How does nutrient runoff contribute to eutrophication in aquatic ecosystems?

How does nutrient runoff contribute to eutrophication in aquatic ecosystems?

• A. It reduces algal growth and increases oxygen
• B. It introduces high nutrients that foster algal blooms; after blooms, decomposition consumes oxygen creating hypoxic zones
• C. It sterilizes the water by killing all microorganisms
• D. It lowers water temperature, reducing metabolic rates

Answer: (B)

Nutrient runoff introduces essential nutrients like nitrogen and phosphorus into aquatic systems, acting as fertilizers for algae and phytoplankton. This kickstarts rapid algal blooms, turning the water dense with living algae. When these blooms die, bacteria break down the dead organic matter, and their activity uses up a lot of dissolved oxygen in the water. With less oxygen available, the environment becomes hypoxic, and aquatic organisms—especially fish and invertebrates—struggle or die. It’s useful to remember that while photosynthesis by algae can produce oxygen during daylight, the overall effect of a bloom followed by decomposition is a net loss of oxygen due to higher respiration and microbial activity. This sequence—nutrients fueling blooms, followed by decomposition-driven oxygen consumption—produces the low-oxygen conditions characteristic of eutrophication. The other ideas don’t fit because nutrient input doesn’t sterilize the water, reduce algal growth, or lower water temperature in a way that would promote healthier ecosystems.

Nutrient runoff introduces essential nutrients like nitrogen and phosphorus into aquatic systems, acting as fertilizers for algae and phytoplankton. This kickstarts rapid algal blooms, turning the water dense with living algae. When these blooms die, bacteria break down the dead organic matter, and their activity uses up a lot of dissolved oxygen in the water. With less oxygen available, the environment becomes hypoxic, and aquatic organisms—especially fish and invertebrates—struggle or die.

It’s useful to remember that while photosynthesis by algae can produce oxygen during daylight, the overall effect of a bloom followed by decomposition is a net loss of oxygen due to higher respiration and microbial activity. This sequence—nutrients fueling blooms, followed by decomposition-driven oxygen consumption—produces the low-oxygen conditions characteristic of eutrophication.

The other ideas don’t fit because nutrient input doesn’t sterilize the water, reduce algal growth, or lower water temperature in a way that would promote healthier ecosystems.