Growth and population dynamics of Posidonia oceanica on the Spanish Mediterranean coast:elucidating seagrass decline

The growth and population dynamics of Posidonia oceanica were examined in 29 meadows along 1000 km of the Spanish Mediterranean coast (from 36"46' to 42"22' N). oceanica developed the densest meadows (1141 shoots m-') and the highest aboveground biomass (1400 g DW m") between 38 and 39"N. P. oceanica shoots produced, on average, 1 leaf every 47 d , though leaf formation rates in the populations increased from north to south (range 5 7 to 8 .9 leaves shoot-' yr-'). P oceanica is a long-living seagrass, with shoots able to live for at least 30 yr P, oceanica recruited shoots at low rates (0.02 to 0.5 In units yr-l) which did not balance the mortality rates (0.06 to 0.5 In unlts yr.') found in most (57 %) of the meadows. If the present disturbance and rate of decline are maintained, shoot density is predicted to decline by 50% over the coming 2 to 24 yr. Because P. oceanica rhizomes grow very slowly (1 to 6 cm yr-' apex-'), maintenance of existing meadows is essential, and our results suggest bad future prospects for P. oceanica in the Spanish Mediterranean Sea like most other seagrass species in the world oceans.