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Seasonal blooms of the invasive ctenophore Mnemiopsis leidyi have been recorded throughout the Adriatic Sea, affecting the ecosystem and its pelagic resources. Gelatinous organisms such as M. leidyi release significant quantities of excreta as labile and surface active dissolved organic carbon (DOC) that supports the microbial activity. The effects of M. leidyi's presence on heterotrophic prokaryotes (HP), cyanobacteria (CB) and heterotrophic nanoflagellates (HNF) were investigated at five locations in the coastal waters of Rovinj-Rovigno (northern Adriatic) during intensive blooms in September 2019. Additionally, circulation in the study area was examined using available current measurements and a hydrodynamic model. In September, low-pass filtered currents near the surface were mostly directed toward S-SE reaching 25 cm/s, indicating the presence of the Istrian Coastal Countercurrent (ICCC). Ctenophores displayed a patchy spatial distribution from 0.05 to 100 M. leidyi individuals per m3. When ctenophore density in the swarms surpassed 1 individual per m3, the change in the abundances of microbial groups became evident. The lowest microbial densities and lowest concentrations of dissolved and particulate organic carbon (DOC and POC) and surface-active substances occurred in the absence of M. leidyi, while the highest were within the swarms. We hypothesise that HP utilises the available DOC enhancing the remineralisation of organic matter. Finally, greater availability of HNF's prey (HP and CB) leads to an increase in their abundance. This study illustrates how the release of surface-active DOC during intensive M. leidyi's blooms can stimulate the microbial loop. The current system with the ICCC prevailing in September enables the spreading of M. leidyi from its hypothetic origin in an open sea gyre towards the coast. Bora wind episodes may initiate and favour this type of circulation. In these conditions, M. leidyi agglomerates in large numbers along the coast, enabling the accumulation of organic matter and altering the abundance of pico- and nanoplankton.

The present paper is a contribution to the first initiative of the Port Baseline Survey (PBS) for Non-indigenous species (NIS) in the Mediterranean Sea. It presents a report on the soft-bottom macrobenthos from the five Adriatic ports: Bari, Ancona (Italy), Koper (Slovenia), Pula, Rijeka (Croatia), with a focus on the presence and contribution of NIS to native assemblages. Out of 451 species identified, only four were common to all ports. A total of eight NIS were recorded, five in surveyed ports and three in the lagoon connected to the Port of Koper. The highest number of NIS was recorded in Bari, and the highest abundance in Ancona and Bari. Generally, the number, abundance and contribution of NIS seems too low to cause a substantial impact on native communities in surveyed ports. The suitability of methods adopted for PBS for soft-bottom NIS was discussed and suggestion for methodological improvement is provided.

The intense shipping traffic characterising the Adriatic Sea favours the spread of marine organisms. Yet, a study of 12 Adriatic ports (4 on the western side and 8 on the eastern side of the basin) found that non-indigenous species (NIS) accounted for only 4% of the benthic communities settled on hard substrates. The cirripeds Amphibalanus amphitrite and Balanus trigonus , found in 8 harbours, were the most common invaders followed by Amphibalanus eburneus , the ascidian Styela plicata , and the bivalve Magallana gigas . The highest percentage of NIS was recorded in Venice and Ploce, the harbours with the least rich native communities; the lowest percentage was retrieved in Trieste, Koper, Pula, and Rijeka, the harbours hosting the highest species diversity. In contrast, the ports of Bari and Ancona showed both high NIS percentages and highly diversified communities.