In the frame of international projects AL:PE2 (Acidification of Mountain Lakes) and MOLAR (Mountain Lake Research) the Tatra Lake Nižné Terianske pleso was chosen as investigated high mountain lake in Slovakia. Diatom assemblages from the sediment core of the Lake Nižné Terianske pleso were studied and pH reconstruction was calculated. Epilithic diatoms and diatoms from sediment traps of this lake were studied too. • Study Site • Lake: Nižné Terianske pleso • Location: High Tatra Mts., Slovakia • Valley: Nefcerka • Position: 49°10'11"N, 20°00'51"E • Altitude: 1941 m a.s.l. • Lake area: 4.8 ha • Max. depth: 44.4 m • Mean depth: 18.4 m • pH: 6.48 • Retention time: 293 days • Catchment area: 1.1 km2 • Catchment geology: granite • Catchment vegetation: alpine meadows: bare rock (1:1) • Ice-free period: 4 month (July – October) • The lake has an inflow and an outflow. Diatoms from the sediments of the lake Nižné Terianske pleso(High Tatra Mts., Slovakia)Elena ŠtefkováDept. Hydrobiology,Institute of Zoology Slovak Academy of Sciences, Bratislava, Slovakia (e-mail: firstname.lastname@example.org) Methods Two cores - TERI93/2 (35 cm) and TERI96/7 (30 cm) have been analyzed for diatoms. The cores were taken in the years 1993 and 1996 from the deepest area of the Lake Nižné Terianske pleso. The cores were subsampled at every 2 mm and these samples were used for various paleolimnological analyses including analyses for diatoms. Diatom samples were prepared by the method Battarbee (1986) and Battarbee & Kneen (1982). Epilithic diatoms were scraped from the stones 3-times yearly (1996-1997) in the ice-free period. Photo V. Kubovčík Reconstructed pH Summary diatom diagram - sediment core TERI93/2 Sediment cores Diatom analysis of the cores show that the assemblages in these cores have a diverse mixture of both planktonic and benthic taxa. From both cores 28 levels have been analyzed for diatoms. From both cores together a total 121 diatom species were identified, of these 93 agreed. Comparing both cores there were just little differences between them, regarding only abundance of some species. A summary percentage diatom diagrams clearly illustrate the most important changes throughout the cores TERI93/2 and TERI96/7. Fairly constant from the base to the top of the sediment surface were some diatoms such as Achnanthes minutissima, Cymbella minuta, Denticula tenuis, Gomphonema angustum and Fragilaria brevistriata. In the bottom part of the cores (to 16 cm) the most abundant were species Asterionella formosa, Fragilaria pinnata, F. brevistriata, F. capucina var.capucina, Denticula tenuis, Navicula schmassmannii, Achnanthes minutissima and A. curtissima. From these species Asterionella formosa (typical plankton species)reached abundance higher than 20% while in the upper part (from 8cm) was rare (less than 1%) or absented. Diatom assemblages started to change in the part from 9 cm to the top. By some species (e.g. Asterionella formosa, Fragilaria capucina, F. pseudoconstruens, F. construens, F. pinnata) the clear decrease was registered and some of them practically disappeared. On the other hand another group of diatoms (Achnanthes spp., Navicula schmassmannii, Orthoseira roeseana) which previously were not present or were very rare started to increase in its abundance. In the upper part of the core (3 – 0 cm) diatom assemblages were composed mainly from species such as Achnanthes marginulata, A. levanderi, A. subatomoides, A. helvetica, Fragilaria brevistriata, Orthoseira roeseana, Neidium bisulcatum, Navicula digitulus, N. schmassmannii and Aulacoseira distans. The most abundant in layers of sediment core TERI96/7 were Navicula schmassmanni, which reached in the upper part abundance nearly 40% and Orthoseira roeseana with abundance about 15%. In the core TERI93/2 reached just 16% abundance. Reconstructed pH The reconstructed pH of the core TERI93/2 was stable at approximately 6,9 from the base of the core to about 2 cm depth. The pH then declines to 6,6 at 0,4 cm depth, recovering to 6,8 at the surface. The reconstructed pH of the core TERI96/7 was approximately 7.04 from the base to 6.66 in the upper part of the sediment core. In the part of 30 – 9 cm pH varied about values 7. From 9 cm to the top of the sediment pH declined to 6,66. Epilithic diatoms Together 7 epilithic samples were taken from thelake Nižné Terianske pleso, a total of 56 species representing 20 genera, were recorded. The most common species were several species of genera Achnanthes (e.g. A. helvetica, A. lapidosa, A. minutissima, A. subatomoides), Cymbella minuta, Denticula tenuis, Fragilaria capucina, Navicula gallica var. perpusilla and Tabellaria flocculosa. Summary diatom diagram -sediment core TERI96/7 Reconstructed pH Species new for the territory of Slovakia Fragilaria pseudoconstruens Marciniak Gomphonema minutum (C. Agardh) C. Agardh Gomphonema pumilum (Grunow) Lange-Bert.et Reichert Navicula digitulus Hust. Navicula lapidosa Krasske Navicula medioconvexa Hust. Navicula pseudokotschyi Lange-Bert. Navicula stroemi Hust. Nitzschia alpina Hust. Surirella bohemica Maly Sediment traps Sediment traps were exposed in the Lake Nižné Terianske pleso. From four sediment traps samples a total of 25 genera with 74 diatom species were recorded. Species composition of such community was dependent on season and time of exposition of the sediment traps in the lake. The most abundant were several species of Achnanthes, Denticula tenuis, Fragilaria crotonensis, Orthoseira roeseana and Tabellaria flocculosa.