Ecophysiological responses of Posidonia oceanica along a depth gradient: implications for ecological restoration

Posidonia oceanica meadows are ecosystems of extreme ecological value covering more than 2% of Mediterranean seabeds and providing several ecosystem services such as supporting biodiversity, contributing to the global carbon balance, and preventing coastal erosion. In recent decades, a growing number of ecological restoration strategies have been developed to counteract the increasing loss of P. oceanica meadows due to anthropogenic activities. However, these strategies are often ineffective, and a better understanding of P. oceanica ecology could facilitate the development of more successful restoration approaches. In this context, we studied the ecophysiological responses of this species along a depth gradient (5-9-16-20m) in an intact meadow in southern Italy (Tyrrhenian Sea), focusing on morphological adaptations, epiphyte colonisation, photosynthetic pigments and carbohydrate allocation. Both orthotropic and plagiotropic rhizomes, along with their shoots, were collected during a summer sampling campaign, with nine shoots and rhizomes selected per depth. Leaf basal and apical parts were analysed separately to assess ecophysiological responses to epiphyte colonization, higher in the latter. Morphological measurements and photosynthetic pigment analyses were carried out on fresh material within 1 day from sampling, whereas soluble sugars and starch were analysed on oven-dried and pulverised samples. Results showed clear acclimation and adaptation processes with depth, with non-linear trends in most of the analysed parameters, and suggest the capability of P. oceanica to balance the acquisition and consumption of trophic resources in the form of soluble sugars in both leaves and rhizomes. Indeed, both the de-crease in light availability and the increase in epiphyte cover with depth, appear to induce compensatory photosynthetic mechanisms, highlighted by an increase in chlorophyll concentrations, ensuring adequate rates of carbon fixation even at greater depth. Findings highlight the high ecophysiological plasticity of P. oceanica, providing information for the mechanistic understanding of its interaction with the environment and for the development of effective restoration strategies at different levels of ecological complexities.