Ecological interactions driving spring dynamics of Aurelia aurita medusae (Linnaeus, 1758) in the western and north-western Black Sea

This study examined the ecological factors influencing spring fluctuations in the medusa population of the moon jellyfish ( Aurelia aurita ) in the western and north-western Black Sea between 2009 and 2015. Data were collected at 216 sampling stations, mainly covering coastal and shelf waters of Bulgaria, Romania and Ukraine. The north-western Black Sea exhibited the highest mean abundance (2.48 ind/m 3 ± 0.92 SD) and biomass (4.65 g/m 3 ± 3.22 SD) of A. aurita , despite individuals in this region having the smallest mean bell diameter (5.7 cm ± 1.5 SD). In contrast, the western regions (Bulgarian and Romanian waters) showed lower, yet comparable abundance and biomass, with mean values of 1.31 ind/m 3 ± 1.01 SD to 1.33 ind/m 3 ± 1.08 SD and 3.79 g/m 3 ± 3.93 SD to 3.99 g/m 3 ± 3.72 SD, respectively, alongside slightly larger average bell diameters (6.2 cm ± 1.8 SD and 6.3 cm ± 1.7 SD). Generalised Additive Models identified mesozooplankton biomass and sea surface salinity as key predictors of jellyfish biomass, while depth significantly influenced abundance and size. Regional variations in mean size distribution, depth-related patterns and non-linear predator-prey dynamics emphasise the role of local environmental conditions in regulating the A. aurita medusa population during the spring post-strobilation phase.