What can lead to oxygen depletion in aquatic environments?

The correct choice highlights a significant factor that contributes to oxygen depletion in aquatic environments: heavy growths of phytoplankton or blooms. When phytoplankton populations experience a rapid increase, known as a bloom, they can overwhelm an aquatic system. Following the bloom, a die-off occurs, leading to substantial amounts of organic matter sinking to the bottom of the water body. The decomposition of this organic matter by bacteria consumes a considerable amount of dissolved oxygen in the process. This can result in hypoxic or anoxic conditions, severely impacting aquatic life that relies on oxygen for survival.

While other factors, such as natural decay processes of organic matter, can also contribute to oxygen depletion through similar mechanisms, they typically do not lead to the same immediate and dramatic declines in oxygen levels as phytoplankton blooms. Low water temperatures generally promote oxygen solubility in water and do not intrinsically lead to depletion. Lastly, while excessive sediment accumulation can affect water quality by influencing light penetration and habitat availability, it does not directly cause the oxygen depletion that results from high rates of organic matter decay initiated by phytoplankton blooms.