Upwelling and internal waves are known to affect the physical and chemical characteristics of marine ecosystems, yet the processes and mechanisms by which internal waves influence phytoplankton biomass and community composition in upwelling regions are still unclear. In this study, a 72-hour time-series of observations was conducted in an upwelling system in the northern South China Sea during the summer of 2014. The results showed that the intensity of upwelling was affected by internal waves, which caused nutrient fluctuations in the upper water column. The phytoplankton total chlorophyll a responded positively to the increase of nutrient concentrations, but only after a time lag of 12-16 hours. This overall response was the net result of four different types of responses displayed by nine specific phytoplankton groups. All groups that displayed immediate positive responses continued to respond positively at least 12 hours later, whereas all groups that responded negatively showed no time-lagged responses. Based on the vertical distributions of the nine phytoplankton groups and their known physiological traits, we suggest that these different types of response were the net result of a rapid physical transport effect and a time-lagged, physiological effect via bottom-up control.

Phytoplankton community; nitrate assimilation; time lag; mesoscale physical process; Taiwan Strait.