Friday, 7 February 2014

Swept clean?

As well as the coastal erosion and damage to infrastructure caused by the succession of storms and tidal surges this winter, there have been other noteworthy consequences. Anyone who has been to Welsh beaches in the last few weeks will have been struck by the huge amount of litter cast up by the sea. This isn’t just big items like plastic drums and broken fish boxes, but also huge quantities of tiny fragments of expanded polystyrene and other plastics.

In the aftermath of major oil pollution incidents years ago, work was done to model the drift of oil and other things on the sea surface. In general, figures of around 2-3% of the near surface wind speed provided a good approximation for both forecasting drift and hindcasting origins. So, while a waterlogged pallet might drift at about 3% of the wind speed, things with more windage, such as empty drinks bottles with lids, would be expected to travel rather faster. Sometimes, where the origins of objects are known, the speed of travel to reach our shores from distant places can seem quite surprising. An example of this from a few years ago was when identifiable parts of a catamaran, which broke up in a storm off Cape Finisterre in NW Spain, reached an Anglesey beach in 110 days.

Seeing the amounts of debris and particularly the fine fragments of broken up plastic on some Welsh beaches facing the SW wind or where the tidal surges have swept debris up inlets, one is reminded of the poetic musings of the Walrus and the Carpenter, as they walked along a beach before over-exploiting the local stock of oysters. Verses 4 & 5 of the poem as told to Alice in Lewis Carroll’s Through the Looking Glass were as follows:-

The Walrus and the Carpenter
Were walking close at hand
They wept like anything to see
Such quantities of sand:
‘If this were only cleared away,’
They said, ‘it would be grand?

‘If seven maids with seven mops
Swept it for half a year,
Do you suppose’ the Walrus said,
‘That they could get it clear?’
‘I doubt it' said the Carpenter,
And shed a bitter tear.

It only requires a slight substitution of junk for sand in verse 4, to express the nigh on impossibility of removing the smaller fragments of plastic even by armies of volunteers. Seaborne litter should be getting a higher profile and is now included in the EU Marine Strategy Framework Directive. Thought is currently being given to how best to develop monitoring programmes for the various components of marine litter. Actually collecting and disposing of it is quite another matter.

One consequence of the prolonged sequence of depressions swept across the Atlantic by the jet stream may have been a reduction in the “standing stock” of litter actually washing to and fro out at sea with more of it landing up on the shore. Back of envelope calculations would suggest that 40 days or so, with winds as they have been, might have swept ashore most of the litter from about 700 – 1000 nautical miles out in the ocean. Looking at the organisms growing on the litter, it was noticeable that while the lost whelk pots and other items thrown up from the near-shore seabed by the early storms had the usual range of barnacles and saddle oysters on them, the floating litter coming up then had few attached organisms. Debris floating in the open ocean for long enough usually gets colonised by things like stalked or goose barnacles (Lepas spp.). The occurrence of such organisms can give clues as to whether a proportion of the litter has come from out in the open ocean or from more local sources in and around the Irish Sea. It was interesting to see that photographs showed Lepas on the bottom of the boat on which the Mexican fisherman apparently survived, drifting across the Pacific for many months.

Lepas anatifera
Five species of goose barnacle turn up on debris washing up on Welsh shores. Of these the most familiar is Lepas anatifera. Unusually, at least in Llŷn and Anglesey, L. anatifera has been less common this winter than a smaller species, L. pectinata, which can be distinguished by the obvious radial ribbing on the shell plates. Another species, which can be found sometimes is L. hilli. This one has an obvious pale and often bright orange band where the stalk reaches the shelled part. The fourth species, L. anserifera, which seems to be rare, perhaps partly because it is less easy to identify, grows to be similar in size to L. anatifera but has ribbed shell plates. Finally there are a few records of the buoy barnacle Dosima fascicularis which starts off by attaching to small things like feathers but then goes on to secrete a float of its own.

Lepas pectinata, which can be distinguished by
the obvious radial ribbing on the shell plates
The author of this blogpost is Ivor Rees.

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