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LOUGH CARRA, MAYO

Lough Carra is the uppermost in the Carra/Mask/Corrib system that forms a major part of the “Great Western Lakes” of Ireland and that drains a huge catchment in the two counties of Mayo and Galway.

Lough Carra is the uppermost in the Carra/Mask/Corrib system that forms a major part of the “Great Western Lakes” of Ireland and that drains a huge catchment in the two counties of Mayo and Galway. Although relatively small compared to Loughs Mask and Corrib, Lough Carra’s 1,500 hectares is the best example of a shallow, marl lake in Western Europe (marl is the calcium carbonate deposit on the lakebed). It is therefore of enormous ecological and conservation importance, and for that reason was designated as an SAC (Special Area of Conservation) under the European Habitats Directive.

There are good populations of brown trout, perch and pike and the Lough is a well-known wild brown trout fishery. Recent genetic research indicates the pike have been in the Lough for around 8,000 years and are therefore a native species.

The lakeshore holds some of the richest terrestrial habitats in the country. Situated on the carboniferous limestone that is an outlier of the Burren, the lakeshore has a mixture of limestone pavement, grassland, fen, marsh, reed swamp, scrub and woodland. These habitats harbour a fantastic variety of wildlife, including over 400 species of higher plants, 25 species of butterfly, 14 species of dragonfly and 20 of Ireland’s 26 orchid species. Irish naturalist Robert Lloyd Praeger (1865-1953) was clearly impressed with the orchid display and mentioned that he had never seen Marsh Helleborine in such profusion. The Carra lakeshore is probably the richest orchid site in Ireland. The presence of dense-flowered orchid and spring gentian demonstrate the biogeographical link to the Burren.

Over 140 bird species have been recorded at the lough and at least 83 of these have bred there. There is an autumn/winter starling roost in the reedbeds with up to 60,000 birds, but even more important is the swallow roost in August when as many as 30,000 come together each night in the reedbeds or bulrushes.

With just about all the protective legal designations possible, it would be expected that the aquatic environment for which the Lough is so important would be well looked after but, sadly, this is far from reality. Dramatic increases in conversion of semi-natural habitats into improved agricultural grassland (25% of the land area in the catchment in 30 years) with related increases in livestock numbers and the application of chemical fertiliser and slurry have resulted in the nutrient enrichment of the Lough. Between 1970 and 2003, cattle numbers rose by 42%, sheep by 136%, slurry application by 300% and chemical fertiliser use by 90%. It is not at all surprising that nutrient enrichment has become a serious problem.

The process of eutrophication is evident in many aspects of the aquatic ecology but has been masked to some extent by the very nature of the lough. The thick layer of marl on the lakebed has the ability to soak up nutrients, especially phosphates and thus act as a “buffer”, preventing the water from rapidly becoming “pea soup”. However, once the marl is saturated with phosphates, further inputs are likely to produce a more dramatic and long-lasting change in the ecosystem. The evidence collected by a team from Trinity College Dublin suggests that this point has been reached, or is at least very close, which is perhaps why they refer to it as an “ecological time-bomb”.

More recently, Charophyte expert Cilian Roden has surveyed the aquatic macrophytes and reports worrying changes, including loss of water clarity and the spread of at least one invasive alien species. His work led to the National Parks and Wildlife Service recognising that Carra’s ecological status is unfavourable and inadequate and that future prospects are bad and declining.

Lough Carra retains much of its wonderful biodiversity and ecological value, although gradual and continuing degradation of the aquatic environment and terrestrial habitats are cause for considerable concern.

Text by Chris Huxley, ecologist.

More information at:

Lough Carra website: www.loughcarra.org

Irish Times: www.irishtimes.com/news/science/time-is-running-out-for-lough-carra-1.3513993

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