The Mediterranean Sea, known for its rich biodiversity and its significance to regional economies, is facing growing pressures due to climate change and increasing water demand. New research emphasizes the urgent need for integrated water resource management, underscoring the critical link between decreased river flow into the Mediterranean and the health of its marine ecosystems. This research highlights the importance of safeguarding the Mediterranean’s biodiversity and the economies it supports, particularly in light of the growing threat of water scarcity.
Climate Change, Water Stress, and Their Impact on Freshwater Flow
Climate change has profound effects on freshwater resources across Europe, with more frequent and severe droughts exacerbating water stress. Water stress occurs when the demand for water exceeds the available supply, a situation that affects 20% of Europe’s territory and 30% of its population each year. The Mediterranean region, in particular, is facing significant challenges, as climate change is expected to worsen both freshwater flow and water availability, particularly in southern and southwestern Europe.
The Mediterranean Sea, which relies heavily on freshwater input from rivers for its health, is already seeing reduced river flow, which is directly impacting the marine ecosystems. The Po River in Italy, a major river flowing into the Mediterranean, has been a particularly notable example of this trend. Over the past 23 years, the Po River has seen critically low levels of water flow multiple times, with its water volume halved on at least five occasions. The drop in flow reached a historic low of just 39% of its average in 2022. Such reductions in river flow have far-reaching consequences for the Mediterranean ecosystem, which relies on a steady input of freshwater for its biogeochemical cycles, nutrient distribution, and primary productivity.
Investigating the Impact of Decreased River Flow on Marine Ecosystems
In a new study led by scientists at the European Commission’s Joint Research Centre (JRC), the impact of decreased river flow on marine ecosystems in the Mediterranean was analyzed. Published in Nature Communications, the study highlights the need to address the consequences of reduced freshwater input from rivers, which is projected to become more severe under the unchecked climate change scenario.
The study modeled a scenario where global temperatures rise by 4°C above pre-industrial levels, a scenario defined by the United Nations as the RCP 8.5 warming pathway, and found that river flow into the Mediterranean could decrease by up to 41%. This extreme scenario, if realized, would dramatically reduce the supply of freshwater and nutrients into the Mediterranean Sea, altering the biogeochemistry and productivity of its marine ecosystems.
Altered Ecosystems and Economic Consequences
Decreased river flow into seas like the Mediterranean results in a significant alteration of the coastal ecosystems. Freshwater serves a vital role in maintaining the nutrient balance in these ecosystems, supporting primary productivity at the base of the food chain. The loss of freshwater and nutrients would lead to a decline in primary productivity, which would ripple through the food web, impacting not only small organisms but also commercially important species, such as fish and shellfish.
The study reveals that a 41% reduction in river flow could reduce marine productivity by 10% and fish biomass by 6%. For the Mediterranean fishery sector, this translates to an annual loss of approximately €4.7 billion. Such a decline in productivity would have devastating socio-economic consequences, especially for coastal communities and the fisheries that depend on a healthy marine ecosystem.
Particularly vulnerable regions include the Adriatic and Aegean Seas, two of the Mediterranean’s most heavily fished areas. The study forecasts that these regions could see productivity reductions of 12% in the Adriatic and a staggering 35% in the Aegean. These areas are vital to the region’s economy, as they are among the most intensely fished in the Mediterranean. For many fishing communities, this loss could be catastrophic, as it would threaten both livelihoods and food security.
The Role of the JRC’s Blue2 Modeling Framework
The study utilized the Blue2 Modeling Framework (Blue2MF), designed by the JRC, to assess the impact of reduced river flow on the Mediterranean’s biogeochemistry, food web, and socio-economic sectors. This modeling framework allowed the researchers to predict the effects of decreased freshwater input on marine ecosystems and the fisheries that rely on them.
The findings from the study demonstrate that the most productive and heavily fished regions of the Mediterranean, such as the Adriatic and Aegean Seas, face the highest risk of significant losses in marine productivity and fish biomass. In some cases, these biomass losses could exceed the current landings from fisheries, further exacerbating the potential socio-economic impacts.
Water Management: A Critical Element in Safeguarding Mediterranean Ecosystems
This new research provides crucial insights into the potential long-term effects of water scarcity on the Mediterranean’s marine ecosystems. The study underscores the need for integrated water resource management that accounts for the entire water cycle, from source to sea. Currently, many stakeholders involved in water management treat freshwater flowing into the ocean as “lost” water, neglecting its vital role in sustaining marine ecosystems. This oversight has significant consequences for both the environment and the economy.
While EU legislation such as the Urban Waste Water Treatment Directive, the Water Framework Directive, and the Marine Strategy Framework Directive recognizes the importance of environmental flows to maintain healthy coastal ecosystems, discussions around water management often fail to adequately account for the marine ecosystems’ needs. This gap in policy could have far-reaching consequences for both marine life and the human communities that depend on the Mediterranean’s resources.
In response to the growing challenge of water stress, the European Union has set ambitious climate targets to reduce emissions by at least 55% by 2030 compared to 1990 levels, with the goal of achieving climate neutrality by 2050. As part of this effort, the European Commission is preparing a Water Resilience Strategy aimed at improving water security, reducing flood risks, and mitigating the impacts of droughts. However, to ensure the long-term health of the Mediterranean’s marine ecosystems and the economies they support, policymakers must integrate freshwater management into their broader environmental and climate strategies.
Conclusion: The Need for Sustainable Water Management
The Mediterranean is at a crossroads. The effects of climate change, combined with growing demands for freshwater, threaten not only the health of the Mediterranean’s marine ecosystems but also the livelihoods of the people who depend on them. The research highlights the urgency of integrating water resource management into climate action strategies, ensuring that the health of both terrestrial and marine environments is preserved.
With the stakes so high, the time is now to implement sustainable water management practices that protect the entire water cycle. By doing so, we can safeguard the rich biodiversity of the Mediterranean Sea and the fisheries and coastal communities that depend on it, ensuring a more resilient and sustainable future for all.
Reference: Diego Macias et al, The overlooked impacts of freshwater scarcity on oceans as evidenced by the Mediterranean Sea, Nature Communications (2025). DOI: 10.1038/s41467-024-54979-4. www.nature.com/articles/s41467-024-54979-4