Dinoflagellate cysts of sediment traps from June 1992 to February 1993 at two sites in the Somali Basin (northwestern Arabian Sea)

To date, relatively little information is available about factors influencing organic-walled cyst production of tropical dinoflagellates and processes influencing the final burial of cysts in bottom sediments, such as transport and preservation. To extend this information, cyst fluxes were documented for three sediment traps from June 1992 to February 1993 at two sites in the Somali Basin (northwestern Arabian Sea) as well as the cyst association of underlying sediments. By comparing cyst associations of contemporaneously collected trap samples at different depths at one site, information about transport and processes of decay in the water column was obtained. Neither transport nor decay appears to have any detectable influence on cyst association during cyst settlement through the water column. Comparing the trap associations with the underlying sediments indicates that downslope transport appears to have influenced the cyst association on a local scale only. Species-selective decay, probably related to the presence of oxygen in bottom sediments, has influenced the cyst association most pronouncedly at the most offshore site. Relating variations in the trap associations with environmental conditions of the overlying surface waters indicates that highest production of both filled and empty cysts occurs during the SW Monsoon upwelling. Based on this correlation three groups of species can be distinguished: Species with highest fluxes during (1) the first-half of the SW Monsoon (June-August); Bitectatodinium spongium, Echinidinium granulatum, Echindinium transparantum, Echinidnium spp., cysts of Protoperidinium compressum and cysts of Protoperidinium subinerme, (2) the transition between the SW-Monsoon and inter-Monsoon; Spiniferites mirabilis and Spiniferites spp., (3) no particular season; all other species. Cyst associations of all trap samples are dominated by cyst of Protoperidinium species. Cysts with highest fluxes during the SW-Monsoon form about a third of the associations