Contrasting effects of temperature rise in different seasons on larger and smaller phytoplankton assemblages in a temperate coastal water, Laoshan Bay, northern Yellow Sea, China

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 phytoplankton size classes have not yet been adequately studied. In this study, two cell-sized phytoplankton assemblages (>20 μm, <20 μm) were cultured at a range of temperatures [i.e., ambient temperature (AT), AT+3 °C, AT+6 °C and AT+9 °C] in Laoshan Bay, Yellow Sea, China, at half-month intervals between June 2022 and July 2023. Total chlorophyll a concentration fluctuated between 0.84 and 7.76 μg L−1 throughout the year, with the highest value presented in winter with the lowest proportion of smaller cells. Photosynthetic efficiency (FV/FM) of larger cells, which varied between 0.15 in winter and 0.52 in summer, was 22% higher than their smaller counterparts, while their growth rate (μ, −0.21 to 0.91 d−1) was 60% higher. The slope derived from the linear fit of FV/FM or μ to temperature, an indicator of temperature sensitivity, was positive in winter but negative in summer, depending mainly on ambient temperature. The μ of larger cells was increased more than that of smaller cells by an increase in temperature in winter, but inhibited more in summer, indicating their greater sensitivity to temperature. Our results also showed that the integrated inhibition of a 1 °C temperature increase over one year is 5.45% and 3.68% on the growth of larger and smaller cells, respectively, suggesting a negative effect of temperature increase on phytoplankton community in Laoshan Bay.