# The functioning of an iconic Mediterranean ecosystem, the Posidonia oceanica meadow: The keys to a success story

Charles-François Boudouresque, Patrick Astruch, Thomas Changeux, Michèle

Perret-Boudouresque, Sandrine Ruitton

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Charles-François Boudouresque, Patrick Astruch, Thomas Changeux, Michèle Perret-Boudouresque, Sandrine Ruitton. The functioning of an iconic Mediterranean ecosystem, the Posidonia oceanica meadow: The keys to a success story. Journal of Black Sea/Mediterranean Environment, 2025. hal-05468814

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# The functioning of an iconic Mediterranean ecosystem, the *Posidonia oceanica* meadow: The keys to a success story

Charles-François Boudouresque<fnref n="1" />*, Patrick Astruch<fnref n="2" />,
Thomas Changeux<fnref n="1" />, Michèle Perret-Boudouresque<fnref n="2" />,
Sandrine Ruitton<fnref n="1" />

ORCID IDs: C.F.B. 0000-0001-82067169; P.A. 0000-0002-6299-9484; T.C. 0000-0002-0418-3321; S.R. 0000-0003-4485-9979

## Abstract

*Posidonia oceanica* is a seagrass endemic to the Mediterranean Sea, where it is widespread and forms vast meadows down to 25-45 m depth. On the basis of its life history traits and the functioning of the key ecosystem of which it is the ecosystem engineer, the authors attempt to explore the reasons for the overwhelming success of this ecosystem and the ecosystem services it provides in a highly oligotrophic sea. The thickness of the matte, the structure formed by weakly degradable rhizomes and roots and by the sediment that fills the interstices, increases over time, so that the matte sequesters large amounts of carbon. Certain characteristics of the ecosystem make it similar to terrestrial forests, such as the very high plant biomass, the importance of the necromass, the preeminence of the detritivore pathway over the herbivore pathway, and the association of roots with fungi. A sophisticated strategy of collecting, trapping, storing, preserving, and even producing (thanks to association with N₂-fixing bacteria) inorganic nitrogen likely emerged as a response to the extreme oligotrophy of Mediterranean waters. The juxtaposition of two sets of primary producers, the seagrass itself that features weakly palatable and slowly degradable material, and leaf epiphytes that feature highly palatable material, constitute a further key to the success of the ecosystem. Like coral reefs, but through a very different strategy, the *P. oceanica* ecosystem thus supports lush life in nutrient-poor waters. Finally, The *P. oceanica* ecosystem provides humans with a variety of goods and ecosystem services of paramount importance; the most outstanding are (i) carbon sequestration partially mitigating human CO₂ emissions, (ii) the production of organogenic sand derived from the calcareous remains of organisms that lived in the meadow, part of which feeds the beaches, (iii) a significant direct and indirect contribution to fisheries landings, and (iv)

<footnote n="1">¹Aix-Marseille University and University of Toulon, MIO (Mediterranean Institute of Oceanography), IRD, CNRS, campus of Luminy, 13288 Marseille cedex 9, FRANCE</footnote>

<footnote n="2">²Aix-Marseille University, OSU Pytheas, GIS Posidonie, campus of Luminy, 13288 Marseille cedex 9, FRANCE</footnote>

<footnote>*Corresponding author: charles.boudouresque@mio.osupytheas.fr</footnote>
a major role in protecting beaches against erosion, directly through the reduction of wave and swell strength, and indirectly through the deposition of dead leaves on the beach. Overall, by interacting with all other coastal ecosystems, e.g., *via* the exportation of huge amounts of dead leaves, the *P. oceanica* ecosystem constitutes a sort of Mediterranean hub.

**Keywords:** Carbon sequestration, ecosystem functioning, ecosystem services, Mediterranean Sea, nitrogen, *Posidonia oceanica*

**Received:** 24.11.2025, **Accepted:** 04.12.2025

## Introduction

The seagrass *Posidonia oceanica* is endemic to the Mediterranean Sea (Boudouresque and Verlaque 2008). It occurs throughout the Mediterranean, with the exception of the Levantine coast, the northernmost and north-western Adriatic Sea, most of the Gulf of Lions (from the Rhône River mouth to French Catalonia) and the immediate vicinity of the Strait of Gibraltar; very localized populations have been reported from the Sea of Marmara (Meinesz et al. 2009; Pergent et al. 2012; Öz et al. 2014; Azcárate-García et al. 2023; Gönülal et al. 2023; Mateo-Ramírez et al. 2023). All other species of the genus *Posidonia* dwell in Australia, e.g., *P. australis*, *P. ostenfeldii* and *P. sinuosa* (Kuo et al. 1990; Spalding et al. 2003).

*Posidonia oceanica* forms vast meadows between the sea level and 25-45 m depth, depending upon water transparency (Molinier and Picard 1952; Augier and Boudouresque 1979; Boudouresque and Meinesz 1982; Pergent-Martini et al. 1999; Tomasello et al. 2001; Borg et al. 2006; Marbà and Duarte 2010) (Figure 1). For the entire Mediterranean, the surface area of *P. oceanica* meadows has been estimated as between 12,247 km² (Telesca et al. 2015) and 19,021 km² (Traganos et al. 2022). This represents 23% of the Mediterranean seabed (between the sea level and 50 m depth) (Pasqualini 2005) or 25% (between the sea level and 35 m depth) (Agulles et al. 2024), and up to 60% in the case of Corsica (between the sea level and 40 m) (Monnier et al. 2019; Pergent et al. 2019).

Here, on the basis of the life history traits of *Posidonia oceanica* and the functioning of the key ecosystem of which it is the ecosystem engineer, we propose avenues for understanding the overwhelming success of the ecosystem, its resilience, the considerable ecosystem services it provides and the crossroad it constitutes in a highly oligotrophic sea, the Mediterranean.

## A very long history

Seagrasses are Magnoliophyta, i.e., flowering plants, with roots and seeds; their ancestors were land plants. It was around 100-70 million years ago that they conquered the marine realm (Kuo and Hartog 2000; Orth et al. 2006); they arrived
there with all the 'technology' acquired on land, e.g., flowers, seeds, roots and an
efficient conduction system for long-distance transport of sap.

The genus *Posidonia* was present in the Tethys, an east-west ocean that stretched
from the Caribbean to eastern Asia. The divergence between Mediterranean and
Australian species would have occurred between 40 and 68 Ma ago (Aires *et al.*
2011; Cantasano 2023).

As for *P. oceanica*, it survived the Messinian crises (6.0-5.3 Ma), when the closure of communication with the Atlantic caused the partial drying up of the Mediterranean (Schroeder and Chiggiato 2023; Agiadi *et al.* 2024). The location of the refuges remains unknown: inside the Mediterranean, close to river freshwater inputs preventing hypersalinity, or outside in the nearby Atlantic? (Por 2009; Taviani 2002). Subsequently, *P. oceanica* also survived ~30 glaciation-deglaciation cycles, since 2 Ma, with a cooling of Mediterranean waters of 4-9°C during the last glacial maximum (LGM), more marked in the western than in the eastern Mediterranean; winter sea surface temperature (SST) was locally as low as 7°C (Cacho *et al.* 2002; Kuhlemann *et al.* 2008). At the same time, 18,000-21,000 years ago, the sea level dropped by 120-140 m (Lichter *et al.* 2010; Vacchi *et al.* 2017).

<figure><img src="image_2.png" /><figcaption>**Figure 1.** A *Posidonia oceanica* meadow. Cap d'Ail, French Riviera, in 2022. Photo © Thomas Schohn, GIS Posidonie</figcaption></figure>

Overall, *P. oceanica* is a species that generally outcompetes most benthic
organisms for substrate occupation in the infralittoral zone (*sensu* Pérès and
Picard 1964). Furthermore, contrary to the perceptions of some authors, whose
views are based on a short-term perspective, the species is very resilient to
disturbances on a long-term perspective (decades, centuries) and geological scale.
Probably excluded, due to low winter temperatures, from the northern
Mediterranean during the LGM, *P. oceanica* subsequently reoccupied this area;
at the same time, it shifted upwards to follow the rapid Holocene rise in sea level
(Boudouresque *et al.* 2021).

Some original features of the *Posidonia oceanica* ecosystem

A characteristic of the *P. oceanica* meadows is that the rhizomes and roots are not very degradable (Piñeiro-Juncal *et al.* 2020) and are preserved, after their death, so that the bottom rises slowly, from 10 to 100 cm per century (Molinier and Picard 1952; Lo Iacomo *et al.* 2008; Mateo *et al.* 2010; Rovere *et al.* 2010; Boudouresque *et al.* 2016) (Figure 2). The structure formed by rhizomes and roots, living and dead, and by the sediment that fills the interstices, is called ‘matte’ (Molinier and Picard 1952). The thickness of the matte increases over time; it is up to 8 m in eastern Corsica (Monnier *et al.* 2019), over 8 m in the Gulf of Hyères (Provence) (Jeudy de Grissac and Boudouresque 1985) and 16 m in Liguria (Italy) (Rovere *et al.* 2010) (Figure 3).

<figure><img src="image_3.png" /><figcaption>**Figure 2.** The rising of the *Posidonia oceanica* matte over time. a: Recently established meadow. b, c, d: Autochthonous and allochthonous sediment is trapped by the leaf canopy. To resist being buried, erect orthotropic) rhizomes grow vertically, which results in the thickening of the matte. Green: *P. oceanica* leaves. Ochre: living parts of the rhizomes. Red: dead parts of the rhizomes. Yellow: sediment. Grey: initial substrate (sand or reef).</figcaption></figure>
Four categories of ecosystems can be distinguished on the basis of nutrient availability (proxy: nitrogen – N) and primary production (proxy: chlorophyll – C), both roughly distinguished into low (L) and high (H) levels (Figure 4): LNLC (e.g., Mediterranean pelagic ecosystems and terrestrial forests), HNLC (e.g., Antarctic pelagic ecosystems), HNHC (e.g., upwelling pelagic ecosystems and agricultural systems) and LNHC. Mediterranean *P. oceanica* meadows belong to the latter category (Gobert *et al.* 2002; Boudouresque *et al.* 2006, 2014): a lush and highly productive ecosystem thriving in a highly oligotrophic sea. The mechanisms for this paradox will be analysed below. Another ecosystem shares this feature, the coral reef ecosystem, through an altogether very different strategy (Boudouresque *et al.* 2017a).

<figure><img src="image_4.png" /><figcaption>**Figure 3.** An erosion scarp in a *Posidonia oceanica* meadow, in Provence (France), showing the thickness of the *matte*. Note the wrecked leaf shoots, at the foot of the scarp, which shows that erosion of the scarp, due to the hydrodynamics, is still active. Photo © Sandrine Ruitton</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 4.** Categories of ecosystems, on the basis of nutrient availability (proxy: nitrogen – N) and primary production (proxy: chlorophyll – C), both roughly distinguished into low (L) and high (H) levels. Redrawn from Boudouresque *et al.* (2014).</figcaption></figure>
The most ‘terrestrial’ among marine ecosystems

The *P. oceanica* ecosystem can be considered as the most ‘terrestrial’ among marine ecosystems (Boudouresque *et al.* 2006, 2024). This is due to a set of characteristics considered to be unique in the marine realm, while more common on land, such as the relatively high biomass of primary producers (PPs), the low herbivore pressure, the importance of the detritus-feeder pathway and the most nutrients remaining inside the ecosystem, through their recycling (Boudouresque *et al.* 2014).

(1) The biomass of PPs in the *P. oceanica* ecosystem (rhizomes, roots, leaves, leaf epibiota and rhizome epbiota) can reach values similar to those recorded in land forest ecosystems but far higher than those recorded in the sea, except in some other seagrass ecosystems such as Australian *Posidonia* spp. and Indo-Pacific *Thalassodendron* spp. (Gomis *et al.* 2025). Underground biomass (rhizomes and roots) in the *P. oceanica* ecosystem can reach 1.8 kg Dry Mass (DM)/m² (Guidetti *et al.* 2002), 3.8 kg DM/m² (Francour 1985) and even 4.4 kg DM (Boudouresque and Jeudy de Grissac 1986); it decreases with depth (Olesen *et al.* 2002). Leaf biomass can reach 0.9 kg DM/m² in the Balearic Islands (Barrón and Duarte 2009), 1.3 kg DM/m² in Algeria (Boumaza and Semroud 1995) and 1.6 kg DM/m² in mainland Spain (Romero *et al.* 1998). Overall, including rhizome and leaf epibiota, the biomass of PPs can reach 6.4 kg DM/m² in Corsica (Boudouresque and Jeudy de Grissac 1986).

(2) The herbivore pathway (HP) corresponds to the percentage of direct consumption of the primary production; the detritus-feeder pathway (DFP) corresponds to the percentage of the primary production that is consumed in the form of dead products within the producer ecosystem. Land forest ecosystems are characterized by DFP, while subtidal marine ecosystems are usually characterized by HP. In the *P. oceanica* ecosystem, as in land forest ecosystems, consumption by macro-herbivores, mainly the teleosts *Sarpa salpa* and *Sparisoma cretense* and the sea urchin *Paracentrotus lividus*, usually represents less than 10% of the PP (Mazzella *et al.* 1993; Cebrían *et al.* 1997; Peirano *et al.* 2001; Vizzini 2009; but see Pergent *et al.* 1993; Prado *et al.* 2007; Ferrari *et al.* 2008). As a result, necromass accumulates (Boudouresque *et al.* 2016).

(3) The importance of the DFP in the *P. oceanica* ecosystem is a consequence of the low pressure of herbivores and the resulting availability of necromass (e.g., Gambi *et al.* 1997; Vizzini 2009; Boudouresque *et al.* 2016; Mascart *et al.* 2018). This feature further contributes to bringing it closer to that of land forests.

(4) In land forest ecosystems, most nutrients remain inside the ecosystem, either through their transfer from old leaves before shedding to perennial tissues, then to young leaves, or through recycling of the litter, the soil and the tree roots (Boudouresque *et al.* 2014). In the *P. oceanica* ecosystem, part of the dead leaves
does not remain within the litter and is exported to other ecosystems (e.g., Boudouresque et al. 1990; Pergent et al. 1994; Fourt and Goujard 2012; Boudouresque et al. 2016, 2017a; Oudin and Boudouresque 2023; Di Montanara et al. 2024; Boudouresque et al. 2025); export also occurs in the form of dissolved organic carbon (DOC) (Champenois et al. 2024); however, a variety of mechanisms maximize nutrient conservation within the ecosystem (see below).

## Further keys to success

The reasons for the paradox displayed by *P. oceanica*, very high primary production despite the very low nutrient concentrations which characterise most of the Mediterranean, may lie in the functioning of the nutrient machine within the ecosystem (Figure 5).

(1) The extraction by roots of nutrients buried within the sediment; these roots can go down to a depth of at least 70 cm (Francour 1985). (2) The association of roots with fungi (the ascomycete *Posidoniomyces atricolor*) (Vohník et al. 2015; 2016, 2017; Borovec and Vohník 2018; Vohník et al. 2019) increases their effectiveness in nutrient extraction; such an association is quite common among land trees but unusual in marine environments, with the exception of other seagrass ecosystems, such as the *Zostera marina* meadow (e.g., Ettinger and Eisen, Wainwright et al. 2019). (3) The uptake of nutrients both by roots and by leaves; their respective importance varies depending on the sites and the seasons (Kraemer and Mazzella 1996; Invers et al. 2002; Gobert et al. 2005). (4) The 'luxury uptake' of inorganic nitrogen, taken up in excess with respect to the plant's requirements, at a season when it is relatively abundant in the water column while other primary producers are still not very active due to the lack of light, e.g., in autumn and winter; the nutrients are then stored within the rhizomes for subsequent utilisation (Mateo et al. 1997; Invers et al. 2000; Lepoint et al. 2000; Vizzini et al. 2003; Romero 2004); very few primary producers practice luxury uptake in other marine ecosystems (Ricklefs and Miller 2005). (5) The trapping by the *P. oceanica* canopy of POM (Particulate Organic Matter) from the water column and its subsequent remineralization (Duarte et al. 1999). (6) Nutrients released by the POM remineralization, together with those from the *P. oceanica* detritus are trapped within the canopy water; the latter acts as a nutrient reservoir for living tissues of *P. oceanica* (Gobert et al. 2002). (7) The internal recycling of part of the nutrients from senescent leaves towards young leaves; this provides 20% of *P. oceanica*'s annual nutrient requirement (Lepoint et al. 2000); leaf nutrient resorption is a common practice among seagrasses as well as among land trees (Hemminga et al. 1999). (8) The presence of N₂-fixing mutualistic bacteria within the rhizosphere; *Candidatus Celerinatantimonas neptuna* provides *P. oceanica* with ammonium and amino acids in exchange for carbohydrates (García-Martínez et al. 2005; Mohr et al. 2021); N₂ fixation by bacteria is a common feature in the rhizosphere of seagrass ecosystems (Patriquin 1972; Capone 1983; Valiela 1991). (9) The occurrence of N₂-fixing cyanobacteria
within the phyllosphere of seagrasses is also a common feature (Capone and Taylor 1977; Iizumi 1994; Hamisi *et al.* 2009); their occurrence in the *P. oceanica* ecosystem has been suggested (Bethoux and Copin-Montegut 1986; Bethoux *et al.* 1992; Boudouresque *et al.* 2006). Overall, N₂-fixing bacteria of both the rhizosphere and the phyllosphere could account for up to 50% of the nitrogen requirements in seagrasses (Capone 1983; Short *et al.* 1990).

<figure><img src="image_6.png" /><figcaption>**Figure 5.** Nutrient and carbon fluxes in the *Posidonia oceanica* ecosystem. Other compartments (leaf epibiota, both autotrophic and heterotrophic) and fluxes (DIN and DOC leaching) are represented. Yellow: the belowground compartment.</figcaption></figure>

Green: *P. oceanica* leaves. Light brown: rhizomes and roots. Blue: N₂-fixing bacteria. Red numbers: see text. Updated from Boudouresque *et al.* (2006).

This sophisticated strategy of collecting, trapping, storing, preserving, and even producing inorganic nitrogen likely emerged as a response to the extreme oligotrophy of Mediterranean waters. It is possible that the sensitivity of *P. oceanica* to pollution is less direct than indirect: in a polluted environment, nutrient availability is no longer a limiting factor, and the advantage that *P. oceanica* previously enjoyed disappears.

Within the *P. oceanica* ecosystem, there is a juxtaposition of two sets of primary producers. (i) The seagrass itself, which features material that is weakly palatable by herbivores, requires months or years to be degraded and enters the detritus food webs. (ii) Photosynthetic epibiota, belonging to macroalgae (mainly red and brown algae), whose material is generally more easily degraded and quickly enters the herbivore food webs (Boudouresque *et al.* 2006, 2015; Personnic *et al.*
2014) (Figure 6). Most land ecosystems only rely on the first type of primary producers, while most marine benthic photophilous ecosystems are only based upon the second. The first type of primary production ensures a secure, long-term resource that fluctuates little between seasons and years. The second type of production causes seasonal (spring and summer) population blooms of herbivores and their predators, and provides resources for reproduction and juveniles through the food web.

<figure><img src="image_7.png" /><figcaption>**Figure 6.** The juxtaposition of two sets of primary producers: the long and narrow seagrass leaves, and the clumps of filamentous macroalgae attached to the leaves. West of Saint-Tropez marina, 16 m depth, August 2007. Photo © Sandrine Ruitton</figcaption></figure>

An interconnected system of gas spaces (aerarium) crosses the whole plant (from leaves to roots), as in all seagrasses (Kuo and McComb 1989; Acunto *et al.* 2000; Klap *et al.* 2000). The aerarium makes possible the recycling of CO₂ (from photorespiration) and the leaking by the roots of O₂ (from photosynthesis) (Pregnall *et al.* 1984; Enriquez *et al.* 2001; Romero 2004; Boudouresque *et al.* 2006).

In a *P. oceanica* meadow, a number of individuals, each originating from a single seed germination event, can coexist; they are genetically distinct and are referred to as 'genets'. Within a 6.4 ha meadow at Ischia Island (Campania, Italy), 126 genets were identified; the largest was 100 m long (Migniaccio *et al.* 2005). A section of rhizome can live between 20 and perhaps 50 years (see Marbà *et al.* 2002; Boudouresque *et al.* 2009); after which it dies so that a genet is fragmented
into several ramets; a ramet is therefore a set of leaf shoots interconnected by live rhizomes, constituting a functional unit. The ramets belonging to a given genet constitute a clone (Boudouresque *et al.* 2009). Within a ramet, photosynthates from a leaf shoot are first exported towards rhizomes, then allocated to other shoots of the ramet (Libes and Boudouresque 1983, 1987; Marbà *et al.* 2002). For a given shoot, this constitutes a kind of insurance: should its photosynthetic budget go into deficit (e.g., due to the overcovering of leaves with epibiota, or to overgrazing), its needs will be supplied by other shoots (Boudouresque *et al.* 2009) (Figure 7).

<figure><img src="image_8.png" /><figcaption>**Figure 7.** A ramet of *Posidonia oceanica*. Photosynthates are exported towards rhizomes, then allocated to other shoots of the ramet. For the leaf shoot in the centre of the figure, whose photosynthetic budget is in deficit, this constitutes a kind of insurance (red arrows).</figcaption></figure>

## A bundle of ecosystem services

The *P. oceanica* ecosystem provides humans with a variety of goods and ecosystem services of paramount importance (e.g., Vassallo *et al.* 2013; Boudouresque *et al.* 2016; El Zrelli *et al.* 2023; Boudouresque *et al.* 2024; Capasso *et al.* 2024; Scemama *et al.* 2024; Addamo *et al.* 2025; Caparrós-Martínez *et al.* 2025; Dentamare *et al.* 2025) (Figure 8).

Among the most outstanding services, carbon sequestration should be placed first (Figure 8: 4). Between 11% and 47% of the primary production is stored within the matte (Pergent *et al.* 1994; Mateo and Romero 1997; Pergent *et al.* 2012; Monnier *et al.* 2022). Large amounts of carbon are therefore sequestered for a long time (at least millennia): 9 to 112 g C/m²/a (Laffoley and Grimsditch 2009). At the scale of the Mediterranean Sea, carbon sequestration is estimated to range between 2.4 and 3.2 Mt C/a (Pergent *et al.* 2019). This carbon sink partly mitigates the anthropogenic release of CO₂: around the Balearic Islands (Spain), the equivalent of 9% of the annual anthropic emissions of CO₂ are trapped in the matte (Maccord and Mateo 2010). Overall, *P. oceanica* is the seagrass which most
sequesters the carbon (Serrano *et al.* 2016), and seagrass beds are among the ecosystems that globally sequester the most carbon, along with peat bogs, mangroves and saltmarshes (Mateo *et al.* 1997; Duarte and Chiscano 1999; Duarte *et al.* 2005; Fourqurean *et al.* 2012; Krause *et al.* 2025).

Another ecosystem service worth highlighting is the role of *P. oceanica* meadows as sand factories (Figure 8: 7). It corresponds to organogenic sand derived from the calcareous remains of organisms that lived in the meadow, e.g., molluscans, sea urchins, bryozoans, foraminiferans, and calcified macroalgae; part of this sand feeds the beaches (Basterretxea *et al.* 2004; De Falco *et al.* 2017; Martín Prieto *et al.* 2018; Simeone *et al.* 2018; Monnier *et al.* 2019; De Luca *et al.* 2020; Bialik *et al.* 2024). In a context where rivers bring less terrigenous sand to the sea, because it is trapped by dams, the relative importance of organogenic sand is increasing (Sharaf El Din 1977; Menéndez *et al.* 2002; Poulos and Collins 2002; Khadr 2003; Syvitski *et al.* 2005; Provansal *et al.* 2009; Boudouresque *et al.* 2022).

<figure><img src="image_9.png" /><figcaption>**Figure 8.** Main ecosystem services provided by the *Posidonia oceanica* ecosystem. Updated from Boudouresque *et al.* (2009).</figcaption></figure>

Species associated with seagrasses (i.e., mainly *P. oceanica*) were estimated to contribute, directly and indirectly, 30-40% to the value of commercial fisheries landings and approximately 29% to recreational fisheries expenditure (Jackson *et al.* 2015) (Figure 8: 1-3).
Finally, *P. oceanica* meadows play a major role in protecting beaches against erosion (Figure 8: 9 and 10). Beach protection is achieved in two ways, directly and indirectly. (i) The direct protection is due to the reduction in swell and wave strength by the meadow (Boudouresque and Jeudy de Grissac 1983; Gacia and Duarte 2001; Cavallaro et al. 2010; Sierra et al. 2023). The decrease in wave height is between 10 and 40%, on average 24% (Shirinov et al. 2025), 30-40% (Jeudy de Grissac and Boudouresque 1985), 60% over a 7 m depth meadow (Infantes et al. 2012), or 60-75% over a 2 to 14 m depth meadow (Gacia et al. 1999). Unsurprisingly, the degradation of the meadow increases wave height and current speed (El Allaoui et al. 2016; Vu 2018). (ii) The indirect protection of beaches is due to the deposition of dead *P. oceanica* leaves, during storms, on the foreshore (Figure 9); these accumulations of dead leaves, 10 to 200 cm thick (up to 300 cm), are called ‘banquettes’ (De Falco et al. 2002; Simeone and De Falco 2013; Boudouresque et al. 2017b, 2025; Otero et al. 2018; Roig Munar et al. 2019; Otero et al. 2022; Vandarakis et al. 2022; Oudin and Boudouresque 2023; Astudillo-Gutierrez et al. 2025a, b; Dentamare et al. 2025).

<figure><img src="image_10.png" /><figcaption>**Figure 9.** A banquette of *Posidonia oceanica*. Stagnolu beach, south-western Corsica, September 2023. Photo © Charles-François Boudouresque</figcaption></figure>

## The hub of Mediterranean coastal ecosystems

The *Posidonia oceanica* ecosystem, by exporting huge quantities of dead leaves to all coastal ecosystems, from beaches to the deep waters of the bathyal zone,
plays a pivotal role in the Mediterranean Sea (Boudouresque et al. 2016, 2023)
(Figure 10).

Dead leaves constitute a significant food source in the Dune-Beach-Banquette (DBB) ecosystem (Boudouresque et al. 2017b, 2025), photophilous reef macroalgal forests (Verlaque and Nédélec 1983; Thibaut et al. 2017), sandy infralittoral ecosystems (Dimech et al. 2006; Cresson et al. 2012; Cresson 2013; Remy et al. 2018; Boncagni et al. 2019), the coralligenous ecosystem (Boudouresque et al. 1990; Astruch et al. 2025), coastal detrital bottoms (Astruch et al. 2023; Cocozza di Montanara et al. 2024) and deep bathyal canyons (Bourcart 1952; Fourt and Goujard 2012) (Figures 11 and 12).

<figure><img src="image_11.png" /><figcaption>**Figure 10.** Interactions between the *Posidonia oceanica* ecosystem and some other coastal ecosystems of the Mediterranean. Red arrows: export of dead *P. oceanica* leaves. Black arrows: other flows. From Boudouresque *et al.* (2023).</figcaption></figure>

In addition, the *P. oceanica* ecosystem interacts with these ecosystems via a
variety of processes, e.g., (i) a spawning area or a nursery for species whose adults
dwell in other ecosystems (García-Rubies and Macpherson 1995; Harmelin-
Vivien et al. 1995; Jimenez et al. 1996; Cheminée et al. 2021); (ii) a habitat for
species whose nursery is located outside; (iii) the importation of fish (e.g.,
*Chromis chromis* and *Spicara smaris*), plankton and Particulate Organic Matter
(POM) from the pelagic ecosystem (Harmelin-Vivien 1982; Templado 1984;
Templado et al. 2004); (iv) and export of Dissolved Organic Carbon (DOC) to
the pelagic ecosystem (Jørgensen et al. 1981; Velimirov 1984, 1986; Vizzini
2009) and photophilous reef macroalgal forests (Champenois et al. 2024).
<figure><img src="image_12.png" /><figcaption>**Figure 11.** Dead leaves and a drift rhizome with two shoots of *Posidonia oceanica*. Toulon Canyon (Provence, France), 363 m depth, April 2010. Some leaves are still green, which means that the pulling out of the rhizome was not very ancient.</figcaption></figure>

The scale bar corresponds to the centre of the photo.

Photo © Maïa Fourt and Adrien Goujard, courtesy of the authors.

<figure><img src="image_13.png" /><figcaption>**Figure 12.** A block of *Posidonia oceanica* matte (20 cm x 20 cm x 10 cm). Juan Canyon (French Riviera), 519 m depth, May 2009. The scale bar corresponds to the centre of the photo. Photo © Maïa Fourt and Adrien Goujard, courtesy of the authors.</figcaption></figure>

## Conclusion

The *Posidonia oceanica* ecosystem is a fascinating one. It exhibits unique features, or a unique combination of features, within the biosphere. It has sometimes been described as 'the most terrestrial among marine ecosystems'.
The *P. oceanica* ecosystem is dominant in the infralittoral zone of the Mediterranean Sea, on both hard and soft substrates. Although it has declined due to human activity, mainly coastal development, bottom trawling, anchoring and pollution, around large urban centres (e.g., Meinesz et al. 1991; Ramos-Esplá et al. 1994; Montefalcone et al. 2006, 2007; Boudouresque et al. 2009; Montefalcone et al. 2009; Holon et al. 2015; Telesca et al. 2015; El Zrelli et al. 2016; Burgos et al. 2017; Di Muro et al. 2018; Guresen et al. 2019; Karayali et al. 2022), it is much more resilient than previously thought (Boudouresque et al. 2009; Pergent-Martini et al. 2013; Boudouresque et al. 2021). It has also survived major geological and climatic events, such as the partial drying up of the Mediterranean during the Messinian crises and some thirty glacial and interglacial periods.

We have tried to understand the keys to such a success, which has lasted for millions of years. These keys are numerous. Some refinements were acquired more than 70 million years ago by their terrestrial ancestors. Others were invented more recently, after their colonization of the marine realm. The complex strategy of trapping, preserving, and even producing nutrients in a highly oligotrophic sea, for example, is remarkably effective.

In a shamelessly anthropomorphic way, might we dare suggest that the ultimate success of *P. oceanica* is to have offered humans what they call 'ecosystem services', so that humans will ensure its protection?

## Acknowledgements

This article is based on a talk presented at the seminar 'Conservation and Restoration of Seagrasses' which took place at the Institute of Marine Sciences and Technology, Dokuz Eylül University, Izmir, Türkiye. The authors thank Prof. Harun Özdağ (Director of the Institute of Marine Sciences and Technology) for his kind invitation, Professor Güzel Yücel-Gier, who organized the workshop with remarkable efficiency, Campus France, who covered their travel expenses, and Juliette Bontemps (Institut Français d'Izmir). They also thank the Probiom Foundation, in particular Prof. Emir Berkane and Prof. Rachid Semroud, and the City of Marseille, in particular Liva Andriamamoujy, Yamina Boulkerara and Sabrina Djouri, which initiated this international cooperation project on *Posidonia oceanica*. Finally, they are indebted to Michael Paul, a native English speaker, for proofreading the manuscript.

**Competing interest:** No potential conflict of interest was reported by the authors.

**Ethics committee approval:** There is no necessity for ethical approval for this research.

**Financial disclosure:** This research did not receive any specific grant

**Author contributions:** C.F.B. planned the study. C.F.B. and M.P.B. wrote the initial manuscript draft, with inputs from all other authors (P.A., S.R. and T.C.). All authors contributed to the article and approved the submitted version.
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