Ireland’s Lakes

Ireland has a vast profusion of natural lakes accounting for about 2% of the country’s surface area, with close to 6,000 over 1 ha. The majority of these are found in the midlands, west and northwest of the country and there are 18 that exceed 1,000 ha. The largest are Corrib at around 17,000 ha, Derg at 11,600, Ree at 10,500 and Mask at 8,300. This plethora of stillwaters is due to a combination of high rainfall, geomorphology and the underlying geology, with most lakes forming as a result of glacial action during the last ice age. The River Shannon system contains many of Ireland’s lakes and forms the central spine of Irish inland waters. The great western lakes of Corrib, Mask, Carra, Conn and Cullin in Counties Galway and Mayo also stand out as major aquatic ecosystems.

The large number of lakes is matched by a wide variety of types. There are turloughs (seasonal lakes on limestone bedrock which disappear in summer), mountain tarns, deep lakes (Lough Mask is c. 60 metres at its deepest), shallow lakes and everything in between. The majority occur over limestone bedrock and were naturally either oligotrophic or mesotrophic with corresponding ecologies. Lakebeds vary from fine silt to large boulders or bare rock and include some which are characterised by the formation of a fine calcium carbonate deposit known as marl. Aquatic vegetation is also highly variable according to the ecological characteristics of the lake and may include fringes of common reed (Phragmites australis), submerged macrophytes such as pondweeds (Potamogeton spp.) and stoneworts (Chara spp.) and the floating-leaved water lilies.

 Many of the lakes hold important populations of fauna, including mammals (otter Lutra lutra), birds such as gulls, waders and waterfowl, fish (including rare and endangered species such as the arctic char Salvelinus alpinus and the pollan Coregonus autumnalis and invertebrates. The salmonid fish populations of many lakes is also recognised to be important at European level.

The social and economic value of Ireland’s lakes is extremely high. Firstly, many of them provide important supplies of water for domestic, industrial and agricultural purposes. Secondly, they provide a wide range of recreational and leisure opportunities and facilities (including angling, boating, swimming and birdwatching). Thirdly, they are invaluable as sources of educational material. Finally, there is also their value in the provision of less obvious ecosystem services, such as the reduction of flood severity.

However, Ireland’s lakes have come under increasing ecological pressure from a variety of threats, foremost among which is that of eutrophication (nutrient enrichment) deriving primarily from agricultural runoff, but also from domestic effluent from wastewater treatment plants, forestry practices and overgrazing of uplands. Many lakes are also polluted to varying degrees with chemicals discharged from various human activities. Abstraction of water for human use is a problem in some lakes, and is likely to become more serious as the effects of climate change modify rainfall patterns. A major and increasingly serious threat comes from the introduction of non-native species of plants and animals. Many of these exotic species cause little harm to overall ecology, but some are aggressively invasive and do pose a serious threat. These include the zebra mussel Dreissena polymorpha, the curly-leaved pondweed Lagarosiphon major, the American mink Mustela vison and several species of fish.