Monday 4 August 2014

Plastic sinks

No it doesn't, says anyone who walks along the beach fringed at the tide-line by the plastic detritus that I like to rummage through. The sink in the title is a noun, however, as in a Carbon-sink - a reservoir of carbon absorbed by an ocean or a tropical rain-forest that may stop our planet from frying us all to death. The plastic along the sea-shore is, of course a teeny fraction of what is out to sea and ALL of it has been generated and jettisoned in the last 50 years.  When I was a child 50 years ago, soda came in glass bottles and one of the indulgences of Summer on the housing-estate where we lived was the Corona lorry which came through once a week selling fizzy drinks in quart bottles (about a litre).  Me, I liked ice-cream soda but ginger-beer would do nicely as well.  You had to pay a deposit for the bottle, so we exchanged the empty for a full bottle unless The Da was siphoning off the pressurized screw-cap bottles for making his own beer. Milk was also delivered to the door in glass bottles and the customer was expected to rinse out the empty bottles for collection. That business model was driven off the streets by supermarkets in the 1970s. Next time I looked, glass bottles for short shelf-life drinks had disappeared because the supermarkets found it was cheaper to let these containers go to landfill than back to the factory. Jim Average is now consuming 38kg of plastic each year and most of that is going to the dump rather than getting piously recycled.

What we also know is that a staggering amount of plastic finishes up in the North Pacific Gyre which was discovered in 1997 in the middle of the Pacific hundreds of miles from civilisation by yachtsman Charles Moore.  The way this information was presented in the media, you'd think it was possible to walk for miles across the ocean on a carpet of flotsam and jetsam. Its existence had been predicted 10 years earlier by NOAA. Well it turned out that this was one of those things that we know for sure which just ain't so.  Because a team of Spaniards went and obtained some data which (pffffff!) let some air out of that hypothesis.

The Spanish government funded Expedición Malaspina (2010-2011) which sent two ships Hespérides and Sarmiento de Gamboa off in the tracks of  Ale[ss|x|j]andro Malaspina [another Shakepelinge issues here] who went exploring round the world between 1786 and 1794. The 2010-2011 expedition generated scads of data about all sorts of maritime questions but their sexiest finding is that we cannot account for 99% of the plastic garbage that we believe to be in the oceans. It was written up and published in PNAS a month ago.  We have a pretty good (within an order of magnitude) idea of how much plastic is manufactured each year and it is widely accepted that 0.1% of it finished up getting washed out to sea or dumped there directly by large continually consuming ships like the QEII or the aircraft carrier USS Nimitz with a crew of more than 5000 all quaffing coke and sprite like they were back home in New Jersey.  That all amounts to a guesstimate of 1 million tons of long-lived crud.  But the Malaspinistas trawled through the gyres and other parts of the ocean and found levels of only 500g/km^2 which is a long way from being a light dusting let alone solid islands of matter.  Where is the missing tonnage? The Malaspinistas offer a few hypotheses:
  1. There are, as yet unclassified, bacteria that can feast on styrofoam PVC and PTFE
  2. The UV-chemical breakdown is more efficient than previously thought and all the plastic is in nano-particles (and hence very trendy!) smaller than 0.2mm which is the mesh-size on the Malaspina trawls
  3. The plastic may be incorporated in the silica and calcium carbonate shells of small marine organisms like diatoms and foraminifera which die and sink below the trawl depth.
  4. They are treated as food by mesopelagic (200m-1000m deep) fish like filter feeding lantern-fish (Myctophidae) of which there are 250 known species ranging in size from 20-300mm.

Notwithstanding the pictures of turtles and terns being strangled by plastic six-pack rings, the vast majority of the plastic litter gets broken down fairly quickly on exposure to the saltwater and direct sunlight and the process can be simulated in the lab, so that you can predict the distribution of particle size if you input a sample of soda-bottles, Crocs, oil-drums, ball-point pens, Saranwrap and hamburger traylets. The Malaspinistas showed that there is a deficiency from expectation in the size range 1-4mm which corresponds disturbingly well with the size of the plankton which are at the very bottom of the food chain and get eaten by Myctophidae. I was listening to Carlos Duarte, the last-named author on the PNAS paper, being interviewed on the wireless a few days ago and he said that, if you classify the particle data by colour, this deficiency in the 1-4mm size range is much less pronounced for blue plastic. Nobody eats blue food, so this is additional evidence for hypothesis 4.  Although not good enough evidence to be submitted to peer-review in the PNAS paper.

Everything else: seals, skuas, dolphins, whales, squid, penguins, tuna, sharks, eats the mesopelagic chaps either directly or concentrated by some intermediate predator. The current working model therefore is that 50 years worth of plastic has been broken down into particles smaller than 4mm, consumed by the mesopelagic fish in a rather indiscriminate feeding frenzy, and are are now working their way up the food chain.  There are adult whales tooling around the world today which were born before we started to foul their living quarters 50 years ago.  Again, notwithstanding the National Geographic pictures of dead seabirds full of plastic detritus and a quantitative study that found 65% of 171 birds had at least one piece of plastic in their guts [whoop whoop small sample-size alert] the impact may still lie in the future. They reckon there are 600 million tons of mesopelagic fish and they may be able to tolerate a 1% burden of plastic in their diet.

Conclusiom (for now): There may be a huge plastic sink in the large marine mammals that have managed to survive the onslaughts of Norwegian and Japanese 'scientific' whaling, and aren't dead yet from blockage of their guts by indigestible fragments of plastic.  Clearly lots of scope for critical evaluation of the assumptions, the extrapolations, the data, the conclusions of this study but me quito el sombrero to the Spanish team for going out a getting some original data.

1 comment:

  1. possibly a future justification for some more "scientific" whaling