Long-term changes in the benthic macroalgal flora of a coastal area affected by urban impacts (Conero Riviera, Mediterranean Sea)

Seaweed communities are important coastal ecosystems representing good indicators of environmental variation related to climate change and other long-term changes. Long-term variation in the distribution of seaweed communities and individual seaweed species has been intensively investigated; however, long-term studies considering the whole seaweed flora of a geographical area have been infrequently produced, despite of their potential to unravel major changes. The macroalgal flora of a coastal area subjected to urban influences (Conero Riviera, Mediterranean Sea) was investigated comparing the contemporary flora with historical data available for three periods (1941-1946; 1964-1976; 1997-1999). The most evident change was the disappearance of 25-30 species that were present in 1941-1946 (and, in some cases, still in 1964-1976). Such losses are not recent and took place mainly in the years 1940-1960. A general environmental deterioration due to the urbanization of the area of Ancona is suggested as main cause of their disappearance; there is no evidence that their loss was related to climate change. Another major change was the introduction of 9 non-indigenous species, which were probably introduced from the Lagoon of Venice by maritime traffic. One of them, Melanothamnus japonicus, is now very abundant on the Conero Riviera. The results show that present floristic patterns may result from changes that took place at the time scale of many decades, which are rarely considered in studies on benthic vegetation. We suggest that future floristic studies of benthic macroalgae should be based on large-scale programs of DNA barcoding, in order to avoid overlooking invasive cryptic species.