Posidonia oceanica (L.) Del is a seagrass plant, endemic to the Mediterranean Sea, belonging to the family Posidoniaceae. It grows up as large underwater meadows both on sandy and rock sea bottoms. It is made up of roots, through which it clings to sea bottom, a woody trunk acting as body reserve (rhizome), and long ribbon-like leaves that are grouped into tufts. The leaves are bright green and they are up to 1.5 metres long. The younger leaves born mainly in Spring, are inside the tuft and remain active until Autumn when they lose their photosynthetic capacity, become brownish and begin to detach from the rhizome.
Meadows of posidonia are included in the list of the priority habitats protected by the Habitats Directive of the European Community (Council Directive 92/43/EEC). The European countries that have been designated as Sites of Community Importance (SCIs) for the protection of the priority habitat of “posidonia meadows” are Italy, Spain, Greece, France, Cyprus, Slovenia and Malta. Meadows of Posidonia oceanica (L.) Del play a key role in the marine ecosystem and ecology of the shoreline for the favorable habitat offered to a wide range of fauna that makes the meadows provide a sort of protection and shelter from predators, they can produce enormous amounts of oxygen and seagrass biomass. In addition, the meadows of posidonia in good health play a role of mechanical protection from the coastal erosion and are an indication of the excellent quality of coastal waters since they can respond to many factors that cause water pollution, including human activities. Nowadays the meadows, at least in some areas of the Mediterranean, are in strong regression due to many human interventions such as, for example, trawling, anchoring operations for pleasure crafts, and works to realize port and tourism facilities; a further threat is still represented by water pollution. In the autumn-winter period the vegetation of posidonia renews itself and part of the senescent leaves are transported by the currents and wash up on the beaches next to the meadows where residues deposit in large amounts as detritus, giving rise to the natural phenomena of putrefaction. Along the coasts concerned it is possible to find heaps of residues of posidonia as leaves and fibers.
The heaps of stranded posidonia residues and other biomass (seaweeds and seagrasses) play a key role in the shoreline ecosystem. For example, since these residues retain significant amounts of sand, they reduce the effects of coastal erosion along the stretches of sandy beaches and contribute to the formation and maintenance of the coastal dune system. Moreover, with their natural mechanical degradation and decomposition they provide an important source of nutrients for the formation of the dune vegetation. The deposits make bathers, tour operators and owners of pleasure crafts feel uneasy. For this reason, in the bathing facilities, in marinas for pleasure crafts and in the cities, municipalities remove these deposits at regular intervals during the ordinary and extraordinary cleaning of beaches, and landfill the material collected. However, these practices have some negative implications: firstly, the removal of accumulations of posidonia harms the fragile coastal ecosystem. In fact, stranded posidonia is an important element for the life cycle of the beaches and it influences the dynamics of the coastal profile. In addition, landfilling is a problem because of the heavy environmental and economic impacts involved. Landfilling should be severely limited in favor of alternative forms of management. Thanks to the project “Posidonia Residues Integrated Management for Eco-sustainability” (P.R.I.M.E.) some criteria for the sustainable management of these biomasses were identified, directing the scientific activity towards the re-use of these residues as an alternative to landfilling. In particular, some studies were carried out on the potential use of posidonia residues for the production of compost.