What is the BEST explanation for the rapid decrease in aquatic life following an algal bloom?

The rapid decrease in aquatic life following an algal bloom is primarily due to the process of decomposition that occurs after the bloom collapses. When an algal bloom occurs, it leads to a significant increase in organic material in the water as the algae grow and later die off. Decomposers, such as bacteria, are responsible for breaking down this organic matter. This decomposition process is highly oxygen-consuming, resulting in a dramatic reduction of dissolved oxygen levels in the water.

As oxygen levels drop, aquatic organisms—particularly fish and other aerobic life forms—struggle to survive. Fish and invertebrates require a certain level of dissolved oxygen to breathe and maintain their metabolic functions. When oxygen is depleted, these organisms may experience stress, and many may die off, leading to a rapid decrease in aquatic life.

Other options do not capture the primary mechanism leading to the significant loss of life. While fish movements and temperature changes might contribute to shifts in populations or stress levels, they don't directly account for the immediate and lethal oxygen depletion caused by decomposers breaking down the excess organic matter from the algal bloom. Insufficient food from algae also doesn't explain the immediate danger posed by the lack of oxygen following the bloom. Hence, the consumption of oxygen by decom