Mediterranean shelf shaped by algal reef (Coralligenous)

Marine bioengineers are organisms able to create rigid biostructures at the seafloor. Such organisms consequently literally shape the seascape. The most famous are hermatypic corals forming the coral reefs, where their growing not only develop a very special marine habitat, but also drives the geomorphological development of coasts and atolls. Outside the tropics, no analogues are reported for shallow water, but it does not mean that the same phenomenon is not in act. In fact, large areas are dominated by mobile and hard substrates of biological origin. No more hermatypic corals, but coralline algae act as bioengineers. They are sciaphilous algae with skeletal thalli. They encrust hard and mobile substrates, or generate new ones. They request dim-light conditions, and consequently colonize both shallow water with low level of irradiance, and relatively deep bathymetric interval. In relatively shallow water (continental shelf), the Mediterranean Sea hosts abundant coralline algae forming algal reefs, referred to as coralligenous. BIOMAP project was one pioneer regional project aimed at the mapping of the coralligenous within 100 m of water depth along the Apulia region coasts. The main outcome of this project is that coralligenous dominates the Apulia shelf (38% of seascape cover). It develops diverse morphotypes, ranging from discrete reliefs, made of small isolated build-ups with a thickness up to 50 cm, up to tabular reliefs, several kilometers large and with a thickness up to 4 m, depending on diverse bathymetric intervals or type of substrate. Therefore, the seascape has been literally drawn by the activity of coralline algae, primary bioengineers of these monumental structures. Algal reef is a very important topic, as biodiversity hotspot in the framework of European Community Directives. Moreover, its carbonate hard structure contributes to the geomorphological development and protection of the coasts, becoming relevant under the scenario of marine acidification.