Spatial Variability of Phytoplankton and Environmental Drivers in the Turbid Sanmen Bay (East China Sea)

Sanmen Bay (SMB) is a typical semi-enclosed turbid bay in the East China Sea. Environmental factors in the bay (e.g., salinity, transparency, and nutrients) vary spatially, and this variation may lead to spatial variability of phytoplankton. Four seasonal cruises were conducted during 2015-2016 to investigate the distribution and composition of phytoplankton community and their relationships with environmental factors in SMB. Historical data was also collected to explore the response of phytoplankton community to environmental changes. Results showed that diatoms were the most dominant group in this study area, followed by dinoflagellates and cryptophytes. Nonmetric multidimensional scaling and analysis of similarity demonstrated significant heterogeneity in community composition across three different subregions of SMB (inner bay, bay mouth, and outer bay). Redundancy analysis and variation partitioning analysis indicated that nutrients most influenced phytoplankton community. Light availability in spring and salinity in summer had nonnegligible effects. Abundance of water-collected phytoplankton in SMB increased by a factor of 5 from 2002 to 2015 and abundance of net-collected phytoplankton in SMB increased by a factor of 10 from the 1980s to the 2010s. These increases were closely associated with the increases in dissolved inorganic nitrogen and phosphorus. Increases in N/P and temperature induced a decrease in the diatom-dinoflagellate ratio and a subsequent shift in the net-collected dominant species (e.g., warm-water species increase) since the 1980s. This study contributes to a better understanding of phytoplankton dynamics and the effects of environmental drivers on phytoplankton in a highly turbid coastal bay.