Different responses of plankton community to mesoscale eddies in the western equatorial Pacific Ocean

The western equatorial Pacific Ocean exhibits a complex pattern of currents and mesoscale eddies. However, the impacts of dynamic hydrological systems on plankton community remain uncertain due to the low resolution of field observation. In this study, we conducted a comparative study involving regional high-resolution field observations, satellite observations, and the numerical model. Our aim was to clarify the spatial distribution characteristics and underlying factors influencing phytoplankton communities in the western equatorial Pacific Ocean under different currents and mesoscale eddies. The combination of field observations and numerical model suggested that the Mindanao Eddy (cyclonic eddy) exhibited a significantly higher total chlorophyll a concentration than the North Equatorial Current, and the contributions of diatom increased as well. In the subtropical gyre, although the intensity of a transient cyclonic eddy was weak, it still lifted the subsurface phytoplankton up and increased their chlorophyll a concentration. In the Halmahera Eddy (anti-cyclonic eddy), however, Prochlorococcus and haptophytes were the dominant groups, with lower total chlorophyll a compared to the surrounding North Equatorial Counter Current. Generally, the distributions of these phytoplankton communities were primarily influenced by variations in temperature and nutrient concentrations. Interestingly, this study further revealed that both cyclonic and anticyclonic eddies enhanced the grazing rate of microzooplankton through the increasing of prey concentration and temperature, respectively. These factors driven by different eddies affect phytoplankton communities directly or indirectly through bottom-up and top-down controls, and demonstrate the impact of different mesoscale eddies on phytoplankton community in the western equatorial Pacific Ocean, which is a region of great significance to both the regional and global ecosystems.