Seasonal dinoflagellate cyst production and terrestrial palynomorph deposition in the East Asian Monsoon influenced South China Sea: A sediment trap study from the Southwest Taiwan waters

Abstract

The South China Sea (SCS), influenced by the tropical East Asian Monsoon (EAM), experiences winter monsoon and summer monsoon shifts in near-surface wind, air temperature, sea-surface temperature, salinity, primary productivity, and other oceanographic conditions. To understand how monsoon seasons influence palynological dynamics and which palynological index could be a reliable indicator of winter or summer monsoons, we studied palynological records of sediment trap samples collected in March–April (winter monsoon season) and July–August (summer monsoon season). Fluxes and assemblages of terrestrial pollen and spores, as well as marine dinoflagellate cysts, were investigated using sediment traps in the southwest Taiwan waters of the SCS. The pollen and spores of 109 taxa and dinoflagellate cysts of 53 taxa were identified in 24 sediment trap samples that were collected at 3-day intervals. The average abundance of Pinus pollen was notably higher in March–April at ~40%, which was double that in July–August. This trend was associated with transport by the northwest wind in March–April when Pinus pollen are produced by the coniferous vegetation in the South China and Taiwan Island. The pollen abundances of Chenopodiaceae/Amaranthaceae and Compositae seemed to be greatly reduced in July–August, with an opposite pattern observed for Poaceae, Artemisia, Cyperaceae, Typha, and fern spores. Fluvial transport is likely the controlling factor since river runoff intensifies in summer. High relative abundances and fluxes of Poaceae pollen are not indicators of summer monsoons but related to cultivated activities.Dinoflagellate cyst assemblages were dominated by cysts produced by heterotrophic taxa, especially Brigantedinium spp. at 68–91%. Proportionally higher increases than those of cysts produced by autotrophic dinoflagellates resulted in lower heterotrophic to autotrophic ratios in July–August. The highest total cyst fluxes occurred in late July of ~20,000 cysts m-2 d-1 together with the highest fluxes of autotrophic Operculodinium centrocarpum, Spiniferites spp., and Lingulodinium hemicystum. The increased fluxes of almost all dinoflagellate cyst taxa during July–August were related to the decreased sea-surface salinity (SSS) due to the greater river water inputs and nutrients.