The waters of Shannon's Lough Derg, so intensely blue and tranquil on a frosty day in December, are the focus of a new ecological alarm. For once it has nothing to do with pollution, but another of man's disruptive habits, the introduction of an invasive species. Unlike pollution, which can usually be cured or cleared, this invasion is here to stay.
Introductions are nothing new in the history of Ireland's rivers and lakes. In the pristine centuries after the Ice Age, the cold-water salmon, trout and char had little competition in the waterways. All of our major coarse fish species - pike, perch, tench, carp, bream, rudd - were introduced for food in the 16th and 17th centuries.
Roach arrived only in the 1960s, as live-bait brought in by English pike anglers, but has since spread in huge numbers through the western catchments and in lakes such as Derravaragh. American mink, escaping from fur farms at about the same time, introduced a new mammalian predator to which our waterways are still adapting.
The new invader enters the foodweb a lot further down, but its impact on Ireland's freshwater ecosystems - not to mention human uses of the rivers - will be considerable. The prodigious reproduction of the zebra mussel, Dreissena polymorpha, can make it a sort of living lava, smothering the beds of lakes and rivers at densities up to 100,000 per square metre. It clogs water-intake pipes, fouls boat-hulls and piers, and, by sheer numbers, introduces a major new competitor for planktonic food.
Ireland has freshwater mussels of its own, in sparse populations. None of them shares the zebra mussel's habit of attaching itself to hard surfaces by tough, byssal threads, like the marine mussels we eat. Dreissena is native to the rivers around the Black, Caspian and Aral seas and spread to western Europe with the early canal traffic of the 1700s.
By 1830 it was in England, and then, remarkably, took until the 1980s to reach the Great Lakes of America, carried in the ballastwater of ships from European freshwater ports.
It arrived in Ireland about three years ago, on the hulls of old canal barges purchased in England and brought across on trailers. The first discovery was made this summer in Lough Derg, where the mussel has already formed some dense colonies. It is also established in Limerick Docks. By last month it had turned up on boats as far north as Carrick-on-Shannon, and on others brought from the Shannon to overwinter in the Grand Canal, as far east as Lowtown, Co Kildare. The mussels were found on barges, houseboats, yachts, cruisers and dinghies.
There is no doubt that Dreissena will increase rapidly in the lower Shannon and eventually spread to navigable waters throughout Ireland: the Erne and Barrow seem certain to be next. But the spread can be slowed and limited if people who move their boats from the Shannon take the trouble to spring-clean them when they lift them out. This is not just a matter of hosing and scrubbing, but also draining the bilge water and bait buckets and any other water that could carry the planktonic freeswimming mussel larvae, called veligers.
Even to guess at the long-term ecological impact of Dreissena would need elaborate computer modelling. As a start, it provides food for other wildlife. Young fish eat the larvae, and crayfish wolf the smallest mussels, a few millimetres long. In some places in Europe, roach eat almost nothing but zebra mussels, and large populations of diving ducks have adapted to feeding on them. People, too, could eat them if they were bigger, easier to harvest and did not tend to accumulate contaminants from sewage outfalls.
The filter-feeding of zebra mussels can have a benefit in human eyes: they clear the water marvellously. Just one mussel, 25 mm long, can filter a litre of water in a day. Unfortunately, most of what it is filtering is food for other organisms - plant plankton which feeds animal plankton which feeds fish. The mussels eat the animal plankton, too. And in a crystal-clear lake, fish in their tiny larval stages may find it harder to hide from predators.
On the beds of lakes and canals, the shells of dead mussels can build up to 15 cm or more. The live mussels cling to aquatic plants, and in Lough Derg small clusters have already been found upon the native swan mussels, Anodonta, showing how the zebras could smother other life. Their waste on the bottom and the clearer water above could encourage new growths of weed and more bottom-dwelling fish and invertebrates.
All this means ecological change and a new balance between species. The arrival in Ireland of the zebra mussel will be a talkingpoint at the big international congress of limnologists - lake scientists - to be held in UCD next August.
Introduced species and conservation of biodiversity are already on the agenda, as well as polluting impacts on lakes, and the management of rivers and canals. This gathering of some 2,000 members of the grandly-named Societas Internationalis Limnologiae will be the biggest single event of its kind in Irish aquatic science (details from XXVII SIL Congress at the UCD Environmental Institute).
Meanwhile, the Marine Institute is distributing leaflets for boatowners, seeking their help in managing the zebra mussel invasion. With its two-tone zig-zag stripes the mussel is unmistakable, and two marine scientists would be glad of reports of any outside the range I have described: Dr Dan Minchin, Marine Institute Fisheries Research Centre, Abbotstown, Dublin 15, tel: 01-8210111; email: dminchin@frc.ie; and Dr T.K. McCarthy, zoology department, University of Galway, tel: 091 750379; email: tk.mccarthy@ucg.ie).
