Mountains of plastic waste afloat in the oceans

Believe it or not, nearly 269,000 tonnes of plastic comprising an estimated 5.25 trillion plastic particles are floating in the world’s oceans, including the Bay of Bengal. This estimate does not take into account the amount of plastic waste found in the shorelines, on the seabed, suspended in the water column and that consumed by marine organisms.

The plastic particles are found in three size ranges — microplastic (less than 4.75 mm), mesoplastic (4.75-200 mm) and macroplastic (above 200 mm). The microplastic particles alone are in two size classes — 0.33-1.00 mm and 1.01-4.75 mm.

Of the 269,000 tonnes of floating plastic waste, the larger plastic items are predominant (an estimated 233,000 tonnes).

The results of a study by Marcus Eriksen (the first author) from the Five Gyres Institute, California, are published today (December 11) in the journal PLOS ONE. The results are based on 24 expeditions undertaken between 2007 and 2013 across all the five sub-tropical gyres — North and South Pacific Oceans, North and South Atlantic Oceans and the Indian Ocean — coastal Australia, Bay of Bengal and the Mediterranean Sea.

With nearly 38 per cent of plastic particles and 35.8 per cent of plastic mass, the North Pacific Ocean is the most polluted ocean in the world. The two northern hemisphere oceans (North Atlantic and North Pacific Oceans) together account for 55.6 per cent of plastic particles and 56.8 per cent of plastic mass found in all oceans.

In the southern oceans, the Indian Ocean has far more plastic particles and mass than the South Atlantic and South Pacific Oceans put together.

The expeditions collected the particles using 680 net tows and also undertook visual surveys of large plastic debris.

Since the amount of plastic in shorelines, ocean bottom and suspended in the water column and consumed by marine animals was not taken into account, the authors state: “We stress that our estimates are highly conservative, and may be considered minimum estimates.”

If plastic waste discarded on land causes several problems including death of cattle due to plastic ingestion, the outcome is worse in the case of the marine organisms.

Plastic debris found in oceans has greater potential to degrade into smaller particles (through the action of light and weathering processes) and spread from the point of origin. Marine organisms and sea birds consume the small plastic particles.

According to the paper, since plastic has a greater propensity to adsorb persistent organic pollutants, organisms that consume the plastic particles end up consuming the pollutants, as well. The amount of microplastic in the oceans was estimated to be nearly 36,000 tonnes.

Compared to earlier studies, the scientists found the amount of microplastic floating in the oceans was far less. According to the authors, there is a 100-fold discrepancy in microplastic weight between this study and the earlier ones.

Missing small particles

The relatively smaller quantities of microplastic on the ocean surface reflect its loss or systematic removal. According to them, UV degradation, biodegradation by bacteria and other microorgansims, ingestion by marine organisms and settling to the ocean bottom due to increased weight may be some of the processes by which microplastic particles are removed from the sea surface.

Though the exact contribution from these removal processes is not known, one thing has become increasingly clear. Contrary to the earlier notion, many more marine organisms have been found to consume microplastic particles.

This could be either from direct ingestion of the particles or by preying on smaller organisms that have already consumed the plastic waste. This could lead to bioaccumulation of plastic in bigger marine animals and sea birds.

Unlike in the case of larger plastic waste floating in the oceans, removing the microsized ones that have settled to the ocean bottom is not feasible. This “reinforces the need for pre-consumer and post-consumer waste stream solutions to reverse this growing environmental problem,” they warn.