Scientists have determined this year’s Gulf of Mexico “dead zone” is 8,776 square miles, an area about the size of New Jersey. This area is called a dead zone because the water is hypoxic. Hypoxic conditions arise when the dissolved oxygen levels in the water fall below 2ppm, which is too low to sustain animal life in the bottom layers of the ocean. It is the largest measured since dead zone mapping began there in 1985. It forms each spring as the Mississippi and other rivers empty into the gulf bringing nutrient rich and sediment filled waters.
In these sediments are agricultural wastes, nitrogen and phosphorus from fertilizers, from over half of the farms in the United States. In addition to agricultural waste, treated (or untreated) sewage and other urban non-point source pollution is also dumped into the river system. The nutrients stimulate the growth of phytoplankton on the surface where sunlight is available causing massive algal blooms. As the plankton die, their remains and fecal matter fall to the ocean floor where they are eaten by bacteria. These bacteria consume excessive amount of oxygen, creating hypoxic conditions. This is called eutrophication.
Hypoxic water may appear normal on the surface, but on the bottom are dead and distressed animals. Often there are layers of stinky sulfur oxidizing bacteria which cause the sediments to turn black. These conditions cause food chain alteration, loss of biodiversity and aquatic species mortality.
The dead zone dissipates when the late summer and fall storms arrive to mix-up the stratified waters, but its size grows every spring. Many people who live along the gulf depend upon the fisheries, but fishing is declining.
For many years, we have attempted to control the Mississippi River. Of course, people live along the river, so their lives need to be protected from flood disasters. If left alone, the river would naturally flood periodically and deposit sediment from up river onto the delta. This natural cycle replenishes the wetlands and marshes in the area. Instead, all that sediment, about a 120 million tons, gets dumped into the Gulf of Mexico and the marshlands recede. The currents move the sediment toward the Texas coast. Ever wonder why the ocean water in Galveston is so murky?
In addition, the petrol-chemical industry has affected the gulf in many ways. Oil spills such as the BP Horizon disaster, are an obvious problem in the gulf. But many offshore drilling sites were never properly shut down and continually leak into the ocean. Being coated in oil is not compatible to life.
The Trump administration said it would allow new offshore oil and gas drilling in nearly all United States coastal waters, giving energy companies access to leases off California for the first time in decades and opening more than a billion acres in the Arctic and along the Eastern Seaboard. I guess turning the rest of the coastal water in the US into a dead zone is not a concern of the current administration. But humans are affecting the ocean ecosystems in profound ways. Our addiction to the short-term benefits of the petrol-chemical industry (which includes herbicides and pesticides) is killing our oceans.