2014, Pp. 245-264 CHANGES in the BENTHIC ALGAL FLORA AND

Naturalista sicil., S. IV, XXXVIII (2), 2014, pp. 245 -264

GIANCARLO BELLISSIMO & C ARLA ORESTANO

CHANGES IN THE BENTHIC ALGAL FLORA AND VEGETATION OF A SEMI-ENCLOSED MEDITERRANEAN COASTAL LAGOON (STAGNONE DI MARSALA, WESTERN SICILY)

SUMMARY

A study of the benthic macroalgal flora and vegetation of the Stagnone di Marsala (Trapani) has been performed in order to provide an up-to-date review of the status of and verify main changes that have occurred in the area over the past three decades. The flora of this study was poorer in taxa than that of literature (94 taxa against 108), with an increase in Chlorophyta and a decrease in Rhodophyta and Phaeophyceae. Only 48 taxa in the present flora have been previously reported, while 46 are new for the Stagnone. Among these, it was noteworthy, from both floristic and ecolog - ical point of view, the record for the first time of Cystoseira compressa (Esper) Gerloff & Nizamud - din and the invasive algae Caulerpa cylindracea Sonder and Lophocladia lallemandii (Montagne) F. Schmitz. On the contrary, 60 taxa previously reported have not been detected during the present study. Among these, Lamprothamnium papulosum (Wallroth) Groves and Cystoseira barbata (Stack - house) C. Agardh f. aurantia (Kützing) Giaccone have been the most significant disappearances. High degree of confinement and sediment composition were the major factors affecting number of taxa and abundance of the phytobenthic communities, with maximum values in the stations charac - terized by a good water exchange and/or presence of Cystoseira assemblages. As a result of the effects of global warming over the past few years, noticeable changes in composition and structure of phytobenthic communities in the lagoon have been observed and some hypotheses of expected changes are here proposed.

Key words : Algal flora, Coastal lagoon, Marine vegetation, Stagnone di Marsala, Transitional waters.

RIASSUNTO

Variazioni nella flora e nella vegetazione macroalgale in una laguna costiera del Mediterraneo (Sta - gnone di Marsala, Sicilia occidentale) . È stato condotto uno studio sulla flora e vegetazione macroal - gale dello Stagnone di Marsala (Trapani) con l’obiettivo di fornire una revisione dei dati riportati in 246 G. B ELLISSMO & C. O RESTANO letteratura ed esaminare i principali cambiamenti che si sono verificati nell’intera area negli ultimi tre decenni. La flora algale è risultata più povera rispetto al passato (94 taxa contro 108), mostrando un incre - mento delle Cloroficee e una riduzione delle Rodoficee e delle Feoficee. Solo 48 taxa sono stati ripor - tati precedentemente, mentre 46 sono nuovi per lo Stagnone. Tra questi, è interessante sottolineare sia da un punto di vista floristico che ecologico, il ritrovamento per la prima volta dell’alga bruna Cystoseira compressa (Esper) Gerloff & Nizamuddin e delle alghe invasive Caulerpa cylindracea Son - der e Lophocladia lallemandii (Montagne) F. Schmitz. Al contrario, 60 taxa riportati precedente - mente non sono stati trovati nel presente studio. In particolare, si annovera la scomparsa dell’alga verde Lamprothamnium papulosum (Wallroth) Groves e dell’alga bruna Cystoseira barbata (Stackhouse) C. Agardh f. aurantia (Kützing) Giaccone. L’elevato grado di confinamento e la natu - ra del substrato sono stati i principali fattori che hanno influenzato il numero dei taxa e l’abbondanza delle comunità fitobentoniche, con valori massimi nelle stazioni caratterizzate da un buon ricambio idrico e/o dalla presenza di comunità dominate da Cystoseira . Nel presente studio v engono proposte alcune ipotesi per descrivere i cambiamenti nella composizione e struttura delle comunità fitoben - toniche all’interno della laguna come conseguenza degli effetti del riscaldamento globale in atto negli ultimi anni.

Parole chiave : Flora algale, Laguna costiera, Vegetazione marina, Stagnone di Marsala, Acque di transizione.

INTRODUCTION

The Stagnone di Marsala is the largest coastal lagoon of Sicily and, from a hydrobiological point of view, represents one of the most characteristic and interesting wetlands of the entire Mediterranean ( MAZZOLA et al ., 2010). For its importance in terms of plant and animal biodiversity ( VIZZINI & SCILIPOTI , 1999; MIRTO et al ., 2004; CALVO et al ., 2009; TOMASELLO et al ., 2009) and as zone of migration and hibernation of aquatic birds ( MASSA , 1973, 1985; LO VALVO & MASSA , 1999), the lagoon has been declared Regional Nature Reserve (Riserva Naturale Regionale delle Isole dello Stagnone di Marsala) since 1984 by the Decree n°215 of 7 July 1984, managed by the Province of Trapani (Decree n°360 of 14 February 1987). Lately, the islands and their depths have been included in the Natura 2000 Network as Sites of Commu - nity Importance called respectively “SCI-ITA010001 - Islands of the Stagnone di Marsala” and “SCI-ITA010026 - Depths of the islands of the Stagnone di Marsala” identified by Habitat Directive 92/43/EC which aims at the preservation of natural habitats. Given the paucity of recent data on both the marine algal flora and veg - etation, a survey was carried out in order to improve and update the knowl - edge regarding the macroalgal biodiversity and phytobenthic communities currently occurring in the Stagnone di Marsala. The aim of this paper is to describe the qualitative and quantitative composition of benthic macroalgal Changes in the benthic algal flora and vegetation of a semienclosed... 247 assemblages and examine temporal changes by comparing the results with previously published data.

MATERIALS AND METHODS

Study site The Stagnone di Marsala is a semi-enclosed oligotrophic brackish coastal lagoon in western Sicily (Fig. 1) about 20 km 2 in area and geomor - phologically divided into two sub-basins by an emerging Posidonia oceanica (Linnaeus) Delile reef barrier ( CALVO et al ., 2010). The northern sub-basin (14 km 2, mean depth 1 m) communicates north - wards with the open sea via a shallow (mean depth ca . 30 cm) channel 400 m wide. The presence in this sub-basin of the islands Santa Maria, San Pantaleo (more frequently referred to as Mothya) and La Scuola as well as emerging vegetation and the submerged relict of a Phoenician road contribute to reduce the flushing and water circulation with a renewal time calculated at about 65 days ( MAZZOLA & SARÀ ,1995; CALVO et al ., 2005). The northern sub-basin shows greater lagoon characteristics and a higher annual variabili - ty of temperature and salinity than the southern basin ( SARÀ et al ., 1999; VIZZINI et al ., 2002). This last (6 km 2, mean depth 2 m) is characterized by a good water exchange and turnover (about 22 m 3 s-1 ) with the open sea (L A LOGGIA et al ., 2004). Because of the small tidal range (about 0.3 m), the Stagnone lagoon hydrodynamic regime and general overturning circulation are expected to be mainly driven by the intensity and direction of the wind ( MAZZOLA & VIZZI - NI , 2005; DE MARCHIS et al ., 2011). As a result, no stratification exists in the water column. The main physical-chemical parameters recorded within the Stagnone pointed to the good condition of the lagoon since nutrients are pres - ents in low concentrations and the trophic response, in terms of chlorophyll a (1.0 g l -1 ), indicates the relative oligotrophy of the waters ( MAIMONE et al ., 1998; SARÀ et al ., 1999). Hard substrata are characterized by small pebbles, shells and man-made objects. Calcareous outcrops are frequently detected in the area between the islands of Mothya and Santa Maria. A dense monospe - cific meadow of the rooted phanerogam Cymodocea nodosa (Ucria) Ascher - son covers most of the bottom of the northern sub-basin and prevails in areas where hydrological turnover is greater ( CALVO et al ., 1982). A patchy distri - bution of P. oceanica covers the central-southern area forming peculiar atoll- shaped structures between the islands of Mothya and Santa Maria (TOMASEL - LO et al ., 2009). The other two rooted phanerogams Z ostera noltei 248 G. B ELLISSMO & C. O RESTANO

Hornemann and Ruppia sp. are scattered in small isolated patches in the north-eastern and central areas, respectively ( CALVO et al ., 1982, 2009).

Literature analysis Bibliographical research was performed by using peer-reviewed journal articles (searched in Scopus and Science Direct) and books (searched in Dig - ital Libraries) for studies from 1980 to date, while grey literature (available in the Dipartimento di Scienze della Terra e del Mare, University of Palermo) for the oldest ones.

Sampling procedure and data analysis Fieldwork was carried out in summer 2012 (the same sampling seasonal period as that of previous studies) at the northern sub-basin of the Stagnone di Marsala lagoon (Fig. 1), following the sampling design by CALVO et al . (1982). Ten stations were selected and georeferenced using a differential GPS

Fig. 1 — Map of the Stagnone di Marsala lagoon showing sampling stations. Changes in the benthic algal flora and vegetation of a semienclosed... 249 with a marginal error of ca . 1 m (Table 1). At each sampling station inside a homogeneous area of 5 m x 5 m, a small pick was used to remove the vegeta - tion from three randomly placed plots of 1600 cm 2 and 3600 cm 2 on hard and soft substrata, respectively. The material collected was sealed in individual plastic bags and preserved in 4% formalin in seawater for subsequent identi - fication. In the laboratory, algal specimens were carefully sorted and exam - ined by both stereo and optical microscope, while a light microscopy was used to detect the presence of reproductive structures. All the specimens were identified at species level, while taxonomically difficult taxa , due to the absence of reproductive structures and/or diacritical features that are tradi - tionally used to identify a species, were summarized to genus level as ‘spp’.

Table 1 Number of stations, type of substratum and total sample size (cm 2) at the Stagnone di Marsala lagoon (UTM coordinates WGS84).

Number of Type of Total Coordinate Coordinate station substratum sample size XY 1 Hard 1600 276143 4197414 2 Soft 3600 276896 4196890 3 Soft 3600 275913 4196771 4 Soft 3600 276124 4195723 5 Soft 3600 278357 4195006 6 Hard 1600 277310 4195025 7 Soft 3600 276372 4195117 8 Soft 3600 277650 4193352 9 Hard 1600 276225 4193352 10 Soft 3600 276051 4192130

Voucher specimens for most of the species identified were deposited in the Phycological Herbarium of the Dipartimento di Scienze della Terra e del Mare of the University of Palermo, Italy. For comparison with our present work the floristic lists by previous studies were corrected according to the most recent taxonomic literature ( CORMACI et al ., 2012; GUIRY & GUIRY , 2014). Conversely, it was impossible to compare quantitative data with litera - ture ones ( CALVO et al ., 1982) since this study took into account smaller sam - pling surface areas (cm 2 vs . m 2), in order to collect also the numerous small- sized species of marine vegetation. The abundance of sorted specimens was considered as the total percentage of surface covered by each taxon in verti - cal projection ( BOUDOURESQUE , 1971) and the percent cover value was 250 G. B ELLISSMO & C. O RESTANO expressed according to the values of the scale of BRAUN -B LANQUET (1928). The diversity for each sampling station was calculated as number of taxa , total percent cover, Shannon-Weaver (log 2 basis) and Pielou evenness indices.

RESULTS

Literature data The early floristic studies about the benthic marine macroalgae from the Stagnone lagoon date back to the half of the last century by MOLINIER & PICARD (1953) and CAVALIERE (1961). However, these authors do not report lists of species but only some notes and observations on a few taxa . To date, the available information about the benthic algal flora and vegetation are mostly due to phycological studies dating back to the early 1980s. The first survey and mapping of the submerged vegetation as well as information on the distribution and biomass of benthic macroalgae were given by CALVO et al . (1979, 1980a, 1980b, 1982) and SORTINO et al . (1981) who reported 41 taxa for the whole northern basin. CALVO et al . (1981) and CALVO & RAGONESE (1982) carried out the first observations and descriptions on peculiar ball-like forms, known as “aegagropilae”, of unattached seaweeds providing a contri - bution towards knowledge of the hydrodynamic conditions of the northern basin. Later, FRADÀ ORESTANO & CALVO (1985) reported the ball-form phy - tocenoses of the Stagnone detecting 4 taxa previously unrecorded. Moreover, an investigation on the distribution of the phytocenoses and their relationship with physico-chemical composition of sediments was undertaken by GENCHI et al . (1985) who reported 12 dominant species (in terms of cover values) in summer marine vegetation. Finally, a complete revision with a nomenclatural update of taxa found in the Stagnone lagoon up to 1993 is by CALVO et al . (2009) who reported for the whole northern basin 108 taxa of which 21 recorded to genus level as ‘spp’.

Floristic data In the northern sub-basin of the Stagnone di Marsala lagoon, a total of 94 taxa at specific and infraspecific rank was identified: 48 Rhodophyta (51.06%), 12 Phaeophyceae (12.77%) and 34 Chlorophyta (36.17%) (Table 2). In particular, 4 specimens of 3 Rhodophyta ( Polysiphonia , Amphiroa and Audouinella ) were recorded to genus level. On the basis of literature data (Fig. 2), the algal flora of the Stagnone consisted of 108 taxa , comprising 59 Rhodophyta (54.63%), 16 Phaeo - phyceae (14.81%) and 33 Chlorophyta (30.56%). Therefore, the flora of this Changes in the benthic algal flora and vegetation of a semienclosed... 251 study was slightly lower than that reported previously (94 taxa compared to 108), with an increase in Chlorophyta of 5.61% and a decrease in Rhodophy - ta and Phaeophyceae of 3.57% and 2.04%, respectively. Only 48 taxa (27 Rhodophyta, 6 Pheophyceae and 15 Chlorophyta) of the present flora have been previously reported, while 46 taxa (21 Rhodophyta, 6 Pheophyceae and 19 Chlorophyta) are new reports for the Stagnone lagoon (Fig. 2, Table 2). It is remarkable that most of new taxa recorded and those not found in this study are opportunistic species with wide ecological valence. From a floristic point of view, it’s noteworthy the finding for the first time of the invasive Chlorophyta Caulerpa cylindracea Sonder and Rhodophyta Lophocladia lalle - mandii (Montagne) F. Schmitz in the northern (station 1) and southern area (station 10) of the sub-basin, respectively. On the contrary, 60 taxa (34 Rhodophyta, 10 Phaeophyceae and 16 Chlorophyta) previously reported

Fig. 2 — Comparison between the floristic richness of literature and present data showing the num - ber of common, newly recorded and disappeared taxa .

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l o m i e o p b o e . m m y o g e p o m m p h h n n s p t a e h o c a r d t o o r r h d i t r t a a E o d p m r y l o v v p c l d e a A n d e . p e r r l i u l d l l o i i y m y a l e o o t e e l p y t u . l e e r a o r a a o J H S P L C A C U U S S L H F D D P L * * * C * * * V A * * * U P * * L P * * * E * * ( C C ( B 256 G. B ELLISSMO & C. O RESTANO have not been detected during the present study (Fig. 2, Table 3). Among these, the Charophyta Lamprothamnium papulosum (Wallroth) Groves and the Phaeophycea Cystoseira barbata (Stackhouse) C. Agardh f . aurantia (Kützing) Giaccone have been the most significant disappearances. The high - est species richness was found at station 6 (44 taxa ) where hard substrata occurred, while the lowest floristic diversity was recorded at station 7 (8) where muddy sediments were observed (Table 2).

Table 3 Macroalgal species previously reported from the Stagnone di Marsala lagoon and not found in the present study (1 = CALVO et al. , 1979; 2 = CALVO et al. , 1980a; 3 = CALVO et al. , 1980b; 4 = SORTINO et al. , 1981; 5 = CALVO et al. , 1982; 6 = FRADÀ ORESTANO & CALVO , 1985; 7 = CALVO et al ., 2009).

Rhodophyta Acrochaetium leptonema (Rosenvinge) Børgesen: (7) as Audouinella leptonema (Rosenvinge) Garbary Audouinella sp. 1: (7) Audouinella sp. 2: (7) Bangia fuscopurpurea (Dillwyn) Lyngbye: (4) as Bangia atropurpurea (Roth) C. Agardh Botryocladia sp.: (7) Ceramium sp.: (7) Chondria dasyphylla (Woodward) C. Agardh: (3), (5) Chondria scintillans G. Feldmann: (7) Chondria sp.: (7) Corallina officinalis Linnaeus: (7) Corallina sp.: (7) Dasya ocellata (Grateloup) Harvey: (7) Dasya sp.: (7) Gayliella mazoyerae T.O.Cho, Fredericq & Hommersand : (7) Gelidiella ramellosa (Kützing) Feldmann & Hamel: (7) Halopithys incurva (Hudson) Batters: (4) Halurus sp.: (7) Herposiphonia secunda (C. Agardh) Ambronn: (7) Hydrolithon farinosum (Lamouroux) Penrose & Chamberlain: (7) Jania rubens (Linnaeus) Lamouroux: (4), (7) Lithophyllum corallinae (P.L. Crouan & H.M. Crouan) Heydrich : (7) Lythothamnion sp.: (7) Melobesia membranacea (Esper) Lamouroux: (7) Osmundea truncata (Kützing) K.W. Nam & Maggs: (7) Palisada perforata (Bory de Saint-Vincent) K.W. Nam : (7) as P. papillosa (C. Agardh) K.W. Nam Peyssonnelia polymorpha (Zanardini) Schmitz: (4) Peyssonnelia squamaria (S.G. Gmelin) Decaisne: (7) Phyllophora sp.: (7) Pneophyllum fragile Kützing: (4), (5) Polysiphonia brodiei (Dillwyn) Sprengel: (7) Polysiphonia denudata (Dillwyn) Greville ex Harvey: (7) Changes in the benthic algal flora and vegetation of a semienclosed... 257

Polysiphonia tripinnata J. Agardh: (6), (7) Polysiphonia sp. 1: (7) Polysiphonia sp. 2: (7)

Phaeophyceae Cladosiphon cylindricus (Sauvageau) Kylin: (4) Cladosiphon mediterraneus Kützing: (5) Cystoseira barbata (Stackhouse) C. Agardh f . aurantia (Kützing) Giaccone: (5) and (7) as C. barbata f. repens Zinova & Kalugina according to GÓMEZ GARRETA et al ., (2001) Cystoseira foeniculacea (Linnaeus) Greville f. schiffneri (Hamel) Gómez Garreta, Barceló, Ribera & Rull Lluch: (1), (4), (7) as Cystoseira ercegovicii Giaccone Cystoseira sp.: (7) Dictyota fasciola (Roth) J.V. Lamouroux: (7) Dictyota sp.: (7) Ectocarpus siliculosus (Dillwyn) Lyngbye: (7) Giraudia sphacelarioides Derbès & Solier: (5) Sphacelaria sp.: (7)

Chlorophyta Aegagropila linnaei Kützing: (7) as Cladophora aegagropila (Linnaeus) Trevisan Cladophora nigrescens Zanardini ex Frauenfeld: (4) Cladophora pellucida (Hudson) Kützing: (7) Cladophora sp.1: (7) Cladophora sp. 2: (7) Codium bursa (Olivi) C. Agardh: (7) Flabellia petiolata (Turra) Nizamuddin: (7) Lamprothamnium papulosum (Wallroth) Groves: (2), (3), (5), (7) Pringsheimiella scutata (Reinke) Marchewianka: (7) Pseudochlorodesmis furcellata (Zanardini) Børgesen: (4), (7) Ulothrix sp.: (7) Ulva intestinalis Linnaeus: (7) Ulva laetevirens Areschoug: (7) Ulva sp. 1: (7) Ulva sp. 2: (7) Valonia sp.: (7)

The mean number of taxa identified at all sampling stations was 23.4 ±9.7 (Fig. 3a). Higher mean values were recorded at stations 6 (40.7) and 10 (34), while the lower ones at stations 5 (12.7) and 7 (8.3). As regards the total percent cover, the mean value recorded at all sampling stations was 127.3% ±38.8 (Fig. 3b). The highest mean values were observed at stations 6 (185.6%) and 4 (171.1%), while the lowest abundance was found at station 5 (35.9). The mean Shannon–Weaver index was 2.02 ±0.6 and varied irregu - larly at all sampling stations (Fig. 3d). The highest mean values were record - ed at stations 6 (2.76) and 1 (2.71) while the lowest ones were observed at sta - tions 3 (1.19) and 8 (1.18). A similar trend was also observed for Pielou index 258 G. B ELLISSMO & C. O RESTANO that showed at all sampling stations a mean value of 0.46 (Fig. 3e). The high - est mean values were found at stations 4 (0.59) and 1 (0.56), while the lowest ones at stations 8 (0.30) and 3 (0.26).

Fig. 3 — Number of taxa (a), percent cover (b), Shannon-Weaver index (c) and Pielou index (d) with standard deviation bars (± sd) at each sampling station and on average. Changes in the benthic algal flora and vegetation of a semienclosed... 259

Vegetation The photophilic plant association Caulerpetum proliferae Giaccone & Di Martino dominated the central part of the northern sub-basin. In particular, the characteristic species of this association, the Chlorophyta Caulerpa prolif - era (Forsskål) J.V. Lamouroux, was found in association with C. nodosa on well-calibrated fine-grained sands scarcely oxygenated. C. prolifera was the most common and abundant macroalgal species throughout the basin and it was found at almost all stations with the highest percent cover at stations 3 and 8 (Table 2). In this area, due to the lack of hard substrata, many algal species were present and abundant in a unattached form exhibiting typical ball-like forms. This was the case of the Rhodophyta Rytiphlaea tinctoria (Clemente) C. Agardh and the Chlorophyta Cladophora echinus (Biasoletto) Kützing, constituting the plant association Cladophoro-Rytiphloetum tinctori - ae Giaccone, that formed ball-form assemblages remaining lying on the bot - tom or slowly rolling on it. Unattached population of R. tinctoria was also detected with high percent cover in the area near Punta dello Stagnone (sta - tion 10), but here unattached thalli were morphologically different from the conspecific ball-like forms showing a peculiar “intermediate” form (see CALVO & R AGONESE , 1982). Moreover, ball-like forms, known as rhodoliths, of the coralline Rhodophyta Mesophyllum lichenoides (Ellis) Lemoine and Spongites fruticulosa Kützing were also recorded in proximity of southern channel and at the northwest of Mothya Island, respectively. In this latter area, the occurrence of weak bottom currents favored the development of the plant association Chaetomorpho-Valonietum aegagropilae Giaccone with the characteristic species, the Chlorophyta Chaetomorpha linum (O.F. Müller) Kützing and Valonia aegagropila C. Agardh, forming ball-shaped masses freely floating on the bottom. On the scattered hard substrata, the vegetation was characterized by large and mature thalli of the brown algae of genus Cys - toseira genus. The community with Cystoseira spinosa Sauvageau was found in three stations but with high cover values at stations 2 and 6 (Table 2). At station 4, there was a community characterized by Cystoseira barbata (Stack - house) C. Agardh forming erect and branched thalli up to 40 cm long, while a community with Cystoseira foeniculacea (Linnaeus) Greville f. tenuiramosa (Ercegovi) Gómez Garreta et al. occurred at stations 2, 9 and 10 with higher cover values in the latter two stations. Also, a community classified into the Cystoseiretum crinitae subassociation Cystoseiretum compressae Molinier was found at station 6, in correspondence to the Phoenician road, with relatively high cover values of Cystoseira compressa (Esper) Gerloff & Nizamuddin. All the above species of Cystoseira supported communities of epiphytic and understory algae with the phaeophyceans Dictyota implexa (Desfontaines) 260 G. B ELLISSMO & C. O RESTANO

J.V. Lamouroux and Sphacelaria fusca (Hudson) S.F. Gray dominating both qualitatively and quantitatively (Table 2).

DISCUSSION

From the comparison with the available literature it seems that the ben - thic algal flora of the Stagnone di Marsala lagoon has not undergone any dras - tic changes in the last 2-3 decades. The 46 taxa newly reported against 60 taxa not found this time highlighted a slight reduction of species richness with a loss of only 14 taxa . However, the results of this study showed qualitative changes to the ben - thic algal flora that strongly depended on the changing environmental condi - tions. The increase in both confinement and temperature over the last decades could explain the variation of biodiversity within the lagoon ( CALVO et al ., 2001). Natural stress levels due to the shallowness and reduced water exchange raise with increase in confinement resulting in a decrease in the vari - ety of species ( REIZOPOULOU & NICOLAIDOU , 2004). Although this trend was observed in the present study, our data also showed that the highest values of species richness and diversity were observed mostly in the stations character - ized by the presence of Cystoseira -dominated assemblages. Cystoseira spp. are considered the most important habitat-forming algal species in Mediterranean shallow waters, providing shade and shelter for a diversified assemblage of ani - mals and plants ( BALLESTEROS , 1992; BELLISSIMO et al ., 2014). Among Cysto - seira species found in the Stagnone, the record for the first time of C. compressa is noteworthy. Although this euryoecious species could be present and/or misidentified in the past, its current presence would provide evidence of the increased degree of sedimentation and reduced hydrodynamic regime that have occurred in the area over the past two decades. The reduction of the ball-like forms of unattached drifting assemblages detected in this study with respect to the past ( FRADÀ ORESTANO & C ALVO , 1985) would constitute a more indicator of the diminished water circulation within the lagoon. A consistent water movement is actually essential to their formation since it allows the radial growth of the thallus by rolling it on the bottom and thus continuously varying its exposure to light. This hypothesis is supported by the almost complete closure of the northern entrance as a con - sequence of the current coast dynamics that caused the dispersion of sedi - ments coming from the north ( AGNESI et al ., 1993). A generalized state of regression of P. oceanica stands and a concomitant settlement and proliferation of C. nodosa and C. prolifera have been associat - ed with the increase of sea water temperature and salinity due to global cli - Changes in the benthic algal flora and vegetation of a semienclosed... 261 mate changes ( CALVO et al ., 2009). In this context, the high abundance of C. prolifera observed in this study with respect to the past ( CALVO et al ., 1982) should confirm the regression tendency of P. oceanica but, unfortunately, the absence of studies between 1993 and the present makes it impossible to ascer - tain if such changes occurred gradually or more or less abruptly throughout the last twenty years. The disappearance of L. papulosum and C. barbata f. aurantia may be due to the rise of mean sea water temperatures due to global warming that, on the contrary, seems likely to have favored the settlement of invasive species with warm-water affinities like C. cylindracea and L. lallemandii . Both of species are considered ecosystem engineers by being capable to alter ecosys - tem structure and function reducing the biodiversity of macroalgae in the invaded assemblages ( PIAZZI et al ., 2001; BEDINI et al ., 2011). Besides, these species show high potential for dispersal, fast growth and great ability to adapt to new environments ( PETROCELLI & CECERE , 2006), suggesting that warm-water introduced species could spread and become even dominant in stressful environments like coastal lagoons. To conclude, this study pointed out to a certain degree of vulnerability of phytobenthic communities of the lagoon under expected environmental changes in the next decades as a consequence of the effects of global warm - ing. These effects may include an increase in sea water temperature, changes in the hydrodynamism of water masses and in water salinity, an increase in fre - quency of extreme weather events and a loss of marine macrophytes. Therefore, further sampling campaigns in different seasons are in progress to better appreciate the dynamics of the Stagnone di Marsala vege - tation. Moreover, a continuous and exhaustive monitoring program aiming at sampling macroalgal and seagrass assemblages is needed to assess the long- term climate change effects that may determine extremely negative impacts on the lagoon biodiversity.

Acknowledgements — This study was supported by a Research Project of National Interest (PRIN 20104J2Y8M_004) funded by Ministero dell’Istruzione, dell’Università e della Ricerca (MIUR). We are grateful to Prof. S. Calvo, Prof. J. Rull Lluch, Dr. M. Pirrotta and Dr. A. Tomasel - lo for stimulating discussions of the data and critical reading of the manuscript. We also thank Dr F. Luzzu, Dr A. Scannavino, Dr. F. Falsone and Dr. M.L. Geraci for their kind help in the fieldwork.

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Authors’ addresses — G. B ELLISSIMO , C. O RESTANO , Dipartimento di Scienze della Terra e del Mare (DiSTeM), Università di Palermo, Viale delle Scienze, Edificio 16 - 90128 Palermo (I); corre - sponding author: G. Bellissimo, e-mail: [email protected].