Radiolarian assemblages in the eastern tropical Atlantic: patterns in the plankton and in sediment trap samples

Polycystine Radiolaria were studied in 40 Multinet (63 μm) plankton samples collected in February–March 1988 in the eastern tropical Atlantic (10 °N to 17 °S), from depths ranging between 0 and 300 m. A total of 148 taxa were recorded, most of them accounting for very low proportions of the individuals identified. The geographic distribution of polycystine assemblages throughout the area was relatively homogeneous and quite unpatterned; no clear associations with latitude, surface temperature, salinity or chlorophyll a were found. On the other hand, several radiolarians showed fairly distinct vertical profiles, allowing identification of forms preferring the uppermost 25–50 m, forms peaking at 50 to 150 m, and forms with highest percentage contributions below 150 m. Comparison of this planktonic collection with a similar survey carried out in 1983 in the same area indicates good overall agreement. In contrast, radiolarian compositions in sediment trap samples deployed at 853 and 2195 m in close vicinity with some of the plankton stations surveyed are totally different. Subsurface advection of shells produced at higher latitudes and integration of low radiolarian abundances over large depth-intervals are the most likely cause of the inconsistencies.