Nitrate Runoff and Oxygen in a Lake
A lake receives runoff from a dairy farm. Students sample three sites along a transect from a stream inflow to the lake centre. At each site they measure nitrate concentration, light penetration, dissolved oxygen at dawn, and count fish species in three hauls. Results: Site A (inflow): nitrate 8.2 mg/dm3, light penetration 0.4 m, oxygen 3.1 mg/dm3, fish species 2. Site B (mid): nitrate 3.5 mg/dm3, light 1.1 m, oxygen 5.6 mg/dm3, fish species 5. Site C (outlet): nitrate 0.6 mg/dm3, light 2.3 m, oxygen 8.4 mg/dm3, fish species 9. The students also find thick green scum at Site A and rotting vegetation on the shore. A local campaign says the farm should be fined for pollution. The farmer says the lake was always muddy and the scum is natural.
1. Describe the pattern in the data and explain how nitrate runoff could produce it.
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There is a clear gradient from Site A to Site C. Nitrate is highest at the inflow (8.2 mg/dm3) and falls to 0.6 mg/dm3 at the outlet. Light penetration is lowest at A (0.4 m) and highest at C (2.3 m). Dissolved oxygen at dawn rises from 3.1 to 8.4 mg/dm3, and fish species richness rises from 2 to 9. This pattern is consistent with nutrient pollution: nitrate from the farm fertilises algae, causing a bloom at the inflow. The bloom shades underwater plants and algae, so light penetration falls. When the bloom and plants die, aerobic bacteria decompose them and use up dissolved oxygen. Low oxygen and poor light make the inflow unsuitable for many fish species, so only tolerant species survive. The rotting vegetation and green scum support this interpretation.
2. Explain why dissolved oxygen is low at Site A even though algae produce oxygen during the day.
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Algae produce oxygen by photosynthesis during daylight, but they also respire all the time. At dawn, photosynthesis has been low or absent for many hours, while respiration has continued. In a dense bloom, the algae themselves, together with bacteria decomposing dead algae and shore vegetation, consume large amounts of oxygen. Decomposition by aerobic bacteria is especially important because it uses oxygen and releases carbon dioxide. If the bloom is thick, oxygen production during the day may not fully compensate, and oxygen can fall to low levels before sunrise. The measurement at dawn therefore shows the lowest daily oxygen concentration, which is why it is a sensitive indicator of pollution.
3. Evaluate the farmer's claim and suggest one management measure with a likely limitation.
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The farmer's claim is not well supported by the data. The nitrate concentration is more than ten times higher at the inflow than at the outlet, which points to the stream entering from the farmed area as a source. The green scum and rotting vegetation are typical of nutrient enrichment, and the low dawn oxygen and low fish diversity at the inflow are consistent with organic pollution. However, the farmer is right that some muddiness and scum can be natural, so a single survey cannot prove the farm is solely responsible; before-and-after data and sampling upstream of the farm would help. A reasonable management measure is a vegetated buffer strip or constructed wetland between the farm and the stream to take up nitrate and trap soil. A limitation is that it may take time to work, needs land, and cannot remove nitrate already stored in lake sediments; fertiliser application should also be reduced and timed to avoid rain.