Biol. Mar. Medit. (2006), 13 (4): 145-148

E. Giovannetti, M. Montefalcone, C.N. Bianchi, C. Morri, G. Albertelli DipTeRis, Dipartimento per lo Studio del Territorio e delle sue Risorse Università di Genova, Corso Europa, 26-16132 Genova, Italia

Structural variability of the epiphytic community in a meadow (Ligurian Sea, NW Mediterranean)

Abstract Several factors influence the epiphytic assemblage of Posidonia oceanica (L.) Delile (Monocotyle- dones, Posidoniaceae) leaves. The aim of this study was to investigate the epiphytic community structure in the P. oceanica meadow of Prelo cove (Ligurian Sea, NW Mediterranean) along a yearly cycle and a depth gradient. Sampling was carried out monthly from September 2003 to August 2004 at three different depths: 4, 9 and 14 m. Each month, 10 randomly selected P. oceanica shoots were collected by SCUBA diving in each station. Percentage cover of the most representative taxa was estimated and subjected to multivariate analysis in order to detect seasonal changes in the epiphytic assemblage along the bathymetric gradient. In all stations, the epiphytic assemblage exhibited cyclic variability, and two major phases were distinguished: an autumnal one dominated by Hydrolithon-Pneophyllum spp, and a spring-early summer one in which the algae were flanked by Electra posidoniae. A poorer but more various assemblage, with the appearance of the erect hydrozoans and bryozoans, was found in the deep station.

Key-words: Posidonia oceanica, epiphytic assemblage, depth gradient, seasonal cycle, Mediterranean Sea.

Introduction The most important in the Mediterranean Sea is Posidonia oceanica (L.) Delile, an endemic species that forms wide meadows. This plant hosts on its leaves and rizhomes a characteristic epiphytic community. Several factors influ- ence the epiphytic assemblage, such as the cyclic phenology of the host plant, hydrodynamic forces, light intensity and anthropogenic disturbances (Mazzella et al., 1989; Dalla Via et al., 1998; Piazzi et al., 2004; Montefalcone et al., 2006). In this work the epiphytic community structure of P. oceanica leaves was inves- tigated in a yearly cycle and along a depth gradient in order to describe its spatial and temporal variation.

Materials and methods The study was carried out in the Posidonia oceanica meadow of Prelo cove (Ligurian Sea, NW Mediterranean). Along a depth transect (directed perpendicu- larly to the shoreline, 270°N), three stations were identified: 4 m (44°20’12” N, 009°13’33” E), 9 m (44°20’11” N, 009°13’36” E) and 14 m (44°20’11” N, 009°13’39” E), where 10 randomly selected P. oceanica shoots were collected by SCUBA diving monthly, from September 2003 to August 2004 (December data are missing). In the laboratory both sides of each P. oceanica leaf blade were analysed using a binocular stereomicroscope to estimate the percentage cover of the following taxa: Hydrolithon-Pneophyllum spp., Myrionema orbiculare, Giraudia sphacelarioides, Electra posidoniae, Aetea truncata, Fenestrulina johannae, Plumu- laria obliqua, Sertularia perpusilla. All the other organisms were grouped in: Other 146 E. Giovannetti, M. Montefalcone, C.N. Bianchi, C. Morri, G. Albertelli

Bryozoans, Serpulids, Other and Other Algae. The cover data per shoot were analysed with multivariate techniques (ANOSIM, MDS, SIMPER) in order to detect spatial and temporal variability of the epiphytic community structure.

Results Two-way crossed analysis ANOSIM test revealed clear differences in the epi- phytic community structure among depths (R = 0.456; P = 0.1 %) and months (R = 0.692;Results P = 0.1 %); the month factor determined the major dissimilarity. The Two-way crossed analysis ANOSIM test revealed clear differences in the epiphytic MDS ordinationcommunity structureshowed, among at three depths depths, (R = 0.456; a cyclic P = 0.1 seasonal %) and months variability (R = 0.692; of the epi- phytic communityP = 0.1 %); the structure month factor (F ig. determined 1). The the vicinity major dissimilarity. of the ending The MDS point ordination (8= August 2004) toshowed, the starting at three depths, point a cyclic (9= Septemberseasonal variability 2003) of the was epiphytic greater community at 14 structurem than at the shallow (Fig.and 1).the Theintermediate vicinity of thestations. ending point (8=August 2004) to the starting point Moreover(9=September the MDS 2003) wasordination greater at 14 showed, m than at theat shallowall stations, and the intermediate differences stations. in dispersion (variability)Moreover of the MDS epiphytic ordination assemblage showed, at all sta intions, each differences month, in dispersion with the (variability) greatest disper- of the epiphytic assemblage in each month, with the greatest dispersion observed in the sion observedautumn-winter in the months. autumn-winter months.

4 m

4 2 5 3 6 1 7 8 11 109

9 m

4 1 2 5 3 6 7 8 11 9 10

14 m

1

3 2 4 5 6 11 7 9 8 10

Fig. 1 - MDSFig. 1. MDS ordination ordination comparing comparing the epiphytic the community epiphytic structure community at the three stations structure (4 m, 9 m, at 14 the m) along three a stations (4yearly m, cycle. 9 m, The 14 three m) depths along have been a yearly plotted separa cycle.tely The(Stress three= 0.14). depthsThe numbers have indicate been the months: plotted from separately September 2003 (9) to August 2004 (8). December data are missing (dashed lines). The centre of the ellipses (Stresscorresponds = to0.14). monthly The means, numbers ellipses depict indicate 95 % confidence the months: intervals. fromArrows Septemberindicate the tendency 2003 to (9)close to August 2004the cycle. (8). The Decembercross indicates the data centre are of the missing axes. (dashed lines). The centre of the ellipses corre- sponds to monthly means, ellipses depict 95 % confidence intervals. Arrows indicate the tendency to close the cycle. The cross indicates the centre of the axes. Structural variability in the epiphytic community of Posidonia oceanica 147

SIMPER, applying the usual cut-off of a cumulative % similarity of 80 %, identified a total of 8 taxa as important in characterizing the epiphytic com- munity structureSIMPER, applying (Fig. the 2). usual The cut-off remaining of a cumulative 4 taxa % similarity (namely of 80 %,Myrionema identified a total orbiculare , Giraudia sphacelarioidesof 8 taxa as important, Fenestrulina in characterizing thjohannaee epiphytic community and Other structure Algae) (Fig. 2). played The a non- remaining 4 taxa (namely Myrionema orbiculare, Giraudia sphacelarioides, Fenestrulina significantjohannae role. and Other Algae) played a non-significant role.

4 m OA 40 BR

30 SP EL % 20 H-P 10

0 Sept Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug

9 m OA 40 BR

30 SP EL % 20 H-P 10

0 Sept Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug

14 m AE 40 ST

30 PL OA % 20 SP 10 EL 0 H-P Sept Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug

Fig. 2 - TheFig. mean2. The mean percentage percentage cover cover of the major of theepiphy majortes, determined epiphytes, through SIMPER, determined along a yearly through cycle at SIMPER, alongthree depthsa yearly (4 m, 9cycle m, 14 m). at H-P three = Hydrolithon depths-Pneophyllum (4 m, 9 m, spp, 14 EL =m). Electra H - posidoniaeP = Hydrolithon, SP = Serpulids,-Pneophyllum BR = Other Bryozoans, OA = Other Animals, PL = Plumularia obliqua, ST = Sertularia perpusilla, AE = Aetea spp,truncata EL . = Electra posidoniae, SP = Serpulids, BR = Other Bryozoans, OA = Other Ani- mals, PL = Plumularia obliqua, ST = Sertularia perpusilla, AE = Aetea truncata.

Hydrolithon-Pneophyllum spp., the main epiphytes at all stations, were present all year round. Apart the autumn months, these epiphytes were flanked by Elec- tra posidoniae that, sometimes, exhibited the major percentage cover. At the deep station a poorer but more various community was found than at the shallow and the intermediate stations, with also the appearance of Aetea truncata, Sertularia perpusilla, and Plumularia obliqua. 148 E. Giovannetti, M. Montefalcone, C.N. Bianchi, C. Morri, G. Albertelli

Discussion The epiphytic community structure was mainly influenced by seasonal variabil- ity. The great environmental variability at the two shallower stations was reflected by the more open seasonal cycles compared with the one at the deep station, implying in turn a colonisation delay between shallow and deep meadow (Maz- zella et al., 1986). Possible relationships with size and ages of leaves remain to be investigated. The depth factor as compared to the month factor had a lower influence on the epiphytic assemblage, because of the small bathymetric gap among the sta- tions. The appearance of erect hydrozoans and bryozoans at the deep station should be correlated to the lower water movement there as compared to the shal- lower stations (Piraino and Morri, 1990; Balduzzi et al., 1991) and to the lesser development of Hydrolithon-Pneophyllum spp (Giovannetti, 2004), leaving greater space to other epiphytes.

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