Abstract. The Mediterranean region is a climate change
hotspot. Increasing greenhouse gas emissions are projected to lead to a
substantial warming of the Mediterranean Sea as well as major changes in its
circulation, but the subsequent effects of such changes on marine
biogeochemistry are poorly understood. Here, our aim is to investigate how
climate change will affect nutrient concentrations and biological
productivity in the Mediterranean Sea. To do so, we perform transient
simulations with the coupled high-resolution model NEMOMED8-PISCES using the
high-emission IPCC Special Report on Emissions
Scenarios (SRES) A2 socioeconomic scenario and corresponding
Atlantic, Black Sea, and riverine nutrient inputs. Our results indicate that
nitrate is accumulating in the Mediterranean Sea over the 21st century, while
phosphorus shows no tendency. These contrasting changes result from an
unbalanced nitrogen-to-phosphorus input from riverine discharge and fluxes
via the Strait of Gibraltar, which lead to an expansion of phosphorus-limited
regions across the Mediterranean. In addition, phytoplankton net primary
productivity is reduced by 10 % in the 2090s in comparison to the present
state, with reductions of up to 50 % in some regions such as the Aegean
Sea as a result of nutrient limitation and vertical stratification. We also
perform sensitivity tests to separately study the effects of climate and
biogeochemical input changes on the future state of the Mediterranean Sea.
Our results show that changes in nutrient supply from the Strait of Gibraltar
and from rivers and circulation changes linked to climate change may have
antagonistic or synergistic effects on nutrient concentrations and surface
primary productivity. In some regions such as the Adriatic Sea, half of the
biogeochemical changes simulated during the 21st century are linked with
external changes in nutrient input, while the other half are linked to climate
change. This study is the first to simulate future transient climate change
effects on Mediterranean Sea biogeochemistry but calls for further work to
characterize effects from atmospheric deposition and to assess the various
sources of uncertainty.
The 13C method as a robust alternative to 14C-based measurements of primary productivity in the Mediterranean Sea