The world's oceans are acidifying and warming as a result of increasing atmospheric CO₂ concentrations. In the coastal oceans of China, seawater CO₂ concentration and temperature increased remarkably. In the Taiwan Strait, the surface seawater CO₂ concentrations increased from 310 μatm in 1995 to 460 μatm in 2019; and temperatures ranged from 14.4 ^oC to 29.8 ^oC and increased by 0.29–0.54 ^oC from 1945 to 2010. To investigate the combined effects of ocean acidification and warming on growth rates of the dominant phytoplankton species and carbon fixation of phytoplankton assemblages in the Taiwan Strait, we conducted the indoor and outdoor incubation experiments, respectively. In the range of 12 ^oC to 26 ^oC (or 30 ^oC), ocean acidification significantly increased the growth rates of the dinoflagellate Prorocentrum donghaiense and the diatom Thalassiosira weissflogii. In the range of 14.4 ^oC to 26.4 ^oC, ocean acidification and warming synergistically increased the carbon fixation contents of phytoplankton assemblages in the Taiwan Strait in spring, autumn and winter. However, in summer (in situ temperature: 28.3–29.8 ^oC), increases in seawater temperature by 1.5 ^oC (ocean warming) significantly reduced the carbon fixation contents of phytoplankton assemblages in the Taiwan Strait. And ocean acidification attenuated the negative effect of ocean warming on carbon fixation contents of phytoplankton assemblage in summer. These results provided basic information for evaluating the carbon biogeochemistry in the Taiwan Strait under ocean acidification and warming conditions in the future.

carbon fixation, ocean acidification, ocean warming, phytoplankton