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

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.

Impacts of Marine and Lagoon Aquaculture on Macrophytes in Mediterranean Benthic Ecosystems

“…see Navarro et al, 2011, for Spanish Catalonia). The flow of mussel larvae could be due to the development of offshore mussel farms, off Sète, ∼100 km upstream, transported by the Northern Mediterranean Current toward Catalonia (Blanfuné et al, 2019). A similar negative impact of mussel proliferation on seaweeds has already been reported, e.g., in Sweden (Lundälv et al, 1986).…”

“…The canopy-forming long-lived Carpodesmia mediterranea (Cystoseira mediterranea; Ochrophyta, Stramenopiles), which forms a dense belt on rocky reefs, at the upper part of the infralittoral zone (sensu Pérès and Picard, 1964), from the mean sea level down to 1 m depth, has experienced marked fluctuation of its abundance since the 1970s in French Catalonia (Gros, 1978;Blanfuné et al, 2019); the most realistic explanation for the episodes of sharp decline of C. mediterranea seems to be the invasion of the habitat by dense stands of the mussel Mytilus galloprovincialis. Mussels could reduce the survival rate of C. mediterranea recruits that settle on its shell because of the vulnerability of this substrate (prone to be pulled out by waves) and the low longevity of the mussels.…”

“…In addition, there is a light deficit for the recruits of C. mediterranea that grow directly on the rocky substrate. Extreme storms are not uncommon in French Catalonia (Figure 7; Thibaut et al, 2005;Blanfuné et al, 2019; FIGURE 7 | A stand of Carpodesmia mediterranea (= Cystoseira mediterranea), in French Catalonia, severely disrupted by mussels of which the recruits probably come from offshore mussel farms.…”

Clunio boudouresquei sp. n. and Thalassosmittia ballestai sp. n., two Tyrrhenian marine species occurring in Scandola Nature Reserve, West Corsica (Diptera: Chironomidae)

“…This sea mussel significantly predominates when changes in water quality and level of pollution become increasingly high (seashores at Banyuls, SW-France, Figs 8-9), where populations are intensely enlarging and reinforcing their potential expansion in occupying up to 70-80% of the living L. byssoides original cover. Such situations are also highlighted in southern France by Blanfuné et al (2019) for the 'Canopy-forming Seaweeds' of Cystoseira mediterranea Sauvageau, 1810, where an important decline of local populations with risk of extinction are reported; Linares et al 2010and Garragou et al (2017) report a similar scenario for the Mediterranean Red Coral, suggesting that constructive plans and management measures for conservation and preservation of autochthonous Tyrrhenian elements must be implemented.…”

Coexistence of the reef-building coral <em>Cladocora caespitosa</em> and the canopy-forming alga <em>Treptacantha ballesterosii:</em> Description of a new Mediterranean habitat

“…Most species of Mediterranean Fucales are undergoing a severe decline (e.g. Bianchi et al 2014, Thibaut et al 2015, Mariani et al 2019, although this is not always the case for all the species (Thibaut et al 2014, Blanfuné et al 2019 or all places (Sales and Ballesteros 2010, Thibaut et al 2016. Pollution is the main threat affecting the survival of Cystoseira s.l.…”