Habitat account - Freshwater habitats


3150 Natural eutrophic lakes with Magnopotamion or Hydrocharition-type vegetation 

Background to selection

Distribution of SACs/SCIs/cSACs with habitat 3150 Natural eutrophic lakes with Magnopotamion or Hydrocharition-type vegetation.  Click image for enlarged map.
Distribution of SACs/SCIs/cSACs with habitat 3150 Natural eutrophic lakes with Magnopotamion or Hydrocharition-type vegetation. Click image for enlarged map.

Description and ecological characteristics

 

Natural eutrophic lakes have nutrient levels that are higher than those of oligotrophic, dystrophic or mesotrophic lakes, resulting in higher natural productivity, and are typically species-rich. However, many such lakes have been damaged by over-enrichment with nutrients, resulting in hypertrophic conditions and a reduction in species-richness.

 

In the UK natural eutrophic lakes typically contain aquatic macrophyte communities dominated by pondweeds Potamogeton spp., spiked water-milfoil Myriophyllum spicatum, yellow water-lily Nuphar lutea, and occasionally by associations of stoneworts Chara spp. Except in the most northerly areas, many eutrophic lakes are fringed by reedmace – common reed Scirpo – Phragmitetum associations. More northern shorelines may have reed-canary grass – shoreweed – spike-rush Phalaris – Littorella – Eleocharis associations. Most eutrophic lakes are formed on soft rocks but wave-washed rocky shores form an important part of the habitat on larger lakes.

 

Three main sub-types of eutrophic lake can be identified:

 

     

  1. southern eutrophic lakes
  2. northern or western eutrophic lakes
  3. coastal eutrophic lakes.

European status and distribution

 

Natural eutrophic lakes are now uncommon in the EU, due to pollution, but the exact status of the habitat type is unknown.

UK status and distribution Click to view UK distribution of this habitat

 

In the UK natural eutrophic lakes are comparatively rare, although they have a wide and scattered distribution. Very few examples occur above 300 m. Hydrocharition-type vegetation is rare in lakes and in the UK seems to be confined to Northern Ireland. In the rest of the UK the most complete expression of this community type is found in the ditch systems of the Norfolk Broads. In eastern England most of the lakes are polluted, but the peripheral grazing marsh and ditch systems contain relict eutrophic communities.

Site selection rationale

 

Lakes which are naturally eutrophic are very rare in the UK, and the SAC series contains a high proportion of the total UK resource. Sites have been selected to encompass the ecological variation and geographical range shown by the habitat, and all three sub-types are represented in the list of sites selected. The sites selected are among the largest examples, but selection has particularly favoured those lakes that are least-modified by human activity, particularly with regard to water quality, although few lakes of this type remain totally unaffected by artificial nutrient enrichment. The UK selection was based largely on the presence of remnants of the Hydrocharition (mainly duckweeds Lemna spp.) and a good representation of the Magnopotamion. Wider countryside measures, such as agri-environment schemes and river basin management plans prepared under the Water Framework Directive will make a substantial contribution to attaining favourable conservation status for this habitat.


Site accounts

Breckland East Anglia
The Breckland meres in Norfolk represent natural eutrophic lakes in the east of England. They are examples of hollows within glacial outwash deposits and are fed by water from the underlying chalk aquifer. Natural fluctuations in groundwater tables mean that these lakes occasionally dry out. The flora is dominated by stonewort – pondweed Characeae – Potamogetonaceae associations.
Llangorse Lake/ Llyn Syfaddan East Wales
Llangorse Lake is the largest lake in south Wales and is an example of a northern or western natural eutrophic lake of glacial origin, in this case lying on Old Red Sandstone. The flora is dominated by pondweed – yellow water-lily Potamogetonaceae – Nupharetum associations, and the shoreline flora is a good example of the club-rush – common reed Scirpo – Phragmitetum association. Llangorse Lake has a different flora from that of Loch Watten, the other lake of this type selected, being richer in more southern elements such as shining pondweed Potamogeton lucens.
Llyn Dinam West Wales and The Valleys
Llyn Dinam is a coastal eutrophic lake in north Wales. Common reed Phragmites australis, and to a lesser extent common club-rush Scirpus lacustris ssp. lacustris, dominate the shoreline. Rigid hornwort Ceratophyllum demersum is abundant in shallow open water, often in association with autumnal starwort Callitriche hermaphroditica and ivy-leaved duckweed Lemna trisulca. The white and yellow water-lilies Nymphaea alba and Nuphar lutea dominate in a sheltered arm on the west side. Fennel-leaved pondweed Potamogeton pectinatus, perfoliate pondweed P. perfoliatus and lesser pondweed P. pusillus have been recorded. Stoneworts Chara spp. are present. Water chemistry characteristics are consistent with those expected in eutrophic lakes, including relatively high pH, alkalinity and phosphorus levels. Llyn Dinam is the least-enriched of a series of Anglesey Lakes which have been subjected to sediment diatom analysis
Loch a' Phuill Highlands and Islands
Loch a’ Phuill is a large (96 ha) high-quality naturally eutrophic loch situated behind an extensive machair complex of windblown sand and is backed by a raised beach, forming its northern shore. It is located in the south-west corner of the island of Tiree. Loch a’ Phuill is one of the best examples of a coastal eutrophic loch, with a catchment including acid grassland/heathland on peaty soils and machair landforms protecting it from any serious modifications. The loch supports a particularly diverse aquatic plant flora with several species indicative of the sandy substrate and maritime influence on this eutrophic waterbody, particularly in its south-west area that is dominated by stoneworts Chara spp. The loch also supports several species of pondweed including the nationally scarce fen pondweed Potamogeton coloratus, and slender-leaved pondweed P. filiformis. Of further interest is the loch’s very high pH level, indicating the liming influence of windblown shell-sand, and high conductivity reflecting sea-water spray affecting the site.
Loch Achnacloich Highlands and Islands
Loch Achnacloich is an example of a high quality naturally eutrophic loch with a profuse and diverse growth of aquatic plants, particularly on its southern and western edges. It is found towards the top of a small catchment which drains into the Balnagowan River, and then into the Cromarty Firth. Many of the species found in Loch Achnacloich are more commonly associated with more southern and lowland parts of the UK. The loch supports six pondweed Potamogeton species as well as the nationally scarce least water-lily Nuphar pumila.
Loch of Isbister Highlands and Islands
Loch of Isbister is an excellent example of a shallow moderate-sized naturally eutrophic loch. Formerly the Loch of Isbister was more extensive, but encroachment by peripheral vegetation and peat has resulted in the development of a high-quality basin-mire complex, with excellent examples of open-water transition plant communities. The loch supports a rich plant flora typical of the Magnopotamion type, with plants able to grow in the centre of the loch due to its shallow nature, including abundant stoneworts Chara spp. and pondweeds Potamogeton spp. The loch is rich in northern species and is the most northerly site for natural eutrophic lakes in the UK.
Loch of Wester Highlands and Islands
Loch of Wester is a high-quality naturally eutrophic loch which drains a large peatland catchment in Caithness, flowing into the North Sea through Keiss Links. It developed after the sand-bar at its outflow formed an impoundment, indicating its recent geological origin. The eutrophic conditions in the loch are derived from the Old Red Sandstone which is found throughout its catchment. The loch lies in the northern north-east Scottish Highlands where low-intensity land-use has protected it from any serious modifications. The plant species assemblage in the loch indicates that it lies towards the lower end of the nutrient-rich category, adding diversity to the suite of sites selected as Natural eutrophic lakes with Magnopotamion or Hydrocharition-type vegetation. The pondweeds include the nationally scarce slender-leaved pondweed Potamogeton filiformis, as well as various-leaved pondweed P. gramineus, fennel pondweed P. pectinatus, perfoliate pondweed P. perfoliatus and lesser pondweed P. pusillus.
Loch Watten Highlands and Islands
Although not the largest eutrophic lake in Scotland (the largest being Loch Leven), this large loch is one of the least affected by nutrient enrichment in Scotland. It is the largest of a series of kettle hole lochs overlying the Old Red Sandstone of the Caithness plain in the north of Scotland. Loch Watten is representative of a northern or western natural eutrophic lake of glacial origin. The flora contains stonewort – pondweed – water-milfoil Characeae – Potamogeton – Myriophyllum associations, with pondweeds well-represented, and is rich in northern species. The shoreline is fringed by reed canary-grass – shoreweed – spike-rush Phalaris – Littorella – Eleocharis associations.
North Uist Machair Highlands and Islands
This site incorporates large sand dune systems grading into machair landscape, including machair lochs, on the west coast of North Uist. The lochs are therefore surrounded by machair landforms and represent high-quality naturally eutrophic waterbodies that reflect a strong maritime influence and have been protected from any major modifications. Lochs of this type are Loch nam Feithean and Loch Croghearraidh. The lochs support a diverse aquatic plant flora typical of eutrophic lochs that includes the rare slender naiad Najas flexilis, slender-leaved pondweed Potamogeton filiformis, common duckweed Lemna minor (which is rare in the local area) and stoneworts Chara spp.
Roman Wall Loughs Northumberland and Tyne and Wear
The Roman Wall Loughs area contains three natural eutrophic lakes, Crag, Broomlee and Greenlee Loughs. Together the loughs contain 11 species of pondweed Potamogeton including P. lucens, P. pusillus, and P. obtusifolius. P. gramineus occurs in all three loughs in an unusual association with stoneworts Chara spp. The nationally-rare autumnal water-starwort Callitriche hermaphroditica occurs in Crag Lough. Shoreweed Littorella uniflora grows in Broomlee and Greenlee Loughs, and greater bladderwort Utricularia vulgaris in the latter.
South Uist Machair Highlands and Islands
South Uist in the Outer Hebrides contains a series of coastal natural eutrophic lakes formed on calcareous marine sediments on the machair plain. These lakes are the best examples of their type in the UK, owing to the richness of their flora. Stoneworts Chara spp. and pondweeds Potamogeton spp. dominate the aquatic community, and shorelines contain spike-rush Eleocharis associations. Lochs of this type within the site are: Loch Roag; West Loch Ollay; Loch Ardvule; Loch Stilligarry; and Loch na Liana Mσire. The South Uist machair supports a unique transition from oligotrophic lochs on peatland towards the centre of the island, through mixed oligotrophic and mesotrophic lochs where peat ‘blackland’ meets machair, to eutrophic lochs over calcareous sand and brackish lochs on the west coast. This site has been selected for many of these Annex I habitat types in their own right.
The Broads East Anglia
The Broads in East Anglia contain several examples of southern natural eutrophic lakes. Although artificial, having arisen from peat digging in medieval times, these lakes and the ditches in areas of fen and drained marshlands support relict vegetation of the original Fenland flora, and collectively this site contains one of the richest assemblages of rare and local aquatic species in the UK. The stonewort – pondweed – water-milfoil – water-lily Characeae – Potamogeton – Myriophyllum – Nuphar associations are well-represented, as are club-rush – common reed Scirpo – Phragmitetum associations. Some Broads, such as Martham North, Martham South and Upton Broad, have escaped the problem of enrichment that has so affected the flora and fauna on many of the other Broads. Others, such as Hickling Broad, are recovering from these effects as a result of remedial measures. Martham North, Martham South, Upton and Hickling Broad contain holly-leaved naiad Najas marina, a national rarity. The dyke (ditch) systems support vegetation characterised by water-soldier Stratiotes aloides, whorled water-milfoil Myriophyllum verticillatum and broad-leaved pondweed Potamogeton natans.
Tiree Machair Highlands and Islands
The Tiree Machair comprises some of the finest examples of calcareous sand dunes grading into machair plain and machair loch (Loch Bhasapol). The Tiree Machair cSAC comprises four discrete areas located in the central and north western areas of the island of Tiree. Loch Bhasapol and other lochs on the machair plain represent high quality naturally eutrophic waterbodies that reflect a strong maritime influence. The eutrophic condition of the waterbodies is derived from the sand deposits and dunes that are prevalent throughout the site. The lochs support a diverse aquatic plant flora with several species indicative of the maritime influence, including brackish water-crowfoot Ranunculus baudotii that has a restricted UK and European distribution. The site also supports eight species of pondweed, including the nationally scarce fen pondweed Potamogeton coloratus, and slender-leaved pondweed P. filiformis, together with the locally rare fennel pondweed P. pectinatus.
Upper Lough Erne Northern Ireland
Upper Lough Erne in Northern Ireland is a very large natural eutrophic lake situated in a drumlin landscape and has a predominantly limestone catchment. The site is an example of a northern or western eutrophic lake of glacial origin. The lake has a very long shoreline and numerous associated satellite lakes, many of which are included in the site. Aquatic vegetation of the Magnopotamion and Hydrocharition type is extensively-developed. Both club-rush – common reed Scirpo – Phragmitetum and reed canary-grass – shoreweed – spike-rush Phalaris – Littorella – Eleocharis associations are well-developed on the shore. There are transitions to swamp and fen vegetation.

SACs/SCIs/cSACs where this Annex I habitat is a qualifying feature, but not a primary reason for site selection

West Fermanagh Scarplands Northern Ireland
Y Twyni o Abermenai i Aberffraw/ Abermenai to Aberffraw Dunes West Wales and The Valleys
 

Many designated sites are on private land: the listing of a site in these pages does not imply any right of public access.