5. The submerged area of the beach The importance of Posidonia oceanica

The Posidonia oceanica from a biological point of view



Posidonia oceanica is a native Mediterranean sea plant found on both sandy and rocky sea floors. This plant requires soil that contains a minimum amount of organic matter.

The plant is made up of a thick stem (rhizome) that grows both horizontally and vertically, a tuft of 6 to 8 ribbon-shaped leaves found at the top of each rhizome, and densely branching roots that also emerge from the rhizomes. The combined growth of an extremely large number of Posidonia plants is what eventually forms the meadows, where one can appreciate the upper part of the plant with its leaves, and with the stems, rhizomes and compacted sediment below. This base can be up to several meters thick. The meadows are quite stable and long-lived formations (estimating that some are as much as 6,000 years old) and retain a substantial amount of sediment.

The parts of the Posidonia oceanica plant and diagram of the formation of a Posidonia meadow Source: CORBERA, J. Praderas y Bosques Marinos de Andalucía.


Posidonia, like land plants, require light to survive via the process of photosynthesis. In this process, plants use carbon dioxide and light to produce organic matter (for growth) and generate oxygen as a byproduct, which is given off into the atmosphere (land plants) or into the sea (marine plants). For this reason, Posidonia grows primarily in areas with transparent waters, since the clearer the water, the more solar rays penetrate the surface and provide more of the energy necessary for photosynthesis. Hence, light is one of the factors that determines the presence of Posidonia, and this plant can be found from depths ranging from the very surface, down to 30 or 40 meters (occasionally reaching depths of 80, or even exceptionally, 100 meters in some areas of extremely transparent waters around the Balearic Islands.) This plant requires temperatures ranging from 15 to 20 degrees Celsius for growth and does not tolerate significant variations in salinity.

Posidonia plants have a very clearly defined life cycle defined primarily by the seasons:

Autumn

When sea temperatures begin to cool, and due to the effects of the season’s first storms, the plant loses its green leaves that throughout the summer became covered with an abundance of both animal and plant species that used them for food or shelter. The autumn storms cause the characteristic settling of tons of dead leaves upon the beaches, which in turn play an important role in the protection of sandy beaches against the erosive waves during subsequent winter storms (Figure 5.2). New leaves begin to sprout using energy that was stored during the spring and summer months. The meadow takes on a more spacious and sparse appearance having lost its older, longer leaves and now having new, shorter leaves absent from epibiontic organisms (those which grow and live upon the surface of the leaves).

Winter

Growth of the new leaves that emerged in the autumn is much slower, coinciding with the lowest temperatures of the year. The process of meadow formation does, however, continue with the growth of the newer leaves. By the end of the winter, the first fruits can be observed, although these normally do not survive due to low temperatures.

A beach covered with Posidonia on the land area


Spring

In this time of year the Posidonia's growth becomes more active as water temperature rises. This is a period of plant maturity. The leaves reach great lengths and the meadow begins to appear thick and dense. It takes on a bright green color as there are still few epibionts covering the leaves (Figure 5.3) The fruit is germinated in the months of April and May, as these will have been maturing since the end of the winter. From the seeds, small sprouting plants, measuring 8-10 cm, begin to grow and hover over the seabed attempting to set themselves into the sediment.

A mass of young Posidonia, still without epibionts


Summer

During the warmest months of the year, a great many organisms (Hydrozoa, Bryozoa, Mollusca, Polychaeta and Foraminifera) progressively attach themselves to the Posidonia leaves until these are completely covered. These are epibiontic organisms. During this process, the leaves take on a whitish hue due to the sheer number of organisms covering them. Leaf growth during this period is minimal, due primarily to the outer layer created precisely by these organisms, which keeps the leaves from adequately carrying out photosynthesis. The leaves subsequently turn brownish-grey until they eventually die, and with the first storms the leaves will fall from the plant. The falling of leaves does occur throughout the year, although at the end of the summer it occurs on a massive scale.

Detail of old Posidonia leaves, covered in epibionts and pale in color


Posidonia oceanica as the basis of the Mediterranean ecosystem
Posidonia oceanica and its relationship with the sedimentation of beaches
Threats to Posidonia oceanica

Posidonia oceanica as the basis of the Mediterranean ecosystem


The meadows of Posidonia seaweed are one of the most characteristic and important of the Menorcan coast. They occupy large areas and their role as underwater forests make them essential to produce life and oxygen.

Among their leaves and rhizomes a wide diversity of organisms interrelate through a complex food chain by which one feeds the others and so on; all this cycle supplies great stability to this habitat. On the other hand, meadows fix the sandy seabed thanks to the plant roots, and the stable structure formed by the rhizomes that has an important buffer effect against surf and sea currents. Therefore coastal erosion and regression of sandy beaches is prevented. Meadows also have a relevant effect as coastal waters purifiers since they clean them from sediment. The CO2-capturing and the oxygen-producing activity of the Posidonia meadows also help to reduce climate change.

Species habitat
Food source
Species nursery

Transparency of water
Prevents the erosion of beaches
Reduces climate change


As a conclusion, the meadows of Posidonia seaweed house a number of plant and animal species, giving them shelter, food and substrate where they can settle.

Posidonia oceanica and its relationship with the sedimentation of beaches


The relevance of Posidonia seaweed in the preservation of Menorcan beaches is such for two different reasons: On the one hand, they help to form an ecosystem that will contribute to the production of sand on the beaches of Menorca, whilst it also helps reduce the erosion action of surf on the beaches.

On the Posidonia seaweed leaves there is abundance of tiny organisms, whose skeletons end up as part of the sand. Furthermore, the leaves of Posidonia seaweed retain the sand particles from the erosion zones and make them ready to be transported towards the emerged beach thanks to the action of the sea during good weather periods. The leaves detached from the meadows of Posidonia seaweed, mainly in the autumn, settle on the beach building a natural barrier that protects the sand against the violent winter storms. Once on the beach, the Posidonia leaves attract some sand that they release when shifting towards the dunes.

(a) Las hojas de Posidonia son trasladadas desde el fondo del mar hacia la playa emergida y (b) ayudan a mitigar la erosión de los temporales. Source: The author: CARDONA, F.

Threats to Posidonia oceanica


Posidonia serves as a gauge in assessing the quality of the water and the environment. Negative effects on the environment could foster the disappearance of Posidonia. Among these negative effects are:

  • Spillage of wastewater.
  • Spillage from desalination plants.
  • Construction of ports, breakwaters, or similar coastal infrastructure whose construction may increase water turbulence (dredging).
  • Water pollution.
  • Artificial beach regeneration.
  • Indiscriminate anchor dropping.