Response of Photosynthesis-Irradiance Parameters to Rain-Derived Reactive Nitrogen Deposition in the Oligotrophic Subtropical Western North Pacific

Photosynthetic performance was investigated using photosynthesis-irradiance curve experiments conducted during shipboard observations in the western North Pacific. Surface photosynthetic parameters exhibited substantial variability, with maximum photosynthetic rates ranging from 2.33 to 8.46 mg C [mg chl-a]−1 h−1, and initial light-limited slopes ranging from 0.015 to 0.044 mg C [mg chl-a]−1 h−1 (μmol photons m−2 s−1)−1. A significant linear relationship between these parameters was observed in oligotrophic subtropical waters where environmental variability was minimal, indicating that variations in primary productivity were closely linked to physiological responses of phytoplankton. Analysis of rainfall events suggested that nitrogen limitation was temporarily alleviated by sporadic allochthonous inputs, such as atmospheric wet deposition of reactive nitrogen, prior to seawater sampling. This interpretation was further evaluated through additional shipboard observations under pre- and post-rainfall conditions. Photosynthetic parameters were enhanced following an episodic rainfall event, and estimated daily primary production increased from 4.5 mg C m−3 d−1 under pre-rainfall conditions to 6.5 mg C m−3 d−1 under post-rainfall conditions, representing a 1.4-fold increase. Based on the measured concentrations of reactive nitrogen in rainwater, the observed enhancement in carbon fixation could be quantitatively supported by atmospheric wet deposition. This observational study demonstrates that rainfall is an important source of allochthonous nutrients in oligotrophic marine environments and that rain-derived reactive nitrogen deposition can exert a significant influence on primary productivity.