Research turns to acidification and warming in the Mediterranean Sea

The Mediterranean Sea Acidification in a changing climate (MedSeA), a project funded by the European Commission under the 7th Framework Programme, assesses uncertainties, risks and thresholds related to Mediterranean acidification and warming at organism, ecosystem and economic scales. Eighteen institutions in 11 countries, mainly from the Mediterranean, are collaborating to identify where the impacts of acidification on Mediterranean waters will be most severe, taking into account the complete chain of causes and effects, from ocean chemistry through marine biology to socio-economic costs (Fig. 18). Policy measures for adaptation and mitigation that may vary geographically, and at the same time require coordination between regions or countries, will be proposed.

A basin-scale approach is adopted, with emphasis on conveying the acquired scientific knowledge to policy-makers, decision-makers, marine managers and other stakeholders through the formation of the Mediterranean Reference User Group (MRUG). Policy measures for adaptation and mitigation that may vary from one Mediterranean area to another, will be suggested. Thus, for the first time, project managers and other stakeholders will have up-to-date vulnerability maps upon which to design action plans.

Fine-scale regional study to resolve the complexity of the Mediterranean Sea acidification

As atmospheric CO2 levels rise, thermodynamics and air-sea gas transfer processes drive some of the excess CO2 into the ocean surface waters, alleviating climate change. This process leads to shifts in seawater acid-base chemical speciation, resulting in lowered pH, increased concentration of bicarbonate ions, reduced concentration of carbonate ions and calcium carbonate saturation state (in other words “ocean acidification”). This poses a threat to marine ecosystems and could potentially result in large changes in global biogeochemical cycles. This acidification could also have significant socio-economic impacts due to effects on tourism (e.g., as a result of coral degradation or invasion of non-endemic species) or fisheries and aquaculture (resulting from altered life cycles of key surface- and bottom-dwelling organisms, including shellfish). There is growing concern that impacts of anthropogenic acidification could propagate from individual organisms up through marine food webs, affecting commercial fisheries and shellfish industries, thereby threatening protein supply and food security for millions of people. The effects on such marine-based activities could indirectly affect land-based economic activities and jobs on a much larger scale.

Although the general impact of acidification on water chemistry is globally well understood, fine-scale regional models are needed to resolve the complexity of the physical and ecological interactions of small and complex basins like the Mediterranean Sea. The Mediterranean Sea is considered a small-scale ocean with high environmental variability and steep physicochemical gradients within a relatively limited area. Its circulation is characterized by zonal gradients of physicochemical variables, with salinity, temperature, stratification and alkalinity all increasing towards the east. The generally low-nutrient (from oligotrophic to ultraoligotrophic) waters offshore stand in contrast to many near-shore regions, often containing coral and seagrass ecosystems, which are affected by human-induced eutrophication. Thus, acidification is an additional anthropogenic pressure on Mediterranean Sea ecosystems, already suffering from overfishing, increasing sea surface temperatures, and invasions by alien species. To properly project how key biogeochemical and ecosystem processes will change, it is fundamental to adequately represent the general circulation of the Mediterranean basin, i.e., both the fine-scale processes that control it (e.g. eddies and deep convection), and the highly variable atmospheric forcing. With their relatively short residence times, Mediterranean Sea deep water changes are likely to lag behind surface waters by a few decades. Changes in deep-water formation sites, such as characterized by the dramatic shift with the Eastern Mediterranean Transient, are likely to coincide with changes in the hot spots where much of the anthropogenic CO2 is taken up from the atmosphere and transferred into the deep sea (where it is stored for longer periods). The efficiency of carbon uptake and export from the surface waters to the basin interior depends on the relatively rapid time scales for surface-to-deep water exchange and the Mediterranean general circulation. Thus, the combined effect of seawater acidification (absorbing anthropogenic CO2 per unit area) and low tropospheric warming on Mediterranean biogeochemistry, ecosystems and the ecosystem services they support (through direct impacts on its highly adapted calcareous and non-calcareous organisms) may be large.

The MedSeA project

A critical mass of scientists is joining forces in this interdisciplinary project to diagnose the current state of Mediterranean Sea acidification and to project how it will evolve in coming decades in terms of impacts on marine ecosystems and human society. New observational and experimental data on Mediterranean organism and ecosystem responses to acidification will be fed into existing fine-scale models of the Mediterranean Sea, modified to better represent key processes, to project future changes. MedSeA’s strategy is to focus on selected ecosystem and socio-economic variables that are likely to be affected by both acidification and warming, to study the combined effects through ship-based observations, laboratory and mesocosm experiments, physical-biogeochemical-ecosystem modelling and economic analyses. It aims to provide best estimates of future changes and related uncertainties in Mediterranean Sea pH, CaCO3 saturation states and other biogeochemical-ecosystem variables. In addition, changes in habitat suitability of relevant ecological and economically-important species will be assessed.

Dynamics of the Mediterranean carbonate chemistry from interannual to millennial timescales

As in the global ocean, when anthropogenic CO2 penetrates the Mediterranean waters, CO2-driven shifts in the carbonate chemical equilibria occur and seawater pH decreases. The MedSeA project is quantifying the rate of pH decrease and will ultimately produce maps identifying those sectors of the Mediterranean Sea currently most affected by pH changes.

As carbonate system data in the Mediterranean Sea are relatively scarce, new field measurements of carbonate system variables, both in the Western and Eastern basins, are being taken. These new data (from cruises and time-series stations) will complement existing data sets and provide a solid basis for understanding the temporal evolution of the penetration of anthropogenic carbon into the Mediterranean Sea. Data sets from survey cruises will provide the necessary links to the time-series stations and facilitate the construction of spatial gradients, thus providing insight on the penetration of anthropogenic carbon into the various Mediterranean water masses. Furthermore, the rate of increase of the pCO2 in surface seawater is under investigation to determine if it follows that of the atmosphere.

As results from cruises become available, it is expected that the parameterization of the variations of the total alkalinity (AT) and total dissolved inorganic carbon (CT), as a function of parameters such as salinity, temperature, and oxygen, will be improved. The few existing relationships present relatively large uncertainties, and it is anticipated that more accurate three-dimensional distributions of AT and CT fields throughout the Mediterranean Sea will be provided.

As the Mediterranean Sea is relatively unusual (e.g., warm waters throughout the water column, high salinity, non-Redfield ratios) compared with the open ocean, analysis of the (available and new) data sets will provide new insights for developing new way(s) of estimating anthropogenic carbon in the ocean. Initially, the three existing methods (TrOCA, MIX, improved Brewer/ Chen & Millero) will be considered. Results will be cross-compared and critically evaluated and improvements that take specific Mediterranean Sea features into account will be proposed. Current pH variations due to anthropogenic carbon penetration will be estimated from the anthropogenic carbon distributions, thus providing information on areas that could potentially be sensitive to future large pH decreases

Understanding the current and future dynamics and vulnerability of the Mediterranean marine carbonate system, however, requires knowledge of the long-term natural variability of the basin. This can be attained by providing proxy-based reconstructions of seawater pH, carbonate ion concentrations and pCO2, together with the response of marine calcifiers during key intervals of the Late Quaternary. These intervals are taken to represent discrete background states of the functioning of the basin or the climate system in general. Information derived from investigating the last millennium is critical to disclosing the natural range of variability of the Mediterranean carbonate system, before and during the anthropogenic perturbation of the global carbon cycle. The last deglaciation could be instructive as it represents an interval of natural CO2 build up (~80 ppm) in the atmosphere, although the change occurred over several millennia, as opposed to the much faster atmospheric CO2 rise observed in the past two centuries. Finally, the last interglacial period, notably in the eastern Mediterranean, is commonly referred to as a period of severely weakened deep and intermediate water overturning, high export of organic carbon to the deep sea (sapropel S5) and development of a large reservoir of respired CO2 in the subsurface. Accordingly, proxy-based reconstructions for this period could potentially inform the ongoing debate on the role of the basin’s thermohaline circulation and export production on the uptake of anthropogenic carbon.

Ecosystem responses, Mediterranean key-stone species and economic impact

MedSeA will define the susceptibility and resilience of key-stone species and endemic ecosystems to Mediterranean acidification and warming. Analysis of the experimental results will enable projections of changes to the services that these ecosystems and species provide. Some services, such as nursery grounds for fish, coastal protection, tourism, carbon sequestration and climate regulation, are likely to be very sensitive to climate change. To constrain projections, the MedSeA consortium will take an interdisciplinary approach, considering both physiological responses as well as ecological responses to environmental change.

The effects of acidification on selected Mediterranean pelagic and benthic species and on potentially sensitive processes such as photosynthesis and calcification will be assessed (Fig. 19). Geographical variability will be assessed by comparing responses in the Western and Eastern Mediterranean basins and Adriatic Sea. This will involve: 1) plankton monitoring at selected time-series stations and on regional cruises to characterize present conditions, 2) laboratory experiments on the response of single species and strains, 3) mesocosm experiments to determine the biogeochemical and community responses, and 4) natural analogue experiments in areas acidified by CO2 vents to determine the long-term effects of acidification across multiple generations of marine organisms (Fig. 20). The current functioning of pelagic and benthic marine communities will be related to carbonate chemistry and other environmental conditions (temperature, nutrients), over a wide geographic area, to inform predictive tools provided by the MedSeA modelling component.

To investigate the organisms, ecosystems and processes that are most likely to be susceptible to acidification in the Mediterranean, model species were selected regardless of whether or not they are unique or endemic to the Mediterranean Sea, major contributors to habitat building, major contributors to ecological function or species of economic value in the Mediterranean region.

Laboratory experiments investigate the individual and dual impacts of acidification and temperature on key pelagic and benthic organisms (coccolithophores, foraminifera, pteropods, jellyfish and endemic habitat-forming seagrasses, coralline algae, corals and vermetids). The potential effect of low phosphorous concentrations, which characterize the eastern Mediterranean and may alter the local outcomes of acidification, is also examined. The response of selected Mediterranean phytoplankton and zooplankton such as copepods is studied as they play a significant role in nutrient recycling in the water column and on the export of particulate matter out of the photic zone. During the last two to three decades, jellyfish blooms have increased throughout the Mediterranean Sea, often associated with warming caused by climate change. The effects of combined acidification and warming on ephyra viability and adult fitness in selected species will be also examined. Aquarium experiments will test adult fitness and reproductive output, larval viability, recruitment and survivorship of common Mediterranean species and of the invasive jellyfish species in the eastern Mediterranean.

Mesocosom incubations containing natural plankton assemblages will be performed in the Eastern and Western Mediterranean basins to examine the impact of changes in carbonate chemistry and temperature on biological and biogeochemical variables under natural and perturbed conditions. Mesocosm and natural analogue experiments will investigate and compare the impact of acidification on marine communities and processes. The first set of mesocosm experiments will be performed in June-July 2012. These will help to identify the impact of acidification on the planktonic community, including the diversity and succession of species, as well as the role of the microbial loop. It will be the first mesocosm study of the response of Mediterranean pelagic calcifiers to acidification.

Responses of benthic ecosystems: Studying benthic communities is essential for understanding the ecological effects of ocean acidification as 98% of all described marine species live on the sea floor. Key habitat-forming species of coralline algae, seagrass, corals and vermetids (Fig. 19) endemic to the Mediterranean, as well as the commercially important species that these habitats support, are targeted. MedSeA combines laboratory, mesocosm and volcanic CO2 vents to examine the long-term effects of elevated CO2 on the structure and functioning of benthic communities (Fig. 20).

Based on worldwide laboratory studies, calcareous coralline algae (CCA) appear to be among the first organisms to cease calcification due to lowered pH. First CO2 vent studies at Ischia show that similar effects may occur in Mediterranean waters. If so, this endangers a suite of diverse Mediterranean habitats (maerl beds, coralligenous habitats, trottoir, vermetid reefs) that are dependent on these calcifiers. MedSeA measures pH and thermal thresholds under which coralline algae are likely to slow calcification. The endemic Mediterranean seagrass Posidonia oceanica forms rich and diverse ecosystems. Work at CO2 vents indicates that this species may actually benefit from ocean acidification, but shows high sensitivity to elevated seawater temperature. Field assessments and experimental manipulations suggest that seagrass community metabolism and photosynthetic activity may buffer the effects of ocean acidification. Hence, declining seagrass meadows due to climate change may detrimentally affect habitat building, enhance beach erosion and locally moderate the effects of ocean acidification. Surveys of vermetid reefs and their associated CCA Neogoniolithon notarisi at field sites in Italy (Sicily) and Israel, combined with a series of mesocosm experiments, show that these engineering species are at risk and in some locations have already suffered local extinction.

A unique advantage of carrying out ocean acidification research in the Mediterranean is the availability of natural CO2 vent systems. These provide MedSeA researchers with the unique opportunity to examine the effects of reduced seawater pH on natural communities. Furthermore, by transplanting organisms along a pH gradient (function of distance from the vent) it is possible to examine short-term physiological responses to explain the observed long-term shifts in benthic communities.

Projected impacts of acidification on biogeochemistry and Mediterranean target species

It is well known that all future carbon emission scenarios from the IPCC indicate that ocean acidification will intensify. Global-scale models confirm this trend while providing a more realistic regional picture, relative to simple equilibrium calculations, particularly for key areas where there is substantial air-sea disequilibrium. The MedSeA project will expand projections of acidification to include the Mediterranean Sea by relying on modelling tools that aim to bridge experimental results with socio-economic studies.

Future projections of changes in Mediterranean Sea pH, CaCO3 saturation states, and other carbonate-system and biogeochemical-ecosystem variables are done using two ocean models, coupled with state-of-the-art biogeochemical models. They will be driven with forcing fields from coupled climate models to simulate 20th century climate conditions and future climate change scenarios. This strategy should help assess uncertainties of future projected changes in acidification by means of an ensemble of experiments. For the 20th century simulations, models will be forced with fields from regional climate models driven by observed changes in atmospheric greenhouse gases. Model skill and bias will be assessed by comparing simulated biogeochemical variables for the current state to available datasets. The simulated spread in future system responses will include differences between climate scenarios, models and assumptions about processes. The design of the simulations will allow researchers to (1) isolate impacts from increasing CO2 and climate change on the carbonate system variables and (2) examine the role played by pelagic biotic processes. By combining the results of this ensemble of experiments, MedSeA will go a long way towards estimating the uncertainty in future projections. Model projections will allow us to construct basin-scale sensitivity maps to ocean acidification, based on combined changes in key model state variables (e.g., pH, CaCO3 saturation states, O2, temperature, stratification). The integrated analysis of these maps, will help the MedSeA scientists to identify the regions of the Mediterranean Sea that are expected to be more vulnerable to acidification under future climate scenarios. To move this investigation from the level of biogeochemical response to the ecosystem level, one task is dedicated to constructing response functions of selected target species (shellfish, coralligenous organisms, etc.) to environmental parameters. This will produce hybrid habitat suitability patterns, i.e. a range of projected variations of the stress factors for growth and functioning of the target species. This information will be further exploited to assess socio-economic impacts associated with each considered target species, with a special focus on market species such as aquaculture mussels.

Socio-economic effects of Mediterranean Sea acidification and potential adaptation strategies and policy tools

MedSeA is developing a conceptual framework for studying the direct and indirect socio-economic (welfare) impacts of ocean acidification. This requires consideration of relevant dynamics and time scales, regional patterns, mechanisms (costs, prices, labour market, trade, income changes, expenditures, transport, etc.), valuation categories (use and non-use values and various sub-categories), economic demand categories and sectors (supply) and interactions between marine resource-based and other sectors (indirect effects). Much can be learned here from existing integrated economic-ecological studies of ecosystem change and values and meta-analysis of relevant past valuation studies.

Important sectors that could potentially be affected by ocean acidification are tourism, fisheries, aquaculture and jewelry production from red coral. Mediterranean acidification may affect the occurrence of harmful algal blooms, jellyfish distribution patterns, shellfish physiology and major contributors to habitat building. Links between these and other activities need to be established to identify indirect economic effects of ocean acidification. In addition, the capacity of the Mediterranean Sea to sequestrate carbon may be affected by ocean acidification. This can influence future concentrations of carbon dioxide in the atmosphere and thus global warming. The economic price of this can be determined, using available cost estimates of climate change damages.

Economic analyses will use scenarios developed in the MedSeA project, to which assumptions relating to relevant economic developments will be added (income growth, travel costs – climate regulation of air traffic and demand for tourism). In addition, synergetic effects of climate change (sea level rise, temperature change, altering weather conditions) and ocean acidification must also be considered. Direct effects of ocean acidification will be assessed, first with a review of existing studies on tourism and aquaculture for selected areas. This will focus on the Mediterranean, transferring and upscaling results to particular Mediterranean countries. Information obtained from valuation studies will be used to address use and non-use values. Meta-analyses and benefit/value transfer studies may be used in some cases, like the loss of nursery habitat value of seagrasses or corals. In other cases, the assessment will include partial equilibrium analysis (PEA), which addresses both market impacts (notably on tourism and aquaculture) and non-market impacts (ecosystem values, including cultural services and non-use values such as option, existence and bequest values). There is also evidence that the chronic acidification stress may cause major shifts in species composition and thus marine community structure, affecting organisms necessary or important in the diet of seafood species. Consequently, this could result in reduced Mediterranean marine biodiversity, which could potentially affect use and non-use values associated with both species diversity and unique Mediterranean ecosystems. The loss or degradation of coralligenous environments due to pH reduction could also have negative socio-economic impacts in regions that attract tourists for recreational diving, swimming, and viewing from underwater observatories or glass-bottom vessels.

Finally, adaptation strategies and policies will be formulated based on the qualitative and quantitative assessments of the natural and social science studies in the project. The objective is to cost-effectively reduce negative socio-economic impacts (damage costs) as much as possible. Strategies may involve technical, biological, legal, economic and spatial tools and instruments, as well as public versus private actions or some combination of these, such as risk management or insurance. Public actions involve the creation of adequate adaptation incentives for private economic agents through particular public policies. Different strategies may be considered for different areas of the Mediterranean region. To improve their effectiveness, some policies may require supranational cooperation or coordination. The expectation is that much can be learned from the growing literature on adaptation to climate change and associated analyses of policies and strategies.

Progress expected and initial results

The starting point is our current knowledge of the Mediterranean system. There is a lack of field-derived information on the carbonate system of the Mediterranean and related ecosystem components. A substantial effort is required to collate existing data and collect new information from the pelagic and benthic systems. The specific oceanographic features of the Mediterranean basin will be assessed using high-resolution, physical-biogeochemical models to provide basin-wide and surface-to-deep distributions of pH and carbonate-related variables. The models will also enable future projections following established scenarios for atmospheric CO2 emissions. MedSeA experimental and field observations, as well as model simulation output will be coordinated by comprehensive data management using a similar approach to the European Project on Ocean Acidification (EPOCA) and Biological Impact of Ocean Acidification (BIOACID) programme. The combination of the model projections, field and laboratory experiment results and socio-economic analyses will enable the development of vulnerability maps and possible economic impacts due to acidification in the Mediterranean. Emphasis will be put on producing the scientific output in a way that is understandable to policy makers and other key stakeholders. Close collaboration is anticipated with other ocean acidification initiatives (e.g. EPOCA, BIOACID, the UK Ocean Acidification Research Programme (UKOA)).

Initial project results confirm that the concentration of anthropogenic carbon extrapolated from Mediterranean field measurements is high and penetrating the deep sea. This process is changing the water chemistry, not only in the surface but also in the deep Mediterranean. The first results, based on a limited number of data, show that the Mediterranean Sea has undergone a reduction of the pH since the onset of industrialization and the effect is largest in the NW Mediterranean Sea. It is clear that due to the complexity and high variability of the basin, this process will have different regional impacts. This is anticipated to occur in regions where future model projections of sea surface temperature indicate a mean increase of winter and summer temperature of up to 2-4oC by the year 2050, if anthropogenic emissions remain unchanged.

MedSeA scientists are studying the responses of a wide range of organisms to ocean acidification – from free drifting plankton to bottom-dwelling organisms. Some, such as corals, seagrass and shellfish, are habitat builders (engeneering species), so their health is critical to hundreds of other organisms. Some have cultural and commercial value for the human population around the Mediterranean Sea. MedSeA scientists have identified and are currently focusing on ecosystems that are uniquely Mediterranean and are of special socio-economic value. Some of their recent findings highlight the sensitivity of some target benthic ecosystems (e.g. corals, shellfish, coralligenous habitat) to environmental change, suggesting that under the projected ocean acidification and global warming, fragile ecosystems of the Mediterranean Sea may experience loss of biodiversity.

This project is the first to offer a comprehensive view on the physical, biological and socio-economic impacts of ocean acidification in the Mediterranean area using a so-called scale-basin approach. On the basis of this, further evidence of the social costs of human emissions of carbon dioxide can be obtained. In addition, the results will allow us to identify particularly sensitive areas and to formulate effective and cost-effective adaptation policies and strategies at different scale levels.

Project coordinator: Patrizia Ziveri,

Project manager: Rahiman Abdullah,

For more information, please visit or the information outlet on Ocean Acidification, Climate and Environmental Change in the Mediterranean Sea at

Patrizia Ziveri, IMBER Newsletter Issue n°20 – May 2012. Article.

  • Reset


OA-ICC Highlights

%d bloggers like this: