Effects of eddy environments on tintinnid distributions in the tropical western Pacific Ocean

The tropical western Pacific Ocean (TWPO) is a region where mesoscale eddies frequently occur. Tintinnid ciliates are a major component of marine microzooplankton, and they play a critical role in sustaining productive plankton assemblages in pelagic oceanic ecosystems. This study was conducted to clarify the effects of eddies on the spatial distribution of tintinnids in TWPO. Species composition, abundance and associated environments of tintinnids in the water column at the upper 200 m depth in an eddy area of the TWPO were investigated in September 2020 and August 2021. A slightly lower salinity was observed in the upper 50 m layer in 2021, but little changes in the water temperature distribution were observed in both years. Subsurface chlorophyll maximum (SCM) layer formed in the water column between 120 and 200 m water depth in 2020 correlated with the vertical distributions of tintinnid abundance; however, the SCM layers did not correlate with the notable increases in tintinnid abundance that occurred on the surface waters of stations T2n and T2 in 2021. The increase of tintinnid abundance might be caused by the eddy currents flowing toward these stations. Tintinnid distributions should be carefully assessed while considering eddy environments in the TWPO.