Planktonic ciliate community structure and its distribution in the oxygen minimum zones in the Bay of Bengal (eastern Indian Ocean)

Abstract

Oxygen minimum zones (OMZs) are geographically widespread phenomena among world seas, while knowledge regarding planktonic ciliate (microzooplankton) community structure in the OMZ is lacking. In this study, we analyzed community structure of planktonic ciliates in the OMZ waters of the Bay of Bengal, eastern Indian Ocean. Both hydrological characteristics (temperature, salinity, chlorophyll a in vivo fluorescence (Chl a) and dissolved oxygen (DO)) and planktonic ciliate community showed obvious variations in vertical profile. An obvious higher ciliate abundance and biomass occurred in the thermocline zones (surface to 100 m) than in the OMZs (150–800 m). For aloricate ciliates, abundance proportions of small (10–20 μm) and large (> 30 μm) size-fractions showed increase and decrease characteristics from thermocline zones to OMZs, respectively. With regard to loricate ciliates (tintinnids), more species richness and higher abundance occurred in the thermocline zones than in the OMZs. Four vertical distribution patterns were classified for tintinnids. Three species only occurred in the OMZs. Three-dimensional representation of tintinnid lorica length, lorica oral diameter and abundance proportion revealed that small-sized species in thermocline zones decreased more sharply to OMZs compared to large-sized species. Relationship between environmental variables and ciliate abundance showed that temperature, DO and Chl a were main factors determining ciliate abundance. Our results provide fundamental data for better understanding on the ecological role of planktonic ciliates in the thermocline zones and OMZs of the Indian Ocean.