CONVERGENT SUCCESSION OF PHYTOPLANKTON IN MICROCOSMS WITH DIFFERENT INOCULUM SPECIES COMPOSITION

Different initial mixtures of phyto- and zooplankton from different lakes were grown under identical chemical and physical conditions in medium size (8- and 12-1) laboratory microcosm cultures until convergence of phytoplankton species composition was attained. Five such experiments with four (four experiments) or three (one experiment) microcosm cultures were run. Three experiments were performed with weak stirring which permitted sedimentary elimination of the diatoms. Two experiments were conducted with stronger stirring to prevent sedimentation. In the three "sedimentation intensive" experiments, the final phytoplankton community was composed of the filamentous chlorophyte Mougeotia thylespora together with a smaller biomass of nanoplanktic algae. In the two "sedimentation free" experiments the final phytoplankton community consisted of pennate diatoms. Both dissolved nutrient concentrations and the chemical composition of biomass suggested strong nutrient limitation of algal growth rates in the final phase of the experiments. The zooplankton communities at the end of the experiments were composed of species that were apparently unable to ingest the large, dominant algae and that presumably fed on the nanoplanktic "undergrowth" and the bacteria. There was a distinct sequence of events in all experiments: first, the large zooplankton species (Daphnia and Copepoda) were replaced by smaller ones (Chydorus, Bosmina, rotifers); second, all cultures within one experiment developed the same nutritional status (limitation by the same nutrient); and third, the taxonomic composition of phytoplankton of the different cultures within one experiment converged. The last took 7-9 weeks, with is about 2-3 times as long as the time needed in a phytoplankton competition experiment to reach the final outcome.