Almost weekly monitoring of diatom occurrences in the Onomichi Channel, within the Seto Inland Sea of Japan, was conducted from 2016 to 2021. Species diversity peaks were observed in January and/or February, and in September. Cell densities increased from February to March and from August to October, with minor peaks occurring from November to December. Total cell volumes, used as an index of diatom biomass, did not always correlate with cell density but peaked from February to March, August to October, and November to December. The trends in diatom occurrences underwent a drastic change between 2016–2018 and 2019–2020. In the former period, diatoms were abundant throughout the year, with peaks observed during winter (January to March), summer (July to September), and autumn (October to December). Notably, Skeletonema spp. were abundant throughout 2016, and Eucampia zodiacus and Coscinodiscus wailesii blooms occurred during the winters of 2017 and 2018. The prolonged diatom occurrences in 2018 coincided with a period of disastrously heavy rainfall during the summer. Conversely, the winters of 2019 and 2020 were characterized by low diatom densities and biomass, followed by C. wailesii blooms in September 2019 and October 2020, resulting in concentrated occurrences within those years. In 2021, the diatom occurrence trend reverted to those observed from 2016 to 2018, with the exception of increased biomass during autumn caused by C. wailesii.