Organic-walled dinoflagellate cysts and benthic foraminifera in coastal sediments of the last century from the Gulf of Tehuantepec, South Pacific Coast of Mexico

Abstract

Qualitative and quantitative analysis of recent organic-walled dinoflagellate cysts (dinocysts) was performed on surface sediment samples and a core from the continental shelf of the Gulf of Tehuantepec, Mexico, in order to document the spatial distribution of dinocyst assemblages in relation to upwelling and primary productivity, and to assess the environmental history of the last century. The analyses of surface sediment samples show a close relation between dinocyst assemblages and productivity on a regional scale. Polysphaeridium zoharyi and heterotrophic taxa (notably Brigantedinium spp.) dominate in the high productivity zone, whereas Spiniferites delicatus and other phototrophic taxa are more abundant in the lower productivity zone. Sediment in an eighteen cm long gravity core (dated using 210Pb and 137Cs) provided a record of the last century at annual to decadal resolution, thus yielding a unique opportunity to examine variations in dinocyst assemblages associated with environmental changes. Cyst concentrations in the core range between 477 and 2300 cysts g − 1 , giving cyst fluxes between 68 and 494 cysts cm − 2 yr − 1 . Twenty-three phototrophic and heterotrophic cyst taxa were identified. Brigantedinium spp., P. zoharyi and Bitectatodinium spongium are dominant, and are associated with the seasonal upwelling that characterizes the area. Cysts of potentially toxic species such as P. zoharyi (the cyst of Pyrodinium bahamense var. compressum and/or bahamense) occur throughout the core. Despite slight variations in relative abundances of the taxa in the assemblages, there is no evidence for eutrophication following industrial development of the adjacent coastal zone. Core samples were also analyzed for benthic foraminiferal content in order to determine possible effects of high, upwelling-induced productivity on bottom water oxygenation. The benthic foraminiferal assemblages are dominated by Hanzawaia concentrica (over 50%), with less abundant Uvigerina excellens, Cancris spp. , Planulina ornata, Quinqueloculina lamarckiana, Epistominella sandiegoensis, Nonionella basispinata, Cassidulina modeloensis and Textularia foliacea. The benthic foraminiferal assemblages are characteristic of oxygen concentrations above 1 ml l − 1 , indicating that possible changes in productivity did not significantly affect bottom water oxygen concentrations over the last 100 years.