Ecology of cryptophytes coexisting near a freshwater chemocline

Abstract

1. A deep chlorophyll maximum dominated by Cryptomonas phaseolus, Cryptomonas undulata and often also by Cryptomonas rostratiformis was observed near the summer chemocline of the dimictic, meso-eutrophic lake Schlachtensee from 1990 to 1996. 2. The cryptophyte populations occupied a stratified water column of about 2 m thickness just below the oxycline. They were never observed in the summer epilimnion. In their habitat, oxygen concentration was always lower than 1 mg L-1 and light was growth-limiting, whereas nitrogen and phosphorus were available at high concentrations. 3. A very large portion of the populations stayed in the anaerobic, sulphide-containing water layer. Below the chemocline, purple and green sulphur bacteria coexisted. 4. The cryptophyte populations were maintained by in situ growth due to low-light adapted photosynthesis and not due to phagotrophy. 5. Short-term changes in the light climate near the chemocline could explain the coexistence of different cryptophyte species.