Practice a College Board-style free response question on The Living World: Ecosystems. Write your response, then reveal the model answer to see exactly what earns each point.
A farming community applies nitrogen- and phosphorus-based fertilizer to corn fields each spring. A freshwater lake located downstream from the fields has been monitored for five years. Researchers recorded the lake's total phosphorus concentration and its average summer dissolved oxygen (DO) level at a depth of 5 meters.
| Year | Total phosphorus (µg/L) | Summer DO at 5 m (mg/L) |
|---|---|---|
| 1 | 12 | 8.5 |
| 2 | 18 | 7.9 |
| 3 | 27 | 6.0 |
| 4 | 39 | 3.2 |
| 5 | 52 | 1.1 |
The most likely source is agricultural fertilizer runoff from the corn fields, carried into the lake by precipitation and surface water flow. Phosphorus is the nutrient typically tracked in freshwater systems because it is usually the limiting nutrient there — meaning even a small increase in available phosphorus can trigger a disproportionately large increase in algal growth, since algae are otherwise nutrient-starved for phosphorus specifically.
The data show an inverse relationship: as total phosphorus rises from 12 to 52 µg/L over five years, dissolved oxygen at 5 meters falls from 8.5 to 1.1 mg/L. This happens because the added phosphorus triggers eutrophication — excess phosphorus causes algae to grow rapidly (an algal bloom). When the algae die, aerobic decomposers (bacteria) break down the dead biomass and consume large amounts of dissolved oxygen from the water in the process, depleting the oxygen available to fish and other aquatic organisms at depth.
The community could establish a vegetative buffer strip (a band of grasses, shrubs, or trees) between the fields and the lake. Plant roots in the buffer strip would absorb dissolved phosphorus and slow surface runoff before it reaches the water, allowing sediment-bound phosphorus to settle out rather than washing directly into the lake. (Other valid answers: contour plowing, no-till farming, applying fertilizer based on soil testing to avoid over-application, or constructing a retention pond to capture runoff before it enters the lake.)