Difference in the vertical distribution of mesozooplankton in the western Indian Ocean

Abstract

We investigated the difference of vertical distribution of zooplankton community at two stations (A and B) characterized by Seychelles-Chagos Thermocline Ridge (SCTR) in the western Indian Ocean (WIO) in 2023. Zooplankton was collected at five depth strata from surface to 1,000 m with MultiNet (200-µm mesh, 1-m2 mouth-size) every 6h over 24h. Total mesozooplankton abundance was higher at station A corresponding to upwelling zone (2,214±1,309 inds./m3) than station B with no upwelling (1,123±199 inds./m3). The mesozooplankton abundance remained high in surface mixed layer (SML) (1,488±941 inds./m3) for 24h at station A, whereas the abundance was high in thermocline (571±137 inds./m3) except dawn at station B. Most zooplankton occurred higher in SML and thermocline at station A associated with high chlorophyll-a compared to station B. Mesozooplankton have mostly remained in SML and thermocline, while a few mesozooplankton remained constantly below thermocline at both stations. Radiolarians and Paracalanus parvus s.l. were numerically dominant in SML, and radiolarians and Pyrocystis noctilua dominated in thermocline at station A, while P. noctilua and radiolarians were common in SML and thermocline at station B. Most zooplankton did not exhibit diel vertical migration (DVM), whereas some mesozooplankton (>2 mm) showed normal DVM (Pleuromamma abdominalis, Scaphocalanus echinatus and Aidanosagitta neglecta) at both stations. At night, most DVM species ascended to SML at station A, whereas the DVM species ascended to thermocline at station B. These results suggested that SCTR affected variation in the vertical distribution of DVM species as well as total mesozooplankton in WIO during the study.