Phytoplankton species composition in the chemocline of mesotrophic lakes

The vertical distribution of phytoplankton was studied in 16 mesotrophic to slightly eutrophic, small and usually dimictic lakes in north-eastern Germany in 1996. All lakes were characterized by an anaerobic hypolimnion in summer. 14 lakes had a total phosphorus concentration >20 mug l-1 in the summer chemocline and were characterized by a deep chlorophyll maximum (DCM) in that layer. Two lakes with lower total phosphorus concentration in the chemocline had no DCM. The DCM was caused by the high biovolume of populations from a group of 11 different cyanobacterial or cryptophyte species that were never observed in the epilimnion. Pseudanabaena catenata and Cryptomonas phaseolus were most widespread in the chemocline. The DCM was in contact with sulphide. Algal biomass in the chemocline was positively correlated with total phosphorus concentration in the chemocline. The percentage of the biovolume of filamentous cyanobacteria in the chemocline increased with increasing total phosphorus concentration, reaching 100 % when the P concentration was >60 mug l-1. The biovolume of cryptophytes reached a higher percentage only at lower P concentration. When the light supply of the chemocline was above average of the studied lakes however, the percentage of filamentous cyanobacteria was lower than predicted from P concentration and cryptophytes were co-dominant