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 Mediterranean Sea is enclosed by three continents and is surrounded by
mountains, deserts, rivers, and industrialized cities. This evaporative basin
is known as one of the most oligotrophic marine environments in the world
Records of the past evolution of the Mediterranean circulation show that the
Mediterranean has undergone abrupt changes in its circulation patterns over
ancient times. In particular, high stratification events, characterized by
the preservation of organic matter in the sediments, known as sapropels, have
been recorded several times through geological history. The most recent of
such event occurred 10 000 years ago and lasted about 3000 years. This
accumulation of organic matter in the sediments is interpreted as the result
of a strong stratification of the water column leading to suboxic deep layers
Future climate projections with high-emission scenarios for greenhouse gases
simulate warming and reduced precipitation over the Mediterranean region
Primary productivity in the ocean is influenced by its circulation and
vertical mixing that brings available nutrients to phytoplankton
As a semi-enclosed oligotrophic basin, the Mediterranean is highly sensitive
to external nutrient sources. Those sources include coastal runoff, river
discharge
This article is organized as follows. The coupled model, forcings and the different simulations are first described. We briefly evaluate the biogeochemical model in Sect. 3.1 and present the evolution of the physical and biogeochemical forcings in Sect. 3.2. In Sect. 3.3, we expose the temporal evolution of the main nutrients and their budgets in present and future conditions and discuss their impact on the biogeochemistry of the Mediterranean Sea in Sect. 4.
The ocean general circulation model used in this study is NEMO
Air–sea fluxes (momentum, heat, water) and river discharge used to force
NEMOMED8 are prescribed by the atmospheric regional climate model
ARPEGE-Climate
ARPEGE-Climate is, itself, driven by greenhouse gases (GHG) and aerosol
forcings following the observations (up to year 2000) and the SRES-A2
scenario afterwards and by SST (sea surface temperature) coming from a
previously run CNRM-CM coupled GCM (general circulation model) simulation
The NEMOMED8 simulation (ocean physics and forcings) used here corresponds to
one of the simulations used by
The main physical changes (SST, SSS – sea surface salinity, surface
circulation, deep convection and thermohaline circulation, vertical
stratification, and sea level) are detailed in
The average SST of the Mediterranean rises by up to
3
Here, the physical model NEMOMED8 is coupled to the biogeochemical model
PISCES
External nutrient supply for the biogeochemical model includes inputs from
the Atlantic Ocean and from Mediterranean rivers. We did not include
submarine groundwater discharge and direct wastewater discharge, as there is,
to date, no climatology for these sources. Atlantic input is prescribed from
water exchange through the Strait of Gibraltar in the NEMO circulation model
along with the concentrations of biogeochemical tracers in the buffer zone.
Nutrient concentrations in the buffer zone are prescribed from a global ocean
climate projection using the A2 simulation values from IPSL-CM5-LR
Nutrient inputs from rivers, including
There is, to our knowledge, no transient scenario for the evolution of
atmospheric deposition over the Mediterranean Sea. However, in order to
evaluate the potential effects of aerosol deposition on the future
Mediterranean Sea, we used deposition fields of total nitrogen deposition
(
Initial nutrient concentrations in the Mediterranean come from the SeaDataNet
database
All simulations began from a restart of a historical run that started in January 1965 following a spin-up of more than 115 years made with a loop over the 1966–1981 period for the physical forcings and the river nutrient discharge.
All simulations were made for 120 years. The control run CTRL was made with
forcing conditions corresponding to the 1966–1981 period looped over the
simulation period. This period was chosen in order to avoid the years with
excessive warming such as the 1980s and 1990s in the CTRL
In order to separately quantify the effects of climate and biogeochemical
changes, we made two additional control simulations: (1) CTRL_R with
climatic and Atlantic conditions corresponding to present-day conditions (no
scenario for climate change or nutrient fluxes through the Strait of
Gibraltar) and river nutrient discharge following the scenario evolution and
(2) CTRL_RG with present-day climatic conditions but river nutrient
discharge and Atlantic buffer-zone concentrations following the scenario
conditions. Table
Description of the simulations. The years indicate the forcing years throughout the 120 years of simulation. The cycles are repeated in the CTRL simulations.
We made two supplementary simulations, one with total nitrogen deposition (HIS/A2_N) and another with total nitrogen and natural dust deposition (HIS/A2_NALADIN). These simulations include climate change and nutrient fluxes from rivers and via the Strait of Gibraltar that follow the scenario conditions. The results from these simulations should be considered as exploratory. Nonetheless, they provide insight into the potential effects of future aerosol deposition.
NEMOMED8 has already been used in a number of regional Mediterranean Sea
modeling studies, either in hindcast mode
Average surface Chl
The regional NEMOMED physical model has already been coupled to the
biogeochemical model PISCES on a
Average surface Chl
The vertical distribution of nitrate and phosphate over a section crossing
the Mediterranean from east to west as well as Chl
In spite of some underestimation of nutrient concentrations that are probably linked with the features of the simulated intermediate and deepwater characteristics, the PISCES model reproduces the main characteristics of the Mediterranean biogeochemistry, including a salient west-to-east gradient in nutrient concentrations, low surface nutrient concentrations, and a DCM. These performances lend credence to our efforts to investigate the evolution of the Mediterranean biogeochemistry under the A2 climate change scenario with the same modeling platform.
Average SST and SSS evolution in the
entire basin during the CTRL and HIS/A2 simulations are shown in
Fig.
Evolution of average Mediterranean
SST
The nutrient budgets of the semi-enclosed Mediterranean basin are highly
dependent on external sources
In this section, we refer to the period 1980–1999 as the beginning of the century, to the period 2030–2049 as the middle of the century, and to the period 2080–2099 as the end of the century.
The Mediterranean is connected to the global ocean by the narrow Strait of
Gibraltar. Water masses transported through this strait contribute
substantially to its water and nutrient budgets
Evolution of total incoming
In the HIS/A2 simulation, the incoming flux of nitrate decreases from 50 to
35 Gmol month
Outgoing fluxes through the Strait of Gibraltar follow the same trends as
incoming fluxes; total outgoing nitrate and phosphate fluxes decrease from
1980 to 2040, with flux values getting closer to zero and then increasing until
the end of the century. We observe a decreasing trend in the nitrate outgoing
flux in the control from
River discharge is the main external source of phosphate for the eastern part
of the basin
Phosphate discharge decreases by 25 % between the beginning and the end
of the simulation period. As suggested by
Nitrate riverine discharge in the HIS/A2 simulation is substantially higher
than in the CTRL simulation by 30 to 60 Gmol month
Sedimentation removes nutrients from the Mediterranean Sea. In this version
of PISCES, the loss of nitrogen and phosphorus to the sediment is calculated
from the sinking of particulate organic carbon (POC) to the sediment (linked
through the Redfield ratio). Sediment fluxes of phosphorus and nitrogen
during the simulations are shown in Fig.
The nutrient loss to the sediment decreases rapidly during the HIS simulation (1980–1999) and remains low during the 21st century, although it exhibits substantial interannual variability in the sedimentation fluxes. By the end of the 21st century, sedimentation of phosphorus and nitrogen are almost 50 % lower relative to the 1980 fluxes.
Simulated integrated phosphate content (10
Tables
Evolution of total river discharge fluxes of nitrate and phosphate
(10
Total phosphate content in the entire Mediterranean grew in our HIS/A2
simulation by 6 % over the 21st century, as determined by the difference
between CTRL and HIS/A2 simulations between 1980–1999 and 2080–2099. The
increase is larger in the eastern basin than in the western basin. In
particular, there is an 8 % increase in phosphate content in the
Ionian–Levantine sub-basin. Nutrient content in the HIS/A2 simulation is affected by both
climate and nutrient fluxes from external sources (rivers and fluxes via the
Strait of Gibraltar). The effects of changes in riverine input of phosphate
are derived from the CTRL_R simulation (see also Fig.
Simulated integrated nitrate content (10
Table
Evolution of total sedimentation fluxes of N and P
(10
These global nutrient budgets reveal that climate change and external nutrient fluxes to the Mediterranean can influence its nutrient content in different, sometimes even in opposing, directions. In particular, river inputs have large effects on nutrient content in the eastern basin, while input through the Strait of Gibraltar has limited effects on the nutrient content, even in the western basin.
Evolution of yearly average phosphate concentration
(10
In order to observe the continuous evolution of nutrient concentrations in
different layers over the 21st century, we plotted the evolution of phosphate
and nitrate concentrations for the entire simulation period in the western
and eastern basins in the surface (0–200 m), intermediate (200–600 m), and
deep (
Evolution of yearly average nitrate concentration
(10
As shown in Fig.
A slight accumulation of about 0.015 mmol m
Phosphate concentrations in the eastern part of the basin are lower than in
the western part. Figure
In the surface of the western basin, nitrate evolutions in the HIS/A2 and
CTRL_RG simulations are similar, confirming the regulating effects of fluxes
through the Strait of Gibraltar (Fig.
In the eastern basin, the impacts of river discharge of nitrate seem to have
large influence on the nitrate accumulation, as shown by the similar
evolution of HIS/A2 and CTRL_R simulations (Fig.
Evaluating the evolution of nutrient concentrations separately in different
layers of the Mediterranean Sea shows that external nutrient fluxes primarily
affect the surface in the western basin, whereas climate change affects the
entire water column. Also, climate and nutrient fluxes may have opposite
effects on surface nutrient concentration. This leads to different trends in
nutrient concentrations in the surface layer and in the intermediate and deep
layers. In particular, surface nitrate in the eastern basin is observed to
increase as a result of increased river discharge, but climate change effects
lower concentrations in HIS/A2 (see Fig.
Present (1980–1999, top) interannual average surface (0–200 m)
concentrations of nitrate (10
Present (1980–1999,
Figures
Present (1980–1999,
On the contrary, Fig.
Present (1980–1999) and future (2080–2099) interannual average surface (0–200 m) limiting nutrient in the HIS/A2 simulation. N and P co-limitation is considered when limitation factors for N and P differ by less than 1 %. Green zones are P limited, orange zones are N limited, and purple zones are N and P co-limited.
Figure
Present (1980–1999,
Figure
The DCM depth changes little during the simulation, even though salinity and
temperature change. The DCM deepens slightly in some regions such as the
north Ionian and the south of Crete. Although the DCM depth changes little in
the future, the intensity of subsurface productivity is reduced (see
Fig.
Present (1980–1999) and future (2080–2099) interannual average
vertical profiles of total Chl
Figure
In the oligotrophic Mediterranean, the majority of the Chl
Simulated integrated Chl
About 85 % of the future reduction in Chl
Evolution of yearly average nanophytoplankton and diatom
concentration (10
Most of the biological activity in the marine environment is found within the
euphotic layer, which is confined to the upper 200 m. Figure
Evolution of yearly average microzooplankton and mesozooplankton
concentrations (10
The same general evolution is found for zooplankton as seen for phytoplankton
(Fig.
Altogether, the analysis of plankton biomass evolution during the simulation
period suggests that primary and secondary production in the eastern basin
are more sensitive to climate change than in the western basin. The eastern
basin is more isolated from the open Atlantic Ocean than the western basin, as it
receives less nutrients from the Atlantic and from coastal inputs. The
eastern basin is also deeper and less productive than the western basin
Present (1980–1999) and future (2080–2099) relative effects of
total nitrogen
Figure
The study represents the first transient long-term simulations of the Mediterranean Sea with a coupled physical–biogeochemical high-resolution model. It provides a first glimpse of the sensitivity of the Mediterranean Sea biogeochemistry to climate change and to the evolution of external nutrient fluxes. As for all modeling studies, our conclusions are subject to some limitations that we attempt to underline in this section.
Although the physical model adequately represents the Mediterranean
thermohaline circulation
Freshwater runoff in the physical model may also influence the circulation
and nutrient concentrations at the river mouth.
The evaluation of the CTRL simulation showed that NEMOMED8-PISCES is stable
over time, in spite of a slight drift in nitrate concentrations (see
Fig.
In the version of PISCES used in this study, variations in nitrate and
phosphate are linked by the Redfield ratio
Climate change may impact all drivers of biogeochemical cycles in the ocean.
In the case of semi-enclosed seas like the Mediterranean, the biogeochemistry
is heavily influenced by external sources of nutrients
Results from the HIS/A2_NALADIN simulation show that enhanced phosphate
fluxes from aerosols may limit the surface decrease of phosphate
concentrations and limit phosphorus limitation. However, in the
HIS/A2_NALADIN simulation, the surface Mediterranean is still P limited in
most of the Mediterranean, because the atmospheric nutrient fluxes are low in
comparison to riverine nutrient fluxes from rivers and the nutrient flux
through the Strait of Gibraltar
Our results show that the state of the Mediterranean biogeochemistry at the end of the 21st century is the result of the combined evolutions of both climate and external nutrient fluxes. Therefore, it is very difficult to predict the future evolution of the Mediterranean based on the evolution of one of these components only. This is why it is important, in the case of semi-enclosed basins, to produce reliable estimates of the evolution of all the components influencing the biogeochemistry.
Schematic diagrams illustrating the Mediterranean budgets of
phosphate and nitrate. For each component, the three lines represent the average
fluxes (in Gmol yr
Figure
In general, the sum of nitrogen net fluxes into the Mediterranean basin
(riverine, Gibraltar Strait, and sedimentary sources and sinks) increases by
39 % at the end of the century in the scenario (HIS/A2), whereas it is
increased by 23 % in the control (CTRL) in comparison to the beginning of
the simulation. The balance between inputs and outputs of phosphorus
increases by 9 % in the scenario and by 11 % in the control. These
results suggest a substantial accumulation of nitrogen in the Mediterranean
basin over the century when phosphorus fluxes can be considered roughly
stable. The strong decrease in sedimentation (Fig.
Results from our transient simulations show that nutrient concentrations may
evolve differently depending on the region and the depth in response to
climate change and external nutrient inputs. In the surface of the western
Mediterranean, the effects of climate change and enhanced nutrient fluxes via
Gibraltar both concur with the increase in nutrient concentrations
(Figs.
Results from our different control simulations indicate the extent to which
the choice of the biogeochemical forcing scenario may influence the future
evolution of the Mediterranean Sea biogeochemistry. Results from
Table
In some parts of the eastern basin, the effects from riverine nutrient fluxes
on Chl
To our knowledge, this is the first attempt to study the basin-scale
biogeochemical evolution using a transient business-as-usual (A2) climate
change scenario.
The modifications of Chl
This study aims at assessing the transient effects of climate and
biogeochemical changes on the Mediterranean Sea biogeochemistry under the
high-emission SRES-A2 scenario. The NEMOMED8-PISCES model adequately
reproduces the main characteristics of the Mediterranean Sea: the
west-to-east gradient in productivity, the main productive zones, and the
presence of a DCM. Hence, it appears reasonable to use it to study the future
evolution of the biogeochemistry of the Mediterranean basin in response to
increasing atmospheric
Its results illustrate how future changes in physical and biogeochemical conditions, including warming, increased stratification, and changes in Atlantic and river inputs, can lead to a significant accumulation of nitrate and a decrease in biological productivity in the surface, thus affecting the entire Mediterranean ecosystem.
Our results also illustrate how climate change and nutrient inputs from riverine sources and fluxes through the Strait of Gibraltar have contrasting influences on the Mediterranean Sea productivity. In particular, the biogeochemistry in the western basin displays similar trends as that for nutrient input across the Strait of Gibraltar. Therefore, it appears critical to correctly represent the future variations of external biogeochemical forcings of the Mediterranean Sea, as they may have an equally important influence on surface biogeochemical cycles as climate. The biogeochemistry of the eastern basin is more sensitive to vertical mixing and river inputs than the western basin, which is regulated by input through the Strait of Gibraltar. Increased future stratification also reduces surface productivity in the eastern basin.
Although this study does account for the changes in fluxes through the Strait
of Gibraltar and riverine inputs, some potentially important sources are
missing, such as direct wastewater discharge, submarine groundwater input, and
atmospheric deposition. These additional nutrient sources are poorly known,
with a general lack of both measurements and models as needed to build
comprehensive datasets for past and future evolution of these nutrient
sources. The HIS/A2_N and HIS/A2_NALADIN simulations presented in this
study include continued present-day nitrogen and phosphate deposition.
Although these atmospheric fluxes have been evaluated previously and were
shown to correctly represent the deposition fluxes, there is no guarantee
that these fluxes will remain constant over the next century. Results
indicate that the future sensitivity of the Mediterranean to atmospheric
deposition depends on the surface nutrient limitation, which may in part be
influenced by aerosol deposition. However, there is, to our knowledge, no
available transient scenario for the 21st century evolution of atmospheric
deposition and no ensemble simulations to assess the future evolution of the
Mediterranean Sea under different climate change scenarios. A new generation
of fully coupled regional models have been developed and used to study
aerosols climatic impacts
Simulation results are available upon request to the main author.
The comparison of modeled surface Chl
Figure
Average Chl
The overestimated DCM depth may be due to the overestimation of nitracline
and phosphaline as shown by Fig.
The vertical distribution of nitrate and phosphate concentrations along a
west-to-east transect is shown in Fig.
Average phosphate
Average concentrations of nitrate
Authors contribution: CR, JCD, LB, JCO, and FD designed the experiments CR ran the simulations, wrote the manuscript and analysed the results BLV, produced the spin up simulations SS Produced the physical simulation. All authors participated in results interpretation and manuscript correction JCO corrected the English.
The authors declare that they have no conflict of interest.
The authors would like to thank Florence Sevault for the physical simulations of NEMOMED8, Pierre Nabat for the dust deposition with ALADIN-Climat, Yves Balkanski and Rong Wang for the N deposition with the LMDz-INCA model, and Wolfgang Ludwig for the river inputs. This work was funded by CEA (Camille Richon PhD grant) and is part of the MISTRALS project. Simulations were made using HPC resources from the French GENCI program (grant x2015010040). Edited by: Jean-Pierre Gattuso Reviewed by: three anonymous referees