What causes ocean dead zones?

Ocean dead zones are primarily caused by eutrophication, which is the process where excessive nutrients, particularly nitrogen and phosphorus, enter marine ecosystems. These nutrients often come from agricultural runoff, urban wastewater, and industrial discharges that flow into rivers and eventually reach the ocean. When these nutrients accumulate in coastal waters, they can stimulate the rapid growth of algae, known as algal blooms.

As the algae die and decompose, the decomposition process consumes significant amounts of oxygen from the water, leading to hypoxic conditions, or very low oxygen levels. This results in dead zones where aquatic life cannot survive, as most marine organisms require a certain level of dissolved oxygen to thrive.

Understanding this process emphasizes the impact of human activity—particularly agricultural practices and pollution—on marine health. While overfishing, oil spills, and climate change can have detrimental effects on ocean ecosystems, the direct and immediate cause of ocean dead zones is largely linked to the nutrient overloading that leads to eutrophication. This highlights the interconnectedness of land-based practices and marine environments.