Excess fertilizers used in agriculture commonly end up as runoff and cause phytoplankton blooms in rivers. To figure out whether these blooms were driven by ammonium or phosphate fertilizers, researchers cultured a phytoplankton species in multiple samples of unpolluted river water. The samples were divided equally among three treatments: ammonium fertilizer added, phosphate fertilizer added, and no fertilizer added. They then measured phytoplankton density in each of the samples after a week. Phytoplankton densities (in thousands of cells/ml) are reported in the table shown.
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In aquatic ecosystems, nitrogen is often the limiting nutrient for phytoplankton growth, while phosphorus typically plays a secondary role.
Nitrogen is the limiting nutrient for phytoplankton growth.
Phosphorus is the limiting nutrient for phytoplankton growth.
Both nitrogen and phosphorus limit phytoplankton growth.
Neither nitrogen nor phosphorus limits phytoplankton growth.
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The Correct Option isA
Solution and Explanation
Step 1: Understanding the data.
The table provides phytoplankton densities in three treatments: no fertilizer, phosphate fertilizer, and ammonium fertilizer. The data shows that without fertilizer, the density is 6 ± 1, with phosphate fertilizer it is 7 ± 1.5, and with ammonium fertilizer it is 27 ± 2.
Step 2: Analyzing the results.
The density is significantly higher when ammonium fertilizer is added compared to both no fertilizer and phosphate fertilizer. This suggests that ammonium (a source of nitrogen) has a major positive impact on phytoplankton growth. Phosphate fertilizer leads to a slight increase in growth, but not as much as ammonium. This indicates that nitrogen is the limiting nutrient for phytoplankton growth in this case.
Step 3: Conclusion.
The correct inference is (A) Nitrogen is the limiting nutrient for phytoplankton growth, as shown by the large increase in phytoplankton density when ammonium (nitrogen) is added.
