In the spring of 2022, with funding from the Argolic Environment Foundation, the Greek eNGO iSea visited the Argolic Gulf to conduct the first ever high-resolution mapping efforts of the Posidonia oceanica (Neptune grass) meadows off the coasts of the islands of Spetses and Velopoula (aka Parapola).
The final report for this effort will be published in the near future, however below is the first map produced by these efforts. Constructed using Google My Maps, we aim for this to be of use to sailors and other users of the marine environment who wish to help conserve these vital ecosystems.
Posidonia oceanica meadows are as valuable to the Mediterranean as coral reefs are to tropical seas. They are key habitats for a multitude of species including iconic and threatened species such as seahorses. They protect juvenile fish from predation, coastal areas from erosion, and can trap 15 times as much carbon as the same expanse of Amazon rainforest!
Sailors are strongly encouraged to avoid anchoring near Posidonia meadows to avoid their anchors and chains from damaging the meadows. Fishing should also be avoided above the meadows, and no waste (including organic waste) should be discharged into the sea near the meadows.
The mapping of Posidonia oceanica was undertaken as part of iSea’s Reposidonia project, co-funded by the Argolic Environment Foundation, the Ionian Environment Foundation, Blue Marine Foundation and Kyma Sea Conservation and Research
Why we mapped the Posidonia meadows
The islands of the Argolic Gulf have had high tourism development in the past decades, especially for sailing boats, as they are an easily accessible destination from highly urbanized areas (i.e. Athens and Piraeus). These facts were suspected to have an immediate impact on the local meadows, which to date haven’t been properly mapped and evaluated.
The goal of the project is to accurately map the meadow’s extent in the coastal zones of the islands of Spetses and Velopoula. The mapping was accomplished by utilizing high-resolution satellite imagery. The team visited sites suspected to have a high density of meadows. The fieldwork resulted in the acquirement of specific ground-truthing points with healthy, dead and mixed (with other phanerogam plants such as Cymodocea nodosa and Zostera marina) P. oceanica meadows, that were used to train and test the accuracy of a mapping model, based on the coloration of each the satellite image in each location.
The mapping effort will set the road for further management actions and will help identify priority meadows in a region that is heavily disturbed by anthropogenic activities.
About Posidonia oceanica
Posidonia oceanica, is a phanerogam plant that lives in the marine environment and is endemic to the Mediterranean Sea. Also known as Neptune Grass, it is one of the most common species of marine plants in the Mediterranean, along with Cymodocea nodosa, and Zostera marina. The length of its leaves can reach up to 1.2 meters and the density of its leaves up to 10,000/m2. It is found in the Infra-littoral zone at depths of up to 40 meters.
This seagrass is observed in rocky to sandy bottoms where it can root. In Greece, Neptune Grass is present along the majority of the mainland coasts, and the Greek islands. Posidonia oceanica meadows are considered one of the most important marine habitat types of the Mediterranean Sea, providing a multitude of ecosystem services.
They are nursery grounds and living areas for a large number of species providing food sources and hunting grounds in its nutrient and oxygen-rich waters. Neptune Grass is an “ecosystem engineer” as it stabilises the sediment with its roots and changes the hydrodynamic status of the sublittoral zone, by limiting the power of waves and currents.
Both the leaves and the sediment beneath the meadows can withhold large amounts of carbon, making the meadows one of the most important habitats for sequestrated blue carbon.
Threats and protection
The main threat to these meadows is habitat degradation by human activities. The degradation of the meadows is attributed to a series of stresses on the species, these include fishing and aquaculture, coastal development, and uncontrolled anchorage.
Due to its importance, Neptune Grass is protected by the Barcelona Convention, the Bern Convention, the Habitats Directive (92/43/EEC), and the EU Regulation (1967/2006/EC) which prohibits fishing with dynamic fishing gear over the meadows.
Despite all national and international protections, P. oceanica is on the IUCN Red List of Threatened Species since November 2010, with its status listed as “Least Concern”, mainly due to its vast distribution throughout the Mediterranean coasts.
Although it is a long-living plant, its extremely slow growth rate makes it highly vulnerable to external disturbances, thus hindering the natural recovery of the meadows if degraded. In the Mediterranean basin, there has been a decrease in P. oceanica, with estimations averaging between 11% and 50% of area coverage lost in the past 50 years.
Despite the fact that a lot of scientific research has been conducted on P. oceanica, more information on species distribution and its threats in the Eastern and Southern Mediterranean basins is needed.