After reading this passage from a health textbook, answer th…
Questions
After reаding this pаssаge frоm a health textbооk, answer the question that follows it.(1) Things had been looking up for the Great Lakes. The pollution-fouled waters of Lake Erie and the other Great Lakes shared by Canada and the United States had become gradually cleaner in the years following the Clean Water Act of 1970 and a binational agreement in 1972. As government regulation brought industrial discharges under control, people once again began to use the lakes for recreation, and populations of fish rebounded.(2) Then the zebra mussel arrived. Black-and-white-striped shellfish the size of a dime, zebra mussels attach to hard surfaces and feed on algae by filtering water through their gills. This mollusk is native to the Caspian Sea, Black Sea, and Azov Sea in western Asia and eastern Europe. In 1988, it was discovered in North American waters at Lake St. Clair, which connects Lake Erie with Lake Huron. It was brought to this continent by accident when ships arriving from Europe discharged ballast water containing the mussels or their larvae into the Great Lakes.(3) Within just two years of their discovery in Lake St. Clair, zebra mussels had multiplied and reached all five of the Great Lakes. The next year, these invaders entered New York’s Hudson River to the east, and the Illinois River at Chicago to the west. From the Illinois River and its canals, they soon reached the Mississippi river, giving them access to a vast watershed covering 40% of the United States. In just three more years, they spread to 19 U.S. states and two Canadian provinces. By 2010, they had colonized waters in 30 U.S. states.(4) Why all the fuss? Zebra mussels clog up water intake pipes at factories, power plants, municipal water supplies, and wastewater treatment facilities. At one Michigan power plant, workers counted 700,000 mussels per square meter of pipe surface. Great densities of these organisms can damage boat engines, degrade docks, foul fishing gear, and sink buoys that ships use for navigation. Through such impacts, zebra mussels cost Great Lakes economies an estimated $5 billion in the first decade of the invasion, and they continue to impose costs of hundreds of millions of dollars each year.—Withgott et al., Environment: The Science behind the Stories, pp. 77–78The subject of this selection is