Phytoplankton community responses to natural and anthropogenic nutrient inputs in the Western Pacific Ocean: Insight from chemical taxonomy analysis

The Western Pacific Ocean (WPO), a vast oligotrophic system, plays a critical role in global carbon cycling, with its biogeochemical processes heavily influenced by complex hydrodynamics. This study investigated the controlling factors of phytoplankton distribution, biomass, and community composition across nine distinct water masses using HPLC-CHEMTAX (chemical taxonomy) analysis of diagnostic pigments. Prochlorophytes were the dominant phytoplankton community, comprising over 50% of the total biomass in most water masses, followed by Chlorophytes, Haptophytes, Cryptophytes, and Chrysophytes. Nutrient availability, primarily nitrate (NO₃-N) and phosphate (PO₄-P), emerged as the principal controlling factors of phytoplankton community structure. Divergence and episodic divergence event facilitated nutrient upwelling, enhancing phytoplankton biomass. River runoff inputs near the New Guinea Island elevated nitrate concentrations, promoting the aggregation of phytoplankton among the coastal zone. The prevalence of picophytoplankton under oligotrophic conditions underscored a community structure with implications for carbon export efficiency. These findings elucidate mechanistic links between physical forcings, nutrient dynamics, and phytoplankton ecology, providing a process-oriented understanding of natural and anthropogenic nutrient inputs and carbon cycling in the WPO with broader relevance to oligotrophic marine systems.