Dynamics of Phototrophic Microbial Populations in the Chemocline of a Meromictic Basin of Lake Banyoles

Abstract

AbstractThe ecological succession of two microbial planktonic populations in the chemocline of C‐IV, a meromictic basin of Lake Banyoles has been studied during the stratification period of 1989 (June to October). A dense population of deep‐living algae Cryptomonas phaseolus was found growing between 13 and 14 meters depth, forming peaks of 10 to 20 μg.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 the deep‐living algae population and setting the conditions for the development of a population of brown Chlorobiaceae, Chlorobium phaeobacteroides. Although Cryptomonas population abandons the chemocline (13.75 m.) and decreases its biomass, it does not disappear at all, but moves upwards (8–11 m.) leaving the zone to sulphur phototrophic bacteria which find optimal conditions to grow. The sulphide diffusion coming from sediment and the nutrient limitation were the main responsible factors accounting for the upwards migration and the decrease of growth rate of the algal population respectively.