Permanent oligotrophic to mesotrophic waterbody with soft-water species

Oligotrophic to mesotrophic waters of sandy plains and rocky substrates (granites, gravel, stones, till, moraine, clay) containing few minerals. Over large parts of the lake the sediment is covered by a thin layer of detritus and accumulation of mud is sparse. The water layer is carbon deficient and poorly buffered (low alkalinity). The water is weakly acid to circumneutral. Concentrations of nitrogen and phosphorous are low and in the oligotrophic to mesotrophic range. The water is clear, sometimes humic (brown) with a low concentration of chlorophyll. The vegetation is low to moderate in species diversity and is dominated by soft-water species. This soft-water vegetation consists mainly of communities dominated by the isoetid species Plantago uniflora and other soft-water species with other growth forms like Myriophyllum alterniflorum, Potamogeton polygonifolius and P. gramineus. Several soft-water species including Baldellia ranunculoides subsp. ranunculoides, B. ranunculoides subsp. repens and Luronium natans are atlantic and are absent from the boreal zone. In the temperate Atlantic zone boreal and atlantic species overlap. As a consequence, soft-water lakes are relatively richer in species in the temperate atlantic zone. Similar habitats in coastal dune slacks, with Plantago uniflora (= Littorella uniflora) as characteristic species, are considered part of habitat B1.8a. In soft-water lakes the vegetated layer extends from the littoral to lower parts of the sub-littoral zone. The littoral zone has fluctuating water levels and the littoral vegetation might be semi-permanent in the summer period. The vegetation and its substrate are mechanically influenced by water movement, ice sheets and wind exposition. Many large lakes in northern Europe represent this type or the more oligotrophic type C1.1a in terms of water chemistry and abundant isoetid vegetation. Occurrence and abundance of other growth forms (elodeids, aquatic mosses, sometimes also Nitella stands, nymphaeids and helophytes) vary according to shore and bottom material, topography, exposition and lake area exposed to wind and subsurface currents. Exposed shores have sparse stands of aquatic vegetation, in sheltered bays vegetation has clear zonation, but the stands are still open. The lower limit of submerged vegetation reaches typically the depth of 3-6 meters, sometimes close to 10 meters. Due to postglacial history deeper lakes host some glacial relict crustaceans and vertebrates, including salmonid fish and a critically endangered fresh water seal (Pusa hispida subsp. saimensis). Large lakes have diverse waterfowl populations and are important parts of migration routes. This habitat type must not be confused with oligotrophic to mesotrophic ponds only periodically flooded, which is instead typical of the Mediterranean area and dominated by isoetid species of Isoëtetalia and Nano-Cyperetalia (class Isoëto-Nanojuncetea). The vegetation of these habitats is composed of a contribution of annual and ephemeral species. These drier habitats are not part of the habitat described here, but belong to types C1.6b "Mediterranean temporary waters" and C3.5b "Periodically exposed shores with stable, mainly mesotrophic sediments with pioneer and ephemeral vegetation".

Indicators of good quality:

• Large stands of soft-water species
• Absence or very low abundance of peat mosses
• Absence or very low abundance of water plants from eutrophic and alkaline waters
• Low abundance of water plants with large floating leaves (Nymphaeids) or emergent plants (e.g. Phragmites australis, Typha spp., Equisetum fluviatile, Carex spp.)
• Long-term habitat stability, with no rapid successional trends (e.g. no trends in acidification or eutrophication)
• Low concentrations of nutrients and chlorophyll (approximately P < 40 μg/L and chlorophyll < 5 μg/L)
• pH weakly acid to circumneutral ( pH 5.5 - 7.5)
• Alkalinity 0.1 – 2 meq/L
• Thin layer of detritus (no accumulation of organic mud) over large parts of the lake
• Occurrence of conspicuous populations of salmonid fish, but population of roach (Rutilus rutilus) and other Cyprinidae low

Note: Chemical and physical parameters are only indicative, they may change in different geographical area and climatic conditions.

Synthesis of Red List assessment

Confidence in the assessment

Pressures and threats