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Prof. Dr. Ferdinand Schanz

Limnological Station
Institute of Plant Biology
Seestr. 187
CH-8802 Kilchberg
Switzerland

Phone: +41 (0)44 716 12 12

e-mail: fschanz@limnol.uzh.ch

Active populations of Planktothrix rubescens occupy the metalimnion of summer stratified Lake Zürich between July and September. At the end of the period of stable thermal stratification, normally at the end of August or the beginning of September, the filamentous cyanobacterium form a narrow layer 1 to 2 m thick before epilimnic entrainment. In 1999 the peak density of this layer was 12 cm³ m^-3 in the 10.5 to 12.5 m depth range. An overview on the biomass dynamics of Planktothrix is given in the J.Plankton Res. 20:1357-1375 (1998). In September 1999, the daily integral of primary production was determined using photosynthesis versus irradiance curves obtained from laboratory experiments. Daily changes in the photosynthetic coefficients were incorporated in the improved, integrating spreadsheet. This had a profound effect on the calculated production. However, the vertical displacement of filaments by seiche movements and buoyancy has a much more important influence on the productivity, a fact not considered in the literature until now.

Key words: Lake Zurich, photosynthesis, phytoplankton buoyancy, seiche movements, cyanobacteria, Planktothrix rubescens

A lake system in the Gotthard region was chosen to collect samples of phytoplankton populations: at about 2450 m a.s.l.- Lago d’Orsirora, superiore, and Lago di Valletta, inferiore; at 2150 m a.s.l.--Lago di Lucendro; at 1970 m--a small pond near the Gotthard-pass street. For comparisons, additional samples have been taken from Lake Lucerne (430 m a.s.l.). The organisms were concentrated using a hollow fibre filter and then exposed to light under laboratory conditions. Equal phytoplankton samples got either PAR (=photosynthetically active radiation, 400-700 nm); PAR and UV-A (320-700 nm); or PAR, UV-A, and UV-B (290-700 nm). The photosynthetic efficiency of photosystem II was followed using the PAM-fluorometry technique. After photoinhibition, a fast (up to 60 min) and a slow (up to 6 hours) recovery period could be distinguished under dim light conditions. It seems that the physiological properties of the phytoplankton population in lakes are mainly determined by the species composition. Under natural conditions acclimation to UV-B radiation might play a subordinated role.

Key words: High mountain lake, acclimation, UV-B, UV-A, photoinhibition, photoinhibition recovery, phytoplankton