Toxic tide mankind's next great threat

Tourism-related litter is a major contributor to plastic waste in the ocean. Photo / Supplied

For most of us, trouble with plastic at sea tends to mean getting a plastic bag caught around a propeller or having one block a water intake.

However, a recent United Nations environmental programme report known as Yearbook for 2011 paints a gloomy picture on a rather larger scale. Describing plastics "lost" in the marine world as "the world's new toxic time-bomb", the report lists a litany of problems likely to flow from our discarded waste.

These include the reasonably well known: entangling wildlife, or being mistaken for food; and the one that has just been discovered: the fact that floating plastics accumulate and concentrate chemicals such as polychlorinated biphenols (PCBs) and the pesticide DDT (and that these then make their way in to the food chain, affecting birds, marine mammals and, of course, humans).

The problem appears to be that, while plastic doesn't biodegrade, it does something far worse: it "photo-degrades". This is a process by which the various plastics are broken down by sunlight into smaller and smaller pieces, all of which are still plastic polymers. These eventually become individual molecules of plastic: totally invisible and yet still too tough for anything to safely digest.

As with most forms of pollution, things have been getting worse over the last half a century or so as the world has become more industrialised and more countries that were once very poor become more affluent (think China, India, the economies of Southeast Asia).

As more and more of our world becomes made of plastic, more of it finds its way into the sea (according to the UN report, about 80 per cent of marine debris doesn't come from the sea at all but from land).

Major contributors are tourism-related litter at the coast (including litter left by beach goers such as food and beverage packaging, cigarettes and plastic beach toys); and sewage-related debris (garbage such as street litter, condoms and syringes discharged directly into the sea or rivers during heavy rainfall or from waste water outlets).

The other 20 per cent consists mainly of fishing-related debris (including fishing lines and nets, fishing pots and strapping bands from bait boxes that are lost accidentally by commercial fishing boats or are deliberately dumped into the ocean); and wastes from ships and boats (including garbage accidentally or deliberately dumped overboard).

In many places, all this harmful waste remains scattered and diverse. However, in our part of the world, the Pacific, the various currents conspire to attract vast amounts of this discarded plastic debris in an area known as the central Pacific gyre. Also commonly referred to as the Great Pacific Garbage Patch or the Pacific Trash Vortex, it lies in the central North Pacific Ocean, roughly between 135°-155°W and 35°-42°N.

Estimates of its size vary but, according to Curtis Ebbesmeyer, one of the world's leading flotsam experts, it is the size of a continent.

It is here that much of our knowledge on the dangers of discarded plastics originates as, as the size of the problem has become apparent, scientists have flocked to the area to study its effects. Unfortunately, they appear to be far worse than anyone first imagined. One of those who frequently ventures into the area aboard the oceanographic research vessel Alguita is Captain Charles Moore. He is one of those warning about what he calls "the darker side" of our plastic pollution.

"As these fragments float around, they accumulate the various non water-soluble poisons we manufacture," he says. "It turns out that these plastic polymers are sponges for DDT, PCBs and nonylphenols - oily toxics that don't dissolve in seawater. They can accumulate at up to one million times the level of these poisons floating in the water.

"These are not like heavy metal poisons, which affect the animal that ingests them directly. Rather, they are what might be called 'second generation' toxic waste."

Moore says this is because animals, including we humans, have evolved receptors for elaborate organic molecules called hormones. These regulate brain activity and reproduction.

"Hormone receptors cannot distinguish these toxics from the natural estrogenic hormone, estradiol and, when the pollutants dock at these receptors instead of the natural hormone, they have been shown to have a number of negative effects in everything from birds and fish to humans."

Moore, who believes the issue of hormone disruption is becoming one of the biggest environmental issues of the 21st century, points out that hormone disruption has been implicated in lower sperm counts and higher ratios of females to males in both humans and animals.

"Unchecked, this trend is a dead end for any species."

Perhaps the most disturbing issue is that problem appears virtually, if not actually, unfixable.

Moore says it would be easier to vacuum every square inch of the entire United States than to clean up the "Garbage Patch".

"The plastic patch is larger than the US and the fragments are mixed below the surface to a depth of at least 30 metres."