Abstract
Some species of seagrasses (e.g., Zostera marina and Posidonia oceanica) have declined in the Mediterranean, at least locally. Others are progressing, helped by sea warming, such as Cymodocea nodosa and the non-native Halophila stipulacea. The decline of one seagrass can favor another seagrass. All in all, the decline of seagrasses could be less extensive and less general than claimed by some authors. Natural recolonization (cuttings and seedlings) has been more rapid and more widespread than was thought in the 20th century; however, it is sometimes insufficient, which justifies transplanting operations. Many techniques have been proposed to restore Mediterranean seagrass meadows. However, setting aside the short-term failure or half-success of experimental operations, long-term monitoring has usually been lacking, suggesting that possible failures were considered not worthy of a scientific paper. Many transplanting operations (e.g., P. oceanica) have been carried out at sites where the species had never previously been present. Replacing the natural ecosystem (e.g., sandy bottoms, sublittoral reefs) with P. oceanica is obviously inappropriate in most cases. This presupposes ignorance of the fact that the diversity of ecosystems is one of the bases of the biodiversity concept. In order to prevent the possibility of seagrass transplanting from being misused as a pretext for further destruction, a guide for the proper conduct of transplanting is proposed.
Highlights
Seagrasses are part of the Magnoliophyta, which number more than 295,000 species [1]
We have considered the case of the Mediterranean, which is of particular interest because a considerable corpus of data is available on the decline, the non-decline, or the recolonization of seagrasses, and we have examined the justification for and effectiveness of seagrass restoration operations
C. nodosa meadows are expanding, firstly because the species is favored by the sea water warming, and secondly, because it benefits from the decline of the P. oceanica meadows [11,90]
Summary
Seagrasses are part of the Magnoliophyta (flowering plants), which number more than 295,000 species [1]. The vast majority of these species are terrestrial, while freshwater taxa represent ≈2450 species [2] and seagrasses only represent 67 species, including Ruppia maritima [3,4] Despite their small number, seagrasses play a role of paramount importance in coastal habitats worldwide. Extrapolation of the cases of decline to the entire seagrass distribution area constitutes another bias: the “undocumented” areas may correspond to areas where nothing is happening, and regarding which it is difficult to publish in high IF journals. Such extrapolations would result in an overestimation of global seagrass decline. The Mediterranean is often regarded as representative of the world ocean in miniature [44]
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