Status of the Java upwelling area (Indian Ocean) during the oligotrophic northern hemisphere winter monsoon season as revealed by coccolithophores

Abstract

Coccolithophores were used as sensitive indicators for the present-day functioning of the Java upwelling area to better understand the key role of this region in the tropical Indian Ocean dipole mode. Sampling was conducted during the oligotrophic phase of the monsoonal cycle. At all sites low abundance coccolithophore communities were found with surface assemblages either dominated by Gephyrocapsa oceanica or Umbellosphaera irregularis or a diverse mixture including the two named species plus Palusphaera vandelii, Ophiaster hydroideus, Syracosphaera spp. and Calciosolenia spp. with no clear dominance of a single species. Below a well defined thermocline a deep-photic community was dominated by Florisphaera profunda, but also with high abundances of Algirosphaera robusta, Gladiolithus flabellatus and Oolithotus antillarum. In the sediments only 19 taxa were found with the three species F. profunda, G. oceanica, and E. huxleyi making up more than 90% of the assemblages. The observed living communities were part of the recurrent seasonal cycle and corresponded to the environmental conditions. The sediment assemblages distinctly differ from the living communities. Alteration effects appeared to be strong but were probably not the only cause for the biases between the living communities and the sediment assemblages. The communities during ongoing upwelling also significantly contributed to and determined the sedimentary assemblage.