Abstract: As the vital primary producer of marine ecosystem, phytoplankton is the important source of seawater organic carbon while taking in inorganic carbon and nutrients for growth and reproduction. Exploring phytoplankton’s primary productivity and organic carbon contribution make great sense to the study of seawater organic carbon. Based on the survey data of Hangzhou Bay in spring 2019, we analyzed the horizontal distribution of environmental factors, nutrients and primary productivity as well as their influence factors, then estimated the organic carbon storage in water using Thiessen polygon algorithm. Results showed that the physical and chemical environmental factors of seawater present spatial heterogeneous distribution in spring. Nutrients such as nitrogen, phosphorus and silicon were abundant but out of proportions with a trend of higher inside than outside. The concentration of chlorophyll a (Chl a) in surface seawater changed from 0.20 μg/L to 1.00 μg/L, which have significantly positive correlation with nutrients. The primary productivity varied greatly in environmental gradient and showed a trend of higher north-bank than south-bank with an average level of 10.70 mg C/(m²·d). The organic carbon storage of Thiessen polygonal water columns ranged from 5.84×10²−1.62×10⁴ t in spring, with the trend of higher north-bank than south-bank as well as higher outside than inside. Seawater organic carbon storage of investigation area was about 1.14×10⁵ t and phytoplankton standing stock contributed less to it.