Diversity and distribution of micro-phytoplankton and harmful microalgae along the Malaysian coasts of Malacca Strait and South China Sea

This study aims to examine the spatial distribution and diversity of micro-phytoplankton assemblages, with a particular focus on harmful microalgal species, in two different water masses of tropical Western Pacific, the Malacca Strait (MS) and the South China Sea (SCS). The synergetic responses of climate change and anthropogenic pressures in this region may lead to increase geographical expansion of harmful algal blooms. The micro-phytoplankton taxa were identified from 50 stations during a scientific research expedition in March 2022 and the in situ water parameters of temperature, salinity, pH, dissolved oxygen, macronutrients, and chlorophyll- a biomass were determined. Our results revealed 69 taxa (60 genera) of micro-phytoplankton, with Chaetoceros, Coscinodiscus, and Skeletonema predominately present throughout the study sites. High abundances of diatoms, either monospecific or multispecies co-dominance, were observed in environmental niche with high nutrient availability. While dinoflagellates, particularly the harmful species, such as Alexandrium, Prorocentrum, Dinophysis, Akashiwo sanguinea, Karenia, Karlodinium, Margalefidinium, and Noctiluca scintillans, were positively associated with higher ratios of nitrogen to phosphorus. Our findings indicated that spatial heterogeneity in micro-phytoplankton community and the prevalence of certain harmful microalgal species could be explained by the physicochemical environmental gradients in the region.