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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. The Black Sea owes its stagnant depths to nature, but other enclosed seas have us to blame. Northern Europe's Baltic Sea connects to the North Sea only by shallow, tortuous channel that curls around Denmark. The rivers that feed it drain from highly populous countries and run through croplands and intensive pig farms. They receive substances flowing out of heavy industry and paper-pulp mills and sewage from cities great and small By the time the rivers reach the coast they are loaded with nutrients and organic waste. Sediment samples from one bay of the Baltic Sea tell a similar story to those from the Mississippi dead zone. Land was cleared and trees felled for agriculture and industry at increasing rates from 1800 onward, which led to an ever-growing rise in nutrient runoff. Problem from plankton blooms and anoxic bottom water emerged after the 1950s, when artificial fertilizers came into widespread use. The number of people in areas surrounding the Baltic Sea has more than doubled since then, and sewage has compounded the effects of fertilizers. The Baltic is a huge brackish water lake that occasionally takes in saltwater from the denser North Sea. When conditions are right, the North Sea pours in over the underwater ridge around Denmark, staying close to the seabed as it moves. Brackish water ( mix of salt and freshwater)is less dense and thus flows out at the same time along the surface. This density difference restricts mixing between the salty deep and fresher surface layers and so promotes a loss of oxygen when dead plankton sink to the bottom of the sea. Species that need saltwater to thrive are thus confined to the deep layers with dwindling oxygen. One reason why Baltic codfish stocks fell so catastrophically in recent years was that their eggs could not survive in these deep pockets of low-oxygen saltwater. 1.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. |
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