Permanent oligotrophic waterbody with very soft-water species

Oligotrophic waters of sandy plains and rocky substrates (granites, gravel, stones, till, moraines) containing very few minerals. The substrates are covered by a thin layer of detritus. The water is carbon deficient and very poorly buffered (low alkalinity). The water is weakly acid to circumneutral. Concentrations of nitrogen and phosphorus are very low and in the oligotrophic range. The water is clear sometimes humic (brown) but always with a very low concentration of chlorophyll. The vegetation is low in species diversity and dominated by aquatic isoetids, i.e. submerged plants with a small rosette of stiff leaves and an extended root system. Aquatic isoetids are adapted to low carbon availability (low pH) by taking up carbon from the sediment through their extended root system, by an adapted carbon fixation (C4), and by re-using carbon-dioxide produced during respiration. The vegetated layer extends from the littoral to lower parts of the sub-littoral zone. The vegetation and its substrate are mechanically influenced by water movement, ice sheets and wind exposition. Sometimes these oligotrophic water bodies can be almost totally unvegetated. In the temperate, Atlantic zone the characteristic isoetid vegetation occurs in lakes fluctuating in water levels where they may be semi-permanent in the summer periods (Arts, 2002; Dierssen, 1996). Apart from the dominant isoetid aquatic macrophytes (i.e. Isoëtes spp.), other plants may be present in addition (Rørslett & Brettum 1989). Important accompanying isoetid species are Lobelia dortmanna, Subularia aquatica in Northern Europe, Eriocaulon aquaticum in Northern-Atlantic Europe or Luronium natans and Isolepis fluitans in Western Europe zones. Subularia aquatica also extends into the mountainous areas of Iberia in this habitat type (Molina et al., 1999).

Many large lakes in northern Europe represent this type or the more mesotrophic type C1.1b in terms of water chemistry and abundant isoetid vegetation. Occurrence and abundance of other growth forms vary according to shore and bottom material, topography, exposition and fetch. Exposed shores have sparse stands of aquatic vegetation, in sheltered bays vegetation has clear zonation, but the stands are still open (Mäkirinta 1978, Rintanen 1982, Toivonen & Lappalainen 1980). 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 saimensis). Large lakes have diverse waterfowl populations and are important parts of bird migration routes  Main differences between this habitat type and the C1.1b is that this one is constantly in the oligotrophic range and therefore is generally poorer than C1.1b in species and communities. Indicators of good quality isoetid species are the most characteristic species of this habitat.

The following characteristics may be used as indicators of a favourable quality:

• Large stands of isoetid 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 other growth forms than the isoetid growth form, e.g. floating or emerged plants e.g. Potamogeton species or Sparganium angustifolium stands or helophytes (Phragmites australis, Eleocharis 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 < 15 μg/L and chlorophyll < 3 μg/L.);

• pH weakly acid to circumneutral ( pH 5.5 - 7), alkalinity < 0.5 meq/L. (Note: Chemical and physical parameters are only indicative, they may change in different geographical area and climatic conditions).

• Thin layer of detritus (no accumulation of organic mud);

• No/ little impact from acidification or regulation;

• Absence or very low abundance of submerged or floating mats of macrophytes e.g. Juncus bulbosus;

• Occurrence of conspicuous populations of salmonid fish, but population of roach (Rutilus rutilus) and other Cyprinidae low.

Synthesis of Red List assessment

Confidence in the assessment

Pressures and threats