Experimental Assessment of Ultraviolet Radiation Impact on the Primary Production of Phytoplankton in the East/Japan Sea

Solar radiation, particularly ultraviolet radiation (UVR, 280–400 nm), is known to play a significant role in driving primary production in marine ecosystems. However, our understanding of the specific effects of UVR on the primary production of natural phytoplankton communities is still limited. We utilized the ¹³C stable isotope to quantify primary production and conducted experiments using different types of incubation bottles (polycarbonate and quartz bottles) to compare the primary production in the absence and presence of UVR. Although we observed a weak inhibitory effect at the surface of the water column, UVR exposure resulted in an approximately 1.5-fold increase in primary production over the euphotic zone. The enhanced primary production during the study period can be attributed to the combined effect of low UVB (280–320 nm) dose and abundant nutrient conditions. Notably, our size-fractionated measurements revealed that UVR exposure led to a two-fold increase in primary production in large cells (>2 μm) compared to the exposure of solely photosynthetically active radiation (PAR). In contrast, there was no significant difference in the primary production of small cells (<2 μm) between the absence and presence of UVR. These findings highlight the advantages of large cells when exposed to UVR, emphasizing the importance of phytoplankton cell size in determining their response to UVR. However, it is important to note that the effects of UVR on phytoplankton are influenced by various environmental factors, which interact with solar radiation, shaping the dynamics of phytoplankton responses to UVR.