Habitat in grasslands of Oceanic Posidonia

Photo by Alessandro Tommasi

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

I rhizome they grow both horizontally (plagiotropic rhizome) and vertically (orthotropic rhizome). The plagiotropic rhizomes have the function of anchoring the plant to the substrate, thanks to the presence of roots on the underside, and of allowing the colonization of new areas. On the other hand, the orthotropic rhizomes, growing in height, contrast the progressive silting up due to sedimentation. The vertical development determines a progressive raising of the bottom, which gives rise to a typical formation called matte. The matte it consists of the intertwining of several layers of rhizomes and roots of old plants and the sediment trapped between these elements.

Le leaves they are deep green in color, ribbon-like and with rounded tips. The leaves are arranged in bundles, each of which contains about 6 or 7, and are distributed in a fan shape: the oldest, of greater length, are located outside the bundle, while the younger, smaller in size, are located in the internal.

P. oceanica has asexual and sexual modes of reproduction. Asexual reproduction occurs through stolonization, that is, through the multiplication and growth of plagiotropic and orthotropic rhizomes. Sexual reproduction occurs through the production of inflorescences bearing 3-5 hermaphrodite flowers. Come on fiori mature i fruits which, detached from the plant, float until the pericarp breaks, which releases the seed, from which a new plant will develop.

When P. oceanica meets favorable environmental conditions, colonizes vast areas of the seabed, forming large expanses called grasslands. The grasslands extend from the surface to about 30-35 meters deep, going beyond 40 meters in particularly clear waters. They are characterized by variable density, usually decreasing with increasing depth.

P. oceanica it is commonly present on mobile substrates such as sand, sometimes mixed with mud, but it can also be found on detrital and rocky bottoms. Regarding the extent, a prairie is defined by a upper limit and one lower limit. The upper limit corresponds to the most superficial bathymetry at which the prairie begins and is always very clear, while the lower limit, that is the deepest bathymetry at which the prairie ends, can have different conformations (Meinesz and Laurent, 1978; Pergent et al., 1995).

Le grasslands di P. oceanica play an extremely important role in protecting the coastal environment, as they stabilize movable bottoms and they defend the coasts from erosion. The waves and currents are in fact damped by the braking action of the matte and leaves and the sediment in transit is retained in part by the leaves and the rhizome system.

The prairies of P. oceanica they also represent a very 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 superposition of different populations: the photophilic one associated with the leaf layer, and the sciaphilous ones associated with the rhizomes and the matte (Mazzella et al., 1989; stems et al., 1992; Dark et al., 2003). The species associated with the leaf layer are often present only on P. oceanica; the species associated with the rhizomes, on the other hand, do not present such peculiar exclusive and characteristic elements, as they are similar to the sciaphilic species of the infralittoral or circalittoral coralligenous, depending on the corresponding depth and quantity of light (Boudouresque, 1968; Piazzi et al., 2002). The ecosystem a P. oceanica also constitutes area of nursery for fish fry and represents a refuge for a large number of organisms, including numerous species of fish, molluscs, cephalopods and crustaceans, also of considerable economic importance (Francour, 1997).

The prairies of P. oceanica they are finally characterized by high oxygen production, high plant biomass and a primary production among the highest in the world, for the marine environment (Pergent et al., 1994; Pergent-Martini et al. 1994). In addition, the prairies of P. oceanica they are capable of sequestering large amounts of carbon and therefore large amounts of carbon dioxide from the atmosphere (Mcleod et al. 2011), thus managing to significantly oxygenate the coastal waters.

The main causes of regression of the prairies are to be connected to growing anthropogenic pressure agent on the coastal environment (such as water pollution, the construction of coastal works, the laying of submarine cables and pipelines) which determines effects on the prairie essentially attributable to changes in turbidity of the water column and variations in sedimentation rates . In particular, theincreased turbidity, with the consequent reduction of the transparency of the water, it reduces the photosynthetic capacity of the plant and is one of the most frequent causes of regression of the grasslands (Larkum and West 1983; Duarte 1991). There variation of sedimentary rates below the coast, induced by the construction of coastal works can also create serious problems for the survival of the prairies, in the first case favoring the silting up and consequent suffocation (Marbà and Duarte 1997; Manzanera et al., 1998), in the second by promoting the undermining of rhizomes and thus making the prairie more sensitive to erosive phenomena (Jeudy de Grissac, 1979; Astier, 1984).

Significant impacts on grasslands are also attributable to trawling activities. Specifically, the repeated use of towing gear on the seabed largely reduces the density and coverage of plants, through the breaking of the rhizomes, the eradication of the leaf bundles and the weakening of the matte. In addition, trawling suspends sediments and alters substrate structure, increasing turbidity while affecting photosynthesis. The slow regrowth of seagrasses also prolongs the impact of bottom trawling, which can sometimes last decades (González-Correa et al. 2005; Boudouresque et al., 2006; Luff et al., 2019).

Due to its peculiar ecological characteristics the grassland habitat by P. oceanica represents a Hotspot of Mediterranean biodiversity and is recognized as a "priority habitat" sensu Directive 92/43 / EEC (Habitat Directive), while at the species level Posidonia it is protected by various international conventions (Barcelona Convention-SPA / BIO Protocol and Berne Convention).