Global warming is facilitating the poleward range expansion of plant and animal species. In the Mediterranean Sea, the concurrent temperature increase and abundance of (sub)tropical non-indigenous species (NIS) is leading to the so-called tropicalization of the Mediterranean Sea, while the colder NW sectors of the basin have been said to undergo a process of meridionalization, that is the establishment of warm-water native species (WWN) previously restricted to the southern sectors.

This study evaluated changes in the occurrence of NIS and WWN in the Gulf of Genoa (NW Mediterranean). For this purpose, recent observations of NIS and WWN were collated to update previous similar inventories and the relative occurrence of both groups was monitored by snorkelling between 2009 and 2015 in shallow reefs and compared with the trend in air and sea surface temperatures.

The results show that, while the temperature has kept on increasing for the whole period, the number of WWN increased linearly and that of NIS exponentially. The authors suggest that these results provide little support to the idea of meridionalization of the northern sectors of the Mediterranean Sea and state that, in their opinion, the process of tropicalization also concerns the northern Mediterranean, where it is simply less showy.

The authors conclude that, should the present seawater warming continue in the future, the Mediterranean would undergo a generalized process of biotic homogenization, and the well established differentiation among its distinctive sub-basins would probably fade away. It is highlighted that the implementation of networks of sustained monitoring should be a major concern to understand and manage these changes.

Bianchi C, Caroli F, Guidetti P, Morri C (2018) Seawater warming at the northern reach for southern species: Gulf of Genoa, NW Mediterranean. Journal of the Marine Biological Association of the United Kingdom, 98:1-12.