The Summer Dead Zone at a River's Mouth
Where a large river meets the sea, the water often looks muddy and full of life. In some places, though, a strange thing happens each summer: in the lower layers of the water, oxygen drops so low that most fish and crabs cannot survive there, an area that scientists call a dead zone, and its story begins far upstream.
Rain washes fertilizer from farm fields and lawns into streams, and wastewater from towns adds more nutrients before the streams join the river, which carries everything toward the coast. The nutrients, mostly nitrogen and phosphorus, act as food for tiny floating algae. In the warm, sunny months, the algae multiply into enormous blooms.
Algae live for a short time. When they die, they sink toward the bottom, where bacteria feed on them. These bacteria use up oxygen as they work. At the same time, the fresh water from the river lies on top of the heavier salt water, like a lid, and warm weather strengthens the layering. Little oxygen from the air can reach the bottom layer. As the bacteria keep breathing, the oxygen runs out.
Animals that can swim, such as many fish, usually escape to better water. Creatures that cannot move quickly, such as clams and worms, may not. Because the cause is spread across a huge watershed, solutions must be too. Farmers can plant cover crops and leave strips of plants beside streams to catch extra nutrients, and towns can improve how they treat wastewater. In places where such steps have been taken, the problem has sometimes eased.