Description: To obtain insight in the relationship between the spatial distribution of organic-walled dinoflagellate cysts (dinocysts) and local environmental conditions, fifty-eight surface sediment samples from the coastal shelf off SW Africa were investigated on their dinocyst content with special focus on the two main river systems and the active upwelling that characterise this region. To avoid possible overprint by species-selective preservation, samples have been selected mainly from shelf sites where high sedimentation rates and/or low bottom water oxygen concentrations prevail. Multivariate ordination analyses have been carried out to investigate the relationship between the distribution patterns of individual species to environmental parameters of the upper water column and sediment transport processes. The main oceanographical variables at the surface (temperature, salinity, nutrients chlorophyll-a) in the region show onshore-offshore gradients. This pattern is reflected in the dinocyst associations with high relative abundances of heterotrophic dinocyst species in neritic regions characterised by high chlorophyll-aand low salinity conditions in surface waters. Phototrophic dinocyst species, notably Operculodinium centrocarpum, dominate in the more oceanic area. Differences in the distribution of phototrophic dinocyst species can be related to sea surface salinity and sea surface temperature gradients and to a lesser extent to chlorophyll-a concentrations. Apart from longitudinal gradients the dinocyst distribution clearly reflects regional environmental features. Six groups of species can be distinguished, characteristic for (1) coastal regions (cysts of Polykrikos kofoidii and Selenopemphix quanta), (2) the vicinity of active upwelling (Brigantedinium spp., Echinidinium aculeatum, Echinidinium spp. and Echinidinium transparantum), (3) river mouths (Lejeunecysta oliva, cysts of Protoperidinium americanum, Selenopemphix nephroides and Votadinium calvum), (4) slope and open ocean sediments (Dalella chathamense, Impagidinium patulum and Operculodinium centrocarpum, (5) the southern Benguela region (south of 24°S) (Spiniferites ramosus) and (6) the northern Benguela region (north of 24°S) (Nematosphaeropsis labyrinthus and Pyxidinopsis reticulata). No indication of overprint of the palaeo-ecological signal by lateral transport of allochthonous species could be observed.

Description: To investigate the potential use of the stable isotope composition of the vegetative cysts of the photosynthetic dinoflagellate Thoracosphaera heimii for quantitative palaeotemperature reconstructions a method has been developed to purify T. heimii cysts from sediment samples. Stable oxygen and carbon isotopes have been measured on T. heimii cysts from 21 surface sediment samples from the equatorial Atlantic and South Atlantic Oceans. Calculated temperatures based on the palaeotemperature equation for inorganic calcite precipitation generally reflect mean annual temperatures of the upper water column, notably of thermocline depths. Although the present results suggest that the isotopic composition of T. heimii shells might be formed in equilibrium with the seawater in which the shells are being formed, future investigations are required to determine possible effects of metabolic and kinetic processes on the fractionation process. This pilot study therefore forms the basis for future investigations on the development of this tool and the determination of a species-specific palaeotemperature equation. The wide geographic and stratigraphic distribution of T. heimii cysts in sediments, the stable position of T. heimii within the water column and the high resistance of its cysts against calcite dissolution underline its potential for a wide usability in palaeotemperature reconstructions.

Description: For the optimal use in palaeoceanographic studies of the stable oxygen isotopic signal and elemental composition of the calcareous photosynthetic dinoflagellate Thoracosphaera heimii, it is essential to gain detailed information about its calcification depth and spatial distribution. We therefore studied the vertical and horizontal distribution patterns of T. heimii in the upper water column (0-200 m) along three transects: an inshore-offshore gradient off Cape Blanc (CB), a south-north transect from CB to the Portuguese coast and a north-south transect off Tanzania. We compared concentrations of living cysts (cells with cell content) with chlorophyll-a, salinity and temperature measurements at the sampling depth. In order to explore the seasonal variability in cyst production, three transect off CB were sampled at three different times of the year. Living T. heimii cysts were found in the upper 160 m of the water column with highest concentrations in the photic zone indicating that the calcification of T. heimii occurs in the upper part of the water column. Maximal abundances of living cysts were found relatively often in or just above the deep chlorophyll maximum (DCM), the depth of which varies regionally from about 20-40 m off CB to about 80 m off Tanzania and along the transect from CB to the Portuguese Coast. However, there was no significant correlation at the 95% confidence level between the cyst concentrations and temperature, salinity and chlorophyll-a concentrations at the sampling depths observed. In both the Atlantic and Indian Oceans, the highest abundances of T. heimii were observed in regions where the upper water masses contained relatively low nutrient concentrations that are influenced only sporadically, or not at all, by enhanced photic zone mixing related to the presence of upwelling cells or river outflow plumes at or close to the sampling sites. The seasonal production of cysts by T. heimii appears to be negatively related to the presence of upwelling filaments across the sampling sites. Our study suggests that turbulence of the upper water masses is a major environmental factor influencing T. heimii production.