Vertical Distribution of Picoplankton Across A Cold Eddy in the West Pacific Ocean

Patrichka Chen Wei Yi^1,2Madeline Olivia^1,2Chia Mei Chang²Gwo Ching Gong^2,3Chung-Chi Chen^4,5Sen Jan⁶Clara Natalie Annabel²An Yi Tsai^1,2,3*

• ¹Doctoral degree program in Ocean Resource and Environmental Changes, National Taiwan Ocean University, Keelung, Taiwan
• ²Institute of Marine Environment and Ecology, National Taiwan Ocean University, Keelung, Taiwan
• ³National Taiwan Ocean University Center of Excellence for the Oceans, Keelung, Taiwan
• ⁴National Taiwan Normal University School of Life Science, Taipei City, Taiwan
• ⁵College of Marine Sciences, National Dong Hwa University, Hualien, Taiwan
• ⁶National Taiwan University Institute of Oceanography, Taipei City, Taiwan

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 under explored. In contrast with the surrounding waters, a cyclonic cold eddy can provide nutrients to the photic zone, increasing primary production and altering microbial communities. We conducted a field investigation 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 adjacent water at 100 m depth. Flow cytometry analyses were used to estimate the abundances of picoplankton populations (heterotrophic bacteria and picophytoplankton) in seawater samples collected from the surface to 1000 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 about 2 × 105 cells mL−1 at the surface and increased to >4 × 105 cells mL−1 at 200 m depth at stations of outside 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.