Seasonal dynamics of phytoplankton phosphorus stress in temperate Jiaozhou Bay, North China

Alkaline phosphatase activity (APA) is always used to evaluate the phosphorus (P) status of phytoplankton community and considered as a proxy for phosphate (DIP) stress or deficiency. In this study, we presented seasonal distributions of APA [including free APA (F-APA) and particulate APA (P-APA)] and enzyme kinetics and compared to the distribution of phosphorus concentrations, planktonic biomass and other environmental variables reflecting community P status in moderate DIP-deficient Jiaozhou Bay, North China. Low potential APA (V-max) were detected in winter (mean 51.3 nM h(-1)), which was probably due to low water temperature, while higher APA corresponded well to high phosphate requirement and high biological productivity in other three seasons. Normalized total APA (specified by chlorophyll a and bacteria biomass) showed an inverse hyperbolic relationship with DIP and the threshold phosphate concentration for the transition from high to low APA was around 0.2 mu M in Jiaozhou Bay. Combined with results of APA, P concentrations (including DIP and DOP) and half saturation constant (K-m), our findings suggested that phytoplankton and bacteria in Jiaozhou Bay may be experiencing a relief from P-limitation in winter; while in other three seasons, these organisms were mainly under persistent P stress. Moreover, in order to compensate for phosphate deficiency, we hypothesize that plankton not only by means of promoting enzyme production as indicated by increase of V-max, but also by an improved enzyme affinity as represented by a decrease of the Michaelis constant (K-m). Consequently, higher APA and lower DOP stocks in summer were probably a result of greater utilization of the DOP pool to maintain high primary production.