Effects of a simulated thermocline deepening on the biochemical composition of a Mediterranean seagrass

Frequency and severity of marine heatwaves are progressively increasing worldwide due to climate change, causing the deepening of the lower limit of the thermocline, ultimately posing severe threats to habitat-forming and foundation marine species. Among these, seagrasses, especially those dwelling in the deepest part of the photic layer usually below the summer thermocline, like the Mediterranean endemic Posidonia oceanica , are at high risk. We investigated whether and how the biochemical composition of P. oceanica leaves varies when the plant is exposed to a simulated thermocline deepening. To do this, we carried out a field manipulation experiment that aimed at investigating changes in protein, water-soluble carbohydrate and lipid contents of P. oceanica leaves, as a proxy for either short-term metabolic responses or the leaf fate along the trophic web. The mechanical and handling stress caused by shoots translocation produced significant effects on almost all the investigated biochemical variables. However, the simulated deepening of the thermocline caused short-term metabolic responses that affected both the leaf biopolymeric composition and the C stored into the leaf biomass. Moreover, changes in the biochemical composition of the seagrass leaves also altered their potential availability to consumers (in terms of caloric content), also suggesting that the effects of the thermocline deepening on P. oceanica biochemical composition could extend much further than the metabolism of the seagrass itself, and, altering leaf C storage and caloric content, could exert consequences propagating through the food webs based on its leaves.