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). P. oceanica developed the densest meadows (1141 shoots m^{- 2}) and the highest aboveground biomass (1400 g DW m^-2) 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^-1 yr^-1), 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 ln units yr^-1) which did not balance the mortality rates (0.06 to 0.5 ln units yr^-1) 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^-1 apex^-1), 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.