The biogeochemical cycling of phosphorus in the upper ocean of the East China Sea

Natural activities of 32P (t1=2 ¼ 14:3 days) and 33P (t1=2 ¼ 25:3 days) in total dissolved P (TDP), suspended particulate matter, net-plankton and rainwater during the spring of 2001 were measured by an ultra-low level liquid scintillation counter. A continuous steady-state model was used to estimate residence times of various phosphorus pools and uptake rates of the biological food chain within the upper ocean of the East China Sea. 32P/33P ratios increased from the dissolved pools to the suspended matter and to netplankton, reflecting that the age of P increased as one moved up through the food chain. The residence times of P in the dissolved pools of coastal water and mid-shelf area were very short (3e4 days), which suggests that low phosphorus concentrations could support relatively high levels of primary production. A phytoplankton assimilating flux of 189e814 mg Cm 2 day 1 and a zooplankton grazing flux of 10e161 mg Cm 2 day 1 were also estimated and show that grazing rates of carbon corresponded to 3e20% of primary production. In addition, the particulate organic carbon fluxes from the upper ocean were calculated by directly comparing the measured seawater inventory of 32P and 33P to that measured in rain, and were found to range between 8.5 and 81.0 mmolm 2 day 1. These results show that the naturally produced 32P and 33P can provide much needed information regarding the biogeochemical cycling of P in marine systems and can be of use in the development of ecosystem models that seek to address mechanisms that affect primary production in the ocean.