Aquatic and marine dead zones can be caused by increases in nitrogen and phosphorus in the water from human waste or fertilizer. these substances encourage algal blooms. although algae produce oxygen in the daytime by photosynthesis, during the night hours they continue to undergo cellular respiration and use up available oxygen. when algal blooms die off, more oxygen is used during decomposition of the dead algal cells. both of these processes can result in a significant depletion of dissolved oxygen in the water, creating dead zones. dead zones can be caused by natural or human factors. a. summarize the role of algal blooms in disrupting the health of aquatic ecosystems.

They lead to aging of lakes

Explanation:

When the loads of algae occasioned by the bloom mentioned in the question begin to die, they will need to be decomposed by aerobic organisms. This causes an increase in the oxygen demand. As oxygen demand increases, oxygen is depleted from the water body. This continues until all the oxygen present is depleted and decomposition continues under anaerobic conditions. All the fishes and other aquatic organisms may eventually die due to the lack of oxygen in the water body, decomposition of these continue anaerobically giving out methane and H2S. This continues until the water body turns into a foul smelling area.

it's c. (to reduce the greenhouse effect/carbon gasses)