The World Ocean is "downhill" from all humankind's activities. Coastal seas receive a continuous flow of sewage, municipal wastes, and other pollutants—but it is here where most marine resource develoment takes place. In addition, most of the world's population lives within 50 miles of a coast. It is our "front yard"—and we are fouling our own nest.
Today, there is an increasing global concern about parts of the coastal ocean that are dying. These are the anoxic or "dead" zones, where a lack of oxygen has caused major changes in marine life. Many living things are suffocating.
These zones generally occur where major rivers discharge into the sea. The culprit is nutrient pollution, mostly nitrates, and mainly from agriculture. These fertilizers are flushed away by rain and snow melt. Concentrated in rivers, they flow into the coastal ocean. There also are two lesser sources of nitrates that find their way into the sea: airborne emissions from fossil fuel burning (such as from factories and automobiles) and improperly treated sewage discharged into waterways.
In the sea, as on land, nitrate compounds fertilize and stimulate growth. In the water column, this results in extensive blooms of well-fed algae. As the short-lived algae die, they sink into the depths, where they are disposed of by bacteria. Bacteria, however, take oxygen from the water to do the job. And they use more oxygen than the sea can replace.
Along coastlines where there are freshwater inflows, the ocean often is stratified, with little vertical mixing in the water column. Even though the surface of the ocean is 100% saturated with oxygen, it is not being moved down into the depths. So for living things in these deep regions, the bacteria's oxygen consumption can lead to eventual suffocation. Mobile organisms such as fish and shrimp can move away from the affected area with little difficulty. In contrast, fixed flora and fauna such as clams, oysters, corals, and plants, or slowly moving creatures such as crabs, are affected and can perish.
It is not a trivial problem. The U.N. Environmental Program (UNEP) says there are now 150 known dead zones around the world, a number that has doubled since 1990. Some are larger than Ireland. UNEP's number is certainly conservative, since a full global survey has not been done. Recent research has shown that the worldwide number of dead zones has doubled every ten years since 1960. In the United States, the major sites are the Chesapeake Bay and the northern Gulf of Mexico.
Some zones are permanent or episodic, but most are annual. These are associated with variations in river runoff. In many regions, rivers will have their maximum flows in the spring, as winter snow cover melts and seasonal rains come. In the fall, there is another increase in flow from rainy weather before winter sets in and before precipitation is stored on the ground in the form of snow.
The world's second largest and one of the most studied hypoxic zones is in the northern Gulf of Mexico. On average, the Mississippi River dumps 4.5 million gallons of water per second into the Gulf. This flow is from a watershed area that drains 41% of the lower 48 states, as well as several Canadian provinces, to carry more than one million tons of nitrates each year into the sea.
Prior to 1960 there was little evidence of a hypoxic zone in the Gulf. Now it is present from late winter to early fall. It changes in size according to seasonal river flow. At its maximum, it covers an area the size of New Jersey, and it is increasing.
This is not an irreversible global problem. Farmers can reduce excessive use of nitrogen-based fertilizers. Better cropland science and active government policies can help adjust fertilization application rates to what is required for crops each growing season. On-land mitigation measures can include establishment of wetlands and forested areas near rivers to intercept some of the runoff and use up its nitrate load. This has worked in the Rhine River basin, where cooperation among nations and farmers reduced nitrogen loads going into the North Sea by 37% between 1985 and 2000.
With humankind deriving 17% of its protein needs from the sea, maintaining the health of the coastal oceans is critical. If large regions of the world's coastal oceans continue to die, however, this may replace overfishing as the primary threat to the availability of this food source. For more information see UNEP's Global Environment Year Book 2003 at www.unep.org/geo/yearbook/.