DEAD ZONES

“Dead zones” are deadly: Few or no organisms can survive in their oxygen-depleted, or hypoxic, waters. Often encompassing large swaths of ocean (and even lakes and ponds), dead zones become oceanic deserts, devoid of the usual aquatic biodiversity.

Though hypoxic zones can occur naturally, many more are caused by agricultural practices across the the world—a big problem for wildlife and for people. A 2008 study found more than 400 dead zones exist worldwide—anywhere excess nutrients travel downstream and into a body of water.

The largest dead zone in the world lies in the Arabian Sea, covering almost the entire 63,700-square mile Gulf of Oman. The second largest sits in the Gulf of Mexico in the United States, averaging almost 6,000 square miles in size.

Phytoplankton, microscopic algae that contain chlorophyll, require sunlight to grow, like a plant. But they float near the surface of the ocean, where they can soak up available sunlight. And they love nutrients, especially nitrogen and phosphorus. Luckily for phytoplankton in coastal areas, humans provide superfluous nitrogen and phosphorus in fertilizer form. Phytoplankton uses the supplemental nutrients to grow and reproduce—and scientists can see the huge blooms via satellite images. Phytoplankton produces oxygen during photosynthesis, so why do dead zones occur? Because of food web dynamics and decomposition.

Zooplankton, small animals that spend most of their lives drifting in water, gobble the abundant phytoplankton, and then small fish consume the zooplankton. The rapid increase in algae produces more animal waste—feces and plankton corpses.

Bacteria munch on the animal waste and dying algae that rain onto the ocean floor. The bacterial process of decomposition sucks up oxygen from the water, lowering the amount that remains available. When water approaches two parts per million or less of oxygen—considered low-oxygen conditions—anything mobile, like crabs, snails, and fish, will try to move away. This can make it harder for larger animals like marlin to find their usual prey.

At the same time, immobile organisms can die in the low-oxygen conditions. Animals that develop near low-oxygen waters can also be affected. For example, a study found female Atlantic croaker fish developed reproductive organs more similar to testes instead of ovaries when living in hypoxic conditions.

SOLUTIONS

Although nutrient run-off is the primary factor affecting the size of a dead zone, other factors like wind direction and strength influence how much oxygen reaches the bottom layers of the water column. Increased mixing between the layers from wind allows more oxygen to reach the bottom layer of the ocean and produces a smaller-than-expected dead zone.

To encourage retaining nutrients at their original source—on land. This is done through better farm management practices, like using less fertilizer and using crop covers to help anchor soil in place. In states that use extensive irrigation, recapturing nutrients from irrigation water would help reduce nutrient loading.