Posidonia oceanica is abundant in the
Mediterranean, however there is evidence that the population is declining in the western Mediterranean. Several local studies have shown serious declines in P. oceanica meadows. However, accurate data are generally very localised and are lacking for many parts of the Mediterranean. Therefore, cases of observed regression are not representative of the region as a whole. Relatively healthy P. oceanica meadows, whose limits have changed little since the 1950s, can thrive in highly developed areas. In some areas, there is evidence of recolonization by P. oceanica after the human impact ceased or was reduced, but the process of recolonization is extremely slow, i.e. a few centimetres per year (Pergent-Martini et al. 1995). Considering all Mediterranean seagrass species area combined, Jackson et al. (2006) estimated total losses at 446 km² over the last 100 years. A recent study looking at the population status of six Mediterranean P. oceanica areas showed positive population dynamics in some localities for all studied parameters (González-Correa et al. 2007). It is estimated that the overall decline in area for P. oceanica is less than 10% (Thomas et al. 2005) over three generation lengths (100 years), and declines have mainly occurred near urban areas (Boudouresque et al. 2006).
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