South East England Biodiversity Forum

Mesotrophic lakes

What are they?

Mesotrophic lakes are bodies of standing water characterised by having a narrow range of nutrients and are in the middle of the trophic range (with a pH usually around or slightly below neutral). Planktonic algae sometimes discolour the water. They may be natural lakes or artificial water bodies such as gravel pits and reservoirs, but not canals or ditches.

Standing waters are usually classified according to their nutrient status. There are three main types of standing waters: oligotrophic (nutrient-poor), eutrophic (nutrient-rich) and mesotrophic (intermediate).

Other types of standing water include dystophic (highly acidic, peat-stained water), guanotrophic, marl lakes, brackish water lakes, turloughs and other temporary water bodies.

Please see the UK BAP website or the regional BAP habitat definition or the national BAP habitat definition for more information.

The situation in the South East

Extent in England  unknown
Extent in the SE region  28 sites
Percentage UK resource in the SE  unknown
Extent covered by SSSI designation  data to be added

Rate of Change


 County 1998 extent (sites)  2008 extent (sites)
 Berkshire  unknown  27
 Buckinghamshire  unknown  0
 Hampshire  unknown  
 Isle of Wight  unknown  0
 Kent  unknown  
 Oxfordshire  unknown  
 Surrey  unknown  1
 Sussex  unknown  
 Total  unknown  28
1998 data taken from 'The Biodiversity of South East England - An Audit and Assessment' published by the Wildlife Trusts of South East England and the RSPB, South East and Central Regions   2008 data taken from regional or national BAP habitat inventory. Please note that some of the changes listed here are due to improvements in mapping and habitat definition. 

  

Current threats

The main threats to this habitat are:

  • Pollution. Enrichment by excessive nutrient input (eutrophication) is the main impact. Anthropogenic nutrient inputs can include:   
  1. sewage effluent;
  2. point and diffuse sources associated with agriculture and forestry;
  3. accidental spillages (eg slurry); 
  4. fish farms in the lake and its feeder streams.  
The effects can be exacerbated by excessive water abstraction upstream, leading to a reduction in the quantity of water reaching the lake. This may affect the residence time of water in the lake, increasing the time available for nutrient uptake by aquatic macrophytes and algae, and so enhancing plant production. This is the primary symptom of eutrophication. Other sources of pollution that can have significant impacts are industrial pollution and pesticide losses. Water acidification is also a factor in some upland catchments.
  • Catchment land use. Ploughing up of grassland and surrounding habitats and underdrainage both increase the possibility of soil erosion with a consequent increase in water-borne sediments. Settled sediments may continue to introduce nutrients into the water column. Sediments in suspension cause turbidity and the resulting light attenuation may inhibit the growth of rooted aquatic plants in the spring, increasing the changes of algal dominance. Ploughing associated with afforestation can have a similar effect, as can peat-cutting on moorland catchments.
  • Fisheries management. Introductions of fish to lakes can alter the natural integrity of mesotrophic lakes in various ways: through competition, altering the native species composition if bottom-feeding fish are involved, through continual disturbance of the sediments, leading to turbidity and the mobilisation of nutrients (favouring algal blooms) through altering the structure of the food web, for example leading to increased predation of the invertebrates that graze algae.
  • Recreation. Water-borne traffic can damage aquatic plants at the point of launch, or through bankside wave erosion, passage through strands of vegetation, or the cutting action of propellers. Increased turbidity from boatwash may also compound macrophyte loss. The suppression of macrophyte communities by these mechanisms may favour algal growth.

Vision for mesotrophic lakes

The South East Biodiversity Forum’s vision for this habitat is that there should be:

  • No further loss of existing habitat
  • Good management, including visitor management, on all extant sites
  • No damage to site integrity from activities arising outside the sites, eg. inadequately managed public access
  • Re-creation of mesotrophic lakes on appropriate sites to restore some past losses, including the linking up of fragmented sites
  • Greater public appreciation of mesotrophic lakes and their specialist wildlife, including greater awareness of the impacts of human pressures, such as dog-walking, mountain-biking, dumping of waste
  • Creation of alternative green space around important mesotrophic lakes under pressure for increasing new housing, such as the Thames Basin Heaths

How we can deliver this vision