NATURE The secret life of seaweed

The secret life of seaweed


Marine life
John Paul Tiernan

There doesn’t seem anything more random than the casual covering of a Mayo beach with seaweed or ‘wrack’ at any time of the year.
To the more abstract mind, it might be attractive, and tone up a stormy grey beach in April with welcome lines of olive yellow and toffee brown. Most, though, will struggle to see it as anything more than an occasional disinterested heaping of the ocean’s leftovers. Cynics might see it as a bad smelling mess drawing curses and closed windows on summer days when the wind comes out of the west.
Almost nobody, however, views it as the punctual produce of a methodical ocean. It is utterly predictable too, if you live long enough by the tide. Economic surges come and go and coastal populations ebb and flow unsteadily, but the May ‘purt’ or ‘Mayweed’ always arrives dutifully and astonishingly faithfully on the same date every year – May 1.
The May purt or ‘scotach’ in Connemara, is the old fronds of the kelp Laminaria hyperborea. In late spring, the kelp sheds these fronds, which are the ‘leaves’ of the plant. In winter, the whole plant, including its long stipe, is removed as it weakens toward the end of its life cycle and is washed up (and known from then on as sea rods).
Predictable too, is what exactly will be on the shore, depending on the part of the county you are in.
On a sheltered shore in Clew Bay or the east side of Achill Island a thin, black covering lies at the tide line; crispy to stand on when dry. This is Ascophyllum and Fucus which grow on intertidal rocks in quiet shallow bays and inlets. The Laminaria and other kelps grow in deeper waters, and the massive accumulations of Mayweed and searods will be found on an exposed shore which has a wave breaking somewhere close by.
Their former life might have expired some months ago in a forest 30 feet below the surface, but the seaweeds’ afterlife continues to unfold on the shore. Sitting futile they are not.
A quick headcount of seaweed flies on a laden shore reveals a billion or so per kilometre. Sandhoppers, which are miniature shrimp-like crustaceans, are less numerous, but in the millions. These provide for many seabirds including gulls, oystercatchers and turnstones, who don’t just turn over stones; they also turn pieces of seaweed over to find the crab or sandhopper beneath. 
Detritus from the kelp dries and blows into the nearby nutrient-poor sandy soil, aiding plant growth and keeping unstable coastal defences from offering themselves too quickly to the sea. And there is still the odd person, apologising to a billion seaweed flies undoubtedly, who gathers some May purt, beautiful or otherwise on its timely arrival, to improve a potato patch.