Upwelling
The major upwellings in the ocean are associated with the divergence of currents that bring deeper, colder, nutrient rich waters to the surface. There are at least five types of upwelling: coastal upwelling, large-scale wind-driven upwelling in the ocean interior, upwelling associated with eddies, topographically-associated upwelling, and broad-diffusive upwelling in the ocean interior.
Lake Erie
The clearing of the water column is also partly due to the introduction and rapid spread of zebra mussels from Europe, which had the effect of covering "the basin floor like shag carpeting" with each creature filtering "a liter of fresh water a day," helping to restore the lake to a cleaner state. The 1972 Great Lakes Water Quality Agreement also significantly reduced the dumping and runoff of phosphorus into the lake. The lake has since become clean enough to allow sunlight to infiltrate its water and produce algae and sea weed, but a dead zone persists in the central Lake Erie Basin during the late summer. The United States Environmental Protection Agency has studied this cyclic phenomenon since 2005. There have been instances of beach closings at Presque Isle off the coast of northwestern Pennsylvania because of unexplained E. Coli contaminations, possibly caused by storm water overflows after heavy downpours.
Since the 1970s environmental regulation has led to a great increase in water quality and the return of economically important fish species such as walleye and other biological life. There was substantial evidence that the new controls had substantially reduced levels of DDT in the water by 1979. Cleanup efforts were described in 1979 as a notable environmental success story, suggesting that the cumulative effect of legislation, studies, and bans had reversed the effects of pollution: