<p>The transition from ice-covered to open waters is a recurring environmental feature in seasonal sea ice regions. Yet the differences in the photosynthetic activity of phytoplankton communities during this period are poorly known. The southern Sea of Okhotsk (SSO), a sizeable seasonal ice zone in the northern hemisphere, experiences significant phytoplankton bloom dynamics, mainly composed of diatoms, driven by sea ice retreat in spring. Here, we compared the differences in the expression level of the <i>rbcL</i> gene, which encodes the large subunit of the photosynthetic carbon fixation enzyme RuBisCO, among dominant diatom families in the SSO from winter to spring in 2022 and compared the results with environmental factors. Higher diatoms’ <i>rbcL</i> transcription efficiency in terms of mRNA (cDNA)/DNA copy number ratio was observed in winter coastal and spring waters than in winter under-ice waters, suggesting light availability as a key driver of their photosynthetic activity. In addition, centric diatoms exhibited significantly higher <i>rbcL</i> transcription efficiency than pennate diatoms throughout the study period. The transcription efficiency of three dominant diatom families—Chaetocerotaceae, Thalassiosiraceae, and Bacillariaceae showed strong positive correlations with both the maximum quantum yield of photochemistry in algal photosystem II (<i>F</i>_v/<i>F</i>_m) and chlorophyll <i>a</i> concentrations. This suggests that light-harvesting capacity and carbon fixation activity (mediated by <i>rbcL-</i>encoded RuBisCO) jointly contribute to spring bloom in the SSO. The <i>rbcL</i> cDNA copy number correlated strongly with fucoxanthin, a proxy for diatom biomass. These findings highlight the identification of photosynthetically active diatom groups contributing to high primary productivity in spring blooms and their sensitivity to environmental changes in seasonally ice-covered regions.</p>