OLIGOTROPHIC AND DYSTROPHIC LAKES (UK BAP PRIORITY HABITAT)
Summary
This priority habitat includes lochs, which are larger than 1 ha in surface area and which have low levels of alkalinity and nutrients, and low productivity. This habitat type is associated with areas of hard, acid geology, and areas of peat and poor soils, such as occur in upland areas and the Highlands and Islands. Although lochs of this type may be sparsely vegetated, they are important for their associated plant and animal species, which are restricted to low nutrient environments.
This priority habitat contains the greatest number of examples of the Standing waters and canals broad habitat category and includes some of Scotland’s largest water bodies, such as Loch Ness and Loch Morar, both of which are oligotrophic water bodies.
There is some overlap between this priority habitat and nutrient-poor examples of Ponds; sites that are less than 2 ha in surface area may be considered as examples of the Ponds priority habitat, if they meet the required quality criteria.
What is it?
The Oligotrophic and dystrophic lakes priority habitat occurs throughout Scotland and includes thousands of sparsely-vegetated lochs on acid, generally impermeable geology. It is characterised by water with acid to neutral pH, low levels of alkalinity and low concentrations of easily available nutrients. Oligotrophic lakes have water column total phosphorus (TP) levels of less than 10 µg P L-1 (OECD, 1982). Dystrophic standing waters may have higher TP levels, but P is present in a form that is not readily available to plants.
Oligotrophic sites are more variable than dystrophic standing waters. They range in size from 1 ha up to several hundred hectares in size. Some of Scotland’s largest lochs are examples of this habitat, for example, Loch Morar. They generally have coarse substrates, but large sites may have sheltered bays with soft substrates, as well as rocky, wave-washed shores. A greater range of species may be found in oligotrophic lochs than in dystrophic sites, but overall biomass remains fairly low. There may be extensive stands of sedges in shallow, sheltered bays (typically bottle sedge Carex rostrata). Small, rosette species are often found along rocky shores, including shoreweed Littorella uniflora and water lobelia Lobelia dortmanna. Water colour may be clear, or peat-stained, though not to the intensity of water colour found in dystrophic water bodies.
Dystrophic sites are generally small (with surface areas of between 1 and 5 ha), have peat- stained water and may also have peat substrates. They generally show little habitat variation and their species diversity is very low, typically limited to one or two species, such as bog mosses Sphagnum spp., the submerged form of bulbous rush Juncus bulbosus, or bog pondweed Potamogeton polygonifolius. Sites located in peat may support ‘schwingmoor’, where bog mosses and common cotton grass Eriophorum angustifolium are found together forming a thin mat over the water surface. Marginal and fringing vegetation may be present, but shallow water areas with emergent species may be absent from dystrophic sites.
Owing to the particularly large number of water bodies categorised as oligotrophic and dystrophic, the Oligotrophic and dystrophic priority habitat comprises only important sites in good condition that will be maintained, important sites that are degraded, but suitable for remediation, and lakes for which there are little or no data on importance. Importance is assessed with regard to conservation status (features of SSSIs and SACs) and presence of rare or protected species.
How do I recognise it?
Differentiation from other Priority Habitats
The most similar priority habitats to Oligotrophic and dystrophic lakes are Ponds, Mesotrophic lakes, Eutrophic standing waters and Blanket bog. There can also be some similarities with the Fen, marsh and swamp broad habitat.
The distinction between the Oligotrophic and dystrophic lakes priority habitat and the Ponds priority habitat is based on size and the quality of habitats and species present, but there is overlap between the two categories. Lake priority habitats are defined as having a surface area of 1 ha or more, whilst the Ponds 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 may be considered as Ponds priority habitat only if they meet the quality criteria for that habitat type. Sites classed as important may fall within the Oligotrophic and dystrophic lakes priority habitat.
Oligotrophic and dystrophic lakes priority habitat is distinguished from other lake priority habitats on the basis of alkalinity, nutrient levels and plant species assemblages. Species indicative of nutrient-poor conditions include shoreweed Littorella uniflora, quillwort Isoetes lacustris, water lobelia Lobelia dortmanna, awlwort Subularia aquatica, bladderworts Utricularia spp. and bog pondweed Potamogeton polygonifolius. Dystrophic lakes have a limited flora that typically includes submerged bog mosses Sphagnum spp., and the submerged form of bulbous rush Juncus bulbosus.
The Blanket bog priority habitat is often characterised by numerous water bodies, each less than 2 ha in size. These may be devoid of aquatic plants, or contain sedges, submerged Sphagnum spp. and/or the submerged form of bulbous rush. Whilst these pools are dystrophic in character they are considered an integral part of the Blanket bog priority habitat and would not be considered in the Oligotrophic and dystrophic lakes habitats definition.
The distinction between the Fen, marsh and swamp broad habitat and the Oligotrophic and dystrophic lakes habitat 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 the relevant 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 1, 2 and 3 of the classification of Palmer (1989) & Groups A, B, C1 & C2 of the revised classification (Duigan et al, 2006). Also includes individual examples of Group D. For both schemes, examples that are less than 2 ha in size, and which meet the quality criteria for ponds may be excluded from this priority habitat. NVC Aquatic Communities A7, A9, A13, A14, A22-A24 for oligotrophic sites; A24 for dystrophic sites. Many dystrophic sites are a component part of the Fen, marsh and swamp; and Bogs broad habitats and relate to NVC types M1, M2 and M3). NVC Swamps and tall-herb fens S4, S8-S11, S19b Phase 1 G1.3 Standing Water – oligotrophic (all sites above 2 ha in size, and some in the size range 1 – 2 ha) G1.4 Standing Water – dystrophic (all sites above 2 ha in size, and some in the size range 1 – 2 ha) UKBAP Broad Habitat All examples of this priority habitat are associated with the broad habitat – Standing waters and canals.
Several NVC communities may be found in oligotrophic water bodies. In addition to types characterised by ’oligotrophic’ species, there may be stands of vegetation that would more commonly be associated with water bodies of a higher nutrient status, e.g. where a base-rich inflow enters a site, or in a sheltered bay. In general, the larger the site and its catchment area, and the more irregular its shoreline, the higher the number of different communities may be present.
Definition in relation to legislative classifications
Classification scheme Habitat types belonging to this UKBAP priority habitat Annex I Includes parts of four types: H3160 Natural dystrophic lakes and ponds; H3110 Oligotrophic waters containing very few minerals of sandy plains: Littorelletalia uniflora; H3130 Oligotrophic to mesotrophic standing waters with vegetation of the Littorelletea uniflorae and/or of the Isoeto-Nanojuncetea (part); H3140 Hard oligo-mesotrophic waters with benthic vegetation of Chara species (part). Scottish Biodiversity List Whilst there is no clear link between SBL types and this priority habitat type, communities which may be present include: A7, A9, A14, A22, A23 and A24; Unvegetated freshwater habitats; Tree roots in freshwater aquatic habitats. SSSIs Includes examples of dystrophic lochs; dystrophic and oligotrophic lochs; oligotrophic lochs; oligo-mesotrophic lochs (part), loch trophic range (part); and sites within blanket bog (including dubh lochs/lochans).
Where is it?
Oligotrophic and dystrophic lakes are associated with areas underlain by hard, acid rock types (e.g. granite, schist and gneiss), with nutrient poor soils, and typically with high rainfall. These rocks are often associated with upland areas, but oligotrophic sites can be found from low to high altitude. Dystrophic sites are usually located on peat. The greatest concentration of sites occurs in the north and west of Scotland.
What is special about it?
Species of special conservation status recorded in this priority habitat in Scotland are listed below.
EC Birds Habitats Directive Directive Schedul Scottish Wildlife & UK BAP Annex e I or II Bio- Red Country- Common priority II*, IV* or diversity Data side Act Group name Latin name list V List List (1981) zircon reed beetle Donacia aquatica y y y beetle bird reed bunting Emberiza schoeniclus y y bird black-throated Gavia arctica y y Sch1 diver bird common Melanitta nigra y y y Sch1 scoter red-necked y bird Phalaropus lobatus y y y Sch1 phalarope fern pillw ort Pilularia globulifera y y y Sch8 fish European eel Anguilla anguilla y y y fish vendace Coregonus albula y y Sch5 w hitefish fish Coregonus lavaretus y y y y Sch5 (pow an) Atlantic fish Salmo salar y y y salmon fish brow n trout Salmo trutta y fish arctic charr Salvelinus alpinus y flow ering y* slender naiad Najas flexilis y y Sch8 plant lichen a lichen Aspicilia melanaspis lichen tarn lecanora Lecanora achariana y y y Sch8 Phaeophyscia lichen a lichen y y endococcina channelled liverw ort Riccia canaliculata y y y crystalwort mammal otter Lutra lutra y y y y soprano mammal Pipistrellus pygmaeus y y* y pipistrelle round-leaved moss Bryum cyclophyllum y y bryum Schleicher’s moss Bryum schleicheri y y y Sch8 thread-moss marsh moss Lycopodiella inundata y y y clubmoss moss Scottish pohlia Pohlia scotia y y y slender stonew ort Nitella gracilis y y y stonew ort
There are a number of benthic and planktonic invertebrates which are limited to oligotrophic lakes and are thought to be glacial relicts. For example, the red listed water beetle Oreodytes alpinus was first recorded from Caithness in the 1980s. The fish fauna is dominated by salmonids, particularly brown trout Salmo trutta. Some larger oligotrophic lochs support a variant of brown trout known as ferox trout.
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.
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. The scale and frequency of these fluctuations may be altered if the loch is used as a water supply (e.g. for drinking water or small-scale hydro power generation), so limitations on water use may be put in place.
Maintenance of high quality water
As this priority habitat is characterised by low productivity it is particularly vulnerable to nutrient enrichment, which may lead to eutrophication. Land within the catchment should be managed to avoid a significant increase in nutrient loadings reaching the loch, which would result in long-term changes in the flora and fauna. The nutrients may originate from diffuse inputs – resulting from run- off from agricultural or forestry land - or as a result of direct inputs (e.g. discharges of treated sewage). Lochs may also be used for the freshwater stages of aquaculture, which can also contribute increased nutrient loadings to sites. These artificial inputs, which are usually rich in phosphorus and nitrogen, encourage the growth of algae and increases in algal biomass reduce the light available in the water column for bottom-rooted plants.
Oligotrophic and dystrophic lakes often have poor acid buffering capabilities, and some sites have undergone acidification as a result of acid deposition or proximity to conifer plantations. Management in these circumstances should involve buffering of vulnerable sites and allow for buffer strips between forestry and watercourses.
Maintenance of hydrological linkages
Some sites support migratory species of fish. These may use different areas of the loch to support each stage of their life cycle, and may need unimpeded access from the loch along tributary and outflow streams. Hydrological linkages may also be important for other ecological groups e.g. benthic invertebrates and higher plants.
Maintenance of native species assemblages
There are a number of invasive non-native species of plant and animal which out- compete native species. Examples include Canadian waterweed Elodea canadensis, Nuttall’s waterweed Elodea nuttallii, New Zealand pygmyweed Crassula helmsii and American signal crayfish Pacifastacus leniusculus. Management should be aimed at ensuring that these species are not spread within or between sites, and at containing or eradicating them where possible.
References, Links and Further Information
Duigan, C., Kovach, W. & Palmer, M. 2006. Vegetation communities of British lakes: a revised classification. Peterborough, Joint Nature Conservation Committee.
Duigan, C., Kovach, W. & Palmer, M. 2007. Vegetation communities of British lakes: a revised classification scheme for conservation. Aquatic Conservation: Marine and Freshwater Ecosystems, 17: 147-173.
Duigan, C., Kovach, W. & Palmer, M. 2008. Aquatic macrophyte classification, distribution, and traits in British lakes. Verh. Internat. Verein. Limnol, 30: 477-481.
European Commission DG Environment. 2003. Interpretation manual of European Union habitats version EUR25. European Commission DG Environment, Brussels. http://ec.europa.eu/environment/nature/legislation/habitatsdirective/docs/2007_07_im.pdf
Jackson D.L. 2000. Guidance on the interpretation of the Biodiversity Broad Habitat Classification (terrestrial and freshwater types): Definitions and the relationship with other classifications. Report 307. JNCC. http://jncc.defra.gov.uk/page-2433#download
JNCC. 1989 & updates. Guidelines for the selection of biological SSSIs. JNCC.
Maddock, A. (Ed). 2008. UK Biodiversity Action Plan Priority Habitat Descriptions Oligotrophic and Dystrophic Lakes. http://jncc.defra.gov.uk/pdf/UKBAP_BAPHabitats-39- OligDysLakes.pdf
Palmer, M. A. 1989. A botanical classification of standing waters in Great Britain. Research & Survey in nature Conservation No. 19. NCC Peterborough OECD. 1982. Eutrophication of Waters, Monitoring Assessment and Control, Organisation for Economic Co-operation and Development, Paris.
Palmer, M. 1992. 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.)*
Phillips, G. Undated. Prioritising UK Lakes for protection and improvement. The role of UK BAP Presentation. http://www.coastms.co.uk/resources/869775eb-a5ab-4ee4-b572- 1a6d3d774513.pdf