The sustainability of high-level primary production in Antarctic coastal polynyas has not been adequately addressed. We investigated the relationship between phytoplankton biomass (as reflected by Chl-a) of Antarctic coastal polynyas and cloud cover, which dominantly influences light conditions. Phytoplankton biomass in Antarctic coastal polynyas showed a robust correlation with the photosynthetically active radiation (PAR) and the cloud fraction. However, a different relationship was observed between phytoplankton biomass and the PAR; this depended on both the ultraviolet (UV) radiation and the geographical location of the coastal polynyas. The high intensity UV radiation is suspected to be an inhibitor of phytoplankton biomass. This indicates that phytoplankton biomass is effectively limited by light intensity, which can be reduced by clouds, particularly mid- and high- level clouds, whereas biomass but it is not limited by light under conditions of high-level exposure to UV radiation.