Another Life: When pellucid green turns a murky shade of jade

If mayflies are key signals of the health of our lakes, Ireland’s inland waters are looking increasingly sick

The mayflies that dance above the lakes of late spring are of the insect order called Ephemeroptera: they hatch, fly, mate, lay eggs on the water and die – ephemeral lives, indeed. Their sudden emergence from the water and busy, nuptial swarmings still signal the launch of anglers eager to catch trout with imitation mayflies of their own, or “dapping” with real insects.

That, at least, was long the picture on the great trout lakes of the west. In today’s watchful world of ecology, mayflies are also key signals of the health – or sickness – of their waters. Their larvae, or nymphs, live on the bottom for up to two years, growing and moulting a score of times before surfacing as bright-winged flies and taking to the air. They are vulnerable to pollution; a recent “red list” of their welfare assessed the threat to all 33 Irish mayfly species. Six are in great danger of extinction.

An especially disturbing case of decline is found at Lough Carra, in Co Mayo, a ragged- edged, islanded lake to the north of Lough Mask and Lough Corrib. It is the largest and best of the marl lakes left on Ireland’s limestone after the Ice Age – perhaps, indeed, the best of its kind left in Europe.

It is so charged with calcium that its bottom of white marl gives a special colour to the water – "a wonderful, pale pellucid green" as described by Robert Lloyd Praeger a century ago, but tending now to murky shades of jade as summer wears on.


From a crystal-clear oligotrophic (or nutrient-poor) lake, it is now mesotrophic and perhaps on its way to eutrophic, a word all too familiar in the lexicon of water quality. Carra once had phenomenal hatches of mayfly – so prolific, indeed, that millions of dead insects washed up along the shore.

Today their population has crashed, as confirmed by biologists at Trinity College Dublin, and the air taken over by midges. For all these reasons trout numbers have declined: an average day’s catch at Carra is now, by all accounts, seldom more than a single fish.

The lake's decline is documented in Lough Carra, a striking new book by Chris and Lynda Huxley, ecologists and anglers with 15 years of painstaking research at the lake. It celebrates, first of all, an ever-changing beauty of light and mood, caught by Lynda Huxley's camera and paintings by Deirdre Walsh. Their pictures enrich a natural history with much richness to discuss. Its limestone shores teem with orchids – the Huxleys have mapped more than 26,000 specimens – and its waters still boast a flourishing lakebed growth of calcified plants, the brittle stoneworts or charophytes. "One can peer down into the clear depths," wrote Praeger, in The Way That I Went, "and see the water plants rising like slender bushes or trees from the bottom."

In shallower water the boulders are coated with krustenstein, a cyanobacterial crust that can make them look like cushions of green coral. All this special vegetation is declining as the water clouds with summer plankton, as waterweeds crowd in on the krustenstein, and filamentous algae turn the marl of the lakebed from snowy white to green. As a recent report for the National Parks and Wildlife Service agreed, “Lough Carra is under considerable ecological stress”.

It is fed from small rivers in a farmed drumlin landscape that has trebled its spread of slurry and doubled its fertiliser over the past 30 years. Carra is protected as a natural heritage area, a special protection area (for its birds) and a special area of conservation, with Inland Fisheries Ireland as an extra watchdog. It could have no greater recognition as an Irish ecological treasure, yet the Huxleys still have to plead for an overall management plan.

Their story of insidious change in water quality appears at a critical juncture, as the government Food Harvest 2020 plan sets ambitious targets for intensifying national production. "Acting smart and thinking green" is a PR slogan from on high, and all the right concerns are emphasised on paper. The water framework directive, the nitrates action programme, the policies and activities of Teagasc and the Environmental Protection Agency all promise even tighter rein on the run-off of slurry and fertiliser. Many farmers, too, have invested in safer ways and means. But how many more cows will we need to raise milk output by half? Another 300,000, says the IFA's chairman in Co Meath. Not necessarily, insist the army of agricultural and food advisers in the Agricultural Science Association: just more efficient production – and, of course, more professional advice.

Lough Carra is a long way from the dairyland of the east and south and the pollution of rivers and estuaries from Co Louth to Co Cork. But if pollution can creep up on a once crystal-clear, and now triply protected, lake in the west, “thinking green” will need to be thought a lot harder.

Lough Carra costs €25 from bookshops in Connacht, or €25 plus €7 postage from