Plastic debris "missing"

Plastic pollution in the open ocean is widespread but less than predicted, according to a study published recently in the US journal Proceedings of the National Academy of Sciences (PNAS).

Scientists say that something is taking a percentage of the plastic out of the oceans. It is possibly being ingested by marine life and entering the marine ecosystems.

Co-authored by the Director of The University of Western Australia's Oceans Institute, Winthrop Professor Carlos Duarte, the study suggests that there is an unknown sink for small plastic particles.

Professor Duarte said the study confirmed plastic pollution was widespread in the ocean and verified the prediction, derived from models, that there were five areas of elevated accumulation in each of the five subtropical gyres (large systems of rotating ocean currents) of the ocean with plastic concentrations similar to that in the North Pacific Gyre.

Written by a team of scientists from UWA and universities in Spain and the Kingdom of Saudi Arabia, the study states that mass production of plastics emerged in the mid-1900s, after which scientists began observing accumulations of plastic waste in the world's oceans.

The concentration of plastic debris in ocean samples initially kept pace with increasing plastics production, but levelled off in the 1980s in spite of continued increases in plastics production and disposal. So where did the extra plastic go?

"Yes, animals are eating it," says oceanographer Peter Davison of the Farallon Institute for Advanced Ecosystem Research in Petaluma, California, who was not involved in the study. "That much is indisputable." 

But, Davison says, it’s hard to know at this time what the biological consequences are. Toxic ocean pollutants like DDT, PCBs, or mercury cling to the surface of plastics, causing them to "suck up all the pollutants in the water and concentrate them." When animals eat the plastic, that poison could be going into the fish and traveling up the food chain to market species like tuna or swordfish. 

Or, Davison says, toxins in the fish "may dissolve back into the water … or for all we know they’re puking [the plastic] or pooping it out, and there’s no long-term damage. We don’t know."

It’s impossible to know how much the animals are eating, says Kara Law, a physical oceanographer at the Sea Education Association in Woods Hole, Massachusetts, who was not involved in the work. The estimated amount of plastic entering the ocean that the study uses is almost half a century old, and "we’re desperately in need of a better estimate of how much plastic is entering the ocean annually."

What’s more, both Davison and Law say there are a number of other potential places the plastic could be ending up. It could be washing ashore, and a lot of it could be degrading into pieces too small to be detected. Another possibility is that organisms sticking to and growing on the plastic are dragging the junk beneath the ocean’s surface, either suspending it in the water column or sinking it all the way to the sea floor. Microbes may even be eating the stuff.

Best-case scenario for the fate of the missing plastic? It’s sinking from the weight of organisms sticking to it or in animal feces and getting buried on the ocean floor, Law says. "I don’t think we can conceive of the worst-case scenario, quite frankly. We really don’t know what this plastic is doing."

To investigate this apparent paradox, lead author Andrés Cózar from the Universidad de Cádiz and his colleagues measured plastic debris contained in ocean surface samples collected during the Spanish Circumnavigation Expedition Malaspina 2010, a nine-month expedition to assess the impact of climate change on the world's oceans.

The authors collected 3070 ocean surface samples at 141 sites around the world and found that 88 per cent of the samples contained plastic debris of varying sizes with highest plastics concentrations in the five subtropical ocean gyres.

Particle size analysis revealed a low concentration of plastic fragments smaller than one millimetre in diameter in the samples. In addition, the estimated total ocean plastic content, on the order of tens of thousands of tons, was less than previous estimates predicted.

The results suggest that plastic waste debris in the ocean is widespread, but that an unknown mechanism is removing small plastic fragments at a higher rate than larger fragments.

According to the authors, further research is needed to determine which ocean life or systems are responsible for the disappearance of the small plastic fragments and how they may be affected by exposure to or ingestion of plastic waste.

Photo: Research vessel RV Hesperides