Abstract
The plankton radiolarian community was investigated in the spring season during the two-month cruise ‘Shiyan1’ (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 a 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—Sticholonche zanclea, 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 Sphaerozoum punctatum and Stichonche zanclea 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 a concentration and those of a north–south transect to surface silicate concentrations or mixed-layer nitrate were significantly correlated (p < 0.01) to the radiolarian community. Our results indicate that the silicate and chlorophyll-a concentrations are the two major factors affecting the radiolarian distribution along two of the investigated transects (southern equator and equator) in the study area.
Highlights
Radiolaria are the siliceous-shelled, single-cell, planktonic organisms frequently studied for palaeoceanographic and environmental reconstruction
An environmental dataset was constructed comprising the values for temperature, salinity and chlorophyll a measured during the cruise at the depths (0 m, 50 m, 75 m, 100 m and 200 m) were derived from CTD data (Figure 4a–i)
Nitrate and oxygen are necessary for radiolarian growth so the connection with the variance is likely to be causal. (b) The variance in an equatorial transect (“Lat-0”) is strongly related to the chlorophyll a level in the mixed layer but it is not certain that chlorophyll a is necessary for radiolarian growth
Summary
Radiolaria are the siliceous-shelled, single-cell, planktonic organisms frequently studied for palaeoceanographic and environmental reconstruction. Radiolarian protists have opaline silica (SiO2 -nH2 O), strontium sulfate or calcium carbonate skeletons and are characterized by central capsule shells and axopodia spines [5]. They contribute significantly to the deposition of silicate from the ocean’s upper layer to the bottom sediments [6,7]. They are the second most important producers of biogenic opal and siliceous oozes in the oceans, exceeding the effect of diatoms in equatorial regions [8].
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