Posidonia oceanica, an endemic species of the Mediterranean Sea, is a seagrass whose meadows make it one of the most important habitats in the sea. It was named after the Greek god of the sea, Poseidon, while it is locally known as lažina, voga, purola or purić. It is often mistaken for an algae. But it is a flowering plant whose fruits look like olives and that is why known in Italy as “the olive of the sea”.
Its meadows are oxygen factories, which is why we call them the lungs of the sea. It is estimated that 1 square meter of this seagrass produces as much as 14 liters of oxygen. Over thirty species of algae can be found on the leaves of Posidonia. Posidonia meadows are habitats, hatcheries and feeding grounds for more than 100 species of fish, most of which are of economic importance. Among its leaves, we can find various snails, bivalves, starfish and many other species of invertebrates.
The stem of Posidonia grows 1 cm per year, while it takes a century to develop a colony. Some meadows are over a thousand years old and are one of the longest-lived organisms in the sea. From its leaves scientists receive a range of information from the fields of biology, but also climatology, oceanography, geology and geography.
Due to process of photosynthesis, it needs conditions of high transparency of sea water, so it maintains it itself. It is an indicator of the purity of the sea, and its antibacterial properties contribute to the health of the marine environment, which is also important for us swimmers.
The roots of Posidonia in the sea prevent erosion, so some of the most famous beaches on our coast, such as the Golden Horn in Bol, owe their existence to it. Like terrestrial plants, Posidonia loses its leaves and regenerates it every year, and discarded leaves accumulate on the coast in late summer and early autumn. Alluvial leaf deposits have been shown to be of great importance in mitigating the effects of erosion, ie the removal and wear of sediment from the coast, especially during strong waves. Also, many organisms feed on organic detritus caused by the decay of flooded leaves.
Posidonia, the source of life, a witness to maritime history and a precondition for the future, is under increasing negative pressure. Man destroys its meadows by filling in shores, concreting, building ports and marinas, discharging wastewater, fish and shellfish farms and using certain fishing tools.
The greatest threat to the survival of Posidonia is anchoring. Until the anchor is caught, it retreats along the seabed and plows the Posidonia. After the anchor is attached to the bottom, the anchor chain is pulled along the bottom and the Posidonia is cut. When the anchor is pulled out, new plowing takes place and thus the Posidonia meadow is destroyed.
In addition, invasive species that are spread by ballast water and anchor transmission pose a great danger to seagrass meadows, and climate change and sea warming are affecting their reproduction. Among the invasive species, the algae Caulerpa cylindracea stands out, which penetrates meadows of Posidonia, suffocates it and takes over its habitat.
A prerequisite for the implementation of protection and sustainable management is monitoring. Using the transect and the square the cover and meadow density is measured, and negative impacts are recorded. Such measurements should be carried out continuously in order to monitor changes in the marine environment and adopt protection measures based on them.
As part of the service Monitoring of the Posidonia oceanica flowering plant in the area of Telašćica Nature Park, in the period from 21st to 24th of June 2021, the Sunce Association conducted the fifth consecutive monitoring of the condition of posidonia meadows inside the Telašćica Nature Park.
In the period from 23rd to 27th of August 2021 we had dived inside the National Park Mljet, where we conducted the service Monitoring of seagrass Posidonia oceanica in the area of the National Park Mljet. Monitoring was conducted in the bays of Međuporat, Luka Polače, Lokva and Srednja.