WHEN BP's chief Tony Hayward said in May that the Gulf oil spill was “tiny in relation to the total water volume” — a drop in the ocean — he was pilloried by Barack Obama and the US press, but he was technically correct.

In the 85 days of the leak, the worst accidental spillage in history, nearly 184 million gallons of crude oil are estimated to have gushed into the Gulf of Mexico, the ninth largest body of water in the world. That is a lot, but no more than Americans burn every five hours and 10 minutes. Indeed, in the 24 hours since BP temporarily capped the Deepwater blowout, Americans have used over 840 million gallons. The pollution from all that oil will have soiled the air, land and sea.

The US can now look back on the Deepwater spill and count itself ecologically lucky in many ways. It was not just 1.5km deep, allowing much of the oil to be diffused in the ocean, but it was 80km offshore in a warm sea.

Other spills have done far more harm because oil breaks down much more slowly in cool seas. The 11 million gallons spilt from the Exxon Valdez off Alaska in 1989 still threaten the ecosystem. That spill killed at least 36,000 sea birds. So far, Friends of the Earth US reports, only 1,387 birds, 444 sea turtles, and 53 mammals have been found dead in the Gulf.

Nonetheless, the damage yet to be revealed will be far worse than a few dead birds and tar balls along 800km of coast. Dolphins, whale sharks and sea turtles will almost certainly have been devastated and some populations may not recover for years. Fish and shrimp breeding habitats will have been hit, as may deep coral reefs which can take centuries to grow.

The ecological damage done in the last three months is made far more serious because it comes on top of years of man-made degradation. Many of the wetlands and estuaries which take the brunt of any oil spilt have already been seriously degraded by interference with river flows. These could disappear faster if the oil has got into the roots of the grasses. The oil that gushed from the Macondo well also added to natural leaks that occur all the time the US department of energy estimates that there may be 5,000 active 'seeps' in the northern Gulf alone. One researcher calculated in 2000 that 500,000 barrels of oil naturally get into the Gulf each year but are never cleaned up.

The Gulf is also heavily polluted by nitrogen and phosphorus from fertilisers and livestock waste washed down the Mississippi. Every year an oxygen-starved region known as the “dead zone” develops off the coast of Louisiana. Last month, the US government's national oceanic and atmospheric administration said it expected this year's zone to be between 17,000 sq km and 20,000 sq km, the 10th largest ever. No one knows yet how the oil spill may affect it. Some marine scientists think the oil may make it larger, but others say it could help to limit its size.

Fish will have suffered but paradoxically the US government's decision to ban commercial fishing from 230,000 sq km of the Gulf — nearly a third of the waters — during the cleanup in order to safeguard human health could help regenerate depleted fish stocks. The Gulf is one of the most overfished seas in the world.

— The Guardian, London