Spreading patterns of the invasive Caulerpa cylindracea Sonder along the west Istrian Coast (northern Adriatic Sea, Croatia)

• Caulerpa cylindracea withstands cold northern Adriatic winter temperatures.
• The algal spreading depends on long-term seawater temperature fluctuations.
• C. cylindracea predominantly colonises locations subjected to anthropogenic pressure.
• Both exposure to wave action and substrate type influence algal diffusion.
• Along the west Istrian coast, C. cylindracea is mostly widespread up to 10 m in depth.

The northern Adriatic Sea represents the northernmost and thus the coldest biogeographic sector of the Mediterranean Sea. In 2004, the invasive green alga Caulerpa cylindracea was recorded for the first time in the northern Adriatic at a site of the west Istrian Coast. Until 2010, additional C. cylindracea mats have only formed up to 7 km northward from the first colonisation site. Subsequently, the alga was also recorded at sites widespread along the entire coast. Both the first 2004 colonisation event and the 2011–2014 colonisation of distant sites occurred during periods of winter seawater temperatures higher than 9 °C. In general, algal spreading was markedly slow. Approximately 10 years after the first record, C. cylindracea has affected less than 1% of the entire west Istrian coastline. The colonisation predominantly occurred in ports and urbanised bays (seaside resorts) suggesting that anthropogenic activities might enhance algal diffusion.