Physical Drivers of Monthly Variation in Phytoplankton Community in the Tidal Freshwater Zone of the Macrotidal Qiantang River Estuary, Eastern China: Implications for Reducing the Risk of Harmful Cyanobacterial Blooms

The Qiantang river estuary flows into Hangzhou Bay, on the East China Sea, and has one of the largest tidal bores worldwide. The tidal freshwater zone (TFZ) in this shallow macrotidal estuary is subjected to strong riverine and marine forcings. We investigated monthly variation of phytoplankton and environmental drivers in the upper and lower sections of TFZ during 2016. Large numbers of phytoplankton taxa (422) and genera (161) were identified. Diatoms were dominant in most months, but cyanobacteria abundance reached its greatest in warm months when runoff was low. Surprisingly, relatively high dominances of marine diatom species (e.g., Cyclotella stylorum, Skeletonema, and Thalassiosira) were observed in the TFZ with salinities usually Microcystis contributed up to 50% to phytoplankton abundance in the upper TFZ in September, consistent with upstream bloom events. Abundances of phytoplankton and cyanobacteria were significantly negatively correlated with monthly runoff and sediment fluxes. Generalized additive models suggested that variation in phytoplankton abundance was largely explained by river flow, temperature and nutrients. Phytoplankton community composition varied significantly across different months and sections. Redundancy analysis indicated that temperature and flow rate explained more variation in phytoplankton community than other variables, but nutrients, Secchi depth and salinity also contributed significantly to the explained variation. Variance partitioning analysis confirmed that phytoplankton variation was largely regulated by physical variables rather than nutrients. These findings highlight the unexpectedly great phytoplankton species richness, dominance of marine diatoms, and physical drivers in the eutrophic macrotidal TFZ under strong tidal forcing.