Tidal variations in dissolved nutrients and particulate matter at the mouth of semiclosed Keunso Bay were investigated over four seasons. The role of the bay, whether it acted as a sink or a source, was element-specific and seasonally dependent. Dissolved inorganic nitrogen was imported in spring, summer, and winter due to active denitrification throughout the year and was exported offshore in autumn. For dissolved inorganic phosphorus, the bay served as a sink in spring and winter and as a source in summer and autumn, possibly in association with the redox potential of surface sediments. Similarly, dissolved silicate was imported in spring when a spring bloom occurred, but the bay was a silicate source in summer and autumn. Suspended particulate matter flux also varied seasonally, with tendencies of summer export and winter import, possibly controlled by meteorological conditions and tidal cycle. Particulate organic carbon was exported offshore in summer but imported in winter.