Does increased springtime solar radiation also increase primary production?

We tested the hypothesis that long-term brightening of solar radiation and changes in the underwater light climate would increase the primary production of phytoplankton in the springtime in Lake Kasumigaura, Japan. Global solar radiation fluctuates on a decadal scale, and the solar radiation in May has been increasing since the 1990s in Japan. We developed a model of depth-integrated primary production (gC m−2 h−1) based on long-term monitoring data. The model based on photosynthesis-irradiance curve clarified that primary production increased by 13.2%, from 0.093 gC m−2 h−1 in 1992 to 0.105 gC m−2 h−1 in 2019, because of the combined effects of solar radiation and water temperature increases. Sensitivity analysis of the model showed that primary production was most sensitive to water quality parameters, i.e. the light attenuation coefficient and nutrient effects on Pmax. Nutrient effects were significantly correlated with the particulate organic carbon to nitrogen ratio, indicating that nitrogen shortage in phytoplankton cells may affect primary production. These results suggest that long-term springtime increases in solar radiation and water temperature increase primary production, but that water quality also controls primary production.