Palynomorph Assemblages Dominated by Heterotrophic Marine Palynomorphs in Tropical Coastal Shallow-water Sediments from the Southern Myanmar Coast

Abstract

The goal of this study was to characterize palynomorph assemblages in tropical marine coastal shallow-water sediments collected from the southern coast of Myanmar, and possibly find them as useful tools for reconstructing changes in the tropical coastal environment. These sediment samples were dominated by heterotrophic marine palynomorphs, particularly in microforaminiferal linings and heterotrophic dinoflagellate cysts. In addition, these tropical marine palynomorph assemblages were characterized by low cell/grain concentrations, especially in photo/mixotrophic dinoflagellate cysts. These marine palynomorph characteristics are common to other tropical coastal surface sediments collected from Southeast Asia. These assemblages may reflect a diagnostic food web that characterizes tropical coastal shallow waters. For example, benthic foraminifers (microforaminiferal linings) that are always dominated usually consume prey organisms composed of bacteria, diatoms, dinoflagellates and amorphous organic substances. And bacteria can utilize dissolved organic matter delivered from terrestrial and marine environments via various organisms that inhabit water and sediment surfaces. The dominance of microforaminiferal linings appears to result from both microbial and grazing food webs in tropical coastal shallow-water sediments. Discipline: FisheriesAdditional key words: coastal environment, dinoflagellate cyst, microbial loop, microforaminiferal lining, tinntinomorph IntroductionMarine palynomorphs are microscopic organic-walled organism remains. These remains consist of different groups of planktonic and benthic microorganisms, such as the resting cysts of dinoflagellates, resting cysts and lorica of tintinids (tintinomorphs), organic linings of smaller benthic foraminifers (microforaminiferal linings), appendages, body and resting eggs of crustanceans (mainly copepods), and micro-remains of unknown organisms called acritarchs (Matsuoka et al. 2017). These organic-walled micro-remains preserved in sediments are regarded as a useful proxy of coastal environments and used in reconstructing coastal marine environmental history. In particular, dinoflagellate cysts are known as the most useful marine palynomorphs, typically employed for understanding eutrophication mainly in temperate to sub-boreal regions (Dale 2009, Matsuoka 2011, Pospelova 2005. Also along tropical Southeast Asian coasts, several studies have examined dinoflagellate cysts found in surface sediments of the Philippines (Baula et al. 2011, Furio et al.