Acidification, deoxygenation, and nutrient and biomass declines in a warming Mediterranean Sea

The projected warming, nutrient decline, changes in net primary production, deoxygenation and acidification of the global ocean will affect marine ecosystems during the 21st century. Here, the climate change-related impacts on the marine ecosystems of the Mediterranean Sea in the middle andat the end of the 21st century are assessed using high-resolution projections of the physical and biogeochemical state of the basin under Representative Concentration Pathways (RCPs) 4.5 and 8.5. In both scenarios, the analysis showschanges in the dissolved nutrient contents of the euphotic and intermediate layers of the basin, net primary production, phytoplankton respiration and carbon stock (including phytoplankton, zooplankton,bacterial biomass and particulate organic matter). The projections also showuniform surface and subsurface reductions in the oxygen concentrationdriven by the warming of the water column and by the increase in ecosystemrespiration as well as an acidification signal in the upper water column linkedto the increase in the dissolved inorganic carbon content of the watercolumn due to CO2 absorption from the atmosphere and the increase inrespiration. The projected changes are stronger in the RCP8.5 (worst-case)scenario and, in particular, in the eastern Mediterranean due to the limitedinfluence of the exchanges in the Strait of Gibraltar in that part of thebasin. On the other hand, analysis of the projections under the RCP4.5emission scenario shows a tendency to recover the values observed at thebeginning of the 21st century for several biogeochemical variables in thesecond half of the period. This result supports the idea - possibly based on the existence in a system such as the Mediterranean Sea of a certain buffercapacity and renewal rate - that the implementation of policies for reducingCO(2) emission could indeed be effective and could contribute to the foundation of ocean sustainability science and policies.