01C1
Oligotrophic mountain lakes
Relationships with habitat classifications. EUNIS: C1.1 Oligotrophic mountain lakes, ponds and pools; PAL. CLASS.: 21.12 Mesotrophic water bodies; HD 92/43: 3130 Oligotrophic to mesotrophic standing waters with vegetation of Littorelletea uniflorae and/or of Isoeto-Nanojuncetea.
Conservation status. BDA, BC, HD.
Category. Vulnerable [VU – A1, 2 B2 C2 D2 E1 G2 H2 I J L3].
General characteristics. The lakes that belong to this habitat type have blue to dark blue colour, high transperancy (10–15 m), and low temperatures of 10–12°C. They are holodimictic water bodies with oxygen composition of approx. 10–14 mg/dm3. In their earliest stages of development no macrophytic vegetation exists. In a large number of cases the epilimnion is considerably smaller than the hypolimnion. The banks and the bottoms are covered by rock pieces. The acidity is close to neutral (pH 6,8–7,2). Sandy or sandy-clayey sediments accumulate on the bottom and in the littoral zone in the later stages of their evolution. Primary and secondary production of biomass increases. The water originates from lakes situated above the water bodies or from snow drifts during the spring/summer snow melting. Maritsa, Iskar rivers and some of the large tributaries of Struma and Mesta rivers start from such lakes. During the later stages of the evolution of these lakes, when sediments are accumulated, plant species appear in their littoral zone, as in Pirin lakes (communities of Isoetes lacustris) and in Rila and Pirin lakes (Sparganium angustifolium, Ranunculus aquatilis). The evolution of these lakes leads to the development of bogs and wet meadows with meandering streams. Most of the lakes in Bulgaria are in their second or third stage of development. Only the lakes at the highest altitudes (Gorno Polezhansko lake, Gazey lakes, Upper Krmenesko lake, Georgiisko lake, upper Todorino lake, Tevno and Vassiliashko lakes in Pirin Mts. and Strashnoto, Okoto, upper Marchino, upper Chanakgjolsko lakes and the upper lakes in Urdina river valley in Rila Mts. are in the first stage of development.
Characteristic taxa.
- Plants: algae – the representatives of the following orders prevail in large quantities: Chrysophyta (Dinobryon divergens, D. sociale americanum, Mallomonas spp. etc.), Pyrrophyta (Gymnodinium spp., Peridinium aciculiferum, P. inconspicuum P. umbonatum, etc.), Bacillariophyta (Achnanthidium spp., Eucocconeis spp., Amphora spp., Asterionella formosa, Aulacoseira spp., Cymbella spp., Encyonema spp., Fragilaria tenera, Pinnularia spp., Surirella spp., Tabellaria fenestrata, T. floculosa, etc.), Chlorophyta (Chlamydomonas spp., Pediastrum duplex, Sphaerocystis schroeteri, etc.), Cyanoprokariota (Microcystis spp., Synechocystis spp., Anabaena spp., etc.), Zygnemophyta (Cosmarium spp., Closterium spp., etc.), Cryptophyta (Cryptomonas spp.); Vascular plants – Isoetes lacustris – in Pirin lakes, Sparganium angustifolium, Ranunculus aquatilis – in Rila and Pirin lakes.
- Animals: more than 120 taxa of tecamoebas, approx. 140 taxa of zooplankton, the most widely distributed are Lecane spp., Polyarthra dolichoptera, Keratella cochlearis, Notholca spp. Hexarthra bulgarica (Rotifera) occurs only in Pirin Mts.; of the crustacean plankton, the species Acroperus elongatus, Alona spp., Arctodiaptomus alpinus, Chirocephalus diaphanous, Daphnia rosea, Mixodiaptomus tatricus, Scapholeberis mucronata. Arctodiaptomus niethammeri is extremely rare, only in some of the seven Rila lakes. The bottom fauna is poor in species, mainly larvae of Diptera (fam. Chironomidae) and water oligochaetes (Oligochaeta) from the families Naididae, Lumbriculidae, Enchytraeidae.
Distribution in Bulgaria. The oligotrophic mountain lakes occur in the mountains that have been subjected to glaciations. In Rila Mts. their number is 140, and in Pirin Mts. – 119. They are situated in the glacier valleys from 1850 up to 2710 m alt. in groups of 1–2 to 7–11.
Conservation importance. The lakes of this type are the reservoirs of water of the highest quality. They balance between the maximums and minimums of the water flow of the mountain rivers and streams. They are important for the regulation of the underground waters and the soil humidity in the surrounding terrestrial ecosystems. They are inhabited by a number of glacial relic and endemic plants and animals. They are indicative habitats for global climate changes.
Threats. Tourism: erosion of the banks, pollution with organic and other types of waste (metals, plastics, food remains); construction and exploitation of hotels, chalets, ski tracks, water catchments, mountain stockbreeding. All listed factors increase the speed of the eutrophication of the lakes.
Conservation measures taken. The habitat is included in Annex № 1 of BDA. All lakes of this type are within the borders of Rila and Pirin National Parks, and some of them are within Strict Nature Reserves. They are also in sites of the European Ecological Network NATURA 2000.
Conservation measures needed. Strict control on the implementation of the envisaged protection measures; construction of water-purifying systems in all chalets and hotels; monitoring and limitation of the tourist flow in the areas of the endangered lakes; prohibition of the building of new ski tracks in close proximity to the lakes; restoration of the most eutrophic lakes according international best practices.
References. Ivanov et al. 1964; Naydenov 1979; Beshkova 2000; Kozuharov 2006; Naidenov 2000; Naidenov & Beshkova 2000; Uzunov & Varadinova 2000.
Authors: Dimitar Kozhuharov, Plamen Ivanov
