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The apparent cause of the creeping dead zones is agriculture, specifically fertilizer. While fertilizer is necessary to foster bumper agricultural crops, it also runs off the fields into the streams and rivers of a watershed. When the fertilizer reaches the ocean, it just becomes more nutrients for the phytoplankton, so they do what they do best: they grow and multiply. Which leads to more organic matter reaching the bottom, more bacterial respiration, and more anoxic bottom water.These effects can be magnified by catastrophe. When the heavy rains of Hurricane Floyd caused extensive flooding in North Carolina in September 1999, the heavy load of nutrients (from dead animals, flooded animal waste ponds, and numerous other sources) reached the sounds that lie between the coast and the Outer Banks, oxygen levels in the water plummeted. The picture at the top of the page shows the heavy load of sediments flowing into Pamlico Sound. SeaWiFS captured a remarkable image on September 23, 1999, when the sediment-laden water was carried into the Gulf Stream. In this image, note the turbidity in the sounds and the deep brown color at the river mouths. In some areas of the Neuse River, the water actually turned red.In Europe, the flow of water into and out of the Baltic Sea is naturally restricted by the islands and narrow channels around Denmark. Thus, any increase in nutrients which augments biological productivity can be a problem â€” and thatâ€™s what is being observed in the Baltic. The situation at the mouths of major rivers is similar: the area covered by anoxic bottom water appears to be increasing every year.
The explosion of jellyfish populations, scientists say, reflects a combination of severe overfishing of natural predators, like tuna, sharks, and swordfish; rising sea temperatures caused in part by global warming; and pollution that has depleted oxygen levels in coastal shallows. These problems are pronounced in the Mediterranean. . .. Left unchecked in the Mediterranean and elsewhere, these problems could make the swarms of jellyfish menacing coastlines a grim vision of seas to come. . . .Jellyfish, relatives of the sea anemone and coral that for the most part are relatively harmless, in fact are the cockroaches of the open waters, the ultimate maritime survivors that thrive in damaged environments, and that is what they are doing.