What happens after algal blooms die in a lake or river?

• A. Decomposition consumes dissolved oxygen, causing hypoxia and fish kills; remediation focuses on nutrient reduction.
• B. Algae produce oxygen as they die, improving water quality.
• C. Sediment deposition increases habitat for fish, improving quality.
• D. Increased turbidity reduces sunlight but does not affect dissolved oxygen.

Answer: (A)

When an algal bloom dies, decomposers and other microbes break down the dead algal biomass. This decomposition uses a lot of dissolved oxygen in the water, so the oxygen level can drop sharply, especially in deeper or stratified parts of a lake or river. If oxygen becomes very low or runs out (hypoxia or anoxia), fish and other aquatic life can be stressed or killed, creating dead zones.

Remediation focuses on reducing nutrient inputs—mainly nitrogen and phosphorus—that fuel blooms. Fewer nutrients mean smaller or less frequent blooms, which means less dead biomass to decompose and exhaust the available oxygen. While living algae contribute oxygen through photosynthesis, the decay phase consumes more oxygen than is produced, leading to the observed hypoxia.

Others options don’t fit because producing oxygen occurs during photosynthesis when algae are alive, not as they die. Sediment changes or habitat gains aren’t the direct consequence of bloom decay, and turbidity alone doesn’t capture the primary issue, which is oxygen depletion driven by microbial respiration during decomposition.