Episodic shifts of phytoplankton community and biomass during a restricted summer coastal upwelling event off the Southeast China

Upwelling supports a diversity of organisms in marine ecosystems. Wind-driven upwelling occurs off the southeast China during summertime. Interannual variability in the intensity, extent and central location of this upwelling presented, resulting in spatial heterogeneity in surrounding environment. A restricted coastal upwelling event visible in satellite contours and concave isolines of temperature, phosphate and silicic acid was observed across a geographical area with weak intensity, accompanied by spatial heterogeneity of nutrients in June 2009. Peaks in depth-averaged chlorophyll a (Chl a, 0.62 and 0.83 mg/m3) and phytoplankton abundance (10.51 and 9.23 × 103 cells/L) in two patches near the upwelling center were consistently exhibited, with maxima at 10 m and 30 m, respectively. A total of 122 species from 41 genera of diatoms and 33 species in 14 genera of dinoflagellates were identified by the Utermöhl method, as well as 4 species in Cyanobacteria and Chrysophyta. Rhizosolenia alata was the most dominant species, followed by Thalassionema nitzschioides, Pseudonitzschia pungens, R. alata f. gracillima, T. frauenfeldii and Paralia sulcata. The restricted upwelling caused an upward transport of deep-water associated flora including Heterodinium blackmanii, Pyrocystis pseudonoctiluca, Ceratium gravidum and C. incisum to the surface, and the dispersal of dominant species across the surveyed area. Cluster analysis was performed across three phytoplankton assemblages in relation to upwelling intensity and continental influence. Redundancy and Spearman correlation analyses revealed that phytoplankton variability was mainly associated with phosphate. Furthermore, transect profiles of Chl a and phytoplankton abundance confirmed the origination of summer upwelling was the transportation of deep cold water in South China Sea to the surface layer along the coastline. These results indicate that the spatial variability of the phytoplankton community and biomass were highly controlled by the changes in upwelling location and intensity.