Two planktonic desmid species were compared in some of their ecophysiological characteristics. Staurastrum chaetoceras, well-known for its abundant occurrence in eutrophic lakes, showed a higher photosynthetic capacity and a higher maximum (intrinsic) growth rate than Cosmarium abbreviatum var. planctonicum, a taxon only encountered in oligo-mesotrophic habitats. The two taxa are comparable in cell size. When grown under a stringent continuous inorganic phosphorus (Pi) limitation C. abbreviatum realized a higher growth rate, due to a higher affinity for the uptake of Pi, than S. chaetoceras. On the other hand, under those conditions, S. chaetoceras displayed a two times higher maximum Pi uptake rate (Vmax). 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, these 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 permanently growth-limiting P concentrations. Since in eutrophic lakes during summer algal bloom dissolved inorganic P concentrations can also be low, the ability of phytoplankton to acquire Pi from short-lived pulses (e.g. excretion of P by zooplankton or fish) has to be considered an important additional characteristic in view of competition. Concerning the two desmid species under discussion, S. chaetoceras will have a competitive advantage when Pi is supplied in distinct pulses, due to its higher Vmax values. On the other hand, C. abbreviatum possibly will be superior in competition for organic P substrates.
In the species studied, different strategies were found to benefit optimally from the resource conditions inherent in the trophic state of their habitat.