Algae blooms and dead zones are an annual occurrence for many water bodies, and this year seems to be worse than many. A dead zone developing in the Chesapeake Bay could turn out to be the largest one ever experienced there; it covers about a third of the bay and is still expanding. Another is developing in the Gulf of Mexico, as heavy rains and snowmelt in the Mississippi River Valley are sending more polluted runoff to the gulf than usual.
Dead zones are caused mainly by nutrient pollution. Nitrogen and phosphorus in runoff from urban and agricultural areas cause excessive algae growth; as the algae dies, oxygen in the water is depleted. The lack of dissolved oxygen can be devastating to aquatic life, and the phenomenon has an economic impact as well in places where fishing or recreation and tourism are affected by the algae blooms.
China, too, is currently experiencing a huge algae bloom, covering hundreds of square miles in the Yellow Sea. The thick green algae is washing up on shore in parts of Shandong province (you can see photos here and here). Algae blooms have occurred in this area for the last several years; during the 2008 Olympics, tons of it had to be cleared so that a sailing competition could take place near Qingdao, one of the Olympic host cities.