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Mesotrophic Lakes

Mesotrophic lakes contain a narrow range of nutrients, principally phosphate and nitrate, the concentrations of which are considered to be neither high nor low. Typically mesotrophic lakes have a nutrient concentration of 0.3 - 0.65 mg/l of nitrate and 0.01 - 0.03 mg/l of phosphate.

Mesotrophic lakes potentially have the highest diversity of plants and animals of any lake type. Relative to other types of lake they contain a higher proportion of nationally scarce and rare aquatic plants. They are also important for many types of insect including dragonflies, water beetles and mayflies.

Nationally mesotrophic lakes support some rare fish species, such as vendace. However, none of these occur in the North East Region.

Current status

Mesotrophic lakes are found mainly in the margins of upland areas in the north and west. Several of the largest and most important lakes in the UK were once mesotrophic but have become eutrophic (rich in nutrients) because of pollution.

There has been little monitoring of the trophic status of water bodies within the Region, so the full status of mesotrophic lakes in the North East is unknown. The best examples of mesotrophic lakes are found within the Northumberland National Park and have collectively been notified as the Roman Wall Loughs SSSI and SAC, although these also show some signs of nutrient enrichment.

Threats

  • The main threat to mesotrophic lakes comes from eutrophication (enrichment by excessive nutrient input). This can come from a number of sources including agricultural run-off, sewage effluent and accidental spillages of slurry etc. The important plant communities of mesotrophic lakes may be damaged by relatively small inputs of nutrients.
  • Acidification and pesticide pollution may also affect mesotrophic lakes.
  • Pollution effects can be exacerbated by excessive water abstraction upstream, leading to a reduction in quality of water reaching lakes.
  • Ploughing of land for agriculture or forestry in lake catchments can increase the sediment load in water causing increased turbidity, which in turn decreases the light available to plants for photosynthesis.
  • Grazing of livestock on the water’s edge can destroy marginal vegetation.
  • The introduction of fish to lakes can alter the natural ecosystems of lakes.
  • Recreational use of lakes can increase bankside erosion, damage vegetation and disturb birds and animals.
  • Introduced species, both plant and animal, may disrupt the ecosystems of mesotrophic lakes and lead to localised extinctions. For example, mink is believed to be partly responsible for the decline of water vole populations within the Region.

Opportunities for protection and enhancement

  • Greenlee Lough is managed as a National Nature Reserve by the Northumberland National Park. Grindon Lough is a Northumberland Wildlife Trust nature reserve.
  • A national strategy for the control of eutrophication in England and Wales is being developed by the Environment Agency. This will be implemented largely through eutrophication control action plans at key sites and, more widely, through Local Environment Agency Plans (LEAPs).
  • The EU Water Framework Directive will provide a basis for the improved management of standing waters.