Organic-walled dinoflagellate cysts in western equatorial Atlantic surfacesediments: distributions and their relation to environment

In contrast to the wide range of studies carried out in temperate and high- latitude oceanic regions, only a few studies have focused on recent and Hol ocene organic-walled dinoflagellate cyst assemblages from the tropics. This information is, however, essential for fully understanding the ability of species to adapt to different oceanographic regimes, and ultimately their p otential application to local and regional palaeoenvironmental and palaeoce anographic reconstructions. Surface sediment samples of the western equator ial Atlantic Ocean north of Brazil, an area greatly influenced by Amazon Ri ver discharge waters, were therefore analysed in detail for their organic-w alled dinoflagellate cyst content. A diverse association of 43 taxa was ide ntified, and large differences in cyst distribution were observed. The cyst thanatocoenosis in bottom sediments reflects the seasonal advection of Ama zon River discharge water through the Guyana Current and the North Equatori al Countercurrent well into the North Atlantic. To establish potential link s between cyst distribution and the environmental conditions of the upper w ater column, distribution patterns were compared with mean temperature, sal inity, density and stratification gradients within the upper water column ( 0-100 m) over different times of the year, using correspondence analysis an d canonical correspondence analysis. The analyses show that differences in these parameters only play a subordinate role in determining species distri bution. Instead, nutrient availability, or related factors, dominates the d istribution pattern. The only possible indicators of slightly reduced salin ities are Trinovantedinium applanatum and Lingulodinium machaerophorum. Fou r assemblage groups of cyst taxa with similar environmental affinities rela ted to specific water masses/currents can be distinguished and have potenti al for palaeoenvironmental reconstruction. (C) 2000 Elsevier Science B.V. A ll rights reserved.