Factors governing phytoplankton biomass and production in tropical estuaries of western Taiwan

• Functional traits were used to explore factors governing phytoplankton production.
• Phytoplankton biomass and production were positively correlated with water temperature.
• Phytoplankton biomass and production were negatively correlated with turbidity.
• The threshold of turbidity was 12 NTU.
• Temperature and SiO2 concentration affected the community composition and production.

Factors governing phytoplankton community composition and production in tropical estuaries remain mostly unknown. We aimed to quantify phytoplankton biomass, production, and community composition seasonally in 2 tropical estuaries with different levels of nutrient concentrations and turbidity, and we compared them with an offshore control site on the western coast of central Taiwan for two years. Phytoplankton biomass and production varied with season and site. Annual integrated primary production showed that these three sites were mesotrophic systems. Spearman rank correlations showed that phytoplankton biomass and production were positively correlated with water temperature, but negatively correlated with turbidity. The threshold of turbidity was 12 Nephelometric Turbidity Units (NTU), above which phytoplankton chlorophyll a concentrations were <0.5 mg m−3, and gross production rate was <100 mg C m−3 d−1. The results of nonmetric multidimensional scaling (MDS) showed that the community was primarily structured by season and secondarily by site. The functional traits further showed that turbidity, water temperature, and SiO2 concentration were governing factors for the variations in the community. In summary, turbidity was the main factor governing phytoplankton biomass and production, whereas water temperature and SiO2 concentration had both a direct effect on production and an indirect effect by changing community composition.