Mediterranean seagrass ecosystem
Posidonia oceanica (L.) Delile is a seagrass species endemic to the Mediterranean Sea that forms dense and extensive underwater meadows with leaves that can attain 1 metre in height. These meadows provide important ecological functions and services and support a highly diverse community, including species of economic interest.
Since the 1970´s, a worldwide decline of seagrass distribution and abundance has been detected and causes are mainly attributed to the negative influence of anthropogenic impacts (Orth et al., 2006). P. oceanica is very sensitive to specific impacts such as bottom trawling (Sánchez and Ramos, 1996), anchoring (Francour et al., 1999), coastal constructions (Ruiz and Romero, 2003), chemical wastes (Pergent-Martini and Pergent, 1995), fish farm effluents (Delgado et al., 1999; Ruiz et al., 2001; Pergent-Martini et al., 2006) desalination plants (Gacia et al in press), geodynamic alterations (Badalamenti et al., 2006, biological invasions (Villèle and Verlaque, 1995) and many others. The effect of these impacts, alone or combined; cause either a loss of vegetated areas, a reduction in seagrass abundance (cover and/or shoot density) or a deterioration of plant health.
P. oceanica beds are identified as a priority habitat for conservation under the European Union’s Habitats Directive (Dir 92/43/CEE). Conservation management is mainly focused on protection from physical damage through the installation of artificial reefs and seagrass-friendly moorings for boats, which reduce the erosive pressure of otter-trawling and free anchoring in shallow meadows. The control of invasive species has also been performed recurrently in some P. oceanica beds.
Regressed meadows are prone to invasion by one or more of the potential substitutes for P. oceanica (Bianchi and Peirano, 1995; Montefalcone et al., 2006) such as the other common Mediterranean seagrass Cymodocea nodosa (Ucria) Ascherson, the native Mediterranean green alga Caulerpa prolifera (Forsskal) Lamouroux and the two alien green algae Caulerpa taxifolia (Vahl) C. Agardh and Caulerpa racemosa (Forskal) J. Agardh.
There is a need to further develop regulations for activities that have a negative impact on P. oceanica beds (e.g. pollutants level limits and allowed minimum distances of impact sources to meadows) and to implement them through a vigilance system that is coordinated with the existing seagrass monitoring networks.
Once the cause of habitat perturbation is eliminated, the slow growth of P. oceanica beds means that recovery can take centuries. Measures like remediation of seagrass sediments enriched with organic matter, or transplanting of P. oceanica, are at an experimental stage.
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