Posidonia oceanica (L.) Delile is a marine phanerogamous plant endemic to the Mediterranean Sea; it has roots, stems (called rhizomes) and leaves, and produces flowers and fruits.

Rhizomes grow both horizontally (plagiotropic rhizomes) and vertically (orthotropic rhizomes). Plagiotropic (creeping) rhizomes have roots on the underside which serve to anchor the plant to the substrate, allowing new areas to be colonised. Orthotropic (erect) rhizomes, however, effectively combat the ongoing siltation caused by sedimentation as they grow upward. Vertical development leads to a progressive raising of the soil, which creates a typical formation called “matte” consisting of interlaced remnants of roots, several layers of rhizomes and entangled sediment.

The leaves are dark green, ribboned and with rounded tips. The leaves are arranged in clusters of about 6 or 7, and are fan-shaped: the older, longer leaves are on the outside of the cluster, while the younger, smaller leaves are on the inside. P. oceanica reproduces both asexually and sexually. Asexual reproduction occurs by stolonisation, i.e. the multiplication and growth of plagiotropic and orthotropic rhizomes. Sexual reproduction is by the production of inflorescences with 3-5 hermaphrodite flowers. The flowers ripen into fruits which float detached from the plant until the pericarp breaks, releasing the seed from which a new plant will develop.

When P. oceanica finds favourable environmental conditions, it colonises large areas of the seabed, forming vast expanses known as meadows. The seagrass beds extend from the surface down to a depth of about 30-35 metres, reaching over 40 metres in particularly clear waters. They are characterised by variable density, usually decreasing with depth.

When P. oceanica encounters favorable environmental conditions, it colonizes vast areas of the seafloor, forming extensive meadows. These meadows extend from the surface to approximately 30-35 meters in depth, reaching beyond 40 meters in particularly clear waters. They are characterized by variable density, which normally decreases with increasing depth.

P. oceanica is commonly found on mobile substrates such as sand, sometimes mixed with mud, but can also be found on detrital and rocky bottoms. The extent of a seagrass meadow is defined by an upper and a lower boundary. The upper limit corresponds to the shallowest bathymetry where the seagrass begins and is always well defined, while the lower limit, i.e. the deepest bathymetry where the seagrass ends, can have different conformations (Meinesz and Laurent, 1978; Pergent et al., 1995).

Posidonia oceanica meadows play an extremely important role in protecting the coastal environment by stabilising the shifting seabed and protecting the coastline from erosion. Matte and leaves mitigate the impact of waves and currents, and sediment in transit is partially retained by the leaf and rhizome system.

Posidonia oceanica constitute a highly intricate and structured biocenosis, showcasing a diverse array of plant and animal communities. (Buia et al., 2000). Within this biocenosis, distinct populations overlap: a photophilic one linked to the leaf layer and sciaphilic ones associated with rhizomes and matte. (Mazzella et al., 1989; Gambi et al., 1992; Buia et al., 2003). The species inhabiting the foliar layer are typically exclusive to Posidonia oceanica, whereas rhizome-associated species share similarities with those found in the infralittoral or circalittoral coralligenous habitats, contingent upon depth and light availability. (Boudouresque, 1968; Piazzi et al., 2002). The Posidonia oceanica ecosystem serves as both a nursery area for juvenile fish and a refuge for numerous organisms, including various species of fish, molluscs, cephalopods, and crustaceans, which are also economically important. (Francour, 1997).

Finally, P. oceanica meadows are characterised by high oxygen production, high plant biomass and some of the highest primary production in the world for the marine environment (Pergent et al., 1994; Pergent-Martini et al., 1994). Furthermore, Posidonia oceanica seagrass meadows play a crucial role in sequestering substantial amounts of carbon dioxide from the atmosphere (Mcleod et al., 2011), leading to significant oxygenation of coastal waters.

The primary causes of seagrass meadow regression stem from escalating anthropogenic pressures on coastal environments, such as water pollution, coastal infrastructure development, and submarine cable and pipeline installations. These factors impact seagrass meadows primarily by altering water turbidity and sedimentation rates. Increased turbidity reduces water transparency, diminishing the plant's photosynthetic capacity, and is a common cause of meadow regression (Larkum and West, 1983; Duarte, 1991). Changes in sedimentation rates induced by coastal developments can also pose significant challenges to meadow survival. They may lead to siltation and subsequent suffocation of the meadows, or undermine rhizomes, increasing vulnerability to erosive phenomena (Marbà and Duarte, 1997; Manzanera et al., 1998; Jeudy de Grissac, 1979; Astier, 1984).

Trawling also has a significant impact on seagrass meadows. In particular, the repeated use of trawl gear on the seabed greatly reduces plant density and cover by breaking rhizomes, removing leaf bundles and weakening the matte. Trawling also suspends sediment and alters the structure of the substrate, increasing turbidity and interfering with photosynthesis. The slow regrowth of marine phanerogams also prolongs the effects of trawling, which can sometimes last for decades (González-Correa et al. 2005; Boudouresque et al., 2006; Luff et al., 2019).

Due to their unique ecological characteristics, Posidonia oceanica meadows represent a Mediterranean biodiversity hotspot and are recognised as a 'priority habitat' under Directive 92/43/EEC (Habitats Directive), while at species level Posidonia is protected under several international conventions (Barcelona Convention - SPA/BIO Protocol and Bern Convention).