Beyond ozone formation, how does elevated NOx deposition affect aquatic ecosystems?

• A. NOx deposition increases nitrate in water, promoting eutrophication and algal blooms, leading to hypoxia and fish kills.
• B. NOx deposition decreases nitrate in water, reducing algal growth.
• C. NOx has no impact on aquatic systems.
• D. NOx deposition cools water temperatures.

Answer: (A)

When NOx is deposited from the atmosphere, it adds reactive nitrogen to aquatic systems in the form of nitrate. Nitrate acts as a nutrient, so elevated levels fuel excessive growth of algae and phytoplankton. This rapid, overabundant growth—eutrophication—reduces light for submerged vegetation and, as the blooms die and decompose, microbial respiration uses up oxygen in the water. The resulting low-oxygen conditions create hypoxic zones that stress or kill fish and other aquatic organisms. This is the main way NOx deposition harms aquatic ecosystems beyond its role in ozone formation. The other options don’t fit because NOx deposition increases, not decreases, nitrate; it does affect aquatic systems; it does not cool water.