Abstract:The viability of algal cells is one of the most fundamental issues in marine ecological research, In this work, amethod was designed to identify algal cell viability based on digital holography and deep learning, which dividealgal cells into three categories: active, weak, and dead cells. This method was applied to measure algal cells insurface waters of the East China Sea in spring, revealing about 4.34 %-23.29 % weak cells and 3.98 %-19.47 %dead cells. Levels of nitrate and chlorophyll a were the main factors affecting the viability of algal cellsFurthermore, algal viability changes during the heating and cooling were observed in laboratory experiments:high temperatures led to an increase in weak algal cells. This may provide an explanation for why most harmfualgal blooms occur in warming months. This study provided a novel insight into how to identify the viability ofalgal cells and understand their significance in the ocean.