ORIGINAL ARTICLE Reproduction of the eelgrass Zostera marina at the species southern distributional limit in the Eastern Atlantic Susana Cabac ¸o & Rui Santos Marine Plant Ecology Research Group (ALGAE), Centre of Marine Sciences (CCMAR), Universidade do Algarve, Campus de Gambelas, Faro, Portugal Problem Seagrasses are marine angiosperms that reproduce both vegetatively through clonal growth and sexually through the production of flowers and seeds (den Hartog 1970; Phillips & Men ˜ez 1988). Successful recruitment from sexual reproduction in seagrasses is extremely low (Hem- minga & Duarte 2000). Their reproductive cycle involves several steps from the flowering event to the formation of a new plant, many of them representing bottlenecks (e.g. seed germination, seedling survival) that constrain sexual reproduction success (Hemminga & Duarte 2000; Orth et al. 2006). The processes that influence the establish- ment of a seedling from a seed are often diverse and complex, and include the site suitability (e.g. nutrients) and safety (e.g. currents protection) among others (Orth Keywords Germination; reproduction; Ria Formosa; seedlings; seeds; Zostera marina. Correspondence Susana Cabac ¸ o, Marine Plant Ecology Research Group (ALGAE), Centre of Marine Sciences (CCMAR), Universidade do Algarve, Campus de Gambelas, 8005-139 Faro, Portugal. E-mail: scabaco@ualg.pt Accepted: 5 August 2009 doi: 10.1111/j.1439-0485.2009.00331.x Abstract The plant reproductive effort, the seed germination rate and the seedling sur- vival and development of Zostera marina (eelgrass) were assessed in four popu- lations (Fuzeta, Culatra, Barrinha and Armona) at the species’ southern distribution limit in the Eastern Atlantic, the Ria Formosa lagoon. Germinated seeds were individually placed in Petri dishes with natural sandy sediments and kept in a culture chamber at the same temperature and salinity conditions as the natural environment. In addition, seeds from three different depths of Fuz- eta population were cultivated in outdoor mesocosms. The populations of Fuz- eta and Barrinha showed higher seed production and the seeds produced were heavier than the other populations. The germination of the seeds both in the laboratory and in the outdoor tanks began c. 8–12 weeks after the collection of the flowering shoots at a water temperature of 22 °C. The spontaneous germi- nation in the laboratory (2.4–5.3%) and in the mesocosm experiment (5.6– 8.9%) was low and from all the germinated seeds (n = 20) only three reached the seedling stage. The spontaneously germinated seeds from Fuzeta survived for a longer period than those from Barrinha, but only the germinated seeds of Barrinha reached the seedling stage (one-leaf seedling stage). In outdoor tanks, higher seed germination and earlier seedling emergence (2 weeks after seeding) and survival (for 208 days) occurred for the seeds obtained from the shallow meadow. The reproductive effort of Z. marina populations of Ria Formosa showed that flowering shoots and seed traits are site-specific. The low repro- ductive success indicated by the low germination and seedling survival suggests a bottleneck in the species’ reproductive cycle that may account for the scarce presence of the species in Ria Formosa lagoon. The high water temperatures of Ria Formosa in winter may partly explain this bottleneck. Increased tempera- tures due to climate change may reduce even further the sexual reproduction of Z. marina in its southern distributional limit in the Eastern Atlantic. Marine Ecology. ISSN 0173-9565 300 Marine Ecology 31 (2010) 300–308 ª 2009 Blackwell Verlag GmbH