| Abstract: | When grown under stringent P limitation, affinity for P uptake and growth in Staurastrum pingue Teil. and Staurastrum chaetoceras (Schr.) G. M. Smith (both originating from eutrophic lakes) were of the same magnitude, whereas these parameters for Cosmarium abbreviatum Rac. var. planctonicum W. & G. S. West (isolated from a meso-oligotrophic lake) were significantly higher in value. On the other hand, at all growth rates tested, maximum P uptake rates were lower in C. abbreviatum than in the two Staurastrum species. The outcome of competition between either Staurastrum species and C. abbreviatum in mixed chemostats under different levels of continuous P limitation was in agreement with what could be predicted from the species-specific affinity parameters: Staurastrum was outcompeted at dilution rates lower than 0.012 h?1, calculated to correspond with external inorganic P concentrations lower than 0.02 μM P, but won out at higher dilution rates. When P was added in two pulses of 2.5 μmol · L?1 a week instead of continuously, S. chaetoceras outcompeted C. abbreviatum at a slow rate. When P was supplied as a daily pulse of 0.7 μmol · L?1, a stable coexistence of S. chaetoceras and C. abbreviatum was established, Staurastrum predominating the culture in cell numbers at ca. 85%. The results show that P uptake and growth characteristics of the three species can predict the outcome of competition under various P-limited conditions. Specific growth kinetic parameters as found in this study may also explain distribution patterns of the species observed in the field. |
