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

Atoll-like structures of the endemic Mediterranean seagrass 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 characterized by limited water exchange with the open sea and by a marked seasonal variation of water salinity and temperature, reaching beyond the theoretical tolerance limits of the species. In the present study we determined the genetic 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 atoll-like structures are genetically isolated from plants growing in meadows outside the lagoon. For this purpose we utilized 13 microsatellite markers to genotype single shoots sampled inside and outside the lagoon. Lepidochronological analyses were performed on the same shoots to determine the annual rhizome growth rate, the number of leaves and the inflorescences formed as an estimate of growth- and reproductive performance over the years. The innermost 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 significant, 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 F-st and factorial correspondence analysis confirmed 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 isolated, with a possible selection of genotypes adapted to persistent stressful conditions, consistent with reduced growth and lack of flowering events.