Posidonia oceanica seagrass habitat

Oceanic Posidonia

Oceanic Posidonia (L.) Delile is a sea phanerogam, endemic to the Mediterranean Sea; it is organized into roots, stem (called rhizome) and leaves and produces flowers and fruits.

The rhizomes grow both horizontally (plagiotropic rhizomes) and vertically (orthotropic rhizomes). Plagiotropic rhizomes function to anchor the plant to the substrate, thanks to the presence of roots on the lower side, and allow for the colonization of new areas. On the other hand, orthotropic rhizomes, by growing vertically, counteract the progressive burial caused by sedimentation. The vertical growth leads to a progressive rise of the seabed, giving rise to a typical formation called "matte." The matte is formed by the interweaving of multiple layers of rhizomes and roots of old plants and the sediment trapped between these elements.

The leaves are intensely green, strap-shaped, and have rounded tips. The leaves are arranged in bundles, each containing approximately 6 or 7leaves, and are fan-shaped: the older, longer leaves are located on the outer side of the bundle, while the younger, smaller ones are found on the inner side.

P. oceanica exhibits both asexual and sexual reproduction. Asexual reproduction occurs through stolonization, which involves the multiplication and growth of plagiotropic and orthotropic rhizomes. Sexual reproduction occurs through the production of inflorescences bearing 3-5hermaphroditic flowers. The fruits mature from the flowers, and when detached from the plant, they float until the pericarp breaks, releasing the seed from which a new plant will develop.

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 occur on detrital and rocky bottoms. Regarding their extent, a meadow is defined by an upper limit and a lower limit. The upper limit corresponds to the shallowest depth at which the meadow begins and is always well-defined, while the lower limit, which is the deepest depth at which the meadow ends, can have different configurations (Meinesz and Laurent, 1978; Pergent et al., 1995).

Posidonia oceanica seagrass meadows play an extremely important role in the protection of the coastal environment as they stabilize mobile sediments and defend the coasts from erosion. Waves and currents are dampened by the braking action of the matte (seagrass mat) and leaves, and sediment in transit is partially retained by the leaves and rhizome system.

Posidonia oceanica meadows also represent a highly complex and well-structured biocenosis, characterized by high biological variability of the plant and animal communities that compose it (Buia et al., 2000). This biocenosis consists of the overlapping of different populations: the photophilic community associated with the leaf canopy, and the sciaphilic communities associated with the rhizomes and the matte (Mazzella et al., 1989; Gambi et al., 1992; Buia et al., 2003). Species associated with the leaf canopy are often unique to P. oceanica, while those associated with the rhizomes do not have exclusive and characteristic elements, as they are similar to the sciaphilic species of the infralittoral or the circumlittoral coralligenous, depending on the depth and light conditions (Boudouresque, 1968; Piazzi et al., 2002). The P. oceanica ecosystem also serves as a nursery area for fish fry and provides refuge for a large number of organisms, including numerous species of fish, mollusks, cephalopods, and crustaceans, many of which have significant economic importance (Francour, 1997).

P. oceanica meadows are also characterized by high oxygen production, high plant biomass, and one of the highest primary productions in the marine environment worldwide (Pergent et al., 1994; Pergent-Martini et al., 1994). Furthermore, P. oceanica meadows are capable of sequestering large amounts of carbon and, therefore, substantial amounts of carbon dioxide from the atmosphere (Mcleod et al., 2001), thus significantly oxygenating coastal waters.

The main causes of regression of the meadows are related to the increasing human pressure on the coastal environment, such as water pollution, coastal construction works, and the laying of underwater cables and pipelines. These factors mainly affect the meadows through changes in water turbidity and sedimentation rates. In particular, increased turbidity, resulting in reduced water transparency, diminishes the photosynthetic capacity of the plant and is one of the most frequent causes of regression of the meadows (Larkum and West, 1983; Duarte, 1991). Changes in sedimentation rates induced by coastal construction works can also pose serious problems for the survival of the meadows, promoting burial and subsequent suffocation in some cases (Marbà and Duarte, 1997; Manzanera et al., 1998), or destabilizing the rhizomes and making the meadows more susceptible to erosion (Jeudy de Grissac, 1979; Astier, 1984).

Significant impacts on the meadows are also attributable to bottom trawling activities. Specifically, the repeated use of trawling gear on the seafloor greatly reduces the density and coverage of the plants through rhizome breakage, uprooting of leaf bundles, and weakening of the matte. Moreover, trawling suspends sediments and alters the substrate structure, increasing turbidity and affecting photosynthesis. The slow regrowth of marine phanerogams further prolongs the impact of trawling, which can sometimes last for decades (González-Correa et al., 2005; Boudouresque et al., 2006; Luff et al., 2019).

For its unique ecological characteristics, the P. oceanica seagrass meadow habitat is recognized as a hotspot of Mediterranean biodiversity and is designated as a "priority habitat" under the Habitats Directive 92/43/CEE. In terms of species, Posidonia is protected by several international conventions, such as the Barcelona Convention-SPA/BIO Protocol and the Bern Convention.