Reposidonia in Poros and Methana
GRANTEE
iSea
AMOUNT AWARDED
€17.000
Co-funded by
The Rauch Foundation
AREAS
Methana peninsula & Poros island
START DATE
02/2024
END DATE
12/2024
STATUS
Completed
PARTNERS
- Local fishers and residents – interviews and field insights
- Katheti: A cultural and environmental NGO from Poros
- External scientific collaborators
- Local municipalities of Poros and Methana
THE CHALLENGE
A data gap in a high-pressure coastal region
The coastal areas of Poros Island and the Methana Peninsula represent some of the most pressured and understudied marine environments in the Argosaronic Gulf. Despite hosting Posidonia oceanica meadows -an ecosystem of high ecological and legal importance- these areas lacked detailed, reliable, and up-to-date scientific data on the presence, extent, and condition of this habitat.
Why Poros and Methana were selected
Poros and Methana were selected for this research because they combine high ecological value with intense human pressure. Both areas experience heavy seasonal tourism, frequent recreational boating, and long-standing aquaculture activity. These pressures pose significant risks to Posidonia meadows through physical damage, increased pollution, and long-term seabed degradation
At the same time, parts of the Methana marine area fall within the Natura 2000 network, while both regions are subject to marine spatial planning and aquaculture expansion proposals. However, environmental assessments and planning processes have historically relied on low-resolution or outdated maps, or on assumptions that Posidonia is absent from certain areas – assumptions that had never been verified through systematic fieldwork
The consequences of missing scientific evidence
Important decisions affecting coastal development, aquaculture siting, and conservation were being made without knowing where Posidonia meadows actually exist, how fragmented they are, or how close they are to human activities.
As a result, Posidonia meadows in Poros and Methana risked continued degradation or permanent loss without being properly documented, considered, or protected.
Addressing this gap was essential – not only to understand the current state of Posidonia in these areas, but also to provide the scientific foundation needed for informed management, effective protection, and sustainable planning in one of the most heavily used coastal zones of the Argosaronic Gulf.
THE SOLUTION
Addressing the core barrier to protection
In Poros and the Methana Peninsula, the primary barrier to protecting Posidonia oceanica meadows has been the absence of detailed, local-scale scientific data. To address this gap, iSea implemented a comprehensive mapping and assessment approach designed specifically for these two high-pressure coastal areas.
A science-based mapping approach
The project combined systematic field surveys, underwater observations, and high-resolution satellite imagery to accurately map where Posidonia meadows exist, how extensive they are, and how close they are to human activities. This approach moved beyond outdated or low-resolution maps and provided, for the first time, a reliable and verifiable picture of Posidonia distribution around Poros and Methana
In Methana, where parts of the marine area fall within the Natura 2000 network, the mapping produced concrete evidence of Posidonia presence and condition in relation to existing and proposed aquaculture activities. In Poros, a highly touristic area, the research revealed fragmented and vulnerable meadows in locations subject to intense anchoring and coastal use. In both cases, the data clearly showed that Posidonia meadows are closer to human pressures than previously assumed, and in some areas have already suffered irreversible loss
Revealing long-term and hidden losses
By also examining historical aerial photographs, the research documented where Posidonia meadows existed in the past but have since disappeared, demonstrating long-term degradation that had never been formally recorded. This historical perspective is critical for understanding cumulative impacts and for preventing further loss.
Turning data into action
The outcome of this work is not just a set of maps, but a practical decision-making tool. The data produced can directly support:
- more accurate environmental impact assessments
- improved marine spatial planning and aquaculture siting
- targeted protection and restoration actions
- stronger enforcement of existing legal protections
From invisible habitat to protected asset
For Poros and Methana, this solution transforms Posidonia meadows from an invisible ecosystem into a clearly identified and manageable natural asset. It provides local authorities, policymakers, and communities with the evidence needed to protect what remains and to avoid repeating past mistakes.
THE RESULTS
Building the scientific baseline
Through systematic field surveys, underwater observations, and high-resolution satellite mapping, iSea produced the most detailed and up-to-date picture to date of Posidonia oceanica meadows in the areas of Poros Island and the Methana Peninsula.
Fragmented and vulnerable meadows
The research revealed that Posidonia meadows in the Argosaronic region are fragmented and unevenly distributed, rather than forming continuous underwater forests. Meadows were found mainly along specific coastal stretches, while large areas previously assumed to host Posidonia were either sparsely covered or completely degraded.
Human activities as a key driver of degradation
A key finding was the clear link between human activities and meadow degradation. In areas close to aquaculture facilities, Posidonia was either absent or found in a degraded state, often mixed with “dead meadow” remains – evidence that healthy meadows existed there in the past but have since collapsed. Even in locations where aquaculture activities stopped years ago, recovery was extremely limited or nonexistent, highlighting the very slow or irreversible nature of Posidonia loss.
Impacts extend beyond expected limits
The study also showed that Posidonia meadows are affected well beyond the immediate footprint of human activities. Meadows were found at distances that exceed those typically considered safe in environmental impact assessments, indicating that current buffer zones and planning assumptions may significantly underestimate real impacts
Revealing long-term and hidden losses
By combining present-day mapping with historical aerial photographs, iSea documented substantial long-term losses of Posidonia meadows in several coastal bays. In some cases, areas that once supported extensive meadows have been reduced to small, isolated patches or have disappeared entirely, without showing signs of natural recovery
Making the invisible visible
Overall, the research confirmed that the lack of detailed, local-scale scientific data has masked the true extent of Posidonia degradation. Without this mapping effort, many of these losses would have remained undocumented and unaccounted for in management and planning decisions.
These findings demonstrate why systematic, science-based mapping is essential: it transforms Posidonia from an “invisible” ecosystem into a clearly defined, measurable, and protectable natural asset.
THE OUTCOMES
Detailed habitat maps for Poros and Methana
Completed 2 detailed habitat maps for Poros and Methana, based on 800+ validation points collected via dives, snorkelling, and ROV. Preliminary results revealed larger-than-expected meadows in several areas.
Case study areas linking posidonia degradation with aquacultures
Were analysis showing measurable meadow loss linked to aquaculture (up to 0.065 km² lost in a single bay). Historical evidence of seagrass decline in aquaculture zones was identified through analysis of 1972 aerial photographs and seabed remains.
Residents, fishers, and volunteers were reached
Through direct participation and events.
Local government representatives and NGOs were engaged
in data dissemination
Local government representatives and NGOs were engaged
in data dissemination
Infographic posters
Prepared for public awareness.
Brochures
Prepared for public awareness.
Overall the project successfully created baseline data for long-term monitoring, replication, and effective advocacy. Beyond data generation, the project produced practical tools for decision-makers.
THE DELIVERABLES
Final Report
Layman’s Report
Poros Map
Methana Map
AFTERMATH
Rejection of Aquaculture expansion in Poros
On August 17th, 2025, Greece’s Central Council for Urban Planning and Disputes of the Ministry of Environment (KESYPOTHA) unanimously rejected the proposed establishment of an extensive Aquaculture Development Zone (POAY) on the island of Poros.
ABOUT OUR GRANTEE
iSea is a Greek NGO founded in 2016, dedicated to protecting aquatic ecosystems through science, awareness, and policy advocacy. Its work focuses on vulnerable species, marine protected areas, aquatic litter, and the human–water interface.
iSea has become one of the leading organisations mapping Posidonia oceanica in Greece, combining cutting-edge methods with citizen science and local partnerships. The REPOSIDONIA initiative has already expanded across multiple regions and continues to build momentum for national-level marine conservation policies.