Vertical distribution of picoplankton across a cold eddy in the West Pacific Ocean

Mesoscale eddies are recognized as an important driver of environmental and microbial dynamics in the Pacific Ocean. However, their specific impact on microbial community vertical distribution remains underexplored. In contrast with the surrounding waters, a cyclonic cold eddy can provide nutrients to the photic zone, increasing primary production and altering microbial communities. A field investigation was conducted in the West Pacific Ocean during cyclonic cold eddy propagation to determine the impact of cold eddies on bacteria and picophytoplankton. Within the cold eddy region, dissolved inorganic nitrate concentrations were higher than those in adjacent water at 100-m depth. Flow cytometric analyses were used to estimate the abundances of picoplankton populations (heterotrophic bacteria and picophytoplankton) in seawater samples collected from the surface to 1,000 m. In this study, Prochlorococcus was the dominant component of abundance at depths above 100 m (62% to 95%). Interestingly, when compared to outside of eddy stations, cyclonic eddy-affected regions had the lowest maximum value of Synechococcus and Prochlorococcus. In contrast to Synechococcus and Prochlorococcus, the distribution pattern of picoeukaryotes abundance showed maxima values (>0.4 × 10³ cells mL⁻¹) at 100–200-m depth within cyclonic eddy-affected regions. Overall, the abundance of bacteria was approximately 2 × 10⁵ cells mL⁻¹ at the surface and increased to >4 × 10⁵ cells mL⁻¹ at 200-m depth at stations outside the eddy. However, peaks in bacterial abundance were observed at 50-m depth, and abundance decreased with increasing depth in the cyclonic eddy-affected regions. In the oligotrophic open ocean, our findings contribute to a better understanding of the biological response of mesoscale eddies.