VULNERABILITY OF AQUATIC ECOSYSTEMS
The Sargassum conundrum: very rare, threatened or locally
extinct in the NW Mediterranean and still lacking protection
Thierry Thibaut .Aure
´.Marc Verlaque .
Charles-Franc¸ois Boudouresque .Sandrine Ruitton
Received: 17 October 2014 / Revised: 6 October 2015 / Accepted: 6 November 2015
ÓSpringer International Publishing Switzerland 2015
Abstract Species of the genus Sargassum are large
canopy-forming marine brown algae (engineer spe-
cies) found worldwide from tropical to cold-temperate
latitudes. Among this very diversiﬁed genus (335 taxa
accepted taxonomically), only 9 species (including the
invasive S. muticum) have been reported from the
Mediterranean Sea. We have analysed the changes
over more than two centuries in the Sargassum’s
pattern of distribution along *2,970 km of north-
western Mediterranean coasts, using all available
historical sources (literature and herbarium vouchers)
and 2003–2014 ﬁeld surveys. Though common in the
past, all the long-lived native Mediterranean species
(except S. vulgare) have become extremely rare or
locally extinct, while the invasive S. muticum has
developed large populations in some coastal lagoons.
The increase in water turbidity, trawling and ﬁshing
nets is possibly responsible for the regression of the
deep populations of S. hornschuchii, S. acinarium,
whereas overgrazing by sea urchins and habitat
destruction (coastal development) are probably
responsible for the decline of shallow populations. In
contrast with the terrestrial realm, where thousands of
species are protected, even some relatively common
species, Mediterranean species of Sargassum that
seem to have become extinct in extensive areas, such
as S. acinarium and S. hornschuchii, are, surprisingly,
still lacking proper protection status and inclusion on
the IUCN Red List.
Keywords Sargassum Mediterranean Local
extinction Threatened species
Along the temperate rocky coasts, the large canopy-
forming kelp (Laminariales, Phaeophyceae, Ochro-
phyta) and fucoids (Fucales, Phaeophyceae, Ochro-
phyta) represent the dominant species in unpolluted
environments (Dayton, 1985; Steneck et al., 2002;
Schiel & Foster, 2006). Kelp and fucoids are ecosys-
tem engineers, forming three-dimensional habitats
that provide shelter, food and nurseries for a wide
range of species. They supply high primary production
which plays a role in supporting diversiﬁed functional
compartments and trophic levels (e.g. Foster & Schiel,
Guest editors: Celine Bertrand, Evelyne Franquet,
Ivan Dekeyser & Christophe Piscart / Vulnerability and
Resilience of Freshwater & Marine Ecosystems
Electronic supplementary material The online version of
this article (doi:10.1007/s10750-015-2580-y) contains supple-
mentary material, which is available to authorized users.
T. Thibaut (&)A. Blanfune
C.-F. Boudouresque S. Ruitton
Aix-Marseille University and Toulon University,
Mediterranean Institute of Oceanography (MIO), CNRS/
INSU, IRD, UM 110, Campus of Luminy,
13288 Marseille Cedex 9, France
1985; Harrold & Pearse, 1987; Steneck et al., 2002;
Estes et al., 2004; Schiel & Foster, 2006; Page et al.,
2008; Reed et al., 2008), their biomass can be exported
to adjacent benthic and pelagic ecosystems (Harrold &
Pearse, 1987; Safran & Omori, 1990; Kaehler et al.,
2000; Yatsuya et al., 2007), they constitute a nursery
for rocky reef ﬁsh (Carr, 1989,1994; Garcı
Macpherson, 1995; Aburto-Oropez et al., 2007;
´e et al., 2013) and large species can attenuate
wave action (e.g. Komatsu & Murakami, 1994;
Rosman et al., 2007). The decline of kelp and fucoids
is a worldwide phenomenon due, directly or indirectly,
to human activities, such as eutrophication, increase in
sea water temperature, increase in herbivory and
habitat destruction (Steneck et al., 2002; Diez et al.,
2003; Helmuth et al., 2006; Worm & Lotze, 2006;
Airoldi & Beck, 2007; Coleman et al., 2008; Hawkins
et al., 2008; Wernberg et al., 2010; Phillips &
Blackshaw, 2011; Schiel, 2011; Lamela-Silvarrey
et al., 2012; Raybaud et al., 2013; Bianchi et al.,
2014; Filbee-Dexter & Scheibling, 2014).
Losses of fucoids have been reported all around
the Mediterranean Sea. Most of the studies have
focussed on species of the genus Cystoseira C.
Agardh. Cystoseira species are habitat-forming
species, dominating several benthic assemblages
from the infralittoral fringe down to the lower
sublittoral zone (Feldmann, 1937a,b; Molinier,
1960; Pignatti, 1962; Boudouresque, 1971a,b;Ver-
laque, 1987;Ballesteros,1988,1990a,b; Giaccone
et al., 1994). Their zonation is dependent on different
environmental conditions (light, temperature, hydro-
dynamism and grazing) (Sauvageau, 1912; Ollivier,
´setal.,2009). Their loss is mainly
caused by habitat destruction, eutrophication and
overgrazing, leading to a shift to lesser structural
complexity, such as turf-forming, ﬁlamentous or
other ephemeral seaweeds or barren grounds where
sea urchin density is a driver of habitat homogeniza-
tion (Munda, 1974,1982,1993; Thibaut et al., 2005,
2015; Devescovi & Ivesa, 2007; Airoldi et al., 2008;
Falace et al., 2010; Fraschetti et al., 2011; Giakoumi
et al., 2012;Salaetal.,2012;Bianchietal.,2014;
Templado, 2014). Some Cystoseira species are
locally extinct and other species that played signif-
icant functional roles in coastal communities in the
past can now be considered as functionally extinct
throughout the Mediterranean Sea (Cecere et al.,
1991; Munda, 1993; Cormaci & Furnari, 1999;
Cecere et al., 2001; Thibaut et al., 2005; Falace
et al., 2006;Serioetal.,2006;Buiaetal.,2013;
Tsiamis et al., 2013; Bianchi et al., 2014; Thibaut
et al., 2015). The only non-impacted or weakly
impacted species are C. compressa and C. amen-
tacea, respectively. The former is a highly stress-
tolerant species (Thibaut et al., 2015), while the latter
is preserved from overgrazing by its very shallow
habitat. C. amentacea is restricted to the infralittoral
fringe (i.e. the upper level of the infralittoral zone,
Because of its shallow habitat, this species escapes
overgrazing by most of the large herbivores such as
the sea urchins Paracentrotus lividus (Lamarck,
1816) and Arbacia lixula (Linnaeus, 1758) and the
teleost Sarpa salpa (Linnaeus, 1758), unlike the other
subtidal Cystoseira species (Cormaci and Furnari,
1999; Thibaut et al., 2005; Serio et al., 2006;Verge
et al., 2009). Overall, C. amentacea has remained
relatively stable over the last two centuries and along
several thousand kilometres of coastline in the north-
western Mediterranean Sea (Thibaut et al., 2014).
In contrast with the genus Cystoseira, the status of
the Sargassum C. Agardh species has been poorly
studied in the Mediterranean Sea. Species of the genus
Sargassum are perennial, more rarely annual, large
canopy-forming macroalgae (engineer species), found
worldwide from tropical to cold-temperate latitudes.
Among this highly diversiﬁed genus (335 taxa taxo-
nomically accepted), only nine species have been
reported from the Mediterranean Sea and 6 in the
north-western basin (including the invasive S. muti-
cum) (Cormaci et al., 2012; Guiry & Guiry, 2015).
They are (or were) the main components of various
shallow and deep algal assemblages and some species
can form (or once formed) large populations (Balles-
teros, 1988,1992; Cocito et al., 2000; Piazzi et al.,
2002; De Biasi et al., 2004;S
ˇpan, 2005; Piazzi et al.,
Despite the ecological importance of habitat-form-
ing large seaweeds, such as Laminariales and Fucales,
although frequently mentioned at international con-
ventions, they rarely beneﬁt from true protection status
in most areas (e.g. Phillips, 1998; Airoldi & Beck,
2007; Salomidi et al., 2012; Gianni et al., 2013). Yet
some seaweeds are regarded as locally extinct or
threatened, such as the red alga Vanvoorstia ben-
netiana (e.g. Boudouresque et al., 1990; Phillips,
1998; Millar, 2003; Brodie et al., 2009; Marzinelli
et al., 2014). However, actual protection measures,
such as legal protection status for species or genera,
are still very uncommon (e.g. Boudouresque, 2002;
Approved Conservation Advice for Giant Kelp Forests
of South-East Australia, 2012).
In order to analyse the long-term changes in the
patterns of distribution of Sargassum species, we
collected all historical available data (herbarium
vouchers, published and grey literature) and we
compared them to our ﬁeld surveys conducted from
2003 through 2014. The aims of this study were (i) to
provide an up-to-date exhaustive quantitative map of
the distribution of the Sargassum species along most
of the north-western Mediterranean coasts (French
Catalonia, Languedoc, Provence, French Riviera and
Corsica); (ii) to compare the present distribution
pattern with historical data; (iii) to discuss the
potential causes of change and its consequences at
Mediterranean scale and ﬁnally (iv) to question the
relevance of the current protection status, or the
absence of protection status, of some Sargassum
species, in the context of their current abundance and
Materials and methods
The study area is the French Mediterranean coast
(north-western Mediterranean), including Monaco. It
encompasses *2,970 km of shoreline (measured on a
1:2,500 scale map). From west to east: French
Catalonia, Languedoc, western Provence, eastern
Provence, French Riviera and Corsica.
Historical records of Sargassum species were
analysed from published articles, unpublished reports
and herbarium vouchers. There is a considerable
amount of available historical data dealing with the
genus Sargassum along the French coasts; from the
19th to the early 21st century, numerous naturalists
and phycologists surveyed the French Mediterranean
coasts. They received the support of local natural
history museums and they built up an efﬁcient web of
collectors, providing large quantities of samples.
Vouchers held in herbaria are an exceptional source
of data providing a basis for checking the identiﬁca-
tion of the specimens. We surveyed several thousand
vouchers held in the herbaria of a dozen or so
institutions, and every Sargassum voucher has been
examined in order to verify its identiﬁcation and to
record the data on the sampling location, date,
collectors and additional information, when pertinent
(Supplementary Data: Herbarium collections).
The current distribution pattern was investigated
by ﬁeld surveys from 2003 to 2014 by snorkelling and
scuba diving. We explored the whole of the historical
localities (Supplementary data. Historical records—
Table S1) where Sargassum species were observed
and/or collected, via snorkelling or scuba diving,
except in the case of the invasive Sargassum
muticum, native to Japan, which was not present in
the historical records. For the exploration of the
localities situated in between historical ones, we used
maps of the substrates, from the sea surface, down to
40 m depth (Androme
´anologie, 2014 and
references therein), to identify suitable substrates, i.e.
rocky reefs, coralligenous assemblages and coastal
detritic bottoms, as deﬁned by Pe
`s(1982). In the
north-western Mediterranean, most of the depth
range between the shoreline and a depth of 40 m is
occupied by seagrass meadows (mainly Posidonia
oceanica (Linnaeus) Delile) and infralittoral soft
substrates unsuitable for Sargassum species. For
shallow suitable hard substrates (
\5-m depth), the
coast was explored by closely following the shoreline
in a 6 m boat at low speed (3–4 km h
observation and ground-truthing via snorkelling at
least every 1-km. For deeper suitable substrates,
random scuba dives were performed every 10 km (in
French Catalonia, Languedoc, Provence and French
Riviera) and 50 km (in Corsica); in addition, we
asked local diving clubs and marine biologists for
Sargassum observations and dived to conﬁrm their
observations of the presence or absence of the
species. Sargassum populations were geo-localised
and their abundance was visually estimated to four
levels: absent, isolated individuals, scattered popu-
lation and dense population. Species were identiﬁed
in the ﬁeld, if possible; otherwise specimens were
collected and identiﬁed in the laboratory, using the
appropriate bibliography (e.g. Hamel, 1931–1939;
´mez-Garreta et al., 2000;S
et al., 2012). Vouchers of samples collected by this
study are deposited in the Thibaut Herbarium
(HCOM) held at the Mediterranean Institute of
Oceanography, Aix-Marseille University.
The changes over time were analysed on a GIS
(Geographical Information System) database
). Within the 6 study regions (French
Catalonia, Languedoc, western Provence, eastern
Provence, French Riviera and Corsica), we compared
the presence and absence between historical data and
current data, the date of the last reported sighting
among 6 regions, and assessed the status of species
within those regions before 1950, after 1950 and
currently (2003 through 2014), according to the
following scale: absent, very rare (no more than 1
sighting), rare (2–6 sightings), frequent (many local-
ities, less than *10 individuals per locality and
sighting) and common (many localities, several tens of
individuals per locality and sighting). 1950 was
chosen as a milestone because coastal development,
tourism and pollution conspicuously increased after
that date. For historical data, probable frequency was
estimated through authors’ comments and the number
Sargassum acinarium (Linnaeus) Setchell
Sargassum acinarium is a species usually growing in
the sublittoral zone at 30-40 m depth. Some historical
records were cited under taxonomical synonyms: S.
linifolium C. Agardh and S. vulgare var. linifolium (C.
Agardh). The complete authority of the latter synonym
is unclear (see Guiry & Guiry, 2015). Sargassum
acinarium was historically recorded and/or cited 52
times (Fig. 1; Supplementary data, Table S1) (Duby,
1830; Debeaux, 1873; Bornet & Flahault, 1883;
´lis, 1907,1924b,c; Leblond, 1924; Jahandiez,
1929; Feldmann, 1937a; Molinier, 1960; Verlaque &
´,1979). The species was present in the six north-
western Mediterranean study regions and can be
regarded as frequent in western and eastern Provence
and common in the French Riviera, before 1950
(Table 2). During recent ﬁeld surveys, it was only
found 3 times, in three of the six regions, while the
remaining three regions have no current populations,
with last report as late as 1932 in French Catalonia
(Table 1). Despite being present currently in three
regions, the abundance of this seaweed is now
dramatically reduced as compared to historical reports
(Table 2). We only observed a single specimen in
2005 at the Island of Port-Cros (30 m depth; northern
part of the island), a single fertile specimen stranded
on the beach of Larvotto (Monaco) in 2007 and one
shallow specimen at Poraggia Island (the Lavezzi
islands) in 2008, for eastern Provence, French Riviera
and Corsica, respectively (Fig. 1).
Sargassum ﬂavifolium Ku
Sargassum ﬂavifolium grows in the upper sublittoral
zone in sheltered places. A drift specimen was
collected in eastern Provence (Verlaque & Boudour-
esque, 1981) while only a few specimens were
collected in Corsica (Verlaque & Boudouresque,
1981; Verlaque, 1990; Verlaque et al., 1998) (Fig. 2;
Supplementary data, Table S1). We did not ﬁnd this
species during our surveys.
Sargassum hornschuchii C. Agardh
Sargassum hornschuchii is a species usually growing
in the sublittoral zone at 30-60 metres depth. The
species was historically recorded and/or cited 45 times
(Agardh, 1842; Raphe
´lis, 1907; Decrock, 1914;
Hamel 1931–1939; Feldmann, 1937a; Laborel, 1960;
Molinier 1960; Verlaque, 1990) (Fig. 3; Supplemen-
tary data, Table S1). It was present in the six north-
western Mediterranean study regions, but was rare,
with the exception of French Catalonia where it was
reported as frequent between 15 and 30 m depth,
among Cystoseira spinosa (Feldmann, 1937a). We
never observed this species during our ﬁeld surveys.
The last recorded sighting dates back to 1988 in
Corsica (Fig. 3; Table 2).
Sargassum muticum (Yendo) Fensholt
In the Mediterranean Sea, Sargassum muticum is an
invasive NIS (non-indigenous species) growing in the
upper sublittoral zone, in sheltered localities (lagoons
and open sea ports). Introduced to Europe in the late
1960 s (English Channel, Critchley et al., 1983,1990),
S. muticum was ﬁrst recorded in the Mediterranean Sea
in the 1980s in France (Thau Lagoon, Pe
´rez et al.,
1984). The species was successively reported from
Grau du Roi, Palavas, Grau du Pre
`te, Cap d’Agde, Brossolette, Gruissan, Port-la-
´guy et al., 1985), Salses-
Leucate, Banyuls (Knoeppﬂer et al., 1990), Diana
lagoon in Corsica, (IARE, 1992), Bages-Sigean
(Mouillot et al., 2005) and Ingril (Lauret et al., 2011)
(Fig. 4; Supplementary data, Table S1). Sargassum
muticum was also introduced via oyster culture
(introduced means established, see Boudouresque &
Verlaque (2002) for deﬁnitions) in Spain (Costa
Brava, Knoeppﬂer et al., 1990), Italy (Venice Lagoon,
Gargiulo et al., 1992) and the Balearic Islands (cast
ashore, Verlaque, 1994). Currently, Sargassum muti-
cum is regularly surveyed through various monitoring
programs. We found the species in abundance in all
the coastal lagoons of Languedoc. In Corsica, we
partially explored the Lagoon of Diana and we did not
observe the species (Fig. 4).
Sargassum trichocarpum J. Agardh
Sargassum trichocarpum grows in the sublittoral zone
down to 30 m depth. Records of this species are rare.
In western Provence, the species was collected once in
1891 at Marseille (Lenormand in Ge
MPU); we consider this non-fertile specimen as
doubtful. Verlaque & Giraud (1979) collected four
specimens in the plume of warm water discharged by
the power plant at Martigues-Ponteau in the Gulf of
Fos. The species was also collected at Sausset-les-Pins
Fig. 1 Records of Sargassum acinarium along the French Mediterranean coasts. Gray circle locality where the species was not
recorded (historical records), black circle locality where the species is still present
Table 1 Last records (i.e. the most recent observation in each region) of the Sargassum species reported from north-western
Species French Catalonia Languedoc Western Provence Eastern Provence French Riviera Corsica
S. acinarium 1932 1898 1929 2005 2007 2008
S. ﬂavifolium N N 1982 1979 N 1998
S. hornschuchii 1955 1954 1958 1934 1938 1988
S. muticum N2013 N N N 1992
S. trichocarpum N N 1977 N N N
S. vulgare 1937 1918 2014 2014 2014 2011
Thin font: historical references. Bold font: veriﬁed by the ﬁeld survey
in 1980 and 1982 on hard substrates in an euphotic
zone between 1 and 1.5 m depth (Verlaque &
Boudouresque, 1981. Along the French Riviera, S.
trichocarpum has been cited by Raphe
Sargassum boryanum Montagne, a heterotypic syn-
onym of S. trichocarpum. It was found in a ﬁshing net,
off Cannes (French Riviera). The doubt about the
latter record of this species of warm afﬁnities, never
previously collected in France, arises from the
synonym proposed by Raphe
´lis (1907), S. salicifolium
Bory, which is actually a heterotypic synonym of the
very common S. vulgare. In addition, the specimen is
absent from the Raphe
´lis herbarium. We therefore
consider this record as a probable misidentiﬁcation.
During recent ﬁeld surveys, we could no longer ﬁnd
this species; the last record, therefore, dates back to
1982 in western Provence (Fig. 5; Table 1; Supple-
mentary data, Table S1).
Sargassum vulgare C. Agardh
Sargassum vulgare grows in the sublittoral zone, on
moderately wave-exposed rocks, in rock pools down
to 30-40 m depth. Some historical records were cited
under taxonomical synonym as S. megalophyllum
Montagne or misidentiﬁed as ‘‘Sargassum salici-
folium’’ auct. plur. non Montagne: Hamel in Phe
´es de France,1931–1939: 427. Sargassum
vulgare was historically recorded and/or cited 139
times (Agardh, 1842; Debeaux, 1873; Anonymous,
´lis, 1907; Mouret, 1911; Camous, 1912;
Decrock, 1914; Raphe
´lis, 1918,1924a; Jahandiez,
Table 2 Status of the
Sargassum species from
Mediterranean regions, over
Absent means: not found
Taxon Before 1950 After 1950 Current Comment
S. acinarium Rare Absent Absent Locally extinct
S. hornschuchii Frequent Very rare Absent Locally extinct
S. vulgare Frequent Absent Absent Locally extinct
S. acinarium Rare? Absent Absent Locally extinct?
S. hornschuchii Rare? Rare? Absent Locally extinct?
S. muticum Absent Common Common Invasive species
S. vulgare Rare? Absent Absent Locally extinct?
S. acinarium Frequent Absent Absent Locally extinct
S. hornschuchii Rare Very rare Absent Locally extinct
S. trichocarpum Very rare? Rare Absent Introduction?
S. vulgare Frequent Very rare Very rare Decline
S. acinarium Frequent Very rare? Very rare Decline
S. ﬂavifolium ? Very rare Absent Locally extinct
S. hornschuchii Very rare? ? Absent Locally extinct
S. vulgare Frequent Frequent Frequent Stable
S. acinarium Common ? Very rare Decline
S. hornschuchii Rare ? Absent Locally extinct?
S. vulgare Common Common Rare Decline
S. acinarium Rare? Rare? Very rare Decline?
S. ﬂavifolium ? Rare Absent Locally extinct?
S. hornschuchii Rare? Rare? Absent Locally extinct?
S. muticum Absent Rare Absent? Invasive species
S. vulgare Frequent? Frequent Frequent Stable
Fig. 2 Records of Sargassum ﬂavifolium along the French Mediterranean coasts. Gray circle locality where the species has
disappeared, black circle locality where the species is still present
Fig. 3 Records of Sargassum hornschuchii along the French Mediterranean coasts. Gray circle locality where the species has
disappeared, black circle locality where the species is still present
Fig. 4 Records of Sargassum muticum along the French Mediterranean coasts. Gray circle locality where the species has disappeared,
black circle locality where the species is still present
Fig. 5 Records of Sargassum trichocarpum along the French Mediterranean coasts. Gray circle locality where the species has
disappeared, black circle locality where the species is still present
1929; Hamel, 1931–1939; Feldmann, 1937a,b;
Coppejans, 1972; Meinesz, 1973; Belsher et al.,
1976; Dhont, 1976; Coppejans, 1977,1979; Verlaque,
1990; Frick et al., 1996; Verlaque et al., 1998; Boury-
Esnault et al., 2001) (Fig. 6; Supplementary data,
Table S1). Before 1950, it was present in all the six
study regions and frequent or common everywhere,
except Languedoc (Table 2). During our ﬁeld survey
in this study, the species was only recorded 50 times. It
is absent from French Catalonia (last record in 1937)
and Languedoc (last record in 1918) and in severe
decline in western Provence and French Riviera; in
contrast, in Corsica, S. vulgare is still frequent
everywhere and even common in some places
Two Sargassum records constitute obvious misiden-
tiﬁcations: the strictly Atlantic species S. natans
(Linnaeus) Gaillon (as S. bacciferum (Turner) C.
Agardh) reported from Cannes in Raphe
(Guiry & Guiry, 2015) and the subtropical species S.
dentifolium (Turner) C. Agardh reported from Nice
(Risso in Herbier Lamouroux Herbarium PC) is a
(Guiry & Guiry, 2015). These erroneous records
probably result from a mix-up between specimens of
The loss of species
The overall situation of the Sargassum species, along
*2,970 km of north-western Mediterranean coasts, is
alarming (Tables 1,2). The analysis of historical data
(vouchers and literature) dating back to the early
nineteenth century and the comparison with the
current survey of a large part of the north-western
Mediterranean, from French Catalonia to Corsica, has
evidenced the severe decline of all the native (i.e. not
introduced; see Boudouresque & Verlaque, 2002 for
deﬁnitions) Sargassum species. Sargassum horn-
schuchii has not been recently observed in most of
the study area, in one case for 80 years. S. acinarium is
also unrecorded from much of its former area. When
we have surveyed S. acinarium in this study, we
Fig. 6 Records of Sargassum vulgare along the French Mediterranean coasts. Gray circle locality where the species has disappeared,
black circle locality where the species is still present
observed a single individual. Even for S. vulgare,a
species once regarded as common, recent surveys only
found scattered individuals.
Are the past and present distributions of the species
reliable? The past distribution of a species, from a
large number of collectors who published their data or
left extensive herbariums, is probably biased, at least
for deep water species. Most of these collectors, at
least from the nineteenth century and the ﬁrst half of
the twentieth century, did not dive (whether snorkel-
ling or scuba diving). Indications of abundance, often
based upon a few access points to the sea or on casual
records in ﬁshing nets, grapnels or specimens stranded
on the beach, must be considered with caution. Most
historical data give presence-only information (see
Coleman & Brawley, 2005; Tingley & Beissinger,
2009; Huisman & Millar, 2013 for a thorough
discussion on problems with herbarium specimens).
With regard to the shallow water species, historical
data are probably more reliable. Where comprehen-
sive exploration, lasting seven years and using the
whole range of modern methods and tools, all the year
round, only resulted in a very few sightings of species
which were so common that it was easy to collect,
despite the poor means available before the 1950s, it is
unrealistic to claim that the decline might be only due
to sampling bias. In addition, recent surveys of deep
habitats by means of Remote Operated Vehicles
(ROVs) or deep diving did not reveal unknown
populations of Sargassum (Ge
´rard Pergent pers.
com.; Florian Holon pers. com.), in contrast with
reports of large Sargassum stands at Milos Island
(Cocito et al., 2000; De Biasi et al., 2004) and the
Gorgona Island and in Tuscany (Piazzi et al., 2002,
2010) (Fig. 7).
In the Languedoc, the historical presence of S.
acinarium,S. hornschuchii and S. vulgare is ques-
tionable. The coast is mainly sandy (large beaches)
and therefore not suitable for sessile macroalgae.
Interestingly, all the records correspond to drift
specimens stranded on the beach. These species have
stalked air vesicles that provide buoyancy, so the long-
distance transport, by the westward Northern Current
(see Bergamasco & Malanotte-Rizzoli, 2010), of
drifting specimens coming from some more or less
distant rocky coasts or from deep beach-rocks is
possible. Taking into account the westerly current
along the coast, the origin must be east of Languedoc,
in Provence or further.
Records of S. trichocarpum in western Provence
could account for an unsuccessful introduction from
other Mediterranean regions. This thermophilic
species is known to occur in the southern and eastern
parts of the Mediterranean Sea (Guiry & Guiry, 2015)
and the records date from a period when the power
plant of Martigues-Ponteau was functioning at a high
Comparison with other areas
Similar local extinctions of Sargassum populations
have already been mentioned in the Mediterranean Sea
(Fig. 7). Munda (1993,2000) reported the loss of the
only two species, S. acinarium and S. hornschuchii,
present in Rovinj (Croatia) and Piran (Slovenia),
Northern Adriatic. These two species also disappeared
from Tremiti Islands (Sicily), the Gargano Promon-
tory (southern Adriatic) and Monte Conero (central
Adriatic) (Cormaci & Furnari, 1999; Cecere et al.,
2001; Falace et al., 2006,2010; Perkol-Finkel &
Airoldi, 2010). The loss of S. acinarium and S.
trichocarpum was reported at Linosa Island, off
Tunisia (Serio et al., 2006). Only one ﬂoating spec-
imen of S. acinarium has been collected in the Gulf of
Pozzuoli (Napoli, Italy) (Buia et al., 2013). At Kos
Island (Greece), S. vulgare disappeared at the study
sites between 1981 and 2013 (Bianchi et al., 2014).
Finally, S. acinarium,S. hornschuchii and S. tri-
chocarpum were no longer observed in Algeria (Ould-
Ahmed et al., 2013).
Outside the Mediterranean, there is a concern about
historical and continuing loss of canopy-forming algae
throughout the world’s temperate coastline. In Aus-
tralia, Coleman et al. (2008) evidenced the total loss of
the habitat-forming Fucoid Phyllospora comosa (La-
bill.) C. Agardh, previously abundant around Sydney,
half a century ago. On the Adelaide metropolitan
coast, up to 70% of the canopy-forming algae were
lost within a few decades (Connel et al., 2008). Along
the Indian and Paciﬁc coasts of Australia, the regres-
sion of a number of macroalgae, including canopy-
forming species, was reported on the basis of
comparison between historical and current data
(Wernberg et al., 2011,2014; but see Huisman &
Millar, 2013,2015). In the northern Paciﬁc and
Atlantic, historical overﬁshing led to the recent
collapse of kelp ecosystems. Fish feeding on sea
urchins are overexploited, and the increasing numbers
of sea urchins without predators devastate kelp forests
(Jackson et al., 2001). However, the distribution of
kelp in the northwest Atlantic Ocean has not changed
over the last 50 years (Merzouk & Johnson, 2011).
Possible causes of losses
The historical data are observational but possibly
biased, while current data, also observational but
based on thorough exploration, can be considered as
more reliable. Nevertheless, attempts to explain the
losses are inevitably hypothetical in many cases, in the
absence of experiments which are beyond the scope of
the present study. The possible causes of the loss of
Sargassum species are numerous, probably working in
synergy, and are depth-dependent. Potential driving
factors for regressions of shallow water species
include habitat destruction and overgrazing, while
deep water population declines may be driven by
trawling and ﬁshing nets and water turbidity.
Habitat loss in the region can be attributable to
demographic increases, coastal development that
occurred during World War II. From the early 19th
century, the number of reclamations (creating new
land from the sea) and harbours considerably
increased along the French Mediterranean coast
(Meinesz et al., 2013; MEDAM, 2015). Overall, the
shoreline has undergone a profound process of artiﬁ-
cialisation. The French Riviera is the region that is
most concerned, with the rate of artiﬁcialisation
increasing from 3% of the coastline in 1800 to 28%
currently, with a sharp increase in the 1970s and a
plateau since the 1990s. Other regions have also been
severely hit with increases in artiﬁcialisation from
1800 to currently: 2 to 22% (1960s increase, 1990s
plateau) in western Provence, 1 to 19% (1970s
increase, 1990s plateau) in Languedoc, 2 to 13%
(1960s increase, 1990s plateau) in eastern Provence
and 5 to 11% (1960s increase, 1980s plateau) in
French Catalonia. Corsica is the only region where the
process of artiﬁcialisation of the shoreline has been
moderate, from 0.3% in 1800 to 2.1% today, with an
increase in the 1970s and a plateau since the 1990s
Fig. 7 Locations where local extinctions of Sargassum spp.
have been reported in the Mediterranean Sea (black dot) (data
from Munda, 1993,2000; Cormaci & Furnari, 1999; Cecere
et al., 2001; Falace et al., 2006,2010; Serio et al., 2006; Perkol-
Finkel & Airoldi, 2010; Buia et al., 2013; Bianchi et al., 2014)
and where Sargassum spp. have been reported as abundant (grey
dot) (Cocito et al., 2000; De Biasi et al., 2004; Piazzi et al., 2002;
(MEDAM, 2015) (Supplementary data, Fig. 1S).
Coastal infrastructures, even when not made of
concrete, do not constitute a suitable substrate for
recolonization by canopy-forming algae such as
Sargassum species (e.g. Perkol-Finkel et al., 2012;
Thibaut et al., 2014). Artiﬁcialisation of the shoreline
constitutes the main driver for habitat fragmentation,
especially where shallow water species are concerned.
Overgrazing by sea urchins may be an additional
potential driver of Sargassum regression as this is
known to be responsible for the loss of other canopy-
forming algae both in the Mediterranean Sea (e.g.
Cormaci & Furnari 1999; Thibaut et al., 2005; Serio
et al., 2006; Boudouresque & Verlaque, 2013; Agnetta
et al., 2015) and worldwide (e.g. Ling et al., 2015;
Wallner-Hahn et al., 2015). In the Mediterranean, the
sea urchins Paracentrotus lividus and Arbacia lixula
are involved; the former feeding preferentially on
erect species, such as Fucales (Boudouresque &
Verlaque, 2013), and the latter on encrusting and turf
species, including plantlets, hence preventing new
settlements (Frantzis et al., 1988; Bonaviri et al., 2011;
Wangensteen et al., 2011; Agnetta et al., 2015). As
observed for the Cystoseira species (Thibaut et al.,
2005,2015), the main losses occur in shallow waters.
The proliferation of herbivorous sea urchins is mainly
linked to overﬁshing of their predators, mainly sparids
and labrids (Sala & Zabala, 1996; Sala, 1997) and the
resulting cascading effect results in the depletion of
the canopy-forming seaweeds (Sala et al., 1998;
Steneck, 1998; Ling et al., 2009; Wallner-Hahn
et al., 2015). Organic pollution can be also responsible
for the proliferation of P. lividus. The increase in
phytoplankton through the eutrophication promotes
the increase in the survival rate of sea urchins through
the increasing availability of prey for sea urchin larvae
(Boudouresque & Verlaque, 2013). In addition, pulses
in phytoplankton productivity enhance the recruitment
and subsequently the adult population of sea urchins,
which at some sites may have an impact on habitat-
forming species (Coma et al., 2011; Cardona et al.,
Artisanal ﬁsheries have been frequently mentioned
by collectors as their main source of deep Sargassum
species, caught in ﬁshing nets and in benthic trawls.
Sargassum species are monopodial (i.e. a single axis),
often tall and weakly ﬁxed to the substratum by a small
basal holdfast, so that they can become entangled in
net meshes and be easily pulled up from the substrate.
Sargassum species are reported to have been regularly
brought up with the trawls in Eastern Provence
(Dufour, 1985). We therefore suspect ﬁsheries of
playing a signiﬁcant role in the loss of deep long-lived
species. At the beginning of the 19
addition to the use of trammel nets, longlines and ﬁsh
pots, ﬁshing methods progressed to various techniques
of trawling (locally called gangui) that scrape the sea
bottom. Trawling was used on detrital bottoms as well
as on ﬂat rocks to catch ﬁsh, sea squirts Microcosmus
sabatieri (Roule, 1758) and sea urchins (Faget, 2009;
Supplementary data: Fisheries). Technical upgrading
of the materials led to ﬁshing with longer nets from the
1950s, the use of ﬁshing winches and an increase in the
power of the motors in the 1960s. In Corsica, ﬁshing
has never been as highly developed as in the other
Mediterranean French regions (Miniconi, 1994). Arti-
sanal ﬁsheries began to decline in the early twentieth
century in Corsica and in the 1990s along French
continental coasts (Supplementary data: Fisheries).
But whatever the period, the entire coast has been
ﬁshed for centuries.
For the deep water populations, the decrease in
water transparency could be responsible for their
decline (Thibaut et al., 2005). At Tremiti Islands
(Sicily), the Gargano Promontory (southern Adriatic)
and Monte Conero (central Adriatic), the loss of deep
populations of S. acinarium and S. hornschuchii has
been ascribed to reduced water transparency (Cormaci
& Furnari, 1999; Cecere et al., 2001; Falace et al.,
2006,2010; Perkol-Finkel & Airoldi, 2010). Eutroph-
ication from untreated sewage outfall, resulting in
increased plankton blooms, and turbidity, due to river
discharge and trawling, may account for reduced water
transparency. Changes in water turbidity can be
measured indirectly through the measurement of the
deeper limit of the seagrass Posidonia oceanica beds.
The regression of this limit has been documented since
at least the 1980s in the north-western Mediterranean
and is still in progress in most areas (Boudouresque
et al., 2000,2012; Mayot et al., 2006; Bonhomme
et al., 2010a,b), despite the conspicuous improvement
of the sewage treatment. The establishment of sewage
treatment plants began in the 1980s and is now
widespread (Agence de l’Eau Rho
Over the past 30 years, an increase in the very deep
(beyond 2,000 m depth) water temperature in the
Mediterranean Sea has been detected (Be
´thoux et al.,
1990). Within the depth range of Sargassum species
(less than 100 m depth), the warming has also been
signiﬁcant, with strong differences due to regional and
local conditions and inter-annual ﬂuctuations. Overall,
French Catalonia, Languedoc and western Provence
are cooler than eastern Provence, French Riviera and
Corsica) (e.g. Prieur, 2002; Harmelin, 2004; Romano
& Lugezi, 2007; Shaltout & Omstedt, 2014). The
mean sea surface temperature (SST) anomalies of the
Mediterranean Sea from 1856 to 2000 show a mainly
irregular oscillation (period of 60-70 years and mean
amplitude of 0.4–0.5°C) and a long-term positive
trend (?0.1°C per century) (Moron, 2003). Along the
French Mediterranean coast, the mean SST anomalies
vary through time, with colder periods in 1901–1914
(between -0.6 and -0.2°C) and 1973–1978 (between
-0.4 to -0.2°C), a warmer period in 1936-1948 (from
?0.2 to ?0.4°C) and 1987–1996 (up to ?0.4°C)
(Moron, 2003). A signiﬁcant warming trend over a
30-year period, with an increase of around 0.3°C per
decade (1975–2004), has been recorded (Bensoussan
et al., 2009, 2011). From 1885 to 1967, SST was
measured daily at Marseille, where a signiﬁcant
warming trend was estimated at ?0.66°C over a
century for the mean annual SST, and ?0.76°C for the
warmest months (June–September) (Romano &
Lugrezi, 2007). Major positive anomalies were
detected in 1999, 2003 and 2006 in the north-western
Mediterranean (Harmelin, 2004; Bensoussan et al.,
2009,2010), concomitant with mass mortality of
gorgonians (Garrabou et al., 2001,2009). The loss of
S. acinarium and S. trichocarpum at Linosa Island, off
Tunisia was ascribed to the increase in water temper-
ature (Serio et al., 2006). Changes in seawater
temperature are directly responsible for changes in
forests of temperate large Sargassum species in the
northwest of Kyushu Island (Japan), where they have
been replaced by small tropical species (Komatsu
et al., 2014). At Honshu Island Paciﬁc coast (Japan),
Sargassum forests were devastated due to high
temperature episodes resulting from water intrusions
from the Kuroshio Current and the increase in
herbivorous tropical ﬁshes (Komatsu et al., 2014). A
northward migration of Sargassum horneri, under an
A2 scenario of global warming in the north-western
Paciﬁc, has been predicted (Komatsu et al., 2014).
Are the above-mentioned factors (Table 3) a likely
cause explaining the observed decline or local extir-
pation of Sargassum species in the north-western
Mediterranean? Habitat loss, though severe in some
regions, does not seem to be a crucial factor, as many
suitable substrates are still available, especially in
Corsica where habitat loss has been weak. Seawater
warming probably enhances rather than hinders Sar-
gassum species. In the Mediterranean Sea, most of the
native Sargassum species are regarded as species of
warm afﬁnities (Braune & Guiry, 2011), so that the
current seawater warming (Rixen et al., 2005; Por,
2009) cannot be implicated in the decline of Sargas-
sum in the study area. Pollution and the decrease in
water transparency could have played a role in the
past. However, they have substantially improved over
the last two decades, and Corsica and the extreme east
of Provence have been only weakly impacted. In fact,
the most probable causes of decline are ﬁshing nets,
trawling and the overgrazing by sea urchins resulting
from overﬁshing of their predator ﬁshes, although it is
unclear whether these factors are sufﬁcient to have
resulted in the region-wide extirpation of the species.
The Mediterranean species of the genus Sargassum,
with the exception of the introduced S. muticum, were
all categorised as locally extinct or in more or less
severe decline at all the sites that have been re-
assessed at different time intervals. Previous studies
were dealing with restricted areas and were therefore
not conclusive. Here, for the ﬁrst time, we have
studied an extensive area in the north-western
Mediterranean, encompassing 2,970 km of shoreline
(measured on a 1:2,500 scale map) and two centuries
of historical data. Comparing the earlier results from
relatively local data and our comprehensive analysis
presents a worrying picture. All native species have
declined or seem to be extinct over relatively exten-
sive areas, such as French Catalonia and Provence.
The status of S. acinarium and S. hornschuchii is the
greatest cause for concern.
As emphasised above, the current warming should
beneﬁt some species. Most Sargassum species are
prone to dispersal over long distances, as drift
individuals, ﬂoating by means of their air vesicles.
The decline concerns areas with high anthropization
together with regions where the anthropic impact is far
from obvious. In fact, none of the above-mentioned
causes, for the strong decline or local extirpation of
some Sargassum species, appear to have been decisive
everywhere and at all times, including the recent
period. Populations protected by well-enforced Mar-
ine Protected Areas (MPAs), including No-Take
zones, such as Cerbe
`re-Banyuls (French Catalonia),
Parc Marin de la Co
ˆte Bleue (western Provence), Port-
Cros Island (eastern Provence), Scandola and Lavezzi
Island (Corsica), have neither resisted the overall
regression nor recovered since the establishment of
these MPA 30–50 years ago (Meinesz & Blanfune
Equally surprising is the total absence of national
and/or international protection. Only four Sargassum
species are mentioned in the IUCN Red List of
threatened species (IUCN, 2014); their status is ‘data
deﬁcient’, with the exception of S. setifolium (Gru-
now) Setchell, a doubtful taxon from Gala
Islands, which is ‘endangered’. For the Mediterranean
Sea, an action plan has been implemented by RAC/
SPA (2000) to manage species and ecosystems
dominated by primary producers; while 14 species of
seagrasses (including Posidonia oceanica) and
‘macroalgae’, e.g. Cystoseira amentacea (C. Agardh)
Bory de Saint-Vincent, C. zosteroides C. Agardh and
Schimmelmania schousboei (J. Agardh) J. Agardh are
included, no Sargassum species are currently on this
management list. The same 14 macrophyte species,
with the addition of Laminaria ochroleuca Bachelot
de la Pylaie, are mentioned in Appendix I (strictly
protected ﬂora species) of the Berne Convention
(Berne Convention on the Conservation of European
Wildlife and Natural Habitats) and Annex II (endan-
gered and threatened species) of the Barcelona Con-
vention (Barcelona Convention for the Protection of
the Mediterranean Sea). Since 2010, Annex II has
been amended; it now encompasses a larger number of
‘macroalgae’ (*50), in particular four species of the
genus Sargassum, namely S. acinarium, S. ﬂavifolium,
S. hornschuchii and S. trichocarpum. Although this
new status for a number of severely threatened
‘macroalgae’ is welcome, it is worth emphasising that
these conventions are not binding; as a result, the
species are not legally protected by most Mediter-
ranean countries, including France. Whatever the
protection level of Mediterranean ‘macroalgae’, the
Table 3 Stressors putatively explaining the decline or the possible loss of Sargassum species
Region Declining or
Habitat loss Pollution Decrease in
Fishing nets Trawling Overgrazing Sea water
French S. acinarium - ? ?? ??? ??? - -
Catalonia S. hornschuchii - ? ?? ??? ??? - ?
S. vulgare - ? - ? - ??? -
Languedoc S. acinarium - ? ?? ??? ??? - -
S. hornschuchii - ? ?? ??? ??? - ?
S. vulgare ??? ? - ? –??? -
Western S. acinarium - ?? ? ??? ??? - -
Provence S. hornschuchii - ?? ? ??? ??? - ?
S. vulgare ??? ??? - ? - ??? -
Eastern S. acinarium ? ? ? ??? ??? - -
Provence S. ﬂavifolium ??? ?? - ? - ??? -
S. hornschuchii ? ? ? ??? ??? - ?
S. vulgare ??? ?? - ? - ??? -
French S. acinarium - ?? ? ??? ??? - -
Riviera S. hornschuchii - ?? ? ??? ??? - ?
S. vulgare ??? ??? - ? - ??? -
Corsica S. acinarium - - - ?? ??? - -
S. ﬂavifolium ?-- --? -
S. hornschuchii - - - ?? ??? - ?
-,?,?? and ???: no effect, low effect, moderate effect and high effect, respectively
protection of marine species is greatly inferior to that
of terrestrial species (concerning thousands, some of
them relatively common and widespread). In addition,
all these ‘macroalgae’ are absent from the EU Habitat
Directive of 1992 (Habitat Directive, 1992) and
therefore are not taken into consideration for the
establishment of ‘Natura 2000’ zones (Natura 2000 is
an EU wide network of nature protection areas
established under the 1992 Habitats Directive. The
aim of the network is to assure the long-term survival
of Europe’s most valuable and threatened species and
habitats. The emphasis will be on ensuring that future
management is sustainable, both ecologically and
In the Mediterranean Sea, S. hornschuchii and S.
trichocarpum are endemic (Giaccone, 2003), S. aci-
narium and S. vulgare are reported as cosmopolitan
species, while S. ﬂavifolium has been observed along
the Atlantic coast of France, Portugal, Spain, Morocco
and in the Canary islands, as well as along the
Mediterranean coasts of Tunisia, Corsica, France,
Greece, Italy, Spain and in the Arabian Gulf (Guiry &
Guiry, 2015). In the absence of molecular phylogeny,
the status of these species should be re-assessed in the
Mediterranean Sea, in order to clarify the species
boundaries, in particular the possible occurrence of
cryptic species, within a complex and polyphyletic
genus (Draisma et al. 2010; Dixon et al., 2014). The
management of threatened species obviously depends
upon a clear taxonomical status.
Finally, the Sargassum species urgently need strong
and properly enforced protection status, at an exten-
sive geographical scale in the Mediterranean Sea.
Acknowledgements We thank two anonymous reviewers for
their helpful suggestions which have improved the present
article. This work was funded by the Agence de l’Eau Rho
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