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Marine Ecology. ISSN 0173-9565

ORIGINAL ARTICLE

Seagrass meadows at the extreme of environmental
tolerance: the case of Posidonia oceanica in a semi-enclosed
coastal lagoon

Agostino Tomasello1*, Germana Di Maida1*, Sebastiano Calvo1, Maria Pirrotta1, Marco Borra2 &
Gabriele Procaccini2

1 Dipartimento di Ecologia, Viale delle Scienze, Palermo, Italy
2 Stazione Zoologica Anton Dohrn, Naples, Italy

Keywords Abstract
Coastal lagoon; environmental stress; genetic
isolation; lepidochronology; microsatellites; Atoll-like structures of the endemic Mediterranean seagrass Posidonia oceanica
Posidonia oceanica. were encountered in the innermost area of the Stagnone di Marsala, a semi-
enclosed coastal lagoon along the western coasts of Sicily. The area is character-
Correspondence ized by limited water exchange with the open sea and by a marked seasonal
G. Di Maida, Dipartimento di Ecologia, Viale variation of water salinity and temperature, reaching beyond the theoretical tol-
delle Scienze, Edificio 16, 90128 Palermo, erance limits of the species. In the present study we determined the genetic
Italy. E-mail: gedimaida@unipa.it composition of the atoll-like structures, as well as the growth performance and
flowering rate of these stands. We also assessed whether and to what extent the
*These two authors contributed equally. atoll-like structures are genetically isolated from plants growing in meadows
outside the lagoon. For this purpose we utilized 13 microsatellite markers to
Accepted: 18 January 2009 genotype single shoots sampled inside and outside the lagoon. Lepidochrono-
logical analyses were performed on the same shoots to determine the annual
doi:10.1111/j.1439-0485.2009.00285.x rhizome growth rate, the number of leaves and the inflorescences formed as an
estimate of growth- and reproductive performance over the years. The inner-
most area of the lagoon showed a lower number of alleles, a lower percentage
of polymorphic loci, a lower clonal diversity, but higher heterozygosis excess
with respect to the other areas analysed. Spatial autocorrelation was here signif-
icant, up to slightly below 300 m. Shoots collected in the atolls exhibited a
25% lower vertical growth rate and 16% lower leaf formation in comparison to
those in open-sea meadows. No flowering events were recorded during 24 years
of investigation, whereas inflorescences were observed frequently in meadows
outside the lagoon. Results from Fst and factorial correspondence analysis con-
firmed the expected genetic isolation of the confined atolls with respect to the
meadows outside the lagoon and revealed limited gene flow within the lagoon
itself. Apparently, the enclosed system of the Stagnone lagoon is genetically iso-
lated, with a possible selection of genotypes adapted to persistent stressful con-
ditions, consistent with reduced growth and lack of flowering events.

Problem rainforests (Costanza et al. 1997). Indeed, seagrasses can
be considered ‘ecosystem engineers’ given their ability to
Seagrass meadows are recognized as among the most affect significantly physical, chemical and biological fea-
widespread and productive coastal ecosystems worldwide tures of their environment (sensu Wright & Jones 2006).
(Hemminga & Duarte 2000), providing high-value ecosys-
tem service, comparable with terrestrial habitats such as The endemic Mediterranean seagrass Posidonia oceanica
(L.) Delile (den Hartog 1970) forms dense meadows from

288 Marine Ecology 30 (2009) 288–300 ª 2009 Blackwell Verlag GmbH