3140 Hard oligo-mesotrophic waters with benthic vegetation of Chara spp.
Description and ecological characteristics
This habitat type is characterised by water with a high base content, most often calcium but very rarely magnesium, and is usually confined to areas of limestone and other base-rich substrates, from which the dissolved minerals are derived. In part the rarity of the habitat type is due to the fact that since calcareous rocks are free-draining, waterbodies occur on the surface of these rocks only very rarely. In addition, such waterbodies are characterised by very clear water and low nutrient status. They are therefore largely restricted to situations where the catchment or aquifer from which they are supplied with water remains relatively unaffected by intensive land-use or other sources of nutrients, and they are most often found in areas supporting mosaics of semi-natural vegetation.
Abundant charophytes (stoneworts) are typically the most prominent component of the vegetation; they can occur as dense beds that cover a significant part of the lake bottom over muddy marl deposits.
Hard oligo-mesotrophic waters occur in three main situations:
- Lakes on a predominantly limestone substrate.
- Coastal sites based on calcium-rich shell-sands, also known as machair lochs.
- Lakes with nutrient inputs from other base-rich influences, e.g. serpentine and boulder clays.
The first type is most common in the UK. Hard oligo-mesotrophic waters also develop in abandoned mineral workings, dammed river valleys (e.g. Bosherston Lakes, Pembrokeshire) and other artificial situations. Artificial examples are included within the Annex I definition.
European status and distribution
This habitat type has a very wide European distribution.
UK status and distribution
Hard oligo-mesotrophic waters with benthic stoneworts Chara spp. are scarce in the UK, and the best examples of this habitat type are restricted to the north and west.
Click here view UK distribution of this species
Corsydd Môn/ Anglesey Fens
West Wales and The Valleys
Within Anglesey Fens, Llyn Yr Wyth Eidion is a small active marl-producing lake in north Wales and is an example of a lake on limestone. It is surrounded by the extensive calcareous valley mire of Cors Erddreiniog, which overlies limestone and protects the lake against nutrient enrichment, resulting in water of high quality. Hedgehog stonewort Chara pedunculata and the rare rugged stonewort C. rudis have been recorded at this site.
Craven Limestone Complex
Malham Tarn in northern England is considered the best example of an upland stonewort Chara-dominated lake in England. It is an example of a lake on limestone and is the highest marl lake in the UK. The water drains from surrounding Carboniferous limestone and is calcareous and low in plant nutrients, although the Tarn has a large catchment and some nutrient enrichment to the system has occurred in the past, slightly reducing the floristic richness.
Highlands and Islands
Durness contains a cluster of three marl lochs (Croispol, Borralie and Caladail) on Dalradian Durness limestone in the extreme north-west of Scotland. These are the northernmost examples of marl lakes in the UK and one of the few high-quality occurrences of the habitat type in Scotland. The water is very clear, low in nutrients and with little phytoplankton production. There are extensive growths of submerged macrophytes, including seven species of pondweed Potamogeton and three species of stonewort Chara. In addition, Borralie is the only marl lake in the UK with a population of Arctic charr Salvelinus alpinus.
Kenfig Pool is a shallow lake system within the extensive sand dune system of Kenfig, alongside Swansea Bay in south Wales. The water chemistry is indicative of a coastal, alkaline lake with a moderate nutrient status. High alkalinity, conductivity, sodium and chloride values reflect this marine influence. Elevated calcium values are probably derived from marine shell remains in the sandy substrate. Large stands of common reed Phragmites australis are found on the pool’s seaward side. Grey club-rush Scirpus lacustris ssp. tabernaemontani, sea club-rush Scirpus maritimus, branched bur-reed Sparganium erectum and yellow iris Iris pseudacorus are also present. A sheltered bay supports a plant association dominated by shining pondweed Potamogeton lucens and curled pondweed P. crispus. Hairlike pondweed P. trichoides is locally dominant in the north end and the south end has abundant rigid hornwort Ceratophyllum demersum, Canadian waterweed Elodea canadensis, fan-leaved water-crowfoot Ranunculus circinatus, spiked water-milfoil Myriophyllum spicatum and the charophytes Chara aspera var. aspera and Nitella flexilis var. flexilis. Shoreweed Littorella uniflora can be found growing in association with C. aspera and the aquatic moss Fontinalis antipyretica along the sandy shore section. C. aspera also dominates the substrate off the grazed landward shoreline, to a depth of approximately 1.5 m.
Highlands and Islands
This series of marl lochs (Loch Baile a’Ghoibbainn, Kilchernan Loch and Loch Fiart) is situated on metamorphic Dalradian limestone on the island of Lismore in Argyll. They represent lakes on a limestone substrate and are one of the few high-quality occurrences of this habitat type in Scotland. The component lakes have very clear water and are low in nutrients but with high alkalinity. The clarity of the water is reflected in the occurrence of long-stalked pondweed Potamogeton praelongus in Loch Baile a’Ghoibbainn at unusual depths – in excess of 6 m. The rare stoneworts Chara rudis and C. curtis, as well as the scarce C. pedunculata, have been recorded.
Magheraveely Marl Loughs
These six loughs are small inter-drumlin marl lakes fed by lime-rich water. They are examples of lakes on a predominantly limestone substrate. In comparison with other lakes in this part of Northern Ireland, this site is important because the water has not been influenced by nutrient enrichment and remains clear, with a high lime content and low plant nutrient content. Stoneworts are the dominant submerged vegetation and include several rare and local species, including Chara aspera, C. curta, C. hispida and C. pedunculata.
Moor House - Upper Teesdale
Cumbria, Tees Valley and Durham
This site includes a single small hard oligo-mesotrophic waterbody, Tarn Dub, an upland pool which is impermanent in nature and situated on the slopes of Cronkley Fell. A species-poor flora includes stoneworts Chara spp. in the deeper parts, as well as shoreweed Littorella uniflora, the aquatic moss Fontinalis antipyretica and tubular water-dropwort Oenanthe fistulosa.
Morecambe Bay Pavements
Hawes Water is a lowland lake in northern England within Morecambe Bay Pavements. It is a lake on a predominantly Carboniferous limestone foundation and has a substrate of deep lacustrine shell-marl. The water is highly calcareous and the lake is fed by springs within it. This site is considered to be the best example of a lowland hard oligo-mesotrophic lake with Chara spp. in England, owing to the clarity, low nutrient status and high calcium content of its water. The rare rugged stonewort Chara rudis and scarce species C. aspera, C. hispida and C. pedunculata occur here.
Orton Pit’s extensive pond system, occupying the disused ridge-and-furrow created as a result of clay extraction for the brick-making industry, contains alkaline water low in nutrients. The site supports a total of ten species of charophyte including the main English population of bearded stonewort Chara canescens. C. canescens is an early coloniser of ponds at the site and is rarely found in ponds over 20 years old. It favours brackish conditions, which at Orton Pit are thought to be provided by the release of salts out of the top few millimetres of the clay that becomes oxidised over a period of time. Other nationally scarce stonewort species present include Chara aspera, C. contraria, C. pedunculata and Tolypella glomerata. The distribution of Chara species across the site varies according to the age and stage of succession of the ponds, with few being found in ponds greater than 25 years old.
Pembrokeshire Bat Sites and Bosherston Lakes/ Safleoedd Ystlum Sir Benfro a Llynnoedd Bosherston
West Wales and The Valleys
Bosherston Lakes are an outstanding shallow marl lake system created at intervals in the late 18th and mid 19th centuries by damming a limestone river valley. They are fed in part by a series of calcium-rich springs and are isolated from the sea by a small sand dune ridge. Charophytes are represented by bristly stonewort Chara hispida which forms dense beds up to 1 m high, with individual plants up to 3.5 m long, and by variable quantities of C. globularis, C. virgata and C. vulgaris. Extensive white water lily Nymphaea alba beds also occur, mainly in the western and central arms. In contrast, the eastern arm is characterised by variably dense stands of curled pondweed Potamogeton crispus, fennel pondweed Potamogeton pectinatus, spiked water-milfoil Myriophyllum spicatum and Canadian waterweed Elodea canadensis. Emergent vegetation fringes parts of the system, mostly common reed Phragmites australis, bulrush Typha latifolia, common spike-rush Eleocharis palustris and branched bur-reed Sparganium erectum.
South Uist Machair
Highlands and Islands
South Uist in the Outer Hebrides contains oligo-mesotrophic waters of the machair loch type which derive the calcium content of their nutrient-poor waters from calcareous shell-sand. This complex of high-quality calcareous machair lochs occurs in an intermediate zone between the oligotrophic inland lochs and the eutrophic lochs found nearer the coast. As a result the site is selected for several Annex I habitat types. The lochs are the most extensive and diverse examples of calcareous machair lochs in Scotland and support a number of stoneworts, including the scarce Chara aspera and C. hispida. A strong maritime influence is discernible from their water chemistry and all sites have extensive areas of shell-sand substrate. Owing to their location, these lochs are subject to a mixture of influences from peat and shell-rich sand. The lochs of this type within the South Uist Machair are: Loch Hallan, Grogarry Loch, Loch an Eilean, Mid Loch Ollay and Loch Toronish.
Highlands and Islands
Strath on the Island of Skye includes two lime-rich lochs (Loch Cill Chriosd and Loch Lonachan) situated over predominantly limestone bedrock. The surroundings encompass the most extensive exposure of Durness limestone in Britain, though there are also areas covered by acidic drift. The component lochs represent high-quality hard-water, oligotrophic habitat with water clear to the bottom of the lochs at 4 m depth. The excellent water clarity is reflected by the presence of long-stalked pondweed Potamogeton praelongus. Characteristic of hard-water waterbodies, both lochs support stoneworts Chara spp. Other plants of note include the rare pipewort Eriocaulon aquaticum in Loch Cill Chriosd and six-stamened waterwort Elatine hexandra in Loch Lonachan. In addition, Loch Cill Chriosd and its environs support 34 species of molluscs, three of which occur at their most northerly known locations in Europe.
The Broads is the richest area for charophytes in Britain (Stewart 1996). Twenty species have been recorded, which represents over 65% of the British flora. The core of this interest is the Thurne Broads and particularly Hickling Broad which is the richest site in the UK. Sixteen species have been recorded within Hickling Broad, a large shallow brackish lake. Within the Broads examples of Chara vegetation are also found within fen pools (turf ponds) and fen and marsh ditch systems. The Broads supports a number of rare and local charophyte species, including Chara aspera, C. baltica, C. connivens, C. contraria, C. curta, C. intermedia, C. pedunculata, Nitella mucronata, Nitellopsis obtusa, Tolypella glomerata and T. intricata.
Cornwall and Isles of Scilly
The coastal peninsula of the Lizard in south-west England supports a nationally unique series of oligo-mesotrophic waterbodies in which high base-status is not due to limestone or shell-sand. The Lizard is partly underlain by igneous serpentine rock which gives rise to calcium-deficient ground waters that are rich in magnesium. Groundwater drains from adjoining wet and dry serpentine heaths to feed the oligo-mesotrophic waterbodies in which another unusual feature is the occurrence of stoneworts Chara species typical of calcareous lakes, together with species normally associated with acid conditions, such as bog pondweed Potamogeton polygonifolius. Stoneworts present include three Red Data Book species – Chara baltica, C. curta and C. fragifera.
SACs where this Annex I habitat is a qualifying feature, but not a primary reason for site selection
- Asby Complex Cumbria
Many designated sites are on private land: the listing of a site in these pages does not imply any right of public access.
Please note that the map shows sites where the presence of a feature is classed as ‘grade d’, but these sites are not listed. This is because ‘grade d’ indicates a non-significant presence.