Spatial and temporal changes in the concentration of various phosphorus pools and their possible biogeochemical roles in the oligotrophic subtropical western North Pacific

Abstract

Spatial and temporal variations in the concentration of phosphorus pools, including total phosphorus (TP), reactive phosphorus (RP), and nonreactive phosphorus (NP), were evaluated in subtropical regions (10–30°N) of the western North Pacific Ocean along 137°E through eight sampling periods from summer 2003 to spring 2005. RP was depleted at the water surface throughout our observation, varying less than 0.1 μM. The low concentration of RP was restricted to the surface mixing layer, and the concentration obviously increased concomitant with the decrease in water temperature. NP concentration was generally highest at the water surface and gradually decreased with increasing depth, but the depth and temporal variations were not definite compared with those of RP. NP was further divided into two fractions depending on its reactivity to specific ultraviolet (UV) irradiation. The concentration of UV labile organic phosphorus (UVL OP) was consistently low, being comparable with that of RP; their inventories from 0 to 50 m of RP and UVL OP fluctuated with in the range of 0.9–3.2 and 2.0–3.1 mmol m−2, respectively. As for incubation experiments in coastal waters using glucose‐1‐phosphate, which is fractionated into UVL OP, the UVL OP most likely has a short turnover time due to rapid utilization by microorganisms, indicating a significant role in the phosphorus cycle through the microbial food web. Inventory of the UV stable OP (UVS OP), on the other hand, varied nearly eightfold in the upper 50 m, and the temporal change in TP inventory was exclusively due to that of UVS OP.