Anthropogenic influences and climate change are leading to more frequent and intense heatwaves, which are known to affect marine ecosystems. However, the effects of rising temperatures on in-situ larger and smaller phytoplankton have not yet been sufficiently studied. Here, two cell size-fractioned phytoplankton assemblages (>20 µm, <20 µm) were cultured at four different temperatures in a half-month interval during June 2022 to July 2023 in Laoshan Bay, Yellow Sea, China. Chlorophyll a concentration fluctuated between 0.84 and 7.76 μg L^-1, with the highest value occurring in winter with the lowest proportion of smaller cells . The photosynthetic efficiency (F_V/F_M) of larger cells varying from 0.15 in winter to 0.52 in summer was about 22% higher than that of smaller cells; while the growth rate (µ, -0.21 to 0.91 d^-1) of larger cells was about 60% higher. The slope derived from the linear fit of F_V/F_M or µ to temperature (i.e., temperature sensitivity) was positive in winter but negative in summer, depending mainly on ambient temperature. Increasing temperature stimulated the growth of larger cells more strongly in winter, but inhibited it more strongly in summer, indicating a higher temperature sensitivity of larger than smaller cells. In addition, our results showed that the integrated inhibition of a 1 ^oC temperature rise over a year was 5.45% and 3.68% on growth of larger and smaller cells, respectively, confirming a negative effect of temperature rise on phytoplankton assemblages in Laoshan Bay.
 

Temperature rise, Fluorescence, Growth, Seasonal variation, Phytoplankton assemblages, Laoshan Bay