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
There periodically is a "dead zone", or region of low oxygen, in the lake whose exact location varies. Scientists from the National Oceanic and Atmospheric Administration have been studying the lake's blue-green algae blooms, and trying to find ways to predict when they are spreading or where they might hit landfall; typically the blooms arrive late each summer. This problem was extreme in the mid and late 1960s and the Lake Erie Wastewater Management Study (LEWMS) conducted by the Buffalo District of the US Army Corps of Engineers determined that the eutrophication was due to "point sources" such as industrial outfalls and municipal sanitary and storm sewer outfalls, as well as "diffuse sources", such as overland runoff from farm and forest land. All these sources contribute nutrients, primarily phosphorus, to the lake. Growth of organisms in the lake is then spiked to the point that oxygen levels are depleted. LEWMS made recommendations for reducing point source outflows, as well as reducing farm contributions of phosphorus by changing fertilizer usage, employing "no-till" farming and other conservative practices. Many industrial and municipal sources have since then been greatly reduced. The improved farming practices, which were voluntary, were followed for a while, resulting in remarkable recovery of the lake in the 1970s.