DO LIGHT QUALITY AND LOW NUTRIENT CONCENTRATION FAVOR PICOCYANOBACTERIA BELOW THE THERMOCLINE OF THE OLIGOTROPHIC LAKE STECHLIN

In May 1995, a short-term study on the vertical distribution of phytop lankton in the holomictic, oligotrophic Lake Stechlin revealed a deep chlorophyll maximum (DCM) due to an increase in phytoplankton biomass below the thermocline. The DCM was dominated by centric diatoms, which probably just passed those water layers while sinking to the sediment , and by picocyanobacteria. The DCM was situated well above the 1% lev el of photosynthetically active radiation, but received almost exclusi vely light of wavelengths between 500 and 600 nm. The dominant picocya nobacteria were pre-adapted to this environment by the possession of p hycoerythrin. The competitive advantage of picocyanobacteria compared with other phycoerythrin-containing phototrophs was probably the most efficient nutrient uptake under low nutrient conditions as a consequen ce of the small size of picoplankton.