A brittle geometry spun in stone

What is it about coral that so teases our sense of beauty? Even without the myriad, darting lives that furnish coral reefs in films, their brittle traceries persuade us of their value to the ocean simply by being intricate and exquisite, a swirling fractal geometry spun in stone.

Ireland has no reefs of the tropical, skindiver sort. But, as we know now, the dark troughs at the fringe of the continental shelf hold spectacular mounds of deep-water coral of great ecological importance to the north-east Atlantic: they are the "cities" of undersea life. Irish scientists are part of a European project to study them, with the threats from bottom-trawling and gas-drilling urgently in mind.

Close inshore in our western bays, something else very like coral has also been under study, both for possible commercial profit and as another marine habitat we should, for the most part, leave well alone.

Maerl - a Breton word - is the limy seabed "gravel" created by a number of seaweed species that calcify their cell-walls to build small, rigid, unattached plants. Alive, these are bright pink, like their algal relatives that crust the sides of rock pools. Dead, they fade into petrified sprigs, rather like pieces of cauliflower.

To any innocent eye, these stony sprigs are "coral" (the real stuff is built into permanent reefs by tiny tube-living animals called polyps) and the sparkling "coral strands" of Mannin and Cashla bays in Connemara are made up of fragments of maerl swept ashore. Along the west coast, from Roaringwater Bay in Cork to Mulroy Bay in Donegal, are some 60 sizeable offshore banks, part of a roughly-estimated total mass of around 114 million cubic metres.

Spreading maerl as "coral sand" to lime acid farmland is a centuries-old practice: it was dredged from West Cork bays from the 1700s onwards, until the coming of ground limestone. Today, off Bere Island, Celtic Sea Minerals is licensed to pump up some 5,000 tonnes a year from a dead maerl bed, for sale as a grass-dressing rich in calcium, magnesium and trace elements. Other modern uses are in water filters and dietary supplements.

Comparison with Brittany, where yearly extraction reaches 500,000 tonnes, has often been used to show maerl as a neglected Irish resource. A series of enthusiastic entrepreneurs have sought extraction licences, notably at Kilkieran in Connemara where particularly large and pure deposits of mostly dead material are piled up into large waves in the outer reaches of the bay.

The concerns of the onshore ground-limestone industry have played a part in the story of delay, but ecologically there are reasons to be glad it has happened. As a new study for the Marine Institute* makes clear, beds of maerl host a rich diversity of animals and plants, yet they have had relatively little scientific attention.

Even the EU, which lists the main maerl seaweeds for careful management of exploitation under the Habitats and Species Directive, has failed to specify much rarer species found along Ireland's Atlantic coast (one of these, the beautiful Lithophyllum dentatum, found off Galway, is shown in my drawing).

The two commonest (but without common names) are Lithothamnion corallioides and Phymatolithon calcareum, which make up most of the maerl beds in the greater Galway Bay. The new study is meant to assess the potential for "sustainable extraction", yet it shows that the plants grow so slowly that extraction means obliteration, both of the maerl bed and the plants and animals that live within it.

Coral strands are strikingly barren, but the density and variety of larger animals burrowing into the maerl beds underwater - crabs, sea-urchins, brittle stars and so on - has been termed "spectacular". Shelter for sea urchins is specially important to Paracentrotus lividus, the purple urchin that has been so grossly overfished for the French market. Deep in the maerl, the most abundant animals are the tiny, shrimp-like amphipods, basic to the ocean food-chain.

Few of these animals are particularly special to maerl - they inhabit sea-bed gravel of all kinds. But springing up among the stiff, coralline branches are a whole range of soft, leafy seaweeds that use maerl for shelter and support. This flora, says the new study, is "extraordinarily species-rich and contributes greatly to the high algal biodiversity of the west of Ireland." A very rare and beautiful leafy seaweed, Halymenia latifolia, is found almost exclusively on maerl - one of a number of poorly-known species that thrive in this exceptional habitat.

The study maps a good many of the maerl beds from echo-sounder surveys. These concentrated on the West Cork bays and on Galway, where drifts of maerl abound in the eastern part of Galway Bay and out in the deep, crystal-clear water off the Aran Islands, at depths of up to 30 metres. Much of it is in thin patches over bedrock, but the beds in sheltered areas can be several metres deep.

Significantly, some large deposits excluded from the survey were those in Kilkieran Bay, Greatman Bay and the south-west Connemara coastline. These were left out "as it was considered exploitation was undesirable, as these are proposed Natural Heritage Areas."

Indeed, the importance of maerl beds to the natural inshore ecosystems, and the need for more and urgent research, is unmistakeable from the Marine Institute study. The research was carried out by a team led by scientists in UCC and UCG, among them Dr Michael Guiry, a particular enthusiast for sustainable seaweed exploitation.

They suggest that perhaps three million cubic metres is the real size of the exploitable maerl resource - this mostly in deposits where dead, broken maerl lies mixed with mud, as happens notably in Bantry Bay. It still represents an opportunity for development, they say, "although perhaps not at the level of French extraction". As for proposals to dredge living banks, they spell out minimum terms for site investigations and biodiversity studies.

Personally, I can see little justification for destroying maerl banks at all. They are first of all a virtually non-renewable resource to sea-life: even the dead beds are busy refuges for hundreds of species, part of a food-chain and ecosystem which includes our inshorefish.

A Study of Selected Maerl Beds in Irish Waters and their Potential for Sustainable Extraction is available from the Marine Institute, 80 Harcourt Street, Dublin 2