Why are coral reefs not found in the Mediterranean today?

Abstract Despite continued warming in the Mediterranean Sea, tropical and sub-tropical hermatypic (i.e., reef-building) zooxanthellate scleractinian corals (henceforth referred to as hermatypic corals) have not returned since their disappearance after the Messinian Salinity Crisis almost 6 Mya ago. We hypothesized that environmental factors limit the presence of hermatypic corals and the reefs they form in the Mediterranean. To test this hypothesis, we compared seawater physical and chemical properties (seawater temperature, salinity, chlorophyll, pH, alkalinity, and photosynthetically available radiation (PAR)) derived from in situ and satellite measurements at locations where hermatypic corals are found in the tropics and sub-tropics to those of the Mediterranean. We found that although the environmental conditions in the southern Mediterranean are similar to that of high-latitude coral reefs (e.g., Bermuda and Japan), the flow conditions of the Suez Canal may be limiting the migration of Red Sea hermatypic corals into the Mediterranean and sediment discharge of the Nile River may be reducing the settlement substrate quality in parts of the Southeastern Mediterranean. In the northern Mediterranean, extended periods of cooler winter temperatures coupled with low PAR levels and duration in winter are most likely limiting large tropical-like coral reef development. While it is possible that unintentional introductions and/or species migration of hermatypic corals through the Suez Canal or from aquarium cast-offs could become established and form coral reefs in parts of the southern Mediterranean over time, less PAR and longer cool winters will probably prevent such coral reef establishment in the northern Mediterranean.