Canopy-forming seaweeds constitute marine forests that deliver ecosystem services. The worldwide range shift, sharp decline or loss of many of these forests, caused by the cumulative impact of increasing human pressure and climate change, have been widely documented.

This study assessed the fate of the populations of the brown seaweed Cystoseira mediterranea along a well-explored Mediterranean coastline in the Gulf of Lions. This was done by using a two-century suite of documents (herbarium vouchers, articles) and a 120-year observation period.

The results show that the northernmost population gradually declined and has been extinct since 1980. While the length of shore occupied by the southern population showed a long-term decline trend, with two sharp minima followed by partial recovery.

It is highlighted that, according to the results, in the Mediterranean Sea, no general conclusions on the decline of marine forests can be drawn. The authors conclude that thanks to the rare opportunity of an unusually long observation period in a well-explored coastline area, they were able to show that sharp fluctuations in density and distributions, as well as natural recovery episodes, occurred over time, reflecting a higher than expected resilience and a health status that is better than that reported for many canopy-forming seaweeds in the world ocean.

Blanfuné et al. (2019) The ups and downs of a canopy-forming seaweed over a span of more than one century. Scientific Reports 9:5250.