresh water is warmed by the bottom sediments. D. The incoming tide pushes fresh water downward.
  3. (2 points) Which pair of measurements gives the clearest evidence that the estuary is stratified? A. Air temperature and wind speed. B. Salinity and water temperature measured at several depths. C. Turbidity measured at the surface only. D. Water depth and tide stage.
  4. (2 points) Which action reduces the external nutrient load reaching Ripplebrook rather than the internal load already in the estuary? A. Aerating the bottom water with diffusers. B. Dredging phosphorus-rich bottom sediments. C. Upgrading the Beaumaris wastewater plant for nitrogen removal. D. Adding alum to bind phosphorus in the sediments.
  5. (4 points) Using the table, calculate the average rate of change in dissolved oxygen between 4 meters and 8 meters, in mg/L per meter. Include the sign.
  6. (4 points) Most fish need dissolved oxygen above 5 mg/L. Using the table, list every profiled depth at which the water fails that benchmark, and state how many meters of the water column are affected.
  7. (6 points) Wren Creek enters the estuary at 18 cubic feet per second. Using 1.547 cubic feet per second per million gallons per day, calculate the flow in million gallons per day, rounded to the nearest thousandth.
  8. (6 points) Wren Creek carries total nitrogen at 4.2 mg/L. Using your item 7 flow and pounds per day equal to concentration in mg/L times flow in million gallons per day times 8.34, calculate the nitrogen load in pounds per day, rounded to the nearest tenth.
  9. (4 points) A watershed plan would cut Wren Creek's nitrogen load by 35 percent. Using your item 8 answer, calculate the remaining load in pounds per day, rounded to the nearest tenth.
  10. (4 points) The surface water at Station R-3 holds 8.1 mg/L of dissolved oxygen while the water at 8 meters holds 0.8 mg/L. Explain how the estuary can be producing oxygen at the surface while the bottom water runs low, and name the process that consumes the oxygen down there.
  11. (4 points) Explain why a wet spring, with heavy runoff and high river flow, tends to make Ripplebrook's late-summer oxygen problem worse. Give two separate effects of the extra freshwater and runoff.
  12. (10 points) Beaumaris can fund one of two projects. Project A upgrades the wastewater plant for nitrogen removal, cutting the plant's discharge load sharply within two years. Project B restores streamside buffers across the Wren Creek watershed, cutting runoff-carried nitrogen gradually over a decade at a similar total cost. Using the loading numbers from items 8 and 9 and the oxygen data in the table, recommend one project. State the criterion that decides it, cite one piece of numerical evidence, and name one drawback of your choice and one advantage of the option you rejected.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: C.
    Salinity rises 8.0 parts per thousand between 4 and 6 meters, from 12.0 to 20.0, which is larger than the 1.2, 6.8, and 2.0 changes in the other intervals. That interval holds the halocline.
  2. Answer: B.
    Dissolved salt raises water's density, so at a given temperature the saltier ocean water sinks beneath the fresher river water. Choice A is backwards, since colder and fresher water holds more oxygen at saturation, and choices C and D describe processes that do not occur here.
  3. Answer: B.
    Stratification is a layered difference in density with depth, and salinity and temperature are the two properties that set density, so measuring both at several depths shows the layers directly. Surface-only or single-value measurements cannot reveal a vertical structure.
  4. Answer: C.
    External load is nitrogen and phosphorus arriving from the watershed, and a plant upgrade cuts the load at its source. Aeration, dredging, and alum all address nutrients or oxygen already inside the estuary, which is internal loading.
  5. Answer: -1.15 mg/L per meter.
    (0.8 - 5.4) / (8 - 4) = -4.6 / 4 = -1.15 mg/L per meter. The negative sign shows oxygen falling with depth.
  6. Answer: Two depths, 6 meters and 8 meters; roughly the bottom 4 meters of the profiled column.
    Dissolved oxygen measures 2.1 mg/L at 6 meters and 0.8 mg/L at 8 meters, both below the 5 mg/L benchmark. The 4-meter reading of 5.4 mg/L still clears it, so the affected zone spans roughly the bottom 4 meters of the profiled column, from below 4 meters to the bottom at 8 meters. The 5 mg/L crossing falls between 4 and 6 meters (about 4.2 meters by interpolation), so accept any span from 2 to 4 meters that the student justifies from the table.
  7. Answer: 11.635 million gallons per day.
    18 cubic feet per second / 1.547 cubic feet per second per million gallons per day = 11.6354, which rounds to 11.635 million gallons per day.
  8. Answer: 407.6 pounds per day.
    4.2 mg/L x 11.635 MGD x 8.34 = 407.55 pounds per day, which rounds to 407.6. Working stepwise: 4.2 x 11.635 = 48.867, and 48.867 x 8.34 = 407.55.
  9. Answer: 264.9 pounds per day.
    A 35 percent cut leaves 65 percent: 407.55 x 0.65 = 264.91, which rounds to 264.9 pounds per day. The plan removes about 142.6 pounds per day.
  10. Answer: Stratification cuts the bottom layer off from the surface.
    Algae and plants at the sunlit surface release oxygen, and the surface also exchanges gas with the air, which keeps the top of the column near 8.1 mg/L. The density difference across the halocline blocks mixing, so no oxygen reaches the bottom layer. Meanwhile bacteria decomposing dead algae and other organic matter that sinks into that layer consume the oxygen already there, drawing it down to 0.8 mg/L. Full credit requires naming decomposition, or bacterial respiration, as the consuming process.
  11. Answer: More nutrients arrive and stratification grows stronger.
    First, heavier runoff carries more nitrogen and phosphorus from the watershed, which feeds a larger algae crop whose remains sink and decompose. Second, more freshwater on top of the salt wedge sharpens the density contrast between the layers, which makes the water column harder for wind to mix, so the bottom layer stays cut off longer. Accept either order, and accept a mention of the extra sediment reducing light as a third effect only if two of the main effects are also present.
  12. Answer: Either project, judged on speed against watershed coverage.
    Award points for a recommendation supported by evidence and trade-offs rather than for a particular choice. A strong answer for Project A names the speed of the oxygen response as the criterion, uses the 0.8 mg/L bottom reading to argue that the estuary cannot wait a decade for the watershed cut of about 142.6 pounds per day from items 8 and 9 to arrive, and accepts the drawback that a plant upgrade leaves runoff-carried nitrogen untouched while acknowledging that buffers would eventually address a larger share of the watershed. A strong answer for Project B names the breadth of the source as the criterion, notes that a plant upgrade cannot reduce nitrogen the creek delivers from fields and lawns, and accepts the drawback that buffers act slowly while acknowledging that the plant upgrade delivers a faster, more certain cut. Full credit requires a criterion, one number cited correctly, one drawback, and one advantage of the rejected option.

Differentiation

Support

Provide the depth profile as a labeled graph alongside the table, with the 5 milligrams per liter fish benchmark drawn as a vertical line, so items 5 and 6 can be read as well as computed. Give the pounds formula on a reference card with each variable labeled, and allow item 12 to be answered in four bulleted sentences matching the four required parts.

Extension

Have students calculate the percent reduction in Wren Creek's nitrogen load that would be needed to bring the load below 250 pounds per day, then explain why a load target expressed in pounds per day is more useful to a watershed plan than a concentration target expressed in milligrams per liter.

ELL supports

Pre-teach stratification, halocline, hypoxia, and load with a labeled cross-section showing fresh water riding over salt water and a shaded low-oxygen layer at the bottom. Permit short-answer items 10 and 11 to be answered with a labeled diagram plus two sentences, using the frames "The bottom water loses oxygen because ___" and "A wet spring makes this worse because ___."