Dynamics of phytoplankton community structure in the South China Sea in response to the East Asian aerosol input
Abstract. Recent studies have regarded atmospheric deposition as an increasingly important source of nutrients to the ocean. The South China Sea (SCS), where aerosol loading is among the highest in the world, however, is poorly studied particularly on the in situ changes of phytoplankton community structures in response to atmospheric deposition. By conducting a series of microcosm bioassays at different hydrographical locations and simulating different aerosol event scales, we observed both positive and negative responses induced by the input of East Asia (EA) aerosol with high nitrogen (N) and trace metal contents, in terms of both community structure and physiological characteristics of phytoplankton. High levels of aerosol loading profoundly relieved phytoplankton nitrogen and trace metal limitations in SCS, and thus increased total phytoplankton biomass, enhanced their physiological indicators (e.g. photosynthetic efficiency) and shifted phytoplankton assemblages from being dominated by picoplankton to microphytoplanton, especially diatoms. However, under low levels of aerosol loading, the composition shift and biomass accumulation were not apparent, suggesting that the stimulation effects might be counterbalanced by enhanced grazing mortality indicated by increased abundance of protist grazers. Trace metal toxicity of the aerosols was also an important negative factor to phytoplankton growth, especially picocyanobacteria, implicated by the high copper (Cu) concentration in the microcosm that surpassed the toxicity threshold of marine cyanobacteria. Moreover, the magnitude and duration of the deposition event, as well as the hydrographical and trophic conditions of receiving waters are also important factors when predicting the influence of an aerosol deposition event. Our results demonstrated that the EA aerosol deposition events could profoundly change nutrient and phytoplankton dynamics in SCS and highlighted the need for achieving an accurate comprehension of atmospheric nutrient on the biogeochemical cycles of the oceans.
