Priority Habitats | Wetland | Marl Lakes
Description of Marl Lakes
All marl lakes are low altitude natural lakes, differentiated from other lakes by their high alkalinity. Many lie in inter-drumlin basins receiving water as run-off from the drumlin slopes and sometimes the lakes are also fed by springs and seepage zones, or by inflowing streams.
The partial pressure of carbon dioxide in water strongly influences the balance between carbonate and the insoluble bicarbonate. When carbon dioxide is removed by photosynthesis the consequence is a shift in the balance in these lakes towards carbonate, which is then precipitated out as white clayey marl. This process produces clear, calcareous and base-rich but oligotrophic waters ie waters which offer little to sustain life, in which phytoplankton productivity is very low but macrophyte (submergent or floating aquatic plants) productivity is high due to high water transparency. Marl lake water bodies are therefore characterised by very clear water and low nutrient status.
Historical info about Marl Lakes
Marl Lakes are quite small and are relatively rare in Northern Ireland with Knockballymore Lough in County Fermanagh having been described as amongst one of the best. It includes a number of very rare plant species, extensive submerged beds of stoneworts and the presence of the Atlantic Stream Crayfish, a species that is becoming increasingly rare. In 1995 it was confirmed as an Area of Special Scientific Interest or ASSI. ASSI’s are areas that have been identified as being of the highest degree of conservation value because of the flora or fauna found there, or because of its geological features. Other Marl Lakes that have been confirmed as ASSI’s are Annachullion Lough (1997), Burdautien Lough (1995), Drumacrittin Lough (1997), Kilroosky Lough (1995) and Summerhill Lough (1995) all of which are found in the County of Fermanagh. In County Armagh there is a marl lake in Tullybrick Lough, confirmed as an ASSI in 2002, this lake lies in a small inter-drumlin wetland and is mainly fed by calcareous springs.
Species living in and around Marl Lakes
The unique aquatic plant and animal communities of marl lakes will include at least some vascular plants or water plants intolerant of elevated nutrient-enriched conditions, especially to the compounds of nitrogen and phosphorus. It is these species that are the most vulnerable.
Northern Ireland marl lakes provide the habitat requirements of UK priority species including the otter Lutra lutra, white-clawed crayfish Austropotamobius pallipe (for which the habitat is particularly important) and the lesser bearded stonewort Chara curta. In addition there can be a number of rare waterbeetles found in these areas including Hydaticus seminiger, Coelostoma orbiculare, Agabus affinis and Llybius guttiger. Submerged stonewort vegetation varies from site to site, however at Drumacrittin Lough stonewort, including Bristly Stonewort Chara hispida and the regionally rare Rugged Stonewort C. rudis, can be found.
Beds of common reed Phragmites australis may fringe the open water, with saw-sedge Cladium mariscus in deeper oligotrophic water and tufted sedge Carex elata in shallower areas. Floating aquatic macrophytes may include white water-lily Nymphaea alba, yellow water-lily Nuphar lutea, pondweeds Potamogeton spp., and bladderworts Utricularia spp.. Some inter-drumlin marl lakes are regionally important for the diversity of their pondweeds including rare species like the fen pondweed Potamogeton coloratus.
Marginal habitats of marl lakes may support breeding wader and wildfowl populations, including snipe Gallinago gallinago, lapwing Vanellus vanellus, and curlew Numenius arquata.
Threats to Marl Lakes
Like any natural environment, marl lakes are susceptible to damages caused through a variety of human influences including industry, agriculture, forestry, mining and other recreational activities. Some threats have a direct effect on the quality of the water whilst others have an indirect effect, instead impacting on the quality of the rivers and streams that flow into the lakes. Some impacts such as eutrophication may be great while others of only local significance. In addition, the specific impact of each factor may vary between lake types. Some of these threats include:
- Eutrophication is considered to be the biggest threat to water quality in Northern Ireland, is it when the environment becomes enriched with nutrients and can cause algal blooms. The algal blooms then cause many problems by disrupting the ecosystem. The algae may use up all the oxygen, block sunlight required for photosynthesis and some algae even produce toxins that are harmful to other life forms.
- Agricultural activities ie fertilizers or slurry applications that are often used in farming can be applied inappropriately causing run-off into the surrounding water areas. This can result in an increase of the nutrient status of the water.
- Discharges from waste water treatment works (WWTW), industrial sites and septic tanks can be a point source of nutrient enrichment of lakes.
- Water abstraction. Sand extraction, commercial fisheries, angling, farming and horticulture, water supply, wetland conservation, water sports, bird watching, wildfowling and flood control all rely on the maintenance of an adequate water supply in some of Northern Ireland’s largest lakes. If water is low during the spring salmon run, fish cannot get into rivers to spawn. All fish movements and migrations depend on adequate flows. Low water levels can also cause important wetland habitats, such as marshes and wet woodlands, to dry out.
- Invasive species. Deliberately or accidentally introduced species like the Zebra mussel Dreissena polymorpha,that arrived in Ireland around 1994, can potentially have a harmful effect on the native flora and fauna of lakes.
- Fish introductions can significantly alter the food web in lakes which can impact on the overall ecological status of lakes. In addition, fish introductions can have specific impacts on genetic integrity of established fish populations such as native populations of brown trout Salmo trutta. The threats from introduced species are very complex.
- Large quantities of refuse, including plastics, cans, bottles and fallen animals are deposited each year all around the shores of Northern Ireland’s waterbodies and along the watercourses that discharge into them. Litter can be aesthetically unpleasant, pose a hazard to the water quality, be a source of infection and disease and a direct hazard to wildlife and livestock.
