Environmental factors controlling the dynamics of phytoplankton communities during spring and fall seasons in the southern Sunda Shelf.

Phytoplankton play important roles in marine ecosystems and have major impacts on the global biogeochemical cycles, yet the knowledge of the processes for oceanic environments controlling the phytoplankton dynamics still needs to be improved, particularly in the coastal ecosystems. Monitoring diversity of phytoplankton communities, including micro/nano- and pico-forms, as well as their dynamics in relation to environmental factors, is thus of critical significance. Here, we conducted two cruises in the southern Sunda Shelf during spring and fall 2016 to investigate the community structure and dynamics of phytoplankton assemblages in relation to environmental factors combining microscopy and flow cytometry. Micro/nanophytoplankton communities changed from Trichodesmium spp. and diatoms co-dominance in spring to communities dominated by Trichodesmium spp. in fall. Diatoms were a highly diverse group, with 78 species, followed by dinoflagellates (35 species). Differences in average abundance of micro/nanophytoplankton between spring and fall might be attributed to the shift in dominance of Trichodesmium spp. and diatoms. Picophytoplankton characterized by a great abundance of picocyanobacteria were dominated by Synechococcus. The dynamics of picophytoplankton during spring and fall might be associated with the differences in dominant Synechococcus strains. Environmental variables had significant influence on the dynamics of phytoplankton communities, especially nitrate and silicate. Silicate was a major driver responsible for the variations of micro/nanophytoplankton. Low silicate potentially limited the diatom growth during fall, leading to the overwhelming dominance of Trichodesmium spp. Picophytoplankton showed positive correlations with nitrate and silicate, suggesting that these nutrients collectively controlled variations in picophytoplankton communities during spring and fall.