Poor success of seagrass Posidonia oceanica transplanting in a meadow disturbed by power line burial.

Local disturbances drive the decrease of the area covered by Posidonia oceanica in the Mediterranean. Mechanical impacts during the development of coastal infrastructures alter sea floor and the recolonization of benthic community will depend on the recovery of pre-disturbance environmental conditions and on the intrinsic characteristics of the local community that was disturbed. We transplanted 468 rhizome fragments and 450 seedlings of P. oceanica in a meadow disturbed by the trenching and deployment of a power line to evaluate the suitability of the disturbed sea floor for rehabilitating P. oceanica meadows. We quantify and compare the survivorship and vegetative development of the transplanted/planted (i.e. fragments/seedlings) material in the two types of the unconsolidated substrata left after infrastructure deployment works finished: sand and burlap bags filled with coarse gravel. The latter was used as a corrective measure for topographic restoration. Three experimental plots with sixteen transplanted fragments or twenty-five seedlings were placed at each substratum type at three different depths (i.e. 15, 20 and 25 m). Our results show that the transplanting of P. oceanica rhizome fragments in the disturbed substrata had low survival rates (0-31%) after 40-48 months. The survivorship of seedlings was lower than that of fragments. Our results highlight the importance of substratum for P. oceanica recovery after mechanical impact; disturbed, non-consolidated substrata will preclude P. oceanica rehabilitation through planting. Preservation of meadow substratum (i.e. dead matte) is a critical element that coastal infrastructure projects should consider to enable future recovery of P. oceanica meadows.