nitrate-rich runoff from the Mississippi River valley region, large areas of the Gulf of Mexico are undergoing eutrification. This rather stagnant body of water is being fertilized to the point that large, oxygen-free water masses are becoming a yearly phenomenon.
Eutrification occurs when nutrients allow such lavish growths of plankton that all available oxygen in the water column is used up and cannot be replenished rapidly enough by ocean-air contact to avoid the formation of deadly, oxygen-free water masses. As these stagnant and anoxic body masses move into shallow waters, they kill off all benthic, or bottom-living, invertebrates, irrespective of whether they live on or in the sediment. Many invertebrates have very slow growth rates, and hence these relatively new oxygen-free zones are radically changing the nature of the sea bottom in important ways. Natural selection must be working in the Gulf of Mexico by favoring those organisms that have best adapted for living in low-oxygen conditions, but in the long history of life on Earth no animal has ever evolved a way to live in zero oxygen.
Low-oxygen water masses have been part of the oceans since the beginning of animal life. While low water-oxygen levels were exacerbated during times when the atmosphere itself had lower oxygen content than today, they are always present and thus there is always stimulus for selection for living in lowered-oxygen conditions. Today the oceans are mixed; the heat gradient between the warm tropics and cold polar regions creates the ocean circulation systems, which are composed of both surface currents and deeper, so-called thermohaline circulation systems, where cold, salt-rich, highly oxygenated bottom water is moved through the deeper oceans beneath warmer, fresher-water masses. This movement tends to oxygenate the oceans. But during past times of warmer climates, when there was much less of a heat gradient, the oceans were stratified, just as the Black Sea is today. There was a permanent presence of an oxygenated surface region atop an essentially worldwide, anoxic ocean at depth. Much of the Mesozoic was like this, and those of us who have collected Mesozoic marine rocks can attest to how widespread the dark anoxic ocean sediments from those times are. Usually such strata are fossil-free.