Mesotrophic Lakes - Habitat Action Plan 

Ref 1/H9 Habitat Action Plan 9
Plan Author: English Nature
Plan Co-ordinator: English Nature
Review complete January 2003

Action Plan Summary

Current Status

National Status

  • Mesotrophic lakes (ie those in the middle of the trophic range) are relatively infrequent in the UK and largely confined to the margins of upland areas in the north and west.

Norfolk Status

  • Within the Broads Natural Area three mesotrophic lakes remain in good condition, these being Blackfleet Broad, Martham South Broad and Upton Broad. A further eleven lakes (Brundall Gardens Lake, Calthorpe Broad, Hickling Broad, Horsey Mere, Martham North Broad, Heigham Sound, Ormesby Broad, Alderfen Broad, Rockland Broad, Strumpshaw Broad and Cockshoot Broad) usually possess clear water with luxuriant growth of several kinds of plants, but have higher nutrient levels. These lakes have the potential to be restored to a mesotrophic state.

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Current factors causing loss or decline in Norfolk

  • Enrichment by excessive nutrients (eutrophication) is the main damaging factor, sources of which include sewage effluent, and point and diffuse sources associated with agriculture. These effects can be exacerbated by water abstraction, leading to a reduction in the quality of water reaching the lake. Abstraction may also affect the residence time of water in the lake, increasing the time available for nutrient uptake by aquatic plants and algae, and so enhancing plant production. The resulting prolific plant growth is the first symptom of eutrophication.
  • At some sites water-borne traffic can damage aquatic plants; bankside erosion, weedcutting to enable navigation, passage of boats through stands of vegetation and the cutting action of propellers all contribute. Increased turbidity from boatwash may also compound water-plant loss. The suppression of water plant communities by these mechanisms may favour algal growth.
  • Ploughing of grassland and surrounding habitats, and under-drainage of agricultural land, both increase soil erosion with a consequent increase in water-borne sediment. Sediments raise the nutrient status of the lake and increase turbidity favouring algal dominance.
  • Inappropriate introductions, or modification of fish communities can also adversely affect the natural integrity of mesotrophic lakes. 

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Current Action in Norfolk

The three mesotrophic lakes remaining in the Broads are protected as Sites of Special Scientific Interest, and are also components of The Broads candidate Special Area of Conservation (cSAC) and Broadland Ramsar site. Many of the sites that could be restored to a mesotrophic condition also receive statutory protection, and several of the lakes are managed by conservation bodies. The Broads Authority undertakes extensive monitoring of aquatic plant assemblages throughout the series of broads.

Research in the Broads has established the feasibility of reversing the effects of eutrophication through programmes of nutrient reduction, allied to ‘biomanipulation’ programmes aimed at managing the populations of fish predators. These techniques can also be applied to mesotrophic lakes undergoing nutrient enrichment. In some cases, sediment removal has been necessary. Some mesotrophic lakes are subject to, or will soon be subject to, Water Level Management Plans prepared by flood defence operating authorities. Research has been commissioned to investigate the hydrology and hydrogeology of Upton Broad and Fen. The Environment Agency carries out an extensive programme of water quality monitoring in Broadland.

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Action Plan Objectives and Targets

National

  • Maintain the characteristic plant and animal communities of current mesotrophic lakes. 
  • Identify and implement effective remedial action to address nutrient-enrichment in polluted mesotrophic lakes by 2010. 

Norfolk

  • Maintain the characteristic plant and animal communities of current mesotrophic lakes. 
  • Identify and implement effective remedial action to address nutrient-enrichment in polluted mesotrophic lakes by 2010. 

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