On the ecophysiology of Posidonia oceanica: morphological adaptation and sugar allocation along a depth gradient

Posidonia oceanica meadows represent one of the most important ecosystems of Mediterranean coastal environments, providing a wide range of ecological functions such as the support of biodiversity, carbon storage and prevention of coastal erosion. Several anthropogenic activities, however, are threatening these ecosystems and causing their regression throughout the Mediterranean Sea, prompting the development of ecological restoration strategies with, unfortunately, variable degree of success. The key to improve the effectiveness of restoration activities lies in a better understanding of the ecology of P. oceanica, especially on the processes responsible for the energy balance and the mechanical interaction with the environment. With a view to shedding light on these topics, the research focuses on morphological adaptations and allocation of sugars in P. oceanica shoots along a depth gradient (5, 9, 16 and 20 m) in an intact meadow in Southern Italy (N 40°09’37.92”; E 15°04’04.67”). Overall, 214 shoots were collected on one occasion during a summer sampling campaign and used to measure the number of leaves per shoot and shoot density, before randomly selecting 9 shoots per depth for leaf morphological and biochemical analyses. Leaf length, area, width and thickness were measured on fresh leaves together with chlorophylls, flavonoids and anthocyanins, whereas oven-dried leaves were used for oligosaccharides and starch analyses. Clear trends over depth were observed in the measured traits, interpretable in terms of both population-level competitive interactions among shoots and plant-level compensatory mechanisms for the reduction in light availability. The obtained findings allow not only shedding light on P. oceanica ecophysiological plasticity, but can also orient the development of mechanistic models paving the way for the prediction of the outcomes of restoration activities at different levels of ecological complexities, scaling up from population dynamics to the ecosystems this species is able to create.