COMPETITION FOR PHOSPHORUS AMONG PLANKTONIC DESMID SPECIES IN CONTINUOUS-FLOW CULTURE

When grown under stringent P limitation, affinity for P uptake and gro wth in Staurastrum pingue Teil. and Staurastrum chaetoceras (Schr.) G. M. Smith (both originating from eutrophic lakes) were of the same mag nitude, whereas these parameters for Cosmarium abbreviatum Rac. var. p lanctonicum W. & G. S. West (isolated from a meso-oligotrophic lake) w ere significantly higher in value. On the other hand, at all growth ra tes tested, maximum P uptake rates were lower in C. abbreviatum than i n the two Staurastrum species. The outcome of competition between eith er Staurastrum species and C. abbreviatum in mixed chemostats under di fferent levels of continuous P limitation was in agreement with what c ould be predicted from the species-specific affinity parameters: Staur astrum was outcompeted at dilution rates lower than 0.012 h(-1), calcu lated to correspond with external inorganic P concentrations lower tha n 0.02 mu M P, but won out at higher dilution rates. When P was added in two pulses of 2.5 mu mol . L(-1) a week instead of continuously, S. chaetoceras outcompeted C. abbreviatum at a slow rate. When P was sup plied as a daily pulse of 0.7 mu mol . L(-1), a stable coexistence of S. chaetoceras and C. abbreviatum was established, Staurastrum predomi nating the culture in cell numbers at ca. 85%. The results show that P uptake and growth characteristics of the three species can predict th e outcome of competition under various P-limited conditions. Specific growth kinetic parameters as found in this study may also explain dist ribution patterns of the species observed in the field.