Polycystine radiolarians within oligotrophic waters: higher abundance closer to tropical oceanic islands

We hypothesize that the polycystine radiolarians suffer modifications in density, species richness/diversity, as well as in the assemblage structure with the proximity of the Saint Peter and Saint Paul archipelago at the tropical South Atlantic. To address this, we conducted two oceanographic campaigns throughout the maximum (April) and minimum (November) rainfall period of 2015. A total of 24 stations were delimited around the archipelago, in the proximities of the 50 and 500 m isobaths (closer and farther areas). Water samples were collected between 1 and 100 m with a 10 L Niskin bottle and fixed with lugol. A CTD (conductivity–temperature–depth profiler) was used to obtain temperature and salinity profiles. Surface temperature and salinity presented minor variation and the depth of the onset of thermocline varied between 16 and 63 m. An increase in polycystine radiolarians density was observed with the proximity to the archipelago, with higher values during the minimum rainfall period (two-way ANOVA, P < 0.001). This increase may be associated with a higher concentration of food items in closer areas. The structure of the assemblage was also influenced by distance and rainfall regime (PERMANOVA, P < 0.001), and shifts in the proportion of the relative abundance of rare species were the main cause. A total of 78 taxa were identified and a large portion of the assemblage was composed by juveniles, especially in closer areas, limiting proper identification. Thus, higher densities and a higher proportion of juveniles were observed closer to the archipelago in relation to farther areas.