Long-term population dynamics in a healthy Posidonia oceanica meadow

Wide losses of Posidonia oceanica led some authors to suggest this species showed evidence of decline in a global scale. Our aim was to survey the long-term evolution of a healthy and mature meadow of Posidonia oceanica at Tabarca Island (SE, Spanish coast). We surveyed cover and density at three depths (-4, -12 and -20m) and the dynamics at border meadows (erosion fronts vs adjacent edges) by one transect laid from -0,5m to -12m depth. Sexual reproduction as a repair mechanism was tested by monitoring the survival of seedling in five random sites. At each sampling site, sea-bottom roughness was estimated. Density and cover at sampling depths of -12 and -20 m were steady for 22 years (1988-2010), but both descriptors significantly decreased at upper limit (-4m) in 2010. Erosion fronts measured on transect went back -0.8 +/- 70.75 m y(-1), though this fact was compensated by the progress of its adjacent edges in 0.6 +/- 0.69 m y(-1). Density of survival seedlings has maintained a steady state for two years after settlement and it was strongly correlated with sea-bottom roughness. Our results suggest P oceanica meadows of Tabarca Island could have remained stable long-term. But we detected "hot areas", such as those placed in upper limit or in erosion fronts, where likely a quick decline by natural causes (e.g. waves) was offset by natural repair mechanisms (e.g. settlement of seedlings and vegetative growth).