Permanent dystrophic waterbody

The term 'dystrophic' is applied to a water body that is usually shallow, rich in humus giving its water a brown colour, with variable amounts of nutrients (though the availability of nutrients in most cases is low), and with the deeper water often depleted of oxygen. Most boreal lakes and ponds have humic substances in the water, but only polyhumic ones (with colour >90 Pt mg/L) are recognized as dystrophic here, the humic substances in the water being derived from mires, wetlands or paludified forests. Oligo- (< 30) and mesohumic (30 – 90 Pt mg/L) lakes and ponds are included in types based on their trophic state (habitats C1.1a, C1.1b and C1.2a, C1.2b).  In most dystrophic lakes and ponds the water is acid, (pH 3-6), but some have a higher pH, often caused by eutrophication. Bottom sediments consist of organogenic mud and debris, and the soft bottom can be some metres thick. Shores consist usually at least partially of peat, representing bog and fen communities, often quaking due to overgrowth from pond margins to the open water. Dystrophic pools with a similar appearance occur also in raised bog systems but as the origin of those pools is usually related to the development of mire complexes, they are included in the D habitats. Small dystrophic ponds (usually <10 hectares) and pools are often in contact with swamps and mires, therefore the water near the shores is often characterized by overgrowth of fen and bog vegetation. Floating-leaved plants are constant, elodeids and isoetids sparse. Potamogeton species are often missing, due to low nutrient status and pH. Freely floating and drifting aquatic mosses (Sphagnum spp., Warnstorfia spp., Drepanocladus spp., Fontinalis spp.) can be abundant. Utricularia minor and U. intermedia are characteristic species. The cover of helophytes and vascular shore plants varies, typical species being Carex lasiocarpa, C. rostrata, Phragmites australis, Equisetum fluviatile, Menyanthes trifoliata, Comarum palustre, Calla palustris etc. Moss cover, often dominated by Sphagnum spp., is well developed on shores.

In boreal regions there are many larger lakes with polyhumic water. Beside peaty shores they have mineral bottoms and shores, often of till or glacifluvial origin. Floristically these lakes are close to habitat C1.1b maintaining sparse stands of helophytes (Phragmites, Equisetum fluviatile, Schoenoplectus lacustris, Eleocharis palustris, Carex rostrata, C. lasiocarpa), floating-leaved macrophytes (Nymphaea alba, Nuphar lutea, N. pumila, Sparganium spp.), elodeids (Potamogeton perfoliatus, Myriophyllum alterniflorum) and isoetids (Isoëtes spp., Subularia aquatica, Eleocharis acicularis). Aquatic mosses are common. The vegetation of Lobelion dortmannae is typically occurring in this habitat type in oceanic Europe (e.g. Scandinavia and Ireland) but is absent in north-west European lowlands. Dystrophic water bodies are abundant in the boreal region with large mire areas, occurring typically on watersheds. They occur commonly in oceanic NW Europe as well but in continental Europe and southern Europe they are rare. Due to slow peat formation they are rare in northern boreal, arctic and alpine areas. Dystrophic lakes have deteriorated largely by forestry activities and drainage of peatlands for forestry, peat excavation etc., resulting in increase of humic substances, and in lowering of the water table. This has changed the bottom quality and depleted oxygen. Many lakes and ponds have also eutrophicated because of human habitation, construction activities and air-born nitrogen.

Indicators of good quality:

• Water body has natural hydrology and water chemistry,
• The pH should be < 6, colour >90 Pt mg/L,
• Catchment area has undisturbed hydrology and natural land cover,
• Typical structure of vegetation and co-existence of Utricularids, aquatic mosses, floating-leaved plants,
• Intact shore vegetation,
• Low anthropogenic influence, in terms of drainage, construction activities, forestry, water exploitation, and eutrophication,
• Absence of invasive alien species.

Synthesis of Red List assessment

Confidence in the assessment

Pressures and threats