Where Seaweed Forests Meet Animal Forests: the Examples of Macroalgae in Coral Reefs and the Mediterranean Coralligenous Ecosystem

Abstract

The Mediterranean coralligenous and the intertropical coral reef ecosystems are similar in several aspects, such as their ability to thrive in nutrient-poor waters and the communities associated with them. For example, these ecosystems encompass communities ranging from bioconstructions by calcified blade-forming coralline macroalgae, bioconstructions by calcified hexacorallians, canopy-forming seaweed forests, canopy-forming gorgonian (animal) forests, to turfs of macroalgae. They depend mainly upon available light, temperature, and herbivore pressure. In spatial terms, these communities can constitute a complex mosaic. Over time, they can follow each other throughout ecological successions, i.e., a suite of shifts, or phase-shift events, as a consequence of natural or anthropogenic disturbances. Some of these communities, of which the autogenic ecosystem engineers are esthetically pleasing, large-sized, and long-lived, such as blade-forming corallines, gorgonians, and hexacorallians, are often explicitly or implicitly, but erroneously, regarded as the whole ecosystem, whereas they are in fact only part of it. Both coral reefs and the Mediterranean coralligenous ecosystems dwell in highly oligotrophic waters. Their success hinges upon mutualism with unicellular primary producers (dinobionts), the efficient recycling of nitrogen, and diazotrophy (coral reefs), and upon the input of allochthonous organic matter (coralligenous), rather than on the primary production of macroalgae alone (e.g., Cystoseira, Sargassum, Turbinaria, and turf-forming species). In addition, species diversity is high, which substantially helps to make these ecosystems species diversity hotspots. The close intertwining of the different communities (seaweed forests, heterotrophic animal forests, photosynthetic animal forests – via mutualism – and highly productive macroalgal turfs) within these two ecosystems, together with the massive precipitation of calcium carbonate, makes the structure and functioning of these ecosystems highly original, without counterpart in the terrestrial realm.