Priority Habitats | Wetland | Mesotrophic Lakes
Description of Mesotrophic Lakes
Lakes are characterised by their concentration of nutrients. Mesotrophic lakes have a medium level of nutrients compared to those of eutrophic lakes that are a bit higher.
The main nutrients that this classification is based upon are inorganic nitrogen and total phosphorus. These nutrients occur naturally or come from a number of external sources such as industry, sewage works and chemicals used on agricultural land. Phosphates are produced from animal waste products, including human sewage. Conventional waste treatments do not remove phosphates and lakes in areas with large populations can suffer from this type of pollution. Phosphates can be removed from waste but that is expensive. Nitrogen comes mainly from agricultural sources where it is used as a fertilizer.
In Northern Ireland, examples of the two largest mesotrophic areas of water are Lough Melvin and Upper Lough Erne, both of which straddle the border with the Republic of Ireland.
Historical info about Mesotrophic Lakes
Lakes can be formed by a number of natural or artificial processes. Glaciers or landslides, for example, can be responsible for the formation of natural lakes. Artificial processes such as quarrying can form man-made lakes. Lakes are a popular amenity for people too, for water sports, fishing and general recreational use. This can bring tourists into rural areas and can be beneficial to the local community. Lakes can tell us a lot about levels of environmental pollution and climate change, by measuring the sediment at the bottom of the lake. It is easy for lakes to be polluted through human usage and it is important that action is taken to preserve these valuable habitats.
Species living in and around Mesotrophic Lakes
Mesotrophic lakes supports a higher diversity of submerged aquatic plants (macrophytes) than any other standing water type. Although they support a large and diverse range of these aquatic plants, the relatively clear water of these lakes actually limits the growth of algae. Mesotrophic lakes also contain a high proportion of nationally scarce and rare aquatic plants. Characteristic aquatic plants in mesotrophic lakes include white water-lily Nymphaea alba, yellow water-lily Nuphar lutea, several pondweeds and stoneworts. Plants with a restricted distribution in the British Isles also occur in mesotrophic lakes, for example slender-leaved pondweed Potamogeton filiformis is found in Lough Melvin.
In addition to a diverse range of plant life, these areas can also contain a variety of fish. In general, fish communities in mesotrophic lakes are a mix of coarse and salmon species, but today there are natural assemblages due to mixing with introduced species. In most Northern Ireland lakes several introduced fish species have been established and become an accepted part of the biodiversity associated with these lakes.
Lough Melvin supports the Northern Ireland priority species Arctic char Salvelinus alpinus at its only known site in Northern Ireland. Other Northern Ireland species of conservation concern in Lough Melvin are three distinct races of brown trout Salmo trutta and the Atlantic salmon Salmo salar.
Other species found at mesotrophic lakes include particularly important groups, such as dragonflies, water beetles, stoneflies and mayflies. UK priority species occurring in mesotrophic lakes include otter Lutra lutra and whiteclawed crayfish Austropotamobius pallipes, globeflower Trollius europaeus, chaffweed Anagallis minima and the water beetle Hygrotus novemlineatus.
Threats to Mesotrophic Lakes
There are many pressures upon the aquatic environment resulting from industry, agriculture, forestry, mining and other human activity. These cause environmental impacts which in turn determine the quality of any particular part of the aquatic environment. One or more of the following specific threats may cause a reduction in biodiversity in lakes if not regulated and controlled. Some threats indirectly affect the water quality and quantity (of lakes) by impacting on rivers and streams that discharge into the standing bodies of water. Some of these threats can include:
Eutrophication is considered to be the biggest threat to water quality in Northern Ireland, this is when the environment becomes enriched with nutrients causing algae blooms and reduction of oxygen within the water.
Discharges from waste water treatment works or agricultural activities can also change the nutrient status of the water. For example inappropriate applications of slurry and inorganic fertilisers e.g. during adverse weather conditions, on steeply sloping land, or over-application can result in loss of phosphorus to waterways.
Fish Farms - the temperature, pH and chemical composition of fish farm effluents may differ from that of natural stream water and may contain waste and partly decomposed food, and the metabolic products of fish. This can lead to increased oxygen demand (and hence a low oxygen concentration in the water), increased suspended solids, and enrichment of the receiving waters.
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.
Dredging and the clearance of bankside vegetation as a result of flood control works can also affect downstream water quality by increasing suspended sediment loads and nutrient concentrations.
Afforestation - in the early stages of afforestation, there can be significant changes in stream and river flows that discharge into lake. Contamination of water resources with nutrients can also occur from aerial fertiliser application if not properly controlled and managed. Clear-felling also has an impact on run-off intensity and patterns.
Invasive species - deliberately or accidentally introduced species e.g. Zebra mussel Dreissena polymorpha, can potentially have a deleterious effect on the native flora and fauna of mesotrophic lakes. Introduced Zebra mussels are able to attach to and form large colonies on any submerged hard surface. They are very effective filter feeders and can virtually strip the water column of zooplankton and phytoplankton leading to improved water clarity, although this does not result in a net loss of nutrients from the system.
Mussel plankton grazing may remove food from larval fish, give sight feeding predatory fish an increased competitive edge over their prey, and shift the bulk of biological systems from pelagic to benthic systems associated with mussel beds. Mussel plankton grazing may also clarify water to the point where algal populations change and where species formerly at a disadvantage are favoured and increase macrophyte growth around lake margins including nuisance carpeting growths of attached algae.
