Dead water

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.

Good but bad

This sounds like it is good news for the climate, because marine life absorbs about 10% of man-made carbon dioxide into the ocean. The more marine life there is, the more absorption. But two-thirds of this effect may be offset by the greater release of nitrous oxide, a potent greenhouse gas, from the sea.

Dr Galloway says the aim is to maximise the effectiveness of nitrogen in food production while minimising the effects on the environment and human health. There is room for improvement. Existing technology can remove nitrogen oxides when fossil fuels burn, at a cost. Breeding or genetic modification could increase the efficiency with which animals and plants take up nitrogen. Improving animal management (with better diets and use of manure), would also help. And if only half of the sewage of the 3.2 billion people living in cities were treated, the environment would be spared about 5m tonnes of reactive nitrogen a year. Altogether, such interventions would add up to about 54m tonnes of less reactive nitrogen, about 28% of what was created in 2005.

Already cap-and-trade schemes are springing up in some American watersheds because of concern about the spread of dead zones. They work in the same way as America‘s sulphur-dioxide trading scheme and Europe‘s emissions-trading scheme for carbon. Polluters trade the right to pollute with substances such as nitrogen. Although this can be cost-effective, it is likely to work only when pollutants come from identifiable sources.

Dr Galloway’s next task is to create a nitrogen-footprint calculator on the internet, which would be similar to a carbon-footprint calculator.

Although there seems to be little prospect of any immediate and concerted action to try to restore the Gulf of Mexico’s dead zone, the north-western coastal area of the Black Sea provides an accidental example of how some places might, if given the chance, improve very quickly. After the collapse of the centrally planned economies of eastern and central Europe, the use of manufactured fertilisers declined because they were no longer affordable. Within seven years the Black Sea‘s dead zone had largely vanished and fisheries had recovered. That, at least, is cause for a little spring cheer.