MESOTROPHIC LAKES (UK BAP PRIORITY HABITAT)

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1 MESOTROPHIC LAKES (UK BAP PRIORITY HABITAT) Summary Mesotrophic lakes are standing water bodies with a surface area greater than 1 ha, which have moderate levels of alkalinity and nutrients. Although they may look similar to other standing water types, this is an unusual loch type in Britain. The sites have clear water and support a wide range of species. Mesotrophic lakes occur relatively infrequently, but this category includes several important sites which support rare species, such as slender naiad Najas flexilis. These sites are also often good fishing lochs. What is it? The Mesotrophic lakes priority habitat is characterised by areas of standing water that have moderate alkalinity and nutrient levels, which result in a high diversity of aquatic plant and macroinvertebrate species. Concentrations of total phosphorus (TP) in the water column are expected to be between µg P L -1, in accordance with the trophic classification system of OECD (1982). Sites occur in a variety of sizes and include a range of substrates. They may support species typical of both oligotrophic (nutrient-poor) and eutrophic (nutrient-rich) conditions, or a flora that is typically mesotrophic. A high diversity of species may result in either case. Good examples are characterised by the presence of high numbers of pondweed Potamogeton spp. Sites may also contain acid-loving and base-loving examples of the same genus for example alternate flowered water milfoil Myriophyllum alterniflorum, which is usually associated with acid waters, and spiked water milfoil M. spicatum, which is associated with base-rich sites.

2 Macroinvertebrates are well represented, with particularly important groups being dragonflies, water beetles, stoneflies and mayflies. In general, fish communities in Mesotrophic lakes may include coarse and salmonid species, but today there are few truly natural assemblages due to competition/predation from introduced fish species. How do I recognise it? Differentiation from other Priority Habitats The most similar priority habitats to the Mesotrophic lakes are Ponds, Oligotrophic and dystrophic lakes and Eutrophic standing waters. There can also be some similarities with the Fen, marsh and swamp broad habitat. The distinction between the Mesotrophic lakes priority habitat and the Ponds priority habitat is based on size and the quality of habitats and species present, although there is some overlap between the two categories. Lake priority habitats are defined as having a surface area of 1 ha or more, whilst the Ponds priority habitat covers water bodies less than 2 ha in size, which meet at least one of five criteria relating to the presence of habitats or species of high conservation value. Sites that are 1 2 ha in size and are mesotrophic in character, but which do not meet the quality criteria for Ponds priority habitat, may be considered as Mesotrophic lakes priority habitat. The Mesotrophic lakes priority habitat is distinguished from other lake priority habitats on the basis of alkalinity, nutrient levels and plant species assemblages. Species characteristic of mesotrophic lakes include pondweeds Potamogeton spp., especially broad-leaved pondweed species, but also Shetland pondweed Potamogeton rutilus. As Mesotrophic lakes sit in the middle of the nutrient spectrum between oligotrophic and eutrophic sites, there is some overlap in the species assemblages found in mesotrophic water bodies and those present in oligotrophic and eutrophic sites. In addition, both mesotrophic lakes and naturally eutrophic standing waters are associated with broad-leaved pondweeds. However, the European Protected Species, slender naiad Najas flexilis, is strongly associated with the Mesotrophic lakes priority habitat. The distinction between the Fen, marsh and swamp broad habitat and Mesotrophic lakes is based on the extent of emergent vegetation present. Any stand of marginal emergent vegetation that is greater than 5 m wide, or areas of wetland habitat adjacent to the water body that are greater than 0.25 ha, are included in the Fen, marsh and swamp broad habitat type and so the corresponding priority habitat, i.e. Lowland fens, Reedbeds or Upland flushes, fens and swamps. Areas of wet woodland greater than 0.25 ha are included in the Broadleaved, mixed and yew woodland broad habitat type and the Wet woodland priority habitat, unless the cover of the canopy is less than 30% (Jackson, 2000). Definition in relation to other habitat classifications Classification scheme Habitat types belonging to this UKBAP priority habitat JNCC Standing Water types Types 4 and 5 of Palmer (1989) classification. Groups D & E of the revised classification (Duigan et al 2006). NVC Aquatic Communities: A2, A7, A9 10, A13, A15-16, & A19 20 NVC Swamps and tall-herb fens S2 4, S8 12, S18, S19, & S22. Phase 1 G1.2 Standing Water - Mesotrophic

3 UKBAP Broad Habitat All examples of this priority habitat are associated with the broad habitat - Standing waters and canals Definition in relation to legislative classifications Classification scheme Annex I Scottish Biodiversity List SSSIs Habitat types belonging to this UKBAP priority habitat Mesotrophic lakes do not directly tally with a single category of the EU Annex 1 habitats but some sites are represented within: H3130: Oligotrophic to mesotrophic standing waters with vegetation of the Littorelletea uniflorae and/or of the Isoëto-Nanojuncetea (part). H3140: hard oligo-mesotrophic waters with benthic vegetation of Chara spp (part). Whilst there is no clear link between SBL types and this priority habitat type, communities which may be present include: A2, A7, A8, A9, A10, A13, A16, A19, A20, S2, and S4. Includes examples of oligo-mesotrophic (part), mesotrophic, meso-eutrophic (part), loch trophic range (part); and machair (part) Where is it? Mesotrophic lakes are often located in lowland areas, within agricultural land and/or close to centres of population. They are associated with catchments that comprise a mixture of base-rich and more acid rocks, or with lowland areas with rocks of moderate base-richness, such as old red sandstone. It is a relatively rare standing water type. In Scotland examples are concentrated in the central lowlands, the east lowlands and in the Northern and Western Isles. What is special about it? Species of special conservation status recorded in this priority habitat in Scotland are listed below. Species not native in Scotland have been excluded from the Table. UK BAP priority list EC Habitats Directive Annex II*, IV* or V Birds Directive Schedule I or II Scottish Biodiversity List Red Data List Wildlife & Countryside Act (1981) Group Common name Latin name beetle zircon reed beetle Donacia aquatica y y y bird greater scaup Aythya marila y y y y Sch1 bird reed bunting Emberiza schoeniclus y y bird red-necked y Phalaropus lobatus y phalarope y y Sch1 fern pillw ort Pilularia globulifera y y y Sch8 fish European eel Anguilla anguilla y y y fish vendace Coregonus albula y y Sch5 fish w hitefish (powan) Coregonus lavaretus y y y y Sch5 fish Atlantic salmon Salmo salar y y y fish brow n trout Salmo trutta y fish Arctic charr Salvelinus alpinus y flow ering slender naiad Najas flexilis y y* y Sch8

4 plant flow ering plant flow ering plant liverw ort Shetland pondw eed Potamogeton rutilus y y Scottish dock Rumex aquaticus y y y pitted frillw ort Fossombronia toveolata channelled liverw ort Riccia canaliculata y y y crystalwort mammal w ater vole Arvicola terrestris y y Sch5 mammal otter Lutra lutra y y y y mammal noctule Nyctalus noctula y y* y Sch6 mammal soprano pipistrelle Pipistrellus pygmaeus y y* y moss round-leaved bryum Bryum cyclophyllum y y stonew ort baltic stonew ort Chara baltica y y y stonew ort bearded stonew ort Chara canescens y y y Sch8 Compared to other lake priority habitats Mesotrophic lakes contain a higher proportion of nationally scarce and rare aquatic plants (Maddock, 2008). They are also important for fish species, notably powan Coregonus lavaretus, which occurs at two Scottish sites that are considered to be oligo-mesotrophic (Loch Eck & Loch Lomond) (Maitland, 1994). How do we manage it? Pressures most commonly occurring with respect to all lake types are nutrient enrichment, alteration of hydrological regime and introduction of invasive non-native species. Management for conservation is generally focused on the land within the catchment area and use of the water resource, and may include the following. y Maintenance of a natural hydrological regime Plants and animals within lochs are adapted to fairly stable conditions, with some fluctuation in water levels caused by high rainfall or periods of drought. Mesotrophic lakes are often located close to settlements and may be used for potable water supply. Good management aims to ensure that the scale and frequency of water level fluctuations is consistent with the natural pattern. Maintenance of high quality water Mesotrophic lakes are characterised by moderate nutrient levels and are vulnerable to nutrient enrichment, which may lead to eutrophication. Increased nutrient loadings may originate from diffuse sources, such as run-off from agricultural or forestry land, or occur as a result of direct inputs (for example sewage or industrial discharges). These artificial inputs, which are usually rich in P and nitrogen, encourage the growth of algae, which reduces the light available in the water column for bottom-rooted plants. Maintenance of native species assemblages There are a number of invasive non-native species of plant and animal which outcompete native species. Examples of invasive plant species include Canadian waterweed Elodea canadensis, Nuttall s waterweed Elodea nuttallii and New Zealand pygmyweed Crassula helmsii. Canadian waterweed and Nuttall s waterweed have colonised a number of mesotrophic sites in Scotland. Nutrient enrichment may exacerbate the effect of introduced plant species.

5 Introduced fish species are a threat to the native fish populations in mesotrophic lakes. American signal crayfish Pacifastacus leniusculus is an invasive, nonnative species that would be a threat to any freshwater environment to which it might be introduced. Management should aim to ensure that invasive non-native species are not spread within or between sites, and to contain or eradicate them where possible. References, Links and Further Information Duigan, C., Kovach, W. & Palmer, M Vegetation communities of British lakes: a revised classification. Peterborough, Joint Nature Conservation Committee. Duigan, C., Kovach, W. & Palmer, M Vegetation communities of British lakes: a revised classification scheme for conservation. Aquatic Conservation: Marine and Freshwater Ecosystems, 17: Duigan, C., Kovach, W. & Palmer, M Aquatic macrophyte classification, distribution and traits in British lakes. Verh. Internat. Verein. Limnol, 30: European Commission DG Environment Interpretation manual of European Union habitats version EUR25. European Commission DG Environment, Brussels. Jackson D.L Guidance on the interpretation of the Biodiversity Broad Habitat Classification (terrestrial and freshwater types): Definitions and the relationship with other classifications. Report 307. JNCC. JNCC & updates. Guidelines for the selection of biological SSSIs. JNCC. Maddock, A. (Ed) UK Biodiversity Action Plan Priority Habitat Descriptions Mesotrophic Lakes MesotrophicLakes.pdf Maitland, P. S Fish. Chapter In The Fresh Waters of Scotland. Wiley. OECD Eutrophication of Waters, Monitoring Assessment and Control, Organisation for Economic Co-operation and Development, Paris. Palmer, M. A A botanical classification of standing waters in Great Britain. Research & Survey in nature Conservation No. 19. NCC Peterborough Palmer, M A botanical classification of standing waters in Great Britain and a method for the use of macrophyte flora in assessing changes in water quality. Peterborough, Joint Nature Conservation Committee. (Research and Survey in Nature Conservation, No. 19.) Palmer, M.A., Bell, S.A. & Butterfield, I A botanical classification of standing waters in Britain: applications for conservation and monitoring. Aquatic Conservation: Marine and Freshwater Ecosystems, 2: Scott Wilson Control of invasive non-native species in priority mesotrophic lochs. Final Report. R80014PUR. Scottish Environment Protection Agency, Stirling