Posldonia Oceanlca (L.) DEI,II.E in SARDINIA KNOWLEDGES .AND PERSPECTIVES A

LA POSIDONIA OCEANI CA POSlDONIA OCEANlCA (L.) DEI,II.E IN SARDINIA KNOWLEDGES .AND PERSPECTIVES A. COSSU e V. GAZALE Dipartimento di Botanica ed Ecologia Vegetale - Università di Sassari Summary Asummary of the research regarding Po sidonia oceànica in Sardinia ì presented; this consists of a series of stl.ldies regardihg the phenology of tbe piant and some con siderations on the conservation of thc prai ries. Finally, the Authors de.'icribe a proposi tion for the protection of rhe plant in Sar dinia. Introduction The analogy between terrestrial prairies and those of Posidonia. 15 apparenti hun dreds and sometinles rhousands of hectares of the sea-bottom are covercd by a carpet of green and brown leaves that in the shal Iower deprhs, bend and waver fol1owìng the incessant rhythm of the waves. It con taìns an eXLraordinary variety of life-forms and is considered lO be one of the most ex rensive and productive of the mediterra nean coastal strips. More than a thollsand scientific artides document its importance. In the course of Posidonids evollltion, the re were first terrestrial plants. AboUl ] 20 seeds. Flowering does not always occur: million years ago, it readapted iL'ielf to ma only in certain particular years does it ac rine life and it fol1owed thc 31tering clima quire an explosive and simultaneous cha tical events of the Mediterranean, overco racter. Most exceptional is the fact that ming the drying-up of this sea. After mil many Posidoni-a groves flower in synch Hons of years, ìt ha arrived at a state of cli rony. \Yfe can cite the example of 1992 max with the environment~ il has become when the tlo\vering in Porto Conte coincì a complex eco-system in which the living ded with thar of Caglari, Baunei aneI Ori elements tend to be stable and are in per stano. The origin of this phenomenon has fect equilibrium with each other. a hap not been clarìfied, but it does not appear to pens in the virgin foresrs in terre triai envi be linked ro the temperature or the fotope ronments. Posidonia is a superior plant; il. riodo. The transporting of the seed far therefore, has the fc,Ut>wing struclUre: away fram the mother plant is accompH roots, stem and leaves. As reprodllctive or shed by the fruir, which when mature floats gans ~ it has flowers Ù1at produce fruil and and is capable of being carried by currents Dicernbre 1995 Supplemento ol/a N./d. Lo\. PO, IDONlA OCEANlCA to areas suitable for germination. This ra dex of the aging of the species. It is cJear rcIy result.':i in gem1ìnation. A site in which thar thìs ìs a distinct lin1ìtation; in faet, the good gennination has been observed is the rate of recolonization is verv slow and ha .. marine area of S.Antioco where cert.ains been estimated as being around lO cm a tract<; were covered by young plants (Giac year. Posidollia, as we have already no cone. personal communtcation). The re ted, grows in both a horizontal and venical productive capacity is delegateci mainly to direction, the laller is in re1ation (O the se vegetative propogation. The modaHty of dimemary regime or the site. In sheltered regrowth may be conducted essentially in environments, wìth a low hvdrodvoamic two [0011S. One is dcfined as plagiotropic regime. a rate of vertical gfo~th of ' l cm a growth and refers te the 1113nner of hori year has been tn.ea.. ured. If the rate of . e zontal lengrhening of (he roorstock; the dimentation exceeds the growth capacit)i of other 15 ortotropìe, which ìndicates vertical the plant, it becomes , uffocated and may growth. The vegetative development Ieads be replaced by a more dynamic commu IO the creation of a dense web of roo[ nity, The opposite proces', highly dynamic stocks that have become anastomosed, of sea-t1(x)r currents, may unclermìne the cle ten resulting from the same mother pIane veloprnent as welI. The removal of secli There also exiS1S a type of propagation ment provokes a denudation of the root by scion. Certain rhizornes bv means of apparatus of the plant1 detaching the bor erosion may become detached from the der of the population. This process may be Posidonia grove and carried to other areas. progressive and reduce considerably the In this case, if the subsrrata proves suitable, dimensions or entire prairies. The effecrs of they can take ront and tan a new popula the dynarnìc aetion of water are parucularly tion. evidem in straite; in exposed zones like tho The prevalenee of vegetative ovef gamie se of Fornelli, between the islands of Asi reproduction is interpre(ed as being an in- nara aod Piana (Cossu et al.,1992). In thìs Supplemento alla R .•1-1 . Dicembre 1995 LA POSIDONIA OCEAl~ICA :i. .•..•.• -L.,..Nn/t . C'fSTOIS9>El\JYOHv.wES!!lNF. .C1S1'Oì5C1flmN OH GIWj'f( (!JiP~T\M DJ.COASTAl DEI!I'U5 0$.. ", ~U.flW P!J..VJogenic mal) 0/ Parco Capo Ceraso area ~ Tavo/ara - Capo <..000 CaL'ullo. area the Cllrrent. have modelled rhe ffi::ltte rous oceanographic cnlÌses conducled in (vegetation, foilagc) in a series of islancls the area cast of Sardinia, aimed at the sLUdy with an orientation longtirudìnal totbc coa of both the geomorphìcal character of the st-line. The clcnsity of the strips of matte sea-tloor and ie' bioJogical communities, maintaìns values between 500 and 600 sui have demonstrated dearly the devastaring ps per q.m. Thc island or Posidonia pre action performed by the tools of trawling, sent an eroded type of matte on the ìde ìllustrating a type of exploitation by those exposed lO the Cllrrents with a net limir not vet sensitive (O the corret.1: management of height that varies bet\veen 20 and 60 cm. of fllarine resources. These wounds<' are ra On the apposite side the limit is of a pro reIy healed by the piant and often tend to gressive type. gl't worse \vith lime. Alterations are evident 1\5 a result. of the indications derived at Tavolara, in the archìpeJago La Maddale from the onothropic and plagìotropic rela na. in the area facing the ourhenl sector of tionshìp associated with Ùle granuJometric Spargi, and in the canal between La Mad measurements of sediment, has been ob dalena, Santo Stefano and Caprera, doclI Glined useful informat.ion about the dyna mented bv studies for the creation of these mics of sedimentation in the gulf of Porto marine p~;rks. In the area indudecl berween Come (Cossu. unpublished data).The an Tavola1'a, Capo Ceraso e Capo Coda Caval choling of boat5 used for recreational pur lo the prairie extends in a more or less poses, as welI as ceJtain types of profes lInifonn manner until about two miles from sional fishing. also detachs fragmenrs of the coast, maìnly a sandy Sllbslfala. This rhizome creating a discontinuity in tbe 50- extension 1S interrupted in the channel of lidity of the prairie.. In recent years nllme- Tavolara by a deep muddy paleo-bed. As Dicembre 199.5 Supplemento al/a R ..\1. • LA POSIDONIA OCE~NICA regards distribution limits, the su perior one The data relative to Posidonia were indu is regressying in the area in front of the me dcd in the cartography of the bentonici po re of San Teodoro, while the inferi or one, pulation5; a serious reg.ression was found found to be on avemge ~tw'een the bathy in the Posidonia. groves in the port area re metrics of 30+35 In, is regressying at Capo lative to neighbouring areas. Even the lat Ceraso, in front of Spalmatore di Terra and ter were not found to be in an optimum south ofMolara. state of health. In 1984 an assoeiatìon filter The homogenity of the distriburion is in plant was installed and after only three terrupted locally by granite outcrops years of operation, a study confirmed that between Capo Ceraso, Spalmatore di Terra it has been efficient in its deaning-up and Punta Pedro$u there are a1.50 presento eft()Its. Intérmattes of noteworthy dimensions, co Outside the dockyard area the prairie lonized by éaulerpa prolifera and Cymo showed an average recovery or 80-90% docea nodosa (Cosso and Gazale, unpubli with a densityequal to 300+350 strips per shed data) are also presento sq.m. defined as harbour prairie (Giraud, In the archipelago of La Maddalena, 1977). Thc colonization of leaf epiphytes wìthin the area projected for definition of was notably modest. In the port area ,also, the Intemational Marine Park of the Straits a significant regrowth of Posidonia was of Bonifacio, the largest distriblltion are in registered, but in this case, as well, the the south-eastern versant of the island, in covering of the epiphytes of the leaves was particular on the southern versant of Ca very poor. In some areas, there was a pro- prera. 10unccd a population of alga, which had at Major indications of damage were found tached itself lo the dead rhizomes of the in the internal parts of the rias and in such Posidonia and had replaced it completely. as ports likcj l..a Maddalena, Cala Lunga in Il is evident, thererore, Ùlat projects ai the island of Razzoli, Porto Pozzo and Ar med at safeguarding the environment pro zachena in island Sardinia, or in the depths duce improvement even to areas particu bet\veen Caprera and la Maddalena (Passo larly subject to environmental impact. Simi della Moneta and Stagnali), where the do lady, it is dear that recovery ìs a very slow minant phytocoenosis is l11ade up of Cau process, and a succes fuI out come is not Jerpa prolifera.

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