DYNAMICS OF PHOTOTROPHIC MICROBIAL-POPULATIONS IN THE CHEMOCLINE OF AMEROMICTIC BASIN OF LAKE BANYOLES

The ecological succession of two microbial planktonic populations in t he chemocline of C-IV, a meromictic basin of Lake Banyoles has been st udied during the stratification period of 1989 (June to October). A de nse population of deep-living algae Cryptomonas phaseolus was found gr owing between 13 and 14 meters depth, forming peaks of 10 to 20 mug.l- 1 of chlorophyll a. The physical and chemical properties of the water were continually changing during the studied period and were strongly related to the biological processes developed at the monimolimnion of C-IV. Although the monimolimnion of the basin was initially anoxic (Eh values were around +300 mV and sulphide was not present at detectable levels) soluble iron and sulphide appeared sequentially, displacing t he deep-living algae population and setting the conditions for the dev elopment of a population of brown Chlorobiaceae, Chlorobium phaeobacte roides. Although Cryptomonas population abandons the chemocline (13.75 m.) and decreases its biomass, it does not disappear at all, but move s upwards (8-11 m.) leaving the zone to sulphur phototrophic bacteria which find optimal conditions to grow. The sulphide diffusion coming f rom sediment and the nutrient limitation were the main responsible fac tors accounting for the upwards migration and the decrease of growth r ate of the algal population respectively.