Why the world is no longer an oyster for native molluscs

The ocean never seems bigger than at this time of year. At the north-west horizon, November opens windows of greeny-blue sky that let me see all the way to Iceland. Out towards High Island, the surging swells and bullying wind ride in on the long fetch from the Americas.

One problem in thinking about the Atlantic is relating its hugeness to small, local events. The past 30 years have taught us that, rather than the vast, homogenous pond we may have imagined, the ocean is divided, as if with glass curtains, into sharply defined regions, precincts and inshore ecosystems, each with its own water-mass and patterns of life.

That miraculous BBC series Blue Planet made majestic drama of the food chain that links the invisible diatoms of marine algae, the phytoplankton, to the biggest animals on earth. But unfolding in our sheltered bays and inlets is another phytoplankton story - of plant cells whose toxic blooms can kill fish and seabed animals and poison the success of our shellfish farming.

In Killary Harbour, beside me, tonnes of mussels, clustered on ropes like glossy black grapes, have been closed off from the market since June. Bays in the south-west also suffer frequent closure, as regular monitoring and testing find toxins that shellfish filter out and concentrate, to human discomfort or worse.

All algal blooms need nourishment. Some comes from natural, cold upwellings of nutrients from the sea bed. This happens off our south-west coast, where offshore blooms are carried into bays by landward currents.

Fish farming at Dunmanway Bay, for example, has been abandoned.

Other blooms are fed by nitrogen and phosphate flowing out from polluted rivers. Some small, enclosed sea inlets are suffering the kind of eutrophication that happens in freshwater lakes, and "plumes" of over-enriched water, surging out from our estuaries, can be carried along the coast by local currents.

Little is known about the mechanisms - ecological and oceanographic - that control individual plankton species, or the conditions that result in massive toxic blooms. An international programme of research, GEOHAB, has drawn scientists from Japan to South Africa, and the modelling of nutrients and algae in Irish estuaries has been launched with detailed studies of Wexford and Cork harbours.

The problem is common to much of the developed world, and has been complicated by the "hitch-hiking" of toxic species in the ballast water of ships. One of the most dangerous arrivals in Irish waters is Karenia mikomotoi, now often seen in such dense concentrations in late summer that it colours the sea in a "red tide". It has killed shoreline species such as lugworm and sea anemones as well as farmed fish and shellfish, and recent work links it closely to the species causing the notorious red tides of Florida, which kill even large fish such as groupers and sharks.

New plankton species are still arriving - another one was found in Killary Harbour this year by Dr Cilian Roden, of NUI Galway, an expert on phytoplankton and its nutrients. He is also a challenging theorist on the causes of our coastal problems. In particular, he seizes on the gross overfishing in the 19th century which virtually extinguished the natural oyster beds in our bays and estuaries.

Some of the beds were huge carpets of shells. In the 1860s and 1870s at Murrisk, below Croagh Patrick on Mayo's Clew Bay, up to six million oysters a year were dredged by 20 boats and sold to Britain - say 600 tonnes, which, at today's values would be worth £4 million. On the east coast, where the beds stretched for 30 miles, Arklow alone was selling nearly 3,000 tonnes of oysters a year.

Today, our remaining native flat oysters, Ostreaedulis, yield a tiny amount of the overall Irish commercial harvest - a mere 10 tonnes a year, for example, from the once prolific beds at Clarinbridge in Galway Bay. The rest, about 4,000 tonnes, are intensively cultivated Pacific oysters, Crassostrea gigas. Molluscan shellfish are herbivores, grazing on plankton, filtering it from water. Left alone, an oyster may live and spawn for decades: the many-layered and sculptured shells one sometimes finds on the shore come from veteran individuals. Their larvae join the teeming zooplankton in the warm surface waters, feeding on phytoplankton to grow and multiply. Thus, the great oyster beds of the past helped to regulate whole shallow-water ecosystems.

Our rivers continue to pour out nutrients, flowing from farmland and city sewage pipes. Improvements in sewage treatment has begun to screen them out, and farmers have been urged to use fertilisers less wastefully, but the nitrogen applied to grasslands still doubled in the past two decades.

Coastal salmon farms produce their own pollution (35 kg of nitrogen and 5 kg of phosphorus per year per tonne of fish).

In some sheltered western bays, a lush growth of seaweed may help to lock up the nitrogen and phosphorus produced by finfish farms and thus become essential to a coastal "polyculture" harvesting fish and shellfish along with an annual crop of seaweed. Such integrated systems, says the Marine Institute's MichΘal ╙ CinnΘide, are a key to sustainable aquaculture.

Marine botanist Dr Cilian Roden goes a lot further. He wants to see a restoration of the once vast populations of oysters and other native bivalves - not to be fished but as a work of ecological repair. Ironically, the population explosion of the alien zebra mussel, which could rapidly clear the water of the Shannon, may offer him a helpful model for research.

Eye on Nature

In the hills north of Sliabh an Iarainn, Co Leitrim, at dawn earlier in the year, I heard a sound very like air being blown across the top of a tube. There was a little wind and I was unable to locate the sound as it appeared to move, giving the impression of being a bird. It was both eerie and peaceful.

Martin Reading, Ballinglera, Co Leitrim

You heard the drumming of the male snipe - his mating display - which is caused by the wind vibrating his tail feathers as he descends through the air from a height.

Michael Viney welcomes observations sent to him at Thallabawn, Carrowniskey PO, Westport, Co Mayo. E-mail (accompanied by a postal address): viney@anu.ie