Dead Zones in the Sea
Many cities depend on nearby rivers to carry away waste. It eventually reaches the sea, where it provides nutrients for plankton(small organisms that drift in water), causing plankton blooms(rapid and excessive increases in plankton populations). When plankton die and sink into deeper water, they decompose in a process that uses oxygen. As a result, the water beneath plankton blooms often becomes anoxic (lacking oxygen) and thus devoid of life. These”dead zones”now occur permanently or seasonally at over four hundred places worldwide, and one of the largest forms annually where the Mississippi River meets the sea. Dead zones form most readily where water is stagnant (moves little) and separated into nonmixing layers. Stagnant pools near the seabed eventually lose their oxygen as the sinking plankton build into piles. Despite their massive nutrient flows to the ocean, the Orinoco and Amazon Rivers have no dead zones, because water is pushed offshore too fast for oxygen to be lost Where the flows of once great rivers have been greatly reduced by dams, cities, and crops along their course, areas where rivers flow into seas may hold water long enough for dead zones to form. This is the case, sadly, for rivers like the Loire in France and Po in Italy, when their lack of oxygen suppresses life in thousands of square kilometers of seabed every summer.
Enclosed seas like the Baltic, Adriatic and Black also suffer problems of stagnation. Shaped like a deep bowl that is cut off from the Mediterranen by a shallow underwater ridge in the Bosporus Strait, the Black Sea was a freshwater lake when sea levels went down fairly dramatically during the last glaciation (an extremely cold period during which much of Earth’s water was locked in ice sheets). It refilled eventually, with saltwater, in a catastrophic flood seven thousand years ago, when the Mediterranean rose high enough to break through the Bosporus. Massive floods triggered tremendous migration, and the event may be recorded in the ancient tale of Gilgamesh. Today only a warm, less salty, surface layer is well oxygenated and able to support abundant life. This low-density layer sits over the cooler, saltier waters of the deep basin like a lid and has suffocated life below. Deeper than about 150 meters, the Black Sea is devoid of oxygen.
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