Environmental changes in the Rila Mountains, Southwestern Bulgaria, as recorded by the sediments of a remote lake

Chemical and biological sedimentary records of the high alpine lake "Bubreka", Rila Mountains, Bulgaria, were used to reconstruct palaeoecological conditions and to determine the phases in the ontogeny of the lake. Sediment cores taken in 2000 were dated by Pb-210 and Cs-137 and analysed for percentage of dry weight (%DW), loss-on-ignition (LOI), pigments, diatoms, chrysophyte stomatocysts, cladocerans, chironomids and spheroidal carbonaceous particles (SCPs). The studies on Lake Bubreka show that high mountain lakes are good sites for the study of the evolution and effects of the long range transport of atmospheric pollutants from the industrial lowlands to mountain regions. Four distinct successional phases can be described in the lake history during the last similar to 250 years, revealing complex interactions between anthropogenic impact and climate. Phase 1 (ca. AD 1762-1834) is characterised by stable oligotrophical conditions. During Phase 2 (ca. AD 1834-1928) there is a change from oligotrophic to mesotrophic conditions. Phase 3 (ca. AD 1928 - 1994 +/- 2) shows an increase in LOI due to the high proportion of organic material. The shorter ice cover period in the lake may have created conditions more suitable for the growth of phyto- and zooplankton and changed the overall primary production from benthos to plankton. In Phase 4 (ca. AD 1994 +/- 2 - 2000) there is increased LOI, increasing percentages of periphytic diatoms, less favorable conditions for cladocerans, and rheophilic chironomid assemblages which indicate rapid input of nutrients causing mesotrophication. The SCP record and comparisons with other European mountain lakes suggest there is a moderate impact from atmospheric deposition compared with similar lakes elsewhere in Europe.