The oceans cover over two-thirds of our planet’s surface, making it hard to imagine we humans could do them much damage.
Unfortunately, along with ocean acidification, global warming and the Giant Pacific Trash Vortex, ocean dead zones are increasingly in the news.
Dead zones are the popular name for hypoxic regions, areas in an ocean or large lake depleted of oxygen. When oxygen levels drop too low, fish or other animals that can move fast enough flee the area. Crabs, starfish, clams and other bottom dwellers die off. Over 400 dead zones have been found around the world, with that number expected to double over the next 10 years.
Normally, ocean surface water is warmed by the sun and contains phytoplankton. These tiny organisms produce oxygen during photosynthesis, necessary for fish and other life to survive. As plankton die, they sink to the bottom where dense colder water collects. There, bacteria break down the dead material, producing rich nutrients but consuming oxygen in the process.
Mixing of surface and deep waters by currents and other forces keeps the most habitable zones of the oceans rich in both oxygen and nutrients.
Dead zones, although they can occur naturally, most often appear near coastal areas with high populations and heavy development. Excess nutrients added to surface waters from run-off of agricultural fertilizers, human waste, and other industrial pollutants leads to large blooms, or overgrowth, of surface plankton.
When these algae blooms die off and sink, the huge amounts of decomposing material rapidly remove oxygen from the water and overwhelm an ocean’s natural mixing cycles.
One of the largest dead zones in the world occurs every summer in the northern Gulf of Mexico, tied to spring run-off from the Mississippi River.
Byproducts from farming, industry and cities pour into the Gulf of Mexico each spring, leading to huge algae blooms and oxygen depletion over an area of several thousand square miles.
Closer to home, large numbers of dying bottom fish and crabs in 2002 led to the discovery of a dead zone off the central Oregon coast, which has reappeared every year since. In 2006, at its largest, the involved area covered nearly 1,200 square miles.
The causes of Oregon’s dead zone are still under investigation, since no heavy run-off, extensive agriculture or other common culprits can explain the situation. Most likely, a combination of shifting natural cycles and some element of human impact holds the answer.
Every summer along the Oregon coast, strong north winds combine with the Earth’s rotation to push warmer surface water further offshore. In its place, deep cold water off the continental shelf gets pulled up and in closer to shore in a process called upwelling. This cold water is rich in nutrients, but lower in oxygen.
Historically, these north winds intermittently switch to the south, pushing warm oxygen-rich water back toward shore again to mix with the upwelling cold water. The result is both abundant nutrients and plenty of oxygen, nourishing sea life and contributing to the rich seafood industry off our coast. But without this mixing, the cold water fuels algae blooms which only worsen the low oxygen levels.
Why the longstanding pattern of wind and upwelling has changed now isn’t clear. Periods of south winds have been shorter and weaker in recent years, limiting offshore mixing.
This lack of mixing could also come from changing ocean currents. In addition, researchers have measured a steady drop each year in oxygen levels of the deep colder waters off the West Coast, which almost certainly compounds the problem.
Probably all these factors and others yet to be discovered play a role. Sadly, all could also be connected to global warming. Melting polar ice and ocean heating can affect currents. Warmer surface water hinders mixing with deeper water and leads to lower deep oxygen levels. Higher temperatures inland promote stronger and steadier north winds off the coast.
Scientific study continues on Oregon’s dead zone, and I’m optimistic that researchers will eventually discover what’s behind the phenomenon. What I’m afraid of is that the answer will turn out to be related to climate change, and be one more indication of how far-reaching our actions can be.
Lifelong Oregonian Fred Schubert, a The Dalles biologist, has a lifelong interest in general science and science writing. Feel free to submit any comments on this article or suggestions for new topics to fcscience @qnect.net.