Simulation on Seasonal Variation of the Phytoplankton Biomass in the Bohai Sea

Seasonal variation of phytoplankton biomass in shallow coastal ecosystems is lack of a complete mechanistic explanation usually.In order to explore the mechanisms that may influence the seasonal variation of phytoplankton biomass in the Bohai Sea,we adopted a nitrogen-based box ecological model,which consisted of four compartments:Nutrient(N),Phytoplankton(P),Zooplankton(Z)and Detritus(D).The simulated seasonal pattern of phytoplankton biomass was in reasonable agreement with the observation,with a stronger spring bloom and a weaker summer bloom.Quantitative and qualitative sensitivity analyses of the biological parameters were conducted,and it was indicated that the model was most sensitive to the three parameters:phytoplankton maximum growth rate,phytoplankton maximum specific mortality rate and nutrient half-saturation constant.Numerical experiments forced by different physical/chemical conditions were followed to investigate the effects of main physical/chemical factors on the phytoplankton variation in the Bohai Sea.The conclusions showed that it was the comprehensive function of the physical/chemical factors,e.g.,solar radiation,transparency,temperature and nutrient that controlled the seasonal cycle of the phytoplankton biomass.The realistic water temperature was one of the triggers of the summer bloom and had influence on the spring bloom,despite that it was not the trigger of the spring bloom.Light condition was the most important limiting factor that controlled the onset of the spring bloom while had little influence on the summer bloom.Nutrient was a limiting factor for the summer bloom,the external input of which played a rather crucial role in determining both the occurrence and the strength of the summer bloom.