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

Atoll-like structures of the endemic Mediterranean seagrass Posidonia oceanicawere encountered in the innermost area of the Stagnone di Marsala, a semienclosedcoastal lagoon along the western coasts of Sicily. The area is characterizedby limited water exchange with the open sea and by a marked seasonalvariation of water salinity and temperature, reaching beyond the theoretical tolerancelimits of the species. In the present study we determined the geneticcomposition of the atoll-like structures, as well as the growth performance andflowering rate of these stands. We also assessed whether and to what extent theatoll-like structures are genetically isolated from plants growing in meadowsoutside the lagoon. For this purpose we utilized 13 microsatellite markers togenotype single shoots sampled inside and outside the lagoon. Lepidochronologicalanalyses were performed on the same shoots to determine the annualrhizome growth rate, the number of leaves and the inflorescences formed as anestimate of growth- and reproductive performance over the years. The innermostarea of the lagoon showed a lower number of alleles, a lower percentageof polymorphic loci, a lower clonal diversity, but higher heterozygosis excesswith respect to the other areas analysed. Spatial autocorrelation was here significant,up to slightly below 300 m. Shoots collected in the atolls exhibited a25% lower vertical growth rate and 16% lower leaf formation in comparison tothose in open-sea meadows. No flowering events were recorded during 24 yearsof investigation, whereas inflorescences were observed frequently in meadowsoutside the lagoon. Results from Fst and factorial correspondence analysis confirmedthe expected genetic isolation of the confined atolls with respect to themeadows outside the lagoon and revealed limited gene flow within the lagoonitself. Apparently, the enclosed system of the Stagnone lagoon is genetically isolated,with a possible selection of genotypes adapted to persistent stressful conditions,consistent with reduced growth and lack of flowering events.