DIFFERENT RESPONSE MECHANISMS OF 2 PLANKTONIC DESMID SPECIES (CHLOROPHYCEAE) TO A SINGLE SATURATING ADDITION OF PHOSPHATE

Two planktonic algal species, Staurastrum chaetoceras (Schr.) G. M. Sm ith and Cosmarium abbreviatum Pac. var. planctonicum W. et G. S. West, from trophically different alkaline lakes, were compared in their res ponse to a single saturating addition of phosphate (P) in a P-limited growth situation. Storage abilities were determined using the luxury c oefficient R = Q(max)/Q(0). Maximum cellular P quotas differed, depend ing on whether cells were harvested during exponential growth at mu(ma x) (Q(max), R being 26.7 and 3.1 for C. abbreviatum and S. chaetoceras , respectively) or harvested after a saturating pulse at P-limited gro wth conditions (Q'(max) R being 53.5 and 20.2 for C. abbreviatum and S . chaetoceras, respectively). At stringent P-limited conditions, maxim um initial uptake rates were higher in S. chaetoceras than in C. abbre viatum (0.094 and 0.073 pmol P . cell(-1). h(-1), respectively), but l ong-term (net) uptake rates (over similar to 20 min) were higher in C. abbreviatum than in S. chaetoceras (0.048 and 0.019 pmol P . cell(-1) . h(-1) respectively). Before growth resumed after the onset of a larg e P addition (150 mu mol . L-1), a lag phase was observed for both spe cies. This period lasted 2-3 days for S. chaetoceras and 3-4 days for C. abbreviatum, corresponding with the time to reach Q'(max). Subseque nt growth rates (over similar to 10 days) were 0.010 h(-1) and 0.006 h (-1) for S. chaetoceras and C. abbreviatum, respectively, being only 2 0%-30% of maximum growth rates. In conclusion, S. chaetoceras, with a relatively high initial P-uptake rate, short lag phase, and high initi al growth rate, is well adapted to a P pulse of short duration. Conver sely, C. abbreviatum, with a high long-term uptake rate and high stora ge capacity, appears competitively superior when exposed to an infrequ ent but lasting pulse. These characteristics provide information about possible strategies of algal species to profit from temporarily high P concentrations.