The Gulf of Mexico Dead Zone is a hypoxic area caused by nutrient runoff high in which element?

• A. Phosphorus
• B. Potassium
• C. Ammonia
• D. Nitrogen

Answer: (D)

Excess nutrients in water drive eutrophication, where algal blooms grow rapidly and then decompose, using up the dissolved oxygen in the water and creating hypoxic conditions. For the Gulf of Mexico, the main nutrient culprit is nitrogen carried by the Mississippi River from agricultural runoff. This nitrogen, in the form of nitrates and related compounds, fuels large phytoplankton blooms that, when they die and are decomposed by microbes, consume a lot of oxygen and leave a bottom-wide dead zone. Phosphorus can contribute, but the extent and timing of the Gulf dead zone are most closely tied to nitrogen loading. Potassium isn’t a primary driver of algal growth in this context, and ammonia is just a form of nitrogen, whereas the element at issue is nitrogen itself.