Effects of irradiance and phosphate on growth of nanophytoplankton and picophytoplankton

In situ incubation experiments were conducted to investigate the phosphate uptake and the growth variations of nano- and picophytoplankton under controlled phosphorus concentrations and irradiances in Changjiang estuary and its adjacent sea in China. The results showed that the rates of phosphate uptake were accelerated at high levels (0.60 μmol/L) under the condition of 100% natural irradiance, and the cell densities of nanophytoplankton and Synechococcus spp. obviously increased, whereas picoeukaryote was adapted to low phosphate levels (0.25 μmol/L). Under low irradiance (50% of natural irradiance), uptake of phosphate was restrained at high levels, and the growth of both nanophytoplankton and Synechococcus spp. was also limited. Morerover, nanophytoplankton and Synechococcus spp. grew well at intermediate phosphate levels (0.41 μmol/L), whereas picoeukaryote grew well at low phosphate levels. In addition, the growth period of phytoplankton at intermediate phosphate levels was obviously prolonged, suggesting that the limitation of phytoplankton growth mainly reflected the changes during its growth period. In the absence of irradiance, the addition of phosphate did not affect the release rates of phosphate with a linear increase in the phytoplankton, whereas the growth rates of the phytoplankton showed an exponential decrease, which showed that phosphate regeneration was faster during day than during night; therefore, the irradiance was a significant factor that affected phosphorous biogeochemical cycle in the Changjiang estuary in China.