Can the Ocean Run Out of Oxygen? Understanding Marine Dead Zones

For most of the year, the Gulf of Mexico thrives with diverse marine life, from tiny crustaceans to massive baleen whales. However, every summer, a disaster unfolds. Around May, animals begin to flee, and those that can't swim fast enough suffocate and die in large numbers. This phenomenon transforms thousands of square kilometers along the coast into a marine dead zone, unable to sustain most aquatic life.

This annual curse isn't unique to the Gulf of Mexico; similar dead zones have emerged worldwide. To understand these lethal conditions, we must first understand how a healthy marine ecosystem functions.

The Foundation of a Healthy Marine Ecosystem

In any body of water with sufficient sunlight, plant-like organisms such as algae and cyanobacteria flourish. These organisms form the base of local food chains, and their photosynthesis provides the oxygen necessary for aquatic animals to survive. Key components include:

• Algae and Cyanobacteria: These organisms thrive in sunlight, forming the foundation of the food chain.
• Photosynthesis: This process provides the oxygen aquatic animals need to survive.
• Seaweeds and Seagrass: In shallower regions, these plants cover the ground, contributing to the ecosystem's health.

Eutrophication: A Deadly Surge

Algae growth depends on nutrients like phosphorus and nitrogen, in addition to sunlight and CO2. While these resources are typically limited, the surrounding watershed can sometimes flood coastal waters with these nutrients. For example, a large rainstorm might wash nutrient-rich sediment from a forest into a lake. This influx leads to a massive increase in algae growth known as eutrophication.

However, this surge of growth has deadly consequences. As more algae grows on the surface, it blocks sunlight to the plants below. These light-deprived plants die and decompose, using up the water's already depleted oxygen supply. Over time, this process can reduce the oxygen content to less than 2 milligrams of oxygen per liter, creating an uninhabitable dead zone.

Human Activity and the Rise of Dead Zones

While rare bodies of water rely on natural eutrophication, human activity has made it a regular and widespread occurrence. Nutrient-rich waste from sewage systems and industrial processes often ends up in lakes, estuaries, and coastal waters. The Gulf of Mexico is one of the largest dumping zones on Earth for fertilizer, a particularly harmful pollutant.

American agriculture relies heavily on nitrogen and phosphate-based fertilizers. Thirty-one states, including America's top agricultural producers, are connected to the Mississippi River Basin, and all of their runoff drains into the Gulf of Mexico. Farmers apply most of this fertilizer during the spring planting season, so the nutrient flood occurs shortly after.

The Impact on the Gulf of Mexico

In the Gulf, decomposing algae sinks into the band of cold saltwater near the seafloor. Since these dense lower waters don't mix with the warmer freshwater above, it can take four months for tropical storms to fully circulate oxygenated water back into the gulf. This dead zone currently costs U.S. seafood and tourism industries as much as $82 million a year, and that cost will only increase as the dead zone gets bigger.

On average, the gulf dead zone is roughly 15,000 square kilometers, but in 2019, it grew to over 22,000 square kilometers—approximately the size of New Jersey. Human activity is similarly responsible for growing dead zones around the world.

Solutions: Reversing the Damage

So, what can be done to combat this growing problem?

• Short-Term Solutions:
  • Countries can set tighter regulations on industrial runoff.
  • Ban the dumping of untreated sewage into ocean waters.
  • Plant buffer zones composed of trees and shrubs on farms to absorb runoff.
• Long-Term Solutions:
  • Radical changes to the way we grow food are necessary.
  • Farmers are currently incentivized to use techniques that reduce soil health and rely heavily on nitrogen-rich fertilizers.
  • Restore the soil's natural nutrients by planting diverse crops that manage soil erosion and fertility.

If we don't make these fundamental changes soon, the future of our marine ecosystems may be dead in the water.