Carbon dioxide and light are two major prerequisites of photosynthesis. Rising CO2 levels in oceanic surface waters in combination with ample light supply are therefore often considered stimulatory to marine primary production(1-3). Here we show that the combination of an increase in both CO2 and light exposure negatively impacts photosynthesis and growth of marine primary producers. When exposed to CO2 concentrations projected for the end of this century(4), natural phytoplankton assemblages of the South China Sea responded with decreased primary production and increased light stress at light intensities representative of the upper surface layer. The phytoplankton community shifted away from diatoms, the dominant phytoplankton group during our field campaigns. To examine the underlying mechanisms of the observed responses, we grew diatoms at different CO2 concentrations and under varying levels (5-100%) of solar radiation experienced by the phytoplankton at different depths of the euphotic zone. Above 22-36% of incident surface irradiance, growth rates in the high-CO2-grown cells were inversely related to light levels and exhibited reduced thresholds at which light becomes inhibitory. Future shoaling of upper-mixed-layer depths will expose phytoplankton to increased mean light intensities(5). In combination with rising CO2 levels, this may cause a widespread decline in marine primary production and a community shift away from diatoms, the main algal group that supports higher trophic levels and carbon export in the ocean.National Basic Research Program of China [2009CB421207, 2011CB200902]; National Natural Science Foundation of China [41120164007, 40930846]; Changjiang Scholars and Innovative Research Team project [IRT0941]; Ministry of Science and Technology [S2012GR0290]; United States National Science Foundation Division of Ocean Sciences [0942379, 0962309, 1043748]; German Ministry of Education and Research; 111 project; State Key Laboratory of Marine Environmental Science (Xiamen University); German Academic Exchange Service (DAAD

To better understand seasonal variation in phytoplankton community structure in the East China Sea (ECS), 4 research cruises were carried out between August 2009 and June 2011 over the continental shelf. Phytoplankton community structure and ambient seawater physicochemical parameters were examined. The phytoplankton community in the ECS was mainly composed of diatoms and dinoflagellates, with 242 taxa in 70 genera of diatoms and 104 taxa in 26 genera of dinoflagellates. Several species of Chlorophyta, Chrysophyta, and Cyanophyta were also observed. Chain-forming diatoms were the most dominant phytoplankton group throughout the year, except in spring when dinoflagellates dominated in coastal waters. Monsoon-induced turbulence played a significant role in regulating the dominance of dinoflagellates among seasons, with calm water favoring the growth of dinoflagellates and turbulent water inhibiting them. Phytoplankton abundance was always higher in coastal than offshore waters, mainly determined by nutrient availability. A Prorocentrum dentatum bloom and a Chaetoceros debilis bloom were observed in the Yangtze River estuary in spring 2011 and fall 2010, respectively. The blooms differed greatly in community diversity and cell abundance distribution, potentially affecting organic carbon export. In general, 3 different hydrographic water systems (coastal water, shelf mixed water, and Kuroshio water) were identified from a temperature−salinity plot of the ECS, and phytoplankton abundance showed spatially and temporally different distribution patterns in these 3 water systems. The nutrient N:P ratio, but not the Si:N ratio, played a significant role in regulating the phytoplankton community structure and the formation of the spring coastal dinoflagellate bloom in the ECS.

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