ECOPHYSIOLOGICAL CHARACTERISTICS OF 2 PLANKTONIC DESMID SPECIES ORIGINATING FROM TROPHICALLY DIFFERENT LAKES

Two planktonic desmid species were compared in some of their ecophysio logical characteristics. Staurastrum chaetoceras, well-known for its a bundant occurrence in eutrophic lakes, showed a higher photosynthetic capacity and a higher maximum (intrinsic) growth rate than Cosmarium a bbreviatum var. planctonicum, a taxon only encountered in oligo-mesotr ophic habitats. The two taxa are comparable in cell size. When grown u nder a stringent continuous inorganic phosphorus (P-i) limitation C. a bbreviatum realized a higher growth rate, due to a higher affinity for the uptake of P-i, than S. chaetoceras. On the other hand, under thos e conditions, S. chaetoceras displayed a two times higher maximum P-i uptake rate (V-max). Regarding cellular alkaline phosphatase activity (hydrolysis of the organic P substrate MFP) C. abbreviatum showed both a higher affinity and maximum rate than S. chaetoceras. In a way, the se characteristics reflect the distribution pattern of the two species in the field. For in eutrophic lakes, during the summer algal bloom, species often have to compete for light as the growth limiting factor, whereas species occurring in oligo-mesotrophic lakes usually face per manently growth-limiting P concentrations. Since in eutrophic lakes du ring summer algal bloom dissolved inorganic P concentrations can also be low, the ability of phytoplankton to acquire P-i from short-lived p ulses (e.g. excretion of P by zooplankton or fish) has to be considere d an important additional characteristic in view of competition. Conce rning the two desmid species under discussion, S. chaetoceras will hav e a competitive advantage when P-i is supplied in distinct pulses, due to its higher V-max values. On the other hand, C. abbreviatum possibl y will be superior in competition for organic P substrates. In the spe cies studied, different strategies were found to benefit optimally fro m the resource conditions inherent in the trophic state of their habit at.