VARIATIONS IN DEMOGRAPHY OF ZOSTERA-MARINA AND Z-NOLTII ON AN INTERTIDAL GRADIENT

The dynamics of the seeds, seedlings and mature shoots were studied for two seagrass species at seven elevations on an intertidal site in the southwestern Netherlands from the autumn to the following summer. The numbers of viable and dead seeds in the sediment seed bank, emergence from the seed bank, and seedling survival and growth were documented. Zostera noltii Hornem. grew as a perennial, regenerating from overwintering rhizomes. Shoot densities declined rapidly in the autumn to minima (late March) of 18 per 0.25 m2 at the highest elevation, to < 1 at the lowest elevation. Buried seeds had a peak density of 44 per 0.25 m2, averaged over the site. During winter and spring the seeds became rarer at the lower elevations, but more abundant nearshore. Zostera noltii seeds were viable, but did not germinate in situ; their fate is unknown. Zostera marina L. grew as an annual, the last shoots disappearing in February-March just as the buried seeds showed a burst of germination. About 80% of buried seeds in the early winter disappeared before February; half of the loss was due to autonomous death. About 50% of the viable seeds germinated, but only 13% of the seedlings successfully established. Established genets, representing about 1% of the autumn seed bank, produced vegetative and flowering branch shoots rapidly in the early summer. Seedling mortality was not density-dependent, but was higher at lower elevations. After about 3 months few genets died until the autumn. A small bank of viable Z. marina seeds remained by the summer, about 5% of the annual seed input.