Comment on bg-2022-151

Abstract

Though diel variations are geographically widespread phenomena among phytoplankton and zooplankton, knowledge is limited regarding diel variations in planktonic ciliate (microzooplankton) community structure. In this study, we analyzed diel variations in community structure of planktonic ciliates in the northern South China Sea (nSCS) and tropical Western Pacific (tWP). Hydrological characteristics during day and night were slightly different over both the nSCS and tWP, while ciliate average abundance in night was clearly higher than in day in the upper 200 m. The differences in weighted mean depth (WMD) for aloricate ciliates and tintinnids revealed that they preformed diel vertical migrations. In the nSCS, the WMDs of both aloricate ciliates and tintinnids were higher in day than in night. However, in the tWP, the WMDs of aloricate ciliates were higher in day than in night, whereas it was the opposite for tintinnids whose WMDs were lower in day than in night, indicating that they were in phase opposition with aloricate ciliates. In both the nSCS and tWP, abundance proportions of large size-fraction aloricate ciliates in night were higher than in day. While for tintinnids, abundance proportion of large lorica oral diameter in night were lower than in day. The relationship between environmental factors and ciliate abundance pointed out that depth and temperature were main factors influencing aloricate ciliate and tintinnid abundances in both day and night. For some dominant tintinnid species, chlorophyll a was another important factor influencing diel vertical distribution. Our results provide fundamental data for better understanding the diel vertical migration ecological role of planktonic ciliates in the tropical Western Pacific Ocean.