講演情報
[ACG67-P01]Rainfall-Driven Reactive Nitrogen Deposition Enhances Phytoplankton Photosynthetic Performance in the Oligotrophic Subtropical Western North Pacific
*松本 和彦1、竹谷 文一1 (1.国立研究開発法人 海洋研究開発機構)
キーワード：
大気沈着、降水、一次生産、P-Eカーブ
Photosynthetic performance of phytoplankton was investigated in the oligotrophic subtropical western North Pacific using photosynthesis–irradiance (P–E) curve experiments conducted during two shipboard cruises in summer (MR18-04) and winter (MR21-01). This region is characterized by low nutrient and low chlorophyll (LNLC) conditions, where nitrogen availability strongly constrains primary productivity. During the summer cruise, surface photosynthetic parameters exhibited substantial variability among subtropical sites despite similarly oligotrophic conditions. The maximum photosynthetic rate (PmaxB) ranged from 2.33 to 8.46 mg C [mg chl-a]-1 h-1 and the initial slope (αB) from 0.015 to 0.044 mg C [mg chl-a]-1 h-1 (μmol photons m-2 s-1)-1. These parameters showed a strong linear relationship, suggesting coherent physiological responses of phytoplankton. Because no clear differences in temperature, nutrient concentrations, or community composition were detected among sites, episodic external nutrient inputs were inferred as a potential driver of this variability.
Rainfall histories and reactive nitrogen (Nr; nitrate and ammonium) concentrations in rainwater were analyzed to evaluate atmospheric wet deposition as an external nitrogen source. Differences in estimated Nr deposition prior to sampling were consistent with the spatial variability in surface photosynthetic performance. This interpretation was further tested during the winter cruise by comparing pre- and post-rainfall conditions. Following an intense rainfall event, surface P–E parameters increased and remained elevated for at least two days, despite no detectable increase in surface chlorophyll a concentration. Estimated daily primary production increased from 4.5 to 6.5 mg C m-3 d-1, representing a 1.4-fold enhancement, which was quantitatively supported by the estimated rain-derived Nr input. These results provide direct in situ evidence that short-term rainfall events can enhance phytoplankton photosynthetic performance and primary productivity in oligotrophic subtropical oceans, even when biomass-based indicators remain unchanged.
Rainfall histories and reactive nitrogen (Nr; nitrate and ammonium) concentrations in rainwater were analyzed to evaluate atmospheric wet deposition as an external nitrogen source. Differences in estimated Nr deposition prior to sampling were consistent with the spatial variability in surface photosynthetic performance. This interpretation was further tested during the winter cruise by comparing pre- and post-rainfall conditions. Following an intense rainfall event, surface P–E parameters increased and remained elevated for at least two days, despite no detectable increase in surface chlorophyll a concentration. Estimated daily primary production increased from 4.5 to 6.5 mg C m-3 d-1, representing a 1.4-fold enhancement, which was quantitatively supported by the estimated rain-derived Nr input. These results provide direct in situ evidence that short-term rainfall events can enhance phytoplankton photosynthetic performance and primary productivity in oligotrophic subtropical oceans, even when biomass-based indicators remain unchanged.