Improvement in the state of Posidonia meadows in the Balearic Islands

The results obtained by the Posidonia Monitoring Network show, in general, an improvement in the state of the meadows in the Balearic Islands.

The Network is a project of the Regional Ministry of Agriculture, Fisheries and Natural Environment, carried out through the Species Protection Service and Tragsatec, which aims to determine the state of conservation of the Posidonia meadows in the Balearic archipelago. This information is very relevant to manage them correctly and to evaluate the effectiveness of the measures implemented for their protection.

Thus, all the data obtained between 2002 and 2023 in the 71 study sites located in different areas of the Balearic Islands, in areas with a wide range of different characteristics, show that situations of stability or an increase in the amount of posidonia predominate and that it only occasionally decreases (in approximately 10% of the study sites). Specifically, 21% of the study sites were considered to be in an unfavourable conservation status and 79% in a favourable conservation status.

It should be noted that grasslands can produce new bundles of leaves from living bundles (increasing density and cover on a small scale) much faster than they can grow in extent once the plant has died and only leafless rhizome remnants remain. For this reason, there are meadows with high amounts of dead Posidonia, but which have been stable or recovering since the start of monitoring.

Data collection is always done underwater using relatively simple and non-destructive methods, which include quantification of the cover and density of posidonia and determination of the proportion of dead posidonia, if present. In addition, the presence of invasive algae and nacre is also noted, and water temperature recorders are installed at some survey sites. Approximately ten partner organizations, twenty volunteer dive centers and up to one hundred volunteer recreational divers participate each year.

During 2022, a new collaboration was established through the monitoring of meadows in places of high nautical pressure in Ibiza, an initiative of the Balearic Ornithology and Nature Defence Group (GEN-GOB), whose data were integrated with the rest of the results from the Balearic Islands. The eighteen new study points were established in areas of high traffic pressure and boat anchoring and, therefore, correspond to more deteriorated meadows. The proportions of dead Posidonia found in 2023 are higher than 25% in half of the points, although, for the moment, the time series available is too short to draw conclusions.

This year, the Network continues and dives have already begun to take data from the meadows. In April, the areas of Cala Vadella and Talamanca, in Ibiza, were visited with GEN-GOB, and in May the dives were resumed with dive centers and volunteer divers on the coast of Mallorca. The dives with volunteers will continue throughout the diving season until the end of October and will also be extended to Ibiza and Formentera (in Menorca all dives are only done by technical staff in collaboration with the Socio-environmental Observatory of Menorca). 

It should be remembered that Royal Decree 139/2011, of 4 February, includes oceanic posidonia in the list of wild species under special protection. This legal protection aims to guarantee a favourable conservation status and implies that a periodic evaluation of its conservation status must be carried out. Likewise, Decree 25/2018, of 27 July, on the conservation of Posidonia in the Balearic Islands, entrusts the Ministry of Agriculture, Fisheries and the Natural Environment with the development of a monitoring plan for the general state of the meadows, with special attention to those of high value, meadows to be regulated and those included in protected natural areas or in the Natura 2000 Network. It is within the framework of the commitments and obligations derived from these regulations that the Posidonia Monitoring Network project is inserted.