Interannual variability in flowering of Posidonia oceanica in the North-Western Mediterranean Sea, and relationships among shoot age and flowering

The frequency of Posidonia oceanica past flowering events in a northwestern Mediterranean area was examined using a reconstructive age technique. We also investigated possible relationships among shoot age and occurrence of flowering. Seven flowering events were recorded over the 14-year period examined. Considerable variations in inflorescence frequency occurred among years (0-25.3%), with two years of abundant flowering (1989 and 1993) separated by several years with little or no flowering. Only 2% of the shoots sampled flowered twice; contrary to previous opinion, the interval among two successive flowerings was considerably shorter than 10 years, varying from 1 to 4 years. The minimum reproductive age of shoots was 6 months. The probability of flowering increased with age until 6 years of age, and decreased thereafter. Both the number of leaves and rhizome segments produced by a shoot during a flowering year did not differ significantly from those produced in the years preceding and following a flowering event, suggesting that the cost of flowering could not affect shortterm growth of shoots. On the basis of our results we can speculate that the maximum sexual reproductive output for P. oceanica in the area studied was about 340-460 seeds m(-2) in 1994.