Environmental factors influencing cyst formation and preservation of organic-walled dinoflagellates: an environmental and laboratory study

Abstract

Organic-walled dinoflagellate cysts are often applied to reconstruct palaeoenvironments, notwithstanding the fact that the factors triggering encystment and cyst burial in sediments are only poorly understood. This information is essential if we want to fully exploit the dinoflagellate toolbox for palaeoceanography; i.e. to obtain reliable reconstructions of bygone conditions of the marine environment. Therefore, a large part of this thesis is devoted to the processes leading from cyst formation to cyst burial. Sediment trap samples from the upwelling area off NW Africa were analysed for their organic-walled dinoflagellate cyst content. Upper ocean conditions in the (sub)tropical Atlantic Ocean off Cape Blanc are strongly determined by almost permanent upwelling related to NE trade winds. Upwelling results on the surface in high productivity and lower temperatures. Additionally productivity is influenced by eolian dust from the Sahara, which provides the upper water column with trace elements. Seasonal variations in dust input, upwelling intensity and sea surface temperatures influence the production of organic-walled dinoflagellate cysts. High nutrient content particularly induces high cyst fluxes of organic-walled dinoflagellates. Analyses of cyst associations collected over a period of 4 years indicate that in this region the most important factor affecting encystment is trace elements supply e.g. iron by eolian dust. Cyst production of Protoperidinium monospinum and P. americanum can be positively correlated to the lithogenic input. Off Cape Blanc upwelled water can contain varying admixtures of relatively nutrient rich South Atlantic Central Water (SACW). During periods when SACW feeds the onshore transport of upwelled water, increased production of Lingulodinium machaerophorum is observed. The fluxes of Brigantedinium spp. vary similar to the total diatom flux. Although upwelling is important factor influencing biological production off NW Africa no clear relationship between surface temperatures (SST), upwelling intensity and cyst production is found in the long-term observations. Brigantedinium spp. together with another heterotrophic taxa Protoperidinium monospinum represent the dominating taxa of trap associations. Autotrophic taxa constitute 1 to 4% of the assemblages only. The surface sediments collected under sediment traps are also dominated by heterotrophic taxa with co-domination of Brigantedinium spp. and Protoperidinium monospinum. However, the proportion of heterotrophic taxa decreased, which is a result of species selective aerobic degradation. Although Protoperidinium monospinum constitutes an important element of analysed associations this species is documented for the first time from an upwelling area. Probably because the morphology of this species is such that it might often be mistaken for a species of Echinidinium or Islandinium. Although, paleoecological research usually uses the changes in the composition of cyst associations, many species also vary considerably in their cyst morphology. Observations from natural environment indicate that some of these variations may be induced environmentally. To investigate this problem directly, Tuberculodinium vancampoae (motile affinity Pyrophacus steinii) culture experiments were carried out. Cultures were established by germinating of single cysts from surface sediments. The species was cultured at different temperatures, light intensities and salinities. Observations indicate that encystment of T. vancampoae occurs in a wide range of environmental conditions. Maximal encystment is observed at 27 à ��à ¯Ã ¿Ã ½Ã ��à �à °C, similar to what is known from the field. Temperature seems to have the largest effect on morphology. In extreme temperatures a reduction of processes is observed. Also salinity affects T. vancampoae morphology. The cyst-to-tubercule length ratio increases with increasing salinity between 20 and 40psu. Although earlier observations indicate that this species is heterothallic, our results suggest that homothally occurs in this species as well. A similar experiment on Gonyaulax sp. was prepared. However, cyst production halted during the experiment run and encystment could not be restored. All of the aspects investigated during this work indicate that there is a link between cyst production and environment. Obtained results indicate that encystment coincides with elevated nutrient availability. Despite this fact a lot of research remains to be done to completely understand factors triggering encystment as is clearly illustrated by the laboratory work on Gonyaulax sp. The study also clearly demonstrates that the primary signal can be strongly modified by secondary processes. Therefore, the successful application of organic-walled dinoflagellate cysts for climate reconstruction, still awaits more detailed answers on the processes determining the cyst associations in sediments.