|What is Posidonia ?||
Biology of the species
Posidonia plants have a characteristic life cycle , with clearly defined seasonal variations.
Autumn. Due to the first storms, common towards the end of the summer, the sea water begins to cool down and the plant loses its green leaves which during the preceding summer have been progressively coated by various species of animals and plants that use Posidonia as substrate. The tonnes of dead seaweed washed up on our beaches are produced through the action of these storms, and these seaweed leaves play an important role in the protection of the beaches. New leaves begin to sprout from the plant , using as energy the starch synthesized during the spring and summer months and which is stored in the rhizomes . The seaweed field now appears spacious and not luxuriant as it has lost the oldest leaves. The new leaves grow short and clean, without epibiont organisms (those that grow and live off the leavesí surface), and the field now acquires a healthy look.
During this season growth speeds up as the temperature of the water
increases. This is the period of maturity, the leaves grow very long
and the field acquires a luscious look of intense green for the leaves
are as yet sparsely coated by epibionte organisms.
During the hot months a large amount of organisms (hidrozoans, briozoans,
molluscs, etc.) attach themselves to the leaves, coating them progressively
until they totally cover them. These are the epibiont organisms. During
this process, the leaves turn a whitish colour due to the profusion
of organisms coating them. The growth of the leaves during this period
is minimal because of the crust that epibiont organisms form on the
leavesí surface and which prevents the leaves from carrying out the
photosynthesis, this will in turn eventually make the leaves turn
a brownish colour and die off. When the first storms arrive, the leaves
fall off, detaching themselves from the ligule.
The plantís sexual reproduction is possible thanks to the flowers which hold the sexual masculine organs (stamens) and the feminine ones (pistils). Fertilization happens when the pollen from the stamens is transported by the marine currents and reaches the pistils. To make this possible the stamens produce filaments of viscose pollen that have serrated edges and are able to travel with the currents and reach the pistils of other plants. Once the plant has been fertilized the seeds will begin to form.
On the earth’s
surface, plants develop all shorts of ingenious mechanisms in order
to propagate their seeds: structures that allow seeds to fly in
the wind, animals that swallow the seeds, etc. What kind of resourcefulness
does Posidonia make use of in order to ensure that its seeds travel
in an aquatic environment?
The best environment for Posidonia to reach its optimum development is that of transparent waters. The more transparent the water is, the more the sun’s rays can penetrate the water and these rays are what provide the plant with the energy required to synthesise organic matter through the process of photosynthesis. Light is therefore one of the factors that regulate the presence of Posidonia.
The lowest range in which Posidonia can thrive is around 30-40 metres, although in extremely clear waters it can be found at a depth of up to 80 m. , and exceptionally, some fields exists at depths of 100 m. in certain areas around the Balearic Islands that have very transparent waters. Some authors believe that these still live deep fields are a vestige from the past when the level of the sea was lower than it is now. Density in these conditions is low.
The highest range in which Posidonia fields can settle is regulated by hydrodynamics, i.e. the rhythm of the waves. Waves break the plants up and stir the sea bed in such a way that they make it impossible for the plant to survive. In fields established in coves protected from sea waves, the plants can reach all the way up to the water’s surface, as happens for example at sa Nitja situated on the north coast of Minorca. Nevertheless, the most common shallowness in which fields can settle is that of 3-5 metres of water, where waves are not so strong as to pull plants up from the root.
As to the substrate, Posidonia can be found established in both soft ground of variable grain size ( fine grained sand, rough grained sand, silted sand, etc.) and in rocky ground. It needs only that the bed be of true ground containing a certain amount of organic matter. The optimum temperature for its development is that of between 15 and 20 degrees Celsius, and the plant does not tolerate variations in salinity.
Also, Posidonia fields thrive only in clean healthy
waters of a good environmental quality. This renders the plant very
vulnerable to human activity. Polluted waters make the fields recede
immediately. Therefore, the presence of Posidonia is a guarantee of
the quality of the water. The best quality seal that any coast line
may exhibit is the presence of healthy Posidonia fields, which are
an authentic guarantee—better than a flag of any sort.
Pollution of Sea Water
A large proportion
of waste produced by human activity ends up directly or indirectly
in the sea, and causes an impact of varying degrees on the Posidonia
fields. Empty bottles and discarded plastics and junk, dirty the
sea bed. Sediments coming from the coast (sewage emissions, waste
dumping) increase the murkiness of the water reducing the quantity
of light reaching the plants. Sewage and fertilizers cause an increase
in the level of nutrients and organic matter; the oxidation of the
same reduces the quantity of oxygen in the water and this in turn
can produce seriously harmful consequences for the fields.