The ups and downs of a canopy-forming seaweed over a span of more than one century

Aurélie Blanfuné,Marc Verlaque,Thierry Thibaut,Charles François Boudouresque

doi:10.1038/s41598-019-41676-2

Abstract

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. Contrasting examples, reflecting higher than expected resilience, have been more rarely reported. Here, we took the opportunity of having at our disposal a two-century suite of documents (herbarium vouchers, articles) and a ~120-year observation period, dealing with a long-lived brown seaweed, Cystoseira mediterranea, along a well-explored Mediterranean coastline in the Gulf of Lions, to depict the fate of its populations. In addition, we provided baselines for future surveys, with a high degree of accuracy. The northernmost population, scattered on rare suitable substrates, gradually declined and has been extinct since the 1980s. The length of shore occupied by the southern population showed a long-term decline trend, with two sharp minima followed by partial recovery. The causes of the decline differ between sites and periods: coastal development, pollution, competition with mussels, heatwaves and exceptional storms. Overall, the Gulf of Lions populations reflects long-lasting resilience, higher than expected, and a health status that is better than that reported for many other canopy-forming seaweeds.

Highlights

Canopy-forming seaweeds, generally large brown algae such as kelp and fucoids, dominate shallow rocky coasts of the world’s temperate and cold-water seas[1], providing high levels of delivery of ecosystem services (e.g.1–3)
A total of 87 historical records of Cystoseira mediterranea have been found from the 19th (1817 for the oldest, at Agde) to the 20th centuries; 19 in the Occitanie Region (Sète and Agde) and 68 in French Catalonia (Figs 2 and 3, Table S1)
In April 2007 and 2012, all suitable habitats were thoroughly explored: C. mediterranea was absent everywhere and its habitat was mainly colonized by the articulated coralline Corallina caespitosa and the mussel Mytilus galloprovincialis (Fig. 2 and Table S1)

Summary

Introduction

Canopy-forming seaweeds, generally large brown algae such as kelp and fucoids, dominate shallow rocky coasts of the world’s temperate and cold-water seas[1], providing high levels of delivery of ecosystem services (e.g.1–3). Among the few taxa strictly restricted to dwelling close to sea level on moderately to strongly exposed rocky coasts, Cystoseira mediterranea Sauvageau is a long-lived brown alga described from Banyuls-sur-Mer (French Catalonia)[41], restricted to the Mediterranean Sea[42,43] and the immediate vicinity of the Gibraltar Strait on the Atlantic Morrocan coast[44,45]. It is a single axis plant up to 40–50 cm height that forms extensive photophilous stands on shallow and wave-exposed rocky substrates, tolerating regular emersion caused by wave-movements and tides. The aim of this study was (i) to provide an exhaustive map of the current distribution of C. mediterranea over the entire French Mediterranean coast; (ii) to compare this distribution with historical data to assess losses, gains or stability; (iii) to identify and analyze the dynamic of the populations and the putative causes of fluctuation of its abundance

Objectives

Methods

Results

Discussion

Conclusion