Alien comb jelly Mnemiopsis leidyi is a gelatinous, invasive organism that is since recently is regularly appearing in the northern Adriatic. Its first appearance in the European seas was in the Black Sea, where it arrived in ballast tanks from the Atlantic and soon gained a reputation as a notorious marine invader. Large M. leidyi swarms depleted the entire zooplankton pool, leading to the collapse of fisheries. To depict the spatiotemporal dynamics of the M. leidyi populations and identify their impacts on the plankton community and anchovies (as representatives of planktivore fish) in the northern Adriatic, different chemical, biological and physical parameters were analysed. The results show that the appearance of this zooplankton organism is largely dependent on the geostrophic flow, while its spread and survival limit food availability and water column temperature. Occurrence along the eastern coast is associated with warmer seasons as a result of secondary dispersion from M. leidyi winter refuges in the northwest and maxima abundances in late summer with favourable sea currents. Moreover, M. leidyi simultaneously supports lower trophic levels while interfering with transport to the higher levels. Especially long high-abundance periods provide a large amount of organic matter, in most part labile and, thus, suitable for microbial digestion that, as follows, the release of nutrients. Furthermore, such an imbalance in the system leads to an increased abundance of picoplankton and their predator heterotrophic nanoflagellate, stimulating the microbial loop. Besides, a highly efficient and non-selective comb jelly diet, combined with other stressors on fish stocks, affects the spatial patterns of anchovy spawning areas and the zooplankton community. It is most important to limit M. leidyi spread to prevent adverse consequences; thus, ballast water treatments are proposed in the last part of this thesis.