Spatial heterogeneity of phytoplankton community structure across two distinct frontal systems in the NW Pacific: Insights from in situ high frequency observations

Background: In the Northwestern Pacific, the complex marine environment drives rapid phytoplankton responses to drastic environmental fluctuations. However, how these primary producers are influenced by intricate physical processes remains poorly understood due to the paucity of targeted meso- and fine-scale observations.

Material and methods: To address this knowledge gap, we conducted a longitudinal transect survey in the northwestern Pacific during September–October 2024. Sampling stations were deployed at mesoscale eddy locations identified by the SWOT satellite. Two distinct frontal zones were delineated along the transect: the southern front (F1) from cold- warm eddy interaction, and the northern front (F2) from Kuroshio-Oyashio convergence. High-frequency sampling (30-min intervals) was performed using an in situ CytoSense flow cytometer to characterize phytoplankton abundance and community structure.

Results: Our results revealed 11 distinct phytoplankton groups across the survey region. Picophytoplankton dominated the entire transect, with OrgPicoPro as the most dominant group followed by RedPico. Phytoplankton abundance exhibited distinct spatial gradients: the lowest values were recorded in warm eddies, followed by cold eddies, and the highest in the two frontal zones—particularly at Front F1, where abundance surged to 20–40 times that in warm eddies. Notably, despite the low phytoplankton abundance in warm eddies, the Shannon diversity index was the highest there. Two subgroups of OrgPicoPro were exclusively detected in warm eddies, whereas only one subgroup was present in all other regions; in contrast, RedPicoPro was absent in warm eddies but distributed in other areas.

Regarding community composition, Front F1 showed significant differences from warm eddies but high similarity to cold eddies. Acting as a barrier separating warm eddies from others, F1 explains the pronounced differences in phytoplankton abundance and community composition between warm eddies and other areas. In contrast, Front F2 exhibited less pronounced differences in both phytoplankton abundance and community composition from the adjacent Kuroshio and Oyashio water masses on either side. Additionally, temperature and salinity exerted different regulatory effects on phytoplankton community among different water masses.

Conclusion: Our findings demonstrate that phytoplankton community structure changes drastically across frontal zones and responds differentially to the two frontal systems, driving significant spatial heterogeneity in the NW Pacific. This study highlights the value of in situ high-frequency observations for unraveling fine-scale physical-biological coupling mechanisms, providing critical insights into phytoplankton ecological dynamics in complex marine environments.

Keywords: Spatial heterogeneity, phytoplankton community structure, frontal zone, Northwestern Pacific