Elsevier

Progress in Oceanography

Volume 163, April 2018, Pages 7-20
Progress in Oceanography

Regionalisation of the Mediterranean basin, a MERMEX synthesis

https://doi.org/10.1016/j.pocean.2017.09.016Get rights and content

Highlights

  • Synthesis of nine previous regionalisations of the epipelagic Mediterranean Sea.

  • Nine congruent frontiers, related to circulation and bathymetry.

  • Eleven congruent homogeneous regions, related to large scale circulation.

  • Four heterogeneous regions, associated with dynamical mesoscale circulation.

  • A key step for future spatial planning in scientific studies and marine management.

Abstract

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.

Introduction

Marine ecosystems and associated services are subject to strong climatic and anthropogenic changes. This is especially true in the Mediterranean Sea (Bianchi and Morri, 2000, Coll et al., 2010), due to both its configuration as a semi-enclosed sea and its high and increasing concentration of human activities (Lejeusne et al., 2010, The MerMex Group, 2011). However, the absence of a consensus of its geographical framework complicates the description, management and conservation of Mediterranean marine ecosystems (Coll et al., 2012). Such a regional framework should rely on regionalisation, or the objective definition of ecological and biogeochemical spatial units. Historically, the Mediterranean Sea has been subdivided into geographical entities (Fig. 1) whose limits were set according to geographical frontiers (such as straits) or political delineations. In the 90s, remote sensing allowed to describe the dynamics of primary production, which was used to delineate provinces within basins (Sathyendranath et al., 1995) and then in the global ocean (Longhurst, 1998, Oliver and Irwin, 2008). In Longhurst’s classification, the Mediterranean Sea was distinguished from other regions, without further separation within the system, despite its apparent heterogeneity. Later, a new global system was proposed by Spalding et al. (2007) for coastal and shelf areas: the Marine Ecoregions of the World (MEOW). This biogeographical classification, based on existing global and regional literature, describes a nested system of 12 realms, 62 provinces, and 232 ecoregions. In this framework, the Mediterranean Sea represents a province of the “Temperate Northern Atlantic” realm and is subdivided into seven ecoregions: the Adriatic Sea, the Aegean Sea, the Levantine Sea, the Tunisian Plateau/Gulf of Sidra, the Ionian Sea, the Western Mediterranean, and the Alboran Sea (Fig. 2A). This regionalisation of the Mediterranean Sea corresponds mostly to classical geographical entities but constitutes a first regionalisation attempt that could be used for regional planning. Indeed, its delineations are clear and correspond to different sub-basins with potentially distinct hydrodynamical, hydrological, biogeochemical, and ecological characteristics. However, regionalisation should rather be based on objective criteria for identifying and defining regions with similar characteristics.

In other parts of the world ocean, regionalisation mainly focused on mapping marine habitats, defined as “a recognizable space which can be distinguished by its abiotic characteristics and associated biological assemblage, operating at particular spatial and temporal scales” (ICES, 2005). Although this definition is well-adapted for benthos, it may be more difficult to apply it to the pelagos (Costello, 2009). Nevertheless, some areas have benefited from tremendous efforts in mapping and documenting pelagic habitats (or seascapes), such as the Austral Ocean (Raymond, 2014) or the seas around Australia (Commonwealth of Australia, 2006). Isolated studies have also documented typologies of hydrological structures and pelagic communities at smaller scale, such as in the Bay of Biscay (Planque et al., 2006) or in the English Channel (Delavenne et al., 2013).

In the Mediterranean Sea, several studies have proposed such objective regionalisations by using statistical clustering applied to various variables describing the open waters (Table 1). Briefly, D’Ortenzio and Ribera d’Alcalà, 2009, Palmiéri, 2014, Mayot et al., 2016 focused on phytoplankton phenology; Nieblas et al., 2014, Reygondeau et al., 2017 used climatological averages of key biogeochemical variables (such as temperature, salinity, nutrient concentrations); Berline et al., 2014, Nieblas et al., 2014, Rossi et al., 2014 used the hydrodynamical properties of surface water masses; Reygondeau et al. (2014) used the composition of biological communities, inferred from the modelled habitats of marine species over the basin (each study is described in greater detail in the next section). This high publication rate (eight regionalisation studies in eight years) reveals a strong focus on the Mediterranean Sea compared to other regions of the world. The Mediterranean Sea seems like a good study case because it can be seen as a “miniature ocean [that] can serve as a giant mesocosm of the world’s oceans” (Lejeusne et al., 2010). Indeed, some of the main features of the global ocean, such as a thermohaline circulation influenced by climate and dense water formation, are represented over smaller spatial scales in the Mediterranean Sea (Bethoux et al., 1999). In addition, the Mediterranean region is a hotspot for climate change (The Mermex Group, 2011) and holds high stakes for biodiversity conservation (Coll et al., 2010). A consensus regionalisation of the Mediterranean Sea is currently missing, while it could not only synthesise our knowledge of this basin but also help our understanding of marine ecosystem at global scale, especially in semi-enclosed seas.

In that context, the Work Package 5 of the French research program MERMEX (https://mermex.mio.univ-amu.fr/?page_id=1663) was a transversal action that seeked to synthesize existing hydrographical, hydrological, biogeochemical, and ecological data in the Mediterranean Sea and to put this knowledge in the context of the multiple natural and anthropogenic pressures acting on the Mediterranean Sea. The present work is a contribution of this MERMEX WP5 and aimed to compare these different regionalisations of the Mediterranean Sea and to propose a synthesis that could be used to inform management decisions as well as for future biogeochemical and ecological studies. We focus on surface, open waters only. While Mediterranean coasts are heavily populated, the previous basin-scale regionalisations did not consider the very coastal areas and, therefore, they cannot be considered here either. Furthermore, they are obviously influenced by localised pressures (riverine inputs, impacts from cities and harbours such as pollution, fishery activities, ) and dedicated regionalisations need to be performed at a more pertinent, smaller scale (e.g., Muñoz et al., 2015, for the Gibraltar Strait; or Hattab et al., 2015, for coastal fishes). Similarly, most previous studies focused on the epipelagic domain, probably because it comprises the euphotic zone, where primary production occurs. Most human activities are also concentrated at the surface (Micheli et al., 2013a) and strongly affect the epipelagic ocean. Finally, only one regionalisation has considered the layers below the epipelagic zone and it has showed that the forcing variables (hence the regions) are different among vertical layers (Reygondeau et al., 2017).

The central questions we want to address are: Where do those recent regionalisations agree or disagree? What consensus regionalisation can we propose? How can we link this synthetic view with the structure and functioning of the Mediterranean Sea? To answer these questions, we will review the methods and data used for these regionalisations, quantify the congruence of their frontiers, and propose a new synthetic regionalisation. The following points will then be discussed: What is the interest of considering a consensus regionalisation? How can it be used by scientists and by managers? What are the scientific bottlenecks and the future directions for regionalisation of the Mediterranean Sea? What could be transferred to larger scale regionalisation efforts?

Section snippets

Previous regionalisations

The first statistical regionalisation of the Mediterranean Sea was proposed by D’Ortenzio and Ribera d’Alcalà (2009) (Table 1). Regions were separated according to the phenology (seasonal cycle) of surface chlorophyll concentration. A non-spatial clustering algorithm (k-means) was used to group pixels according to weekly climatologies computed from 10 years of satellite ocean colour data. Seven “trophic” regimes were identified and coherent spatial patterns matched well-known oceanographic

Methods

To analyse comparable regionalisation efforts with statistical tools, to avoid over-representing some processes, and to propose a synthetic view, only epipelagic regions estimated from non-redundant data were kept. It means that when several regionalisations were proposed using the same datasets, only the regionalisation taking into account the highest number of environmental predictors was kept. Following these criteria, the regionalisation based climatologies in Mayot et al. (2016) was

Consensus frontiers

Although the different regionalisations do not often agree on frontier location at the pixel scale (maximum frontier congruence of 5; Fig. 3), some areas are characterised by high frontier congruence and correspond to consensus frontiers (Fig. 4).

About half of these consensus frontiers may be related to hydrodynamical discontinuities that result in strong gradients of hydrological and biogeochemical properties (Fig. 1; Millot, 1999, Millot and Taupier-Letage, 2005). The Almeria-Oran front, at

A synthetic view of the epipelagic Mediterranean Sea

From the review and comparison of the recent regionalisations of the Mediterranean Sea, this study proposes the first regionalisation based on the consensus of several regionalization studies of the epipelagic open seas in the Mediterranean basin. Because it is based on multiple and independent approaches, this new regionalisation should provide a more synthetic and consensual view of the structure of the Mediterranean Sea than any dedicated study. Using the congruence of frontiers among

Acknowledgments

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

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