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.