Oxygen dead zones

What is a dead zone?

A strange thing happens when water from the Mississippi River empties into the Gulf of Mexico. Instead of supporting a rich ecosystem, the water, laden with nutrients from upstream, creates a massive dead zone, so named for its apparent lack of life. No fish or shellfish can be found in an area that, some years, equals the size of New Jersey. It’s the second largest dead zone in the world behind the Baltic Sea, and one of hundreds—perhaps as many as a thousand—that exist along coastlines worldwide.

Dead zones occur when the water lacks oxygen. Like us, marine animals require oxygen to breathe, and when oxygen levels drop too low they can suffocate. Under most circumstances, oxygen is plentiful. At the ocean’s surface, the gas enters water from the atmosphere, then diffuses into lower layers as they are mixed via wind and wave action. Plants and macroalgae growing in shallow areas also release oxygen as they photosynthesize. Animals thrive in these areas where a rich three-dimensional habitat produces abundant shelter, food, and breathable water.

Nutrients from upstream nourish these ecosystems where rivers drain into the ocean, but an overabundance of nutrients can be harmful. On land, people apply fertilizers to agricultural fields and lawns to stimulate plants to grow. These applications are high in nitrogen and phosphorous, two elements plants need to photosynthesize. But when fertilizers are used too often or applied too heavily, or when there is heavy rain, these nutrients wash off the land where they were applied, running downhill until they eventually reach the ocean. Vast quantities of fertilizer wind up in the river and ultimately the Gulf, where it does what it’s intended to do: It stimulates photosynthesis. But here, the photosynthesizers are algae and cyanobacteria.

These single-celled organisms make the most of the influx of nutrients, dividing again and again until an algal bloom forms. When the bloom dies, it sinks to the seafloor, where bacteria and other decomposers start to break it down. The process of decomposition uses up large amounts of oxygen. The more algae decays, the more oxygen is removed from the water. When oxygen levels drop to 2 milligrams per liter of water or lower, most organisms can no longer survive. Those that can leave the area do so. Those that cannot may experience slow growth, become unable to reproduce, and eventually die.

Acropora coral were decimated after severely low-oxygen water shoaled onto a shallow-water reef in Panama over a few days in 2017. (Photo courtesy of Maggie Johnson)