What is a direct consequence of nitrate runoff on aquatic ecosystems?

• A. Improved water quality due to lower nutrient loads
• B. Water becomes less productive for algae
• C. Decreased algae growth and higher oxygen levels
• D. Algal blooms and oxygen depletion in water bodies

Answer: (D)

Nitrate runoff acts as a fertilizer for aquatic plants and algae. When excess nitrates enter streams, rivers, or lakes, they fuel rapid algal growth, causing algal blooms. This sudden surge in algae can reduce water quality by making water appear cloudy or green and can shade underwater plants, disrupting the ecosystem.

The real trouble starts when these algae die. Microorganisms decompose the dead biomass, and this process uses a lot of dissolved oxygen in the water. As oxygen levels drop, aquatic animals like fish and invertebrates struggle to survive, leading to oxygen depletion and sometimes dead zones where life is scarce.

So the strongest consequence is algal blooms followed by oxygen depletion in the water body. The other options describe outcomes that would not happen with nitrate runoff—lower nutrients, less algae growth, or higher oxygen—because the runoff actually increases nutrients and boosts productivity, not the opposite.