Oxygen and <scp>pH</scp> fluxes in shallow bay habitats: Evaluating the effectiveness of a macroalgal forest restoration

Marine macroalgae are important primary producers in coastal ecosystems. Within sheltered and shallow bays in the Mediterranean, various Fucalean macroalgae and seagrasses coexist, creating habitats of high ecological importance. These habitats have historically suffered from various disturbances, and on this basis, active restoration actions have been proposed as potential solutions for their recovery. Here, we assessed the restoration success of a 10-year restored macroalgal forest by evaluating the recovery in terms of oxygen and pH fluxes and comparing those data with those of a healthy marine forest and a degraded habitat counterpart. We estimated the overall changes in dissolved oxygen and pH using light and dark community in situ incubations. We also determined the biomass and composition of macroalgal and macroinvertebrate compartments of each assemblage. During light incubations, the healthy and restored forest assemblages showed similar average net oxygen production, 5.7 times higher than in the degraded one, and a greater increase in pH. More than 95% of the incubated biomass corresponded to macroalgal and seagrass species. The restored forest showed a six-fold increase in biomass, most likely being responsible for the recovery of primary production. This work provides empirical evidence that the restoration of a single structural species, once successful in the early stages, can yield positive results by recovering processes such as primary production and dark respiration. Moreover, these results showcase differences in ecosystem functions between healthy (either mature or restored) and degraded habitats, highlighting the importance of protecting and preserving coastal marine forests.