Regionalisation of the Mediterranean basin, a MERMEX synthesis

Regionalisation aims at delimiting provinces within which physical conditions, chemical properties, and biological communities are reasonably homogeneous. This article proposes a synthesis of the many recent regionalisations of the open-sea regions of the Mediterranean Sea. The nine studies considered here defined regions based on different, and sometimes complementary, criteria: dynamics of surface chlorophyll concentration, ocean currents, three-dimensional hydrological and biogeochemical properties, or the distribution of organisms. Although they identified different numbers and patterns of homogeneous regions, their compilation in the epipelagic zone identifies nine consensus frontiers, eleven consensus regions with relatively homogeneous conditions, and four heterogeneous regions with highly dynamical conditions. The consensus frontiers and regions are in agreement with well-known hydrodynamical features of the Mediterranean Sea, which constrain the distribution of hydrological and ecological variables. The heterogeneous regions are rather defined by intense mesoscale activity. The synthesis proposed here could constitute a reference step for management actions and spatial planning, such as the application of the European Marine Strategy Framework Directive, and for future biogeochemical and ecological studies in the Mediterranean Sea.This study was conducted as part of the WP5 MERMEX/MISTRALS project and is a contribution to the international SOLAS, IMBER and LOICZ programs. The lead authors are grateful to Pr. Philippe Koubbi (MNHN) for initiating (eco)regionalisation studies of the Mediterranean Sea at the Laboratoire d’Océanographie de Villefranche sur mer (LOV, UPMC/CNRS). Some initial thoughts that have led to this synthesis were also supported by the PlankMed action of WP5 MERMEX and by the EC FP7 PERSEUS Project (Policy-oriented marine Environmental Research in the Southern EUropean Seas; Grant. Agr. 287600).Peer reviewe