Agricultural runoff disrupts the nitrogen cycle in aquatic systems by increasing which compound in water, often causing eutrophication?

• A. Ammonia
• B. Nitrates
• C. Nitrogen gas
• D. Organic nitrogen in soil

Answer: (B)

When fertilizer runs off from agricultural land into waterways, dissolved inorganic nitrogen—primarily nitrates—enters the water. Nitrates are highly soluble, so they readily accumulate in streams and lakes and become readily available for aquatic plants and algae. This influx fuels rapid algal growth, or blooms, which then die and are decomposed by microbes that consume a lot of dissolved oxygen. The result is lower oxygen levels in the water, a hallmark of eutrophication. Among the options, nitrates are the form of nitrogen most likely to increase in water from runoff and to drive this process. Ammonia can be present but is often converted to nitrate in oxygen-rich water; nitrogen gas isn’t typically added by runoff; organic nitrogen in soil isn’t the dissolved inorganic form that accumulates in aquatic systems.