Monsoon-Driven Phytoplankton Community Succession in the Southern South China Sea

Abstract

Phytoplankton, the main contributor to ocean carbon and nitrogen fixation, responds rapidly to physicochemical changes caused by hydrodynamics. To understand the response of phytoplankton community succession to monsoon-driven hydrographic changes, we analyzed the phytoplankton and environmental data of four cruises in the southern South China Sea (SSCS) during different monsoon periods. The results revealed that the phytoplankton communities in the monsoon interval (MI) period significantly differed from those in monsoon periods. Specifically, the phytoplankton communities exhibited the highest diversity owing to the pronounced environmental heterogeneity in the sampling time of MI. However, the total abundance of phytoplankton (4.2 ± 4.0 × 10³ cells/L) was the highest in the northeast monsoon (NEM) period due to the nutrient-rich conditions caused by strong vertical mixing possibly induced by the western boundary current. Notably, diazotrophic cyanobacteria Trichodesmium (0.9–21.3 × 10³ cells/L) exhibited high abundances at the edge of the anticyclonic eddy during the southwest monsoon (SWM) sampling period. Moreover, Trichodesmium (0.6–18.0 × 10³ cells/L) remained abundant primarily in the nitrogen-limited areas with sufficient phosphate during the sampling periods of the pre-NEM and NEM. Our study provides new insights into the importance of Trichodesmium in supplying new nitrogen to monsoon-influenced seas and the role of monsoon variability in shaping phytoplankton community succession.