1.4 Light variable

Light in the sea is a major factor for vegetation as these organisms (algae and plants), must perform photosynthesis in order to survive. It is a process that uses light and carbon dioxide dissolved in water to build nourishment and that releases oxygen as waste material.

The intensity of the light radiation with penetration in the sea decreases by 50% in the first 50 m, only 1% reaches 100-150 m and then darkness falls. This tendency can be influenced by the amount of organisms present and the particles in suspension, which in turn change with weather-marine conditions. The amount of light that reaches the water depends on latitude, seasons and sky cover. (Fig.1_SES1.4)

In relation to the penetration of light, a superficial euphotic layer, an underlying disphotic area and a deep aphotic dark area can be distinguished. In the first one the phytoplankton photosynthesis processes take place, in the Mediterranean it reaches an average of up to 100 m. In the second one the light decreases, but animals are still able to see.

In the coastal ecosystem of the Mediterranean Sea, if we remain within the so-called photic zone (which receives the light), are the algae and seagrasses to represent a role of primary importance in the structuring of habitats. In the sandy seabed is a plant, Posidonia oceanica (Linnaeus) Delile, 1813 (Fig.2_SES1.4), to play a key role:

• produces a high quantity of oxygen, up to 20 liters per day per meadow meter;

• contributes to the consolidation of the seabed and beaches, protecting them
erosion;

• is the ideal environment for refuge and the development of many species, such as molluscs, crustaceans and fish.

Currently, the P. oceanica is in strong regression throughout the Mediterranean basin due to numerous factors, including chemical pollution, coastal protection works and the “ploughing” of the seabed caused by boat anchors and trawl fishing. Another threat to P. oceanica is the green alga Caulerpa cylindracea Sonder, 1845 (Fig.3_SES1.4).

C. cylindracea is a green alga native to south-western Australia and introduced into the Mediterranean in the late 90s through propagules transported by the ballast water of the ships. It is observed from the surface up to more than 60 m of depth. It can be found in the Posidonia meadows where can increase the meadow fragmentation, and on rocky bottoms where can alter the local populations. The alga is considered an allochthonous or alien species, which means that it is an organism able to survive and reproduce outside the geographical area where its species has evolved. Its geographical distribution appears to be increasing and it is possible to consult a map on the Reef Check Italia Onlus website at this link: www.reefcheckmed.org/italiano/reef-check-med/webgis/.

In the hard bottoms, the key role is covered by coralline algae (Fig.4_SES1.4), which are able to deposit calcified and permanent thalli. They can create a complexity comparable to that of the tropical coral reef. The habitat formed by these algae is known as coralligenous and that is where many organisms found their ideal habitat.