Posidonia oceanica meadows are not declining globally. Analysis of population dynamics in marine protected areas of the Mediterranean Sea

Many studies have reported a worldwide decline in seagrasses, including Posidonia oceanica, with evidence of regression on a global scale for different reasons. With the aim of assessing the population dynamics of P. oceanica in the Mediterranean Sea, we chose 6 protected and pristine areas, separated by 200 to 3000 km, to determine whether an overall decline is occurring. Within each locality, we randomly selected 2 sites separated by 2 to 4 km. At each site, we sampled seagrass at 2 fixed depths (10 and 20 m). At each sampled depth, 10 principal plagiotropic rhizomes, separated by 1 to 5 m, were collected. We used lepidochronology to study seagrass dynamics, and analysis of variance (ANOVA) and permutational multivariate ANOVA (PERMANOVA) to analyze the data, testing the 3 main factors of locality, site, and depth. Our results showed positive population dynamics of P. oceanica in all localities and for all studied parameters. We observed exceptional vegetative growth around Zembra Island, Tunisia. We did not find statistically different vegetative growth between the northernmost and the warmer localities, showing a high level of plasticity in this species. This study demonstrates that global processes are not causing an overall decline in P. oceanica meadows around the Mediterranean Sea and attributes the decline reported in the literature to cumulative effects derived from different natural and anthropogenic local processes.