Assessing the ecological condition and resilience of the Posidonia oceanica (L.) Delile prairie require the use of phenological and structural traits as reliable indicators. This study investigates P. oceanica prairie inside and outside the Marine Protected Area (MPA) "Costa degli Infreschi e della Masseta" (Southern Tyrrhenian Sea, Italy), to evaluate the effectiveness of MPA protection measures through the prairie conservation parameters. During autumn 2024 and summer 2025, 8 sites (6 inside and 2 outside the MPA), located at a depth of 15 m, were sampled by SCUBA diving. At each site, 3 stations, at least 10 m apart, were sampled with a 40×40cm sampling square. Shoot density was assessed through 3 random replicates. 6 orthotropic shoots were also collected at each site for phenological analysis. Data were analyzed using two-way ANOVA to test differences between areas (inside vs. outside the MPA) and season (autumn vs. summer). Analyses were also performed to detect the relationships among shoot density, leaf length, and number, and photosynthetic tissue.

A significant difference was found between the areas (p = 0.002), with a significantly higher shoot density outside the MPA in both seasons: 392.50 shoots/m² in autumn and 376.67 shoots/m² in summer outside the MPA; 318.82 shoots/m² in autumn and 263.54 shoots/m² in summer inside MPA. Seasonally leaf length and photosynthetic tissue varied, with maximum values in summer, while no significant differences were observed in leaf number. A positive and significant correlation emerged between the length of adult and intermediate leaf (r = 0.56, p < 0.001). In contrast, photosynthetic tissue was negatively correlated with leaf length (r = -0.40, p = 0.124 in adult; r = -0.92, p < 0.001 in intermediate). These results support the role of MPAs in maintaining seagrass also in surrounding areas.