Zooplankton as an essential component in the pelagic food web are directly linked to pelagic ecosystem functioning. Therefore, comprehension of zooplankton functional diversity (FD) and its responses to environmental changes is crucial for ecosystem-based view. To identify FD responses to environmental drivers, we analysed 25 years of summer data on the brackish mesozooplankton community (including rotifers, cladocerans, copepods, and meroplankton) from the eutrophied, shallow Gulf of Riga (Baltic Sea). We established that within the Gulf of Riga, open waters are notably different from coastal regions based on the dynamics of hydrological conditions (temperature, salinity), cyanobacterial dominance, abundance of mesozooplankton functional groups, and mesozooplankton FD indices. Competition over resources in combination with hydrodynamic features and predation by adult herring were seemingly the central structuring mechanism behind the dynamics of FD metrics (richness, evenness, divergence, and dispersion) within coastal mesozooplankton communities. Whereas predation by young herring was an important driver only for the mesozooplankton communities in the open waters. Cyanobacterial dominance, used as a proxy for food quality and availability, had no effect on summer mesozooplankton FD metrics.

Four species of the genus Synchaeta were identified in the waters of Liepaja harbour (coastal Eastern Gotland Basin, Baltic Sea). Synchaeta baltica and S. monopus are common in the Baltic Sea and they co-dominated most of the samples. Synchaeta fennica was abundant during spring, but S. triophthalma was detected in October 2014 for the first time in Latvian waters. During sample analysis particular attention was paid to insufficiently described trophi of S. monopus and S. fennica. Subsequently, brief descriptions were made during analysis and complemented with images.

This data article describes the occurrences of the moon jelly Aurelia aurita 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 A. aurita medusae occurrences in the area is scarce, especially long-term. The data also describe the seasonal dynamics of A. aurita medusae presence. Most frequently, observations occur from August to October in the studied area. However, the earliest observation of A. aurita 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 A. aurita. Moreover, the data can be used as a baseline for future studies on the eastern Baltic Sea addressing climate change and anthropogenic forcing impacts.

In the brackish ecosystem of the Gulf of Riga, the calanoid copepod Eurytemora affinis is a key species, but recently a new non-indigenous species Eurytemora carolleeae was discovered in the region. In the present study, we aimed to validate the presence of E. carolleeae in the southern part of the Gulf of Riga five years after its first discovery. The study area is the closest region to the Rīga harbour — the main source of non-indigenous species arrival in the Gulf. Recent studies have predicted the possible potential of E. carolleeae invasion due to its physiological plasticity. Male and female specimens of Eurytemora were collected in spring, summer, and autumn of 2013, and analysed based on three morphological indicators. Despite the higher reproduction rate of E. carolleeae , this intrusive species does not seem to have succeeded in establishing during the five-year period after its first discovery in the Gulf of Riga, and hence does not present a threat to the native E. affinis population in the study area.