The Horizontal Distribution of Siliceous Planktonic Radiolarian Community in the Eastern Indian Ocean

The plankton radiolarian community was investigated in the spring season during the two-month cruise ‘<i>Shiyan</i>1’ (10 April–13 May 2014) in the Eastern Indian Ocean. This is the first comprehensive plankton tow study to be carried out from 44 sampling stations across the entire area (80.00°–96.10° E, 10.08° N–6.00° S) of the Eastern Indian Ocean. The plankton tow samples were collected from a vertical haul from a depth 200 m to the surface. During the cruise, conductivity–temperature–depth (CTD) measurements were taken of temperature, salinity and chlorophyll <i>a</i> from the surface to 200 m depth. Shannon–Wiener’s diversity index (H’) and the dominance index (Y) were used to analyze community structure. There was a total of 168 plankton species, composed of Acantharia, Phaeodaria, Polycystina, Collodaria and Taxopodida (monospecific—<i>Sticholonche zanclea,</i> Hertwig is the only recognized species). Hence, it included both celestine-based and siliceous organisms, which are also described here for the first time from this region. Total radiolarians ranged from 5 to 5500 ind/m⁻³, dominated by co-occurrences of <i>Sphaerozoum punctatum</i> and <i>Stichonche zanclea</i> species at the south-equator zone (SEQ)-transect 80° E and equator zone (EQ)-transect Lati-0. The possible environmental variables were tested through RDA analysis; although no result was obtained for the full species dataset, the samples from the equatorial transect related strongly to mixed-layer chlorophyll <i>a</i> concentration and those of a north–south transect to surface silicate concentrations or mixed-layer nitrate were significantly correlated (<i>p <</i> 0.01) to the radiolarian community. Our results indicate that the silicate and chlorophyll-<i>a</i> concentrations are the two major factors affecting the radiolarian distribution along two of the investigated transects (southern equator and equator) in the study area.