Dead water

Too much nitrogen being washed into the sea is causing dead zones to spread alarmingly
NEW life generally flourishes in the spring, unless it is marine life in the Gulf of Mexico. Every spring the coastal waters turn into a scene of devastation and death. Known as a “dead zone”, this vast oxygen-depleted area extends along the coast between Louisiana and Texas.

Hundreds of the world's coastal regions have dead zones. They mostly occur when spring rainfall gathers on land, makes its way into streams and rivers, and eventually tumbles down to the ocean. The rivers carry with them a cargo of nutrients, in particular nitrogen, from farms in the watershed. When this nitrogen reaches the sea it causes a brief frenzy of algal growth which depletes the water of oxygen. Fish, clams, shrimp, crabs, entire mussel reefs and other bottom-dwelling animals can be wiped out.

Jane Lubchenco, a marine ecologist from Oregon State University, says this nutrient run-off from land is increasing the number, size, duration and severity of the dead zones. This is mainly because the use of fertilisers in agriculture is increasing. Sometimes the waste from animals or human sewage worsens the blight.

Nitrogen, which, makes up about 78% of the Earth's atmosphere, is an inert gas but it has more reactive forms. One of these comes from making fertilisers, using the Haber-Bosch process which converts nitrogen gas into ammonia. Although some of the fertiliser used on fields is taken up by plants and then by the animals that eat them, most of it accumulates in the soil before being washed to the coast and eventually even to the deep ocean. Another source of nitrogen pollution comes from fossil fuels, which produce nitrogen oxides when they are burnt. These oxides are released into the atmosphere and they can fall back to earth as acid rain.

The release of reactive nitrogen into the environment has a “cascade” effect, according to two papers published in the latest issue of Science. James Galloway of the University of Virginia, the lead author of one of the papers, says that every single atom of reactive nitrogen can cause a cascading sequence of events which can harm human health and ecosystems.

In the lower atmosphere the oxides of nitrogen add to an increase in ozone and small particles, which can cause respiratory ailments. The reactive nitrogen in acid rain kills insects and fish in rivers and lakes. And when it is carried to the coast it contributes to the formation of dead zones and in the creation of red tides (a kind of toxic, algal bloom that can form in the sea). It is then converted to nitrous oxide which adds to global warming.

According to Alan Townsend, of the University of Colorado, Boulder, humans are creating reactive nitrogen at a record pace, and moving it around the world as never before. People create about 190m tonnes of reactive nitrogen a year, compared with 90m-120m tonnes from natural processes, such as nitrogen-fixing bacteria and lightning strikes.

Some of this man-made nitrogen helps to grow food and biofuels, but the nitrogen uptake by plants and animals is so inefficient that only 10-15% of the reactive nitrogen created for food production actually ends up being eaten as food. The rest of the nitrogen goes into the environment. What is worrying is that the production of reactive nitrogen is set to increase according to most predictions.

Doug Capone, of the University of Southern California, says that the increased levels of reactive nitrogen are now responsible for about 3% of the biological production of new marine life in the open ocean. Because there is only a limited supply of nitrogen out in the open ocean, additional amounts of it can have a huge stimulating effect.

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