Coastal micro-phytoplankton community changes during the toxigenic Alexandrium minutum blooms in a semi-enclosed tropical coastal lagoon (Malaysia, South China Sea)

Estuarine lagoons often experience eutrophication due to anthropogenic activities in proximity and poor water exchange. Deterioration of water quality in the lagoon has driven the shifts in micro-phytoplankton communities that often form coastal harmful algal blooms (HABs). This study inves-tigated changes in the micro-phytoplankton community in a tropical semi-enclosed lagoon, Geting Lagoon, for two consecutive years during the study periods of 2014/2015 (September 2014-March 2015) and 2015/2016 (August 2015-March 2016). A total of 38 phytoplankton taxa were identified throughout the sampling periods, with 17 taxa of harmful algae identified microscopically. The diatoms Skeletonema costatum and Chaetoceros were predominant in the lagoon. The lagoon was significantly enriched in macronutrients, and high overall nitrogen and phosphorous concentrations likely supported substantial phytoplankton blooms in the lagoon throughout the studied periods. It was observed that with similar temperatures and nutrient availability between the 2014/2015 and 2015/2016 periods, the density of A. minutum in 2014/2015 did not reach as high as in those of the 2015/2016 period, in which higher precipitations, streamflow, and decreased in salinity stratification index had prevented the development of A. minutum bloom. The findings of this study revealed that the coastal micro-phytoplankton community was dependent on the interplay of environmental variables and unique strategic adaptations of the functional types in the assemblages. This study provides valuable insights into the phytoplankton community dynamics in a tropical eutrophic lagoon. (c) 2022 Elsevier B.V. All rights reserved.