The existence of the zone, which expands every summer before contracting again in winter months, has been known for a long time. Mapping started 22 years ago. But the area affected this year could, it is thought, hit 22,000sq km ? equal to the current record size noted in 2002.
The de-oxygenation occurs when nutrients, largely from agricultural fertilisers but also from soil erosion and sewage treatment plant discharges, enter the Gulf after seeping into rivers, most notably the Mississippi. This boosts the growth of algae, which in turn absorb the oxygen in the sea.
Researchers, working under the Louisiana Universities Marine Consortium, say that the particularly large zone is expected because a 'very high' nitrate loading occurred in the Gulf this May, a crucial period affecting the extent of de-oxygenation during the rest of the summer.
The only thing that might reduce the effect would be a particularly active period of storms, which would have the effect of mixing the sea, dispersing algae and re-introducing oxygenated water.
It has been calculated that the level of nutrients entering the Gulf of Mexico has increased threefold over the past 50 years. Factors have included more intensive farming, including the increased use of fertilisers, and an increase in unusual weather patterns, which can lead to a greater wash-off of nitrates from the land.
The dead zone effect could increase in other parts of the world too, say the researchers, where large-scale, fertiliser-rich farming takes place along the shores of major rivers leading to the sea.
|
