Dead Zone

The term “Dead Zone” was first applied to the Gulf of Mexico when scientists discovered that vast areas of open water south and west of the Mississippi delta contained no dissolved oxygen No animals could exist in the water or on the bottom. The Gulf of Mexico “Dead Zone” has continued to enlarge since its discovery and now occupies an area the size of the state of Delaware. The reason for such an immense volume of water being unable to support animal life was quickly traced to nitrate and phosphate from the Mississippi River. The Mississippi River drains most of North America, including the rich agricultural lands of the mid-west. The source of most of the nitrate and phosphate is agricultural fertilizer, augmented by the discharge from wastewater treatment plants. Fertilizer is leached from the fields by rain, enters the groundwater, and discharges into the rivers. When the Mississippi River empties into the salt water of the Gulf, the fertilizer triggers the rapid growth of tiny suspended marine algae. Because there are not enough animals to eat all the algae, dead cells accumulate on the sea floor and rot, consuming oxygen from the water. In summer, when the water is warm and can only dissolve small amounts of oxygen, and when microbial decomposition of the dead algae (rotting) is rapid, the water can lose all dissolved oxygen, killing animals and worsening the problem because the animal carcasses also rot and consume oxygen from the water.

Chesapeake Bay has always had a “Dead Zone” in summer, in a small volume of water deeper than about 100 feet. But the “Dead Zone” is steadily expanding so that now, in the middle of summer, water deeper than about 30 or 40 feet contains so little oxygen it cannot support life near Smith Point. Water shallower than about 40 feet contains sufficiently low levels of oxygen that animal life is stressed (see the “Fact Sheet” entitled “Chesapeake Bay’s Dead Zone” at www.cbf.org.) Conditions worsen to the north, in Maryland, and they improve toward the Bay mouth to the south. The vast volumes of water in the open Bay that are now periodically “Dead” cannot sustain fish in the water or small organisms such as oysters, worms, crabs, clams etc. that normally live in or on the bottom. “Fish kills” are becoming more and more common in the small creeks and rivers that empty into the open Bay as nitrate and phosphate from local agricultural and homeowner practices create “Dead Zones” in shallow water. The reason there are no longer any oysters on the creek and river bottoms is probably as much due to oysters being killed by low dissolved oxygen concentrations in the summer as it is to the diseases MSX and Dermo.

Unless we begin to actually do something about this problem, water quality will continue to worsen. The Bay Act has been in place since 1989 and yet the health of the Bay, as measured by the size of the “Dead Zone” or the area occupied by Submerged Aquatic Vegetation (SAV), has not significantly improved. 2003 was one of the worst years for recreational fishing in memory. Increasingly bad fishing (and crabbing) is not too surprising when you realize that each decade there is less oxygenated Bay bottom to produce food or provide habitat, and less Bay water with enough oxygen for the fish and crabs to occupy. Bluefish, primarily a deep-water oceanic fish, no longer enter the Bay in the numbers once recorded, perhaps because the water is just too “sour” for them. The paucity of grass beds (SAV) is undoubtedly one reason trout have become so scarce. “The Cell” is about as far north as a fish can swim without encountering “bad” water at depths of about 40 feet or deeper in the summer.

We know what needs to be done regionally to reverse the decline in the health of the Bay and what needs to be done locally to reverse the decline in the health of our local waterways. First, we must reduce the amounts of nitrate and phosphate that enter the groundwater. Homeowners should minimize fertilization and properly maintain their septic systems by not overloading the system with either water or solids and having the septic tank pumped out if necessary. Farmers should use Best Management Practices (BMPs), especially nutrient management plans for both nitrogen and phosphorous.

The second action we need to take is to establish 100 foot-wide buffer strips containing large trees adjacent to all waterways, bordered by marsh grasses if possible. The deep roots of trees, especially trees that can tolerate saturated soils part of the year, remove nitrate and phosphate from the groundwater and promote microbial reactions that convert the nitrate into harmless nitrogen gas. Marshes perform a similar function. Runoff is a vastly less important process of pollution than is groundwater discharge. A buffer strip is the only way we know to cost-effectively remove the nitrate and phosphate once it enters the groundwater. Even if the amount of nitrate and phosphate entering the groundwater today is reduced, it will take more than a decade to flush out the nitrate and phosphate already present in today’s groundwater.

The third action that needs to be taken regionally is to upgrade sewage treatment plants throughout the watershed so that most of the nitrate and phosphate is removed before the water is discharged into waterways and the Bay. Unless we all do all these things it is absolutely certain the “Dead Zone” will continue to expand, SAV will not proliferate, and commercial and recreational fishing will continue to decline.