Vertical distribution patterns of Radiolaria Polycystina (Protista) in the World Ocean: living ranges, isothermal submersion and settling shells

A critical review of information from ~7000 plankton, sediment trap and surface sediment samples from the World Ocean indicates that standing stock and species richness (i.e. number of species) of polycystine radiolarians are highest in the tropics and subtropics at 0–100 m (~1100–1400 ind. m−3), dropping sharply below 400 m (<1 to ~250 ind. m−3). Polewards of 45°N and 45°S, highest densities (~200–500 ind. m−3) occur at 100–400 m. In contrast with plankton samples, sediment traps yield highest radiolarian accumulation rates at the deepest layers, probably because of higher biogenic silica dissolution rates in the shallower traps. Deeper plankton and sediment trap samples yield more species than shallower ones; in polar–subpolar areas, many of these species represent in situ living populations, but in the low latitudes, they are largely represented by dead, settling individuals and by colder water species that submerge under the warm surficial layers. Staining of the protoplasm with Rose Bengal does not allow adequate discrimination of subsurface resident populations from dead or nonviable organisms settling from above. The vertical heterogeneity of radiolarian assemblages has significant implications for biogeographic and paleoecological surveys.