What is Posidonia oceanica and why is it so important to preserve it?

Posidonia is not a seaweed! It belongs to the angiosperms (Angiospermae) i.e. flowering plants. Mediterranean Posidonia (Posidonia oceanica) is a species of aquatic flowering plant (a seagrass) in the Posidoniaceae family, endemic to the Mediterranean Sea (Boudouresque et al., 1984).

The first fossils of European Posidonia date back to the Cretaceous (Posidonia cretacea), 145 to 166 million years ago (Aires et al., 2011; Hemminga and Duarte, 2000) and have changed relatively little since then (Den Hartog, 1970).

Posidonia grows between the surface and a depth of 30 to 40 metres. It forms large meadows (Picard and Molinier, 1952), constituting key ecosystems that play a major ecological role (Campagne et al., 2015; Gacia and Duarte, 2001; Mangos et al., 2010), even though they only represent 1% of the surface area of the mediterranean basin (Boudouresque et al., 2006).

P. oceanica plays an important role in the carbon cycle, particularly in the storage of anthropogenic carbon on the ocean floor. It is estimated that total carbon production by P. oceanica grasslands is 3.5 million tonnes, of which 2.4 million tonnes is exported and stored (Pergent et al., 1997). In addition, there are numerous economic possibilities for using P. oceanica: for coastal fishing, as a natural fertiliser when it washes up on beaches, for wastewater treatment, for carbon storage, for coastal protection, etc. (El Zrelli et al., 2023).

It reproduces both sexually (with flowers and seeds) and asexually (by spreading rhizomes).

Today's cold-tolerant (and therefore heat-sensitive) genotypes were probably selected during the recent Plio-Pleistocene cooling. Indeed, today temperatures above 27◦C cause significant flowering (Diaz-Almela et al., 2007; Marín-Guirao et al., 2019; Ruiz et al., 2018).

The ability of P. oceanica plants to reproduce sexually when water temperature is >27◦C could lead in the future to the selection of heat-loving plants (as heat-sensitive clones decline), with an associated increase in genetic variability in the species gene pool (Martínez-Abraín et al., 2022).

Anthropogenic activity has been shown to have a negative impact on the development and maintenance of Posidonia meadows (McDonald et al., 2023).

Despite legislation, Posidonia meadows are declining rapidly. The estimated regression of meadows amounts to 34% over the last 50 years, showing that this widespread phenomenon should be attributed mainly to the cumulative effects of multiple stress factors (Telesca et al., 2015). This decline is due to local impacts (physical impacts, construction, eutrophication) and global impacts (climate change, presence of invasive species, storms, Labyrintula infections, sea level rise, temperature increase) (Bianchi, 1997; Bianchi and Morri, 2004; Casoli et al, 2021; Chefaoui et al., 2017; Jevrejeva et al., 2012; Jordà et al., 2012; Levermann et al., 2013; Mancini et al., 2019, 2023; Marbà et al., 1996, 2014; Micheli et al., 2005; Montefalcone et al., 2007; Ruiz and Romero, 2003; shukla et al., 2019; Telesca et al., 2015).

P. oceanica recovers very slowly from these disturbances and could take decades to centuries to recover from its losses. Given their vital role in the ecological structure of the coast, it is crucial to protect these meadows from human fragmentation to ensure the resilience of these ecosystems in the face of the climate crisis (Marco-Méndez et al., 2024).

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