Posidonia: The forests of the sea in danger

Overview

Posidonia seagrass beds are protected habitats, which have a fundamental role in the health and productivity of Mediterranean marine ecosystems. Posidonia oceanica, a rooted, flowering plant, endemic to the Mediterranean Sea, is well adapted to the marine environment. It forms extensive meadows from the intertidal zone to depths down to 50-60 m, estimated to occupy between 25000 and 45000km2 of the Mediterranean basin. The NE Mediterranean still supports extensive areas of Posidonia meadows, in comparison with West Mediterranean Sea where they have been damaged or destroyed, to a large extent due to the urbanisation of the coastal areas. However, in the Greek seas, there are no conservation or protection measures. They are therefore one of the most important conservation priorities of Archipelagos.
Importance

Posidonia meadows play a crucial role as breeding and nursery grounds for numerous species. Over 300 species of plants and 1000 species of fauna live within Posidonia meadows, including a large number of commercially important fish species. Apart from supporting productive marine ecosystems and fisheries, seagrass beds also have great socio-economical importance, by providing protection against beach erosion. This is achieved through their rhizomes and leaves; the network of roots and rhizomes stabilises the sediments whereas the high density of leaves reduces wave energy.

Threats

Since Posidonia oceanica has a very slow growth rate (<10cm/year), it is very vulnerable to anthropogenic impacts and it is difficult to recover once destroyed. In the Aegean Sea, the main factors of threat include:

1. Trawl fishing and uncontrolled anchorage on sea grass beds, which cause a direct physical impact on the seabed and the organisms living on it.
2. The damage caused by trawlers and uncontrolled anchorage, gives the chance to opportunistic algae, such as the invasive green algae Caulerpa racemosa, to colonize new patches.
3. Fish farming produces a large amount of organic matter that settles on the seabed, altering the physio-chemical characteristics, which can affect the growth of Posidonia oceanica.
4. Coastal works, such as harbours, reduce currents and increase water turbidity. This minimizes water clarity and consequently reduces seagrass meadows.
5. Water pollution is caused by the increase of tourism and industry in all Mediterranean coasts.
6. Erosion of terrestrial ecosystems is caused by the removal of terrestrial vegetation, due to factors such as uncontrolled construction or intense overgrazing by livestock. In cases of severe erosion, following intense rainfall, the large amount of sediment entering marine ecosystems can cause significant impacts

Conservation

Sea grass meadows are protected by the following international conventions, the European and the Greek legislation:

• Bern Convention Annex I P. oceanica: Extremely protected species of flora.
• EU Habitats Directive 92/43/CEE of 21/05/1992 and posterior adaptation at 97/62 CE of 27/11/1997 include Posidonia oceanica, habitat 1120 as a priority in conservation.
• Barcelona Convention for the Protection of the Marine Environment Annex II P. oceanica as an endangered species.
• EU Fishing legislation: According to council regulation 1626/94 of 27/6/94, defining technical measures for the conservation of fishery resources in the Mediterranean, it is forbidden to fish with bottom trawls, seines or similar nets above Posidonia oceanica beds.
• National legislation: According to ministerial decision 167378, it is forbidden to fish with dredging gear over defined Posidonia oceanica beds.

Due to most of the mapping and ecology studies having been carried out in the western Mediterranean, combined with the lack of any conservation or management framework, protecting Posidonia meadows in the Aegean has been made nearly impossible. Therefore, a detailed knowledge of their location and their status is necessary in order to apply successful management measures, such as:

- Monitoring of fishing activities, and the promotion of sustainable fishing practices through the definition of fishing grounds for towed fishing gear (trawler, purse seiner, beach seiner), away from seagrass beds (as is defined by EU and national legislation).
- Monitoring recreational boating activities, and installation of environmentally friendly mooring systems in bays to solve the direct impact of anchoring.

Public awareness campaigns to schools, local communities and tourists, are also considered a great priority in order to promote the importance of Posidonia meadows and marine ecosystem conservation.