Seawater warming is impacting marine ecosystems, but proper evaluation of change requires the availability of long-term biological data series. Mesco Reef (Ligurian Sea, Italy) offers one of the longest Mediterranean data series on sessile epibenthic communities, based on underwater photographic surveys.

This study aimed to: i) update the existing historical data series in Mesco Reef with new photographic surveys carried out in 2017, in order to see if the community remained stable for a further decade or returned to the previous situation; and ii) compare community trends with pre-existing sea water temperature data.

Multivariate analysis evidenced an abrupt compositional change between 1990 and 1996 and a parallel change was observed in Ligurian Sea temperatures. Data series show that two invasive macroalgae (Caulerpa cylindracea and Womersleyella setacea) became dominant after 1996.

The results show that community diversity was low in 1961 to 1996, rapidly increased between 1996 and 2008, and exhibited distinctly higher values in 2008-2017.

It is highlighted that a novel community emerged from the climate shift to the 1990s, with many once characteristic species lost, reduced complexity, biotic homogenization, greater diversity and domination by aliens. Only continued monitoring will help envisage the possibility for a reversal of the present phase shift or for further transformations driven by global change.

Bianchi et al. (2019) Coincided with a rapid acceleration of sea water warming. Diversity 11:215.