Biogeochemical Controls of Particulate Phosphorus Distribution Across the Oligotrophic Subtropical Pacific Ocean

We investigated basin‐wide distributions of total particulate phosphorus (TPP) and associated biogeochemical parameters in the euphotic zone (EZ) of the subtropical North and South Pacific Ocean. TPP is primarily composed of living microorganisms, and its distribution is likely controlled by nitrogen (N) supply in typical N‐limited Pacific waters as well as phosphorus (P) supply in P‐limited western North Pacific. TPP concentrations showed a vertically uniform distribution (approximately 15 nM) within the EZ at most stations, but at several northern North Pacific stations, TPP peaks (>20 nM) were observed in the lower EZ where nitrate and nitrite concentrations were high. Relationships between TPP and biogeochemical parameters indicate that a possible factor controlling TPP in the upper EZ was dinitrogen fixation and that in the lower EZ was shoaling of the nitracline. Geographical distribution of TPP was relatively uniform compared to that of phosphate, which varied from <10 nM in the western North Pacific to >100 nM in the eastern North and South Pacific. The western North Pacific was a domain characterized by lower dissolved organic P (DOP) concentrations, higher alkaline phosphatase activities, and higher particulate polyphosphate (PpolyP):TPP ratios compared to other domains. These characteristics indicated that active DOP utilization and PpolyP accumulation could play important roles in the maintenance of TPP stocks as alternatives to phosphate utilization in this distinctive domain. Our results demonstrated that TPP distributions were vertically and geographically uniform across the subtropical Pacific Ocean, concealing that TPP stocks were likely underpinned by a range of nutrient supply mechanisms.