Flexible Polyphosphate Allocation Supports Phytoplankton Survival and Growth in the Nutrient‐Depleted Layer of the Northwest Pacific Ocean

Abstract Polyphosphate (polyP) represents a dynamic component of particulate phosphorus (P) in the upper ocean, yet its regulation and ecological significance for phytoplankton remain unclear. Here, we explored these processes in a cross‐latitudinal section (15°N–39°N, 150°E) of the Northwest Pacific Ocean, covering broad gradients in nutrient availability and P stress conditions. Comparative analyses in both horizontal and vertical dimensions revealed that phytoplankton P stress, indicated by biomass‐normalized alkaline phosphatase activity (APA:biomass Pico ), was positively correlated with the ratio of polyP to total particulate P (TPP) in the nutrient‐depleted layer (NDL) of the subtropical gyre. This relationship suggests a P‐stress response of phytoplankton in which elevated polyP:TPP ratios represent greater relative allocation of P to polyP compared with other forms of TPP. In contrast, declining polyP:TPP ratios under reduced P stress (lower APA:biomass Pico ), accompanied by increased abundance and biomass of picophytoplankton (0.2–2 μm), indicate a shift toward lower relative allocation of P to polyP. This flexible strategy was less evident in nutrient‐replete waters below the NDL and in the northward Kuroshio Extension areas, where different nutrient regimes complicated polyP allocation patterns compared with those observed in the NDL. Our results suggest that variation in relative polyP allocation may represent a physiological strategy associated with phytoplankton adaptation to P stress and potentially contribute to sustaining primary productivity in oligotrophic subtropical gyres.