Response of a natural phytoplankton assemblage to pulsed supply of phosphorus in semicontinuous cultures

A natural phytoplankton assemblage from a pond was grown in semicontinuous cultures receiving phosphorus (P) pulses once every 2 or 6 days. The study shows that the structure of phytoplankton assemblage depends on the mode of P supply. Biomass, expressed as total biovolume, was greatest in cultures receiving P pulse once every 2 days. While the total amount of P administer ed to both pulsed cultures remained similar, differences in structure of th e phytoplankton assemblage were observed during the 4 weeks experimental pe riod. The small sized Navicula cryptocephala became dominant in cultures wh ich did not receive P pulses. Therefore, this species seems to be "affinity specialist" well-adapted to a low P concentration. The replacement of the dominant cyanobacterium Gomphosphaeria aponina by the diatoms Navicula cryp tocephala and Synedra ulna in the control and P-pulsed cultures might be du e to another factor. The large sized Phormidium mucosum and Hormidium sp. w ere favoured in cultures pulsed with P at the lowest frequency, which seems to be due mainly to their larger P accumulation capacity. Synedra ulna was favoured to the same extent under the 2 and 6-day pulse periods.