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In the 1970s, the Chesapeake Bay contained one of the planet's first identified marine dead zones, where hypoxic waters were so depleted in oxygen they were unable to support life, resulting in massive fish kills. Large algae blooms, nourished by the runoff of farm and industrial waste throughout the watershed, prevent sunlight from reaching the bottom of the bay.  The resulting loss of marine vegetation has depleted the habitat for many of the bay's animal creatures. One particularly harmful algae is Pfiesteria piscicida, which can affect both fish and humans. The depletion of oysters due to overharvesting and damaged habitat has had a particularly harmful effect on the quality of the bay. The bay's oyster industry has also suffered from two diseases: MSX and Dermo. Oysters serve as natural water filters, and their decline has further reduced the water quality of the bay. Water that was once clear for metres is now so turbid that a wader may lose sight of his feet before his knees are wet.
Efforts of federal, state and local governments, working in partnership through the Chesapeake Bay Program, and the Chesapeake Bay Foundation and other nonprofit environmental groups, to restore or at least maintain the current water quality have had mixed results. One particular obstacle to cleaning up the bay is that much of the polluting substances arise far upstream in tributaries lying within states far removed from the bay itself.
It has been estimated that in pre-colonial times, oysters could filter all the water in the Bay in about 3.3 days, but in 1988 it was calculated that depletion of oyster beds had increased this time to 325 days.[5] Efforts at oyster farming is one method Marylanders are using to clean up the pollution.
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