Spatiotemporal dataset on moon jellyfish incidental observations in the Gulf of Riga and Eastern Gotland Basin, Baltic Sea.

This data article describes the occurrences of the moon jelly medusae in the Gulf of Riga and Eastern Gotland Basin (Baltic Sea) between 1998 and 2023. All data are incidental observations obtained during Latvian national monitoring cruises. Gelatinous zooplankton is not a standard group in regional marine monitoring, and jellyfish are not intentionally monitored within the framework of national marine monitoring by many countries across the Baltic Sea region. Hence, the information about medusae occurrences in the area is scarce, especially long-term. The data also describe the seasonal dynamics of medusae presence. Most frequently, observations occur from August to October in the studied area. However, the earliest observation of medusae was recorded in July and the latest observation was in late November. These data provide relevant contributions to comprehending the seasonal and long-term dynamics of gelatinous zooplankton, specifically . Moreover, the data can be used as a baseline for future studies on the eastern Baltic Sea addressing climate change and anthropogenic forcing impacts.