Impacts of Internal Waves on Phytoplankton Communities in Two Contrasting Habitats of the Northern South China Sea

Internal waves are known to affect physical and chemical conditions of marine ecosystems, yet their effects on the phytoplankton biomass and composition are still not well elucidated. In light of this, we conducted two continuous field observations in two habitats of the northern South China Sea, where vigorous internal waves were frequently observed, in the summer of 2014. These two habitats differed greatly in both nutrient regimes and phytoplankton compositions, with the nutrient-replete habitat (NRH) dominated primarily by diatoms and the nutrient-depleted habitat (NDH) by Prochlorococcus. A positive correlation between the total chlorophyll a (TChl a) specific nitrate assimilation rates and the sunlight transmittance existed only in the NRH, the suggestion being that the nitrate assimilation was light-dependent in the NRH and nutrient-dependent in the NDH. Both the isotherms and isohalines of the subsurface layers in both habitats fluctuated up and down during our observation period, indicating the existence of internal waves. In the NRH, the concentration of TChl a increased 12-hours later, whereas the relative abundance of diatoms did not show significant changes. In the NDH, the relative abundance of Prochlorococcus decreased in 100 m and increased in 25-50 m, whereas the concentration of TChl a remained constant in all layers. Our analyses revealed that the internal wave uplifted nutrient-rich waters to the upper euphotic zone in the NRH, thereby promoting the growth of the originally dominant group, diatoms. In the NDH, however, other groups instead of Prochlorococcus benefited from the increase of nitrate concentration brought by internal waves below the nitracline, whereas Prochlorococcus benefited from recycled nutrients in the upper layers.