Posidonia oceanica in the Marmara Sea A. Meinesz a, *,S ¸. Cirik b,c , B. Akcali c , F. Javel a , M. Migliaccio d , T. Thibaut a , A. Yu ¨ ksek e , G. Procaccini d a University of Nice-Sophia Antipolis, EA 4228 ECOMERS, Lab. Environnement Marin littoral, Parc Valrose, 06108 Nice Cedex 2, France b C ¸anakkale Onsekiz Mart University, Faculty of Fisheries, Terzioglu Kampusu, 17100 C ¸anakkale, Turkey c Dokuz Eylu ¨l University, Institute of Marine Science and Technology, Haydar Aliyev Bulv No. 32 Inciralti 35340 Izmir, Turkey d Stazione Zoologica Anton Dohrn, 80121 Napoli, Italy e Istanbul University, Institute of Marine Science and Management, Muskule sok., Vefa, 34470 Istanbul, Turkey 1. Introduction Posidonia oceanica (L.) Delile (Magnoliophyta) is the most abundant seagrass in the Mediterranean Sea and its distribution is fairly well recorded (Green and Short, 2003). However, its distribution limit is still unclear particularly at the narrow Dardanelle Strait where the Aegean Sea meets the Marmara Sea and previous records appear to be incorrect. The record of P. oceanica reported in the Marmara Sea in The World Atlas of Seagrasses (Green and Short, 2003), was in fact observed at Urla- Iskele, close to Izmir in the Aegean Sea (Short, pers. com.). This endemic Mediterranean species is known to be stenoha- line, living in a salinity range between 36.5 ppt (in the Alboran Sea: Ramirez et al., 2005) and 39.5 ppt (in the Aeagean Sea: Bes ¸ iktepe, 2003). The species can occasionally tolerate very high salinity up to 48 ppt in semi-enclosed environments, such as the Stagnone coastal lagoon at Marsala (Di Maida et al., in press.), but up to now it has not been recorded in low salinity environments. The presence of the species in the Marmara Sea is exceptional because of its brackish water. Moreover, the regular and vigorous intrusion of the Black Sea brackish waters, which flow at the subsurface (0– 25 m depth) since the Holocene (O ¨ zsoy et al., 1995; Tug ˘rul et al., 1995; Polat and Tugrul, 1996; Tu ¨ rkog ˘lu et al., 2004) limits the arrival of floating seeds and/or rhizomes from the Mediterranean Sea, thus probably isolating Marmara Sea meadows. We quantified the distribution of P. oceanica in the Dardanelles Strait and in the Marmara Sea through SCUBA diving observations, beach surveys, discovering large beds in the straits of Dardanelles and isolated populations in the Sea of Marmara. In order to understand the level of genetic isolation of the newly discovered Posidonia meadows we assessed the clonal diversity and the genetic affinity with other Mediterranean populations. 2. Methods The Marmara Sea is about 210 km long and 75 km wide (11,500 km 2 ), with 800 km of coastline and connections to the Black Sea at its eastern side and the Mediterranean Sea (Aegean Sea) at its western side (Fig. 1). High density water coming from the Aegean Sea (salinity ranging 39–39.5 ppt) sink below the surface at the entrance and move along the bottom of the Dardanelles Strait (60–70 m depth) throughout the Marmara Sea, reaching the Black Sea after the Bosphorus Strait (310 km 3 year À1 ). Low density waters (salinity from 20 to 29 ppt) flow at the surface (between 0 Aquatic Botany 90 (2009) 18–22 ARTICLE INFO Article history: Received 8 November 2007 Received in revised form 8 April 2008 Accepted 24 April 2008 Available online 3 May 2008 Keywords: Seagrass Posidonia oceanica Marmara Sea Salinity tolerance Genetic isolation ABSTRACT The seagrass Posidonia oceanica is a stenohaline species endemic to the Mediterranean Sea, where it normally lives at a salinity of between 36.5 and 39.5 ppt. Surveys carried out at the North-eastern distribution limits revealed large beds in the Dardanelles Strait and isolated beds in the Marmara Sea, where the salinity ranges between 21.5 and 28 ppt. Microsatellite analysis of these low-salinity tolerant P. oceanica beds, show different signs of genetic isolation: excess of heterozygosity and a presence of fixed alleles. These particularities are rarely found in the whole distributional range of the species. Moreover, all the populations considered in the analysis have a very low genetic diversity in comparison with most of the meadows sampled throughout the Mediterranean Sea. Taking into consideration the genetic data, rhizome expansion rate and the actual extent of the isolated beds in the Marmara Sea and knowing the reproductive rate and dissemination characteristics of P. oceanica, we hypothesize that the isolated population of the Marmara Sea has been established since the Middle Holocene, before the catastrophic intrusion of brackish water into the Marmara Sea and the strong and persistent flow coming from the Black Sea. ß 2008 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +33 611500404; fax: +33 492076849. E-mail address: meinesz@unice.fr (A. Meinesz). Contents lists available at ScienceDirect Aquatic Botany journal homepage: www.elsevier.com/locate/aquabot 0304-3770/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.aquabot.2008.04.013