Responses of Phytoplankton Communities to Internal Waves in Oligotrophic Oceans

Understanding the potential impacts of internal waves on phytoplankton communities in oligotrophic oceans remains an important research challenge. In this study, we elucidated the impact of internal waves on phytoplankton communities through a comprehensive 154‐hr time‐series of observations in the South China Sea (SCS). We identified distinctive variations in phytoplankton pigment biomass and composition across the upper, middle, and lower layers of the euphotic zone, which we attributed to the perturbations triggered by internal waves. Phytoplankton other than Prochlorococcus in the lower, nutrient‐replete layer likely benefitted from allochthonous nutrients introduced by internal waves, but their growth rates were constrained by light limitation, and their pigment biomass was held in check by microzooplankton grazing. In contrast, in the upper, nutrient‐depleted layer, the relative abundance of Prochlorococcus increased, likely because of the ammonium regenerated by zooplankton. The middle layer, characterized as the deep chlorophyll maximum layer, exhibited a dynamic equilibrium characterized by nutrient and light co‐limitation. This equilibrium resulted in high nitrate assimilation and growth by phytoplankton. The balancing of those rates by significant grazing losses maintained total chlorophyll a concentrations at a high level. Based on these findings, we proposed a three‐layer euphotic zone structure characterized by distinct physiological conditions, nutrient‐light dynamics, grazing pressure, and phytoplankton responses to internal waves. This three‐layer paradigm elucidated the intricate interplay between internal waves and phytoplankton communities and provided insights into the mechanisms that govern primary production and carbon cycling in oligotrophic oceanic ecosystems. Internal waves, a common occurrence in stratified oceans, play a crucial role in oligotrophic oceans by delivering nutrients to the euphotic zone. The enhancement of phytoplankton growth and primary productivity by this nutrient supply impacts the phytoplankton community composition. However, the specific mechanisms behind these changes remain uncertain. Our study, conducted at the SouthEast Asia Time‐series Study station, located at 116°E, 18°N in the northern South China Sea, provided valuable insights. Through satellite imagery, 154‐hr time series observations, and barotropic tide data, we firmly established the presence of internal wave activity in this region. Our observ