The reasons behind the green weed

This is a regular seasonal phenomenon, summer sea temperatures allowing the weed, generally referred to as sea lettuce, to proliferate in inshore waters.

This year, however, exceptional quantities are being deposited on the shore, to the extent that the sands are obscured and drifts of decaying vegetable matter are accumulating at the high tide mark.

Besides giving off the objectionable smell of hydrogen sulphide gas as it rots, the weed is so dense that few people are venturing onto the beach. Naturally enough, those living around the bay, beach concession proprietors, and, of course, visitors are complaining.

Efforts are being made to clear the weed – an essential process given that the poisonous gas given off by particularly dense aggregations in Brittany has caused fatalities – but this can be a thankless task. If the weed is merely moved down the shore, it simply washes up again. It has to be shifted from the beach and dumped. Again naturally enough, no one wants it their back yard.

Elsewhere, notably in Japan, the principal species, Ulva lactuca, is eaten. Here the only feasible answer to the problem of large-scale disposal must be to compost the dying and dead weed.

But that amounts to a stop-gap solution. If future algal blooms of this magnitude are to be avoided a longer term approach is vital. It has been pointed out that nutrients flowing into the sea from streams and outfalls encourage algal growth, but this issue appears not to have been addressed satisfactorily.

Although denitrification technology is used at the Belozanne treatment works it has been acknowledged that it this is far from fully effective in spite of the major investment it represents. It has also been suggested that the equipment is being overwhelmed by a high concentration of agricultural chemicals swept into the drainage system from the many fields now farmed intensively by the potato sector.

Further research may well be necessary to establish not only the cause of prolific weed growth but to identify ways of tackling the consequences. In any event, the present scale of the ‘green plague’ is such that it cannot be ignored.