Temporal dynamics of Zostera marina L. off the Lagoon of Grado (northern Adriatic Sea, Italy)

The temporal dynamics of biomass, phenology and sexual reproduction of Zost era marina were investigated during 1997 in a shallow meadow located off th e eastern sea entrance of the Lagoon of Grade (northern Adriatic Sea). Tota l biomass and the relative contribution of the above- and below-ground comp artments were measured, as well as the shoot density and phenological param eters of vegetative and fertile shoots. Flowering and seed production were also investigated and the stages of spathe maturation were recorded. Total biomass was highest in summer and lowest in autumn; the relative contributi on of the above-ground compartment, mainly composed of leaf blades, was hig hest in summer and lowest in autumn, when it accounted for about 40%; and 7 % respectively. Below-ground organs (rhizomes and roots) accounted for a hi gher fraction during the autumn-winter period. Shoot density and all the ph enological features of vegetative shoots, except the number of leaves per s hoot, changed widely throughout the year with maxima generally occurring in spring-summer and minima in autumn-winter. With regard to the sexual repro duction, the flowering of Z. marina was at the initial stage and the densit y of fertile shoots m(-2) was lower in winter than in spring (6 vs 265 fert ile shoots m(-2)), when ripe fruits occurred on spathes. In summer, the fer tile shoots still present (5 fertile shoots m(-2)) were dark-brown coloured with evident signs of decay. A pollination success of 55% was observed and a potential of seed production of more than 20000 seeds m(-2) was estimate d in spring. The seasonal plant dynamics and the timing of the sexual repro duction of Z. marina at Grade seem to be similar to those observed in the L agoon of Venice and in some areas outside the Mediterranean Sea. The result s of this paper and the published data emphasise the great variability in t he temporal dynamics, phenological features of both vegetative and reproduc tive shoots, and in the potential seed production of Z. marina. In this res pect, the well known phenotypic plasticity of this species may account for its ability to live in variable environmental conditions. The high potentia l of sexual reproduction, however, is apparently in contrast with the absen ce of records of seedlings in the northern Adriatic Sea.