Dinoflagellate cyst distribution along a shelf to slope transect of an oligotrophic tropical sea (Sunda Shelf, South China Sea)

From 51 surface samples collected along a shelf to slope transect of the Sunda Shelf, South China Sea, 36 taxa of organic-walled dinoflagellate cysts are identified. Oligotrophic tropical shelf assemblages on the Sunda Shelf are dominated by gonyaulacoids such as Spiniferites species, Operculodinium centrocarpum and Operculodinium israelianum. Concentrations of dinoflagellate cysts in the shelf sediments are generally low and correlate well with the content of fine-grained (clay and silt fraction) sediments. Detailed comparisons of sediment grain-size distributions to concentrations of dominant dinoflagellate taxa (Spiniferites species, O. centrocarpum and O. israelianum) in the shelf sediments indicate that these taxa behave in water like sediment particles with size range phi 5.75-6.25 (13-18 mum). In contrast, slope assemblages in fine-grained sediments are dominated by protoperidinioids. This may reflect higher nutrient availability as a result of weak winter upwelling. The concentrations of dinoflagellate cysts in the shelf sediments are mainly controlled by transport and winnowing processes and are probably not representative of surface water conditions.