Pillars of Hercules: is the Atlantic–Mediterranean transition a phylogeographical break?
Version of Record online: 1 OCT 2007
Volume 16, Issue 21, pages 4426–4444, November 2007
How to Cite
PATARNELLO, T., VOLCKAERT, F. A. M. J. and CASTILHO, R. (2007), Pillars of Hercules: is the Atlantic–Mediterranean transition a phylogeographical break?. Molecular Ecology, 16: 4426–4444. doi: 10.1111/j.1365-294X.2007.03477.x
Four features dominate the palaeoceanography of the Mediterranean Sea: (i) it is enclosed geographically with the presence of sills separating individual basins; (ii) its connection to the Atlantic Ocean — throughout history the main source of biota for colonization; (iii) the climate regime where temperature determines the natural range of living organisms, and where the temperature and evaporation balance determine seawater density and hence thermohaline circulation; and (iv) the sea level, which determines hydraulic connectivity between basins.
The Mediterranean Sea is a fully enclosed sea, except for a 12.9-km wide and 286-m deep connection with the Atlantic Ocean between Cape Trafalgar and Cape Spartel (Straits of Gibraltar) (). The Mediterranean Sea has a depth of approximately 3400 m in the western basin and 4200 m in the eastern basin; therefore, it can be named a proper ocean basin. It is divided into two units by a shallow sill (350 m deep) in the Strait of Sicily, namely the Eastern and the Western Mediterranean. The Bosporus–Dardanelles sill (40–70 m) separates the Black Sea from the Mediterranean Sea. The Pelagosa sill (160 m) separates the central Adriatic Sea from the Eastern Mediterranean Sea. The man-made Suez Canal (opened in 1869) in the southeastern Mediterranean Sea has led to a steady influx of saline water (and biota) from the subtropical Red Sea.
Connection to the Atlantic Ocean
Apart from the Suez Canal, the only source of oceanic water to the Mediterranean Sea originates from the North Atlantic Ocean, entering through the Straits of Gibraltar. The Mediterranean Sea became stably separated from the Atlantic Ocean during the Messinian salinity crisis (MSC), which occurred in the late Miocene (5.59–5.33 Ma) (Hsüet al. 1973; Krijgsman et al. 1999). The sea level of the Mediterranean Sea dropped dramatically. Once the obstruction to this gateway was removed because of tectonic uplifting, faulting and sea level changes, the Mediterranean Sea was flooded catastrophically and synchronously with oceanic water, one of the most dramatic events of the Cenozoic (Fig. 1a–c). Recurrent, short periods of separation between the Atlantic and Mediterranean waters occurred during the Quaternary in correspondence to cyclic ice ages and the associated sea level changes (Fig. 1d).
The relatively small and marginal basin of the Mediterranean Sea has tracked global climate oscillations faithfully. The Lago-Mare facies during the MSC represented evidence of extensive desiccation and the formation of hypersaline and freshwater sub-basins (Krijgsman et al. 1999). Although large areas of low salinity were present in the proximity of river outflows, few coastal marine taxa survived this period (Penzo et al. 1998; Hrbek & Meyer 2003; Huyse et al. 2004). Termination of the MSC coincided with the catastrophic flooding of the Mediterranean Sea. Following the restoration of the marine ecosystem, the Late-Pliocene epoch saw several climate-induced increases in marine production associated with processional minima and coincidental regional warming (Passier et al. 1999; Haywood et al. 2000; Meyers & Arnaboldi 2005). About 3 Ma, the Mediterranean climate was warmer and wetter than today (Haywood et al. 2000). During the Quaternary, the Mediterranean Sea continues to register and amplify the smallest climatic variations occurring at mid-latitude regions (Sbaffi et al. 2001). Hence, the glacial–interglacial cycling so typical during the Pleistocene is well reflected in the palaeontological record. Relatively warm periods (interglacials) have been dated at ~630 000, ~330 000, ~200 000, ~130 000 (the last interglacial) and 11 500 years ago (the current interglacial, named Holocene). Glacial stadia, including the last glacial maximum (LGM) — 24 000–20 700 years ago (Lambeck et al. 2002) have led to colder (but ice-free) conditions. Local analysis of dated deposits by means of proxies of sea surface temperature, point to a close tracking of the climate by the biota (Zonneveld 1996; Paterne et al. 1999; Sbaffi et al. 2001; Rohling et al. 2002).During the Pleistocene period, circulation in the Mediterranean Sea became so restricted at times that the deeper waters became anoxic as in the Black Sea today. Primary productivity of the basin increased and a series of organic rich sediments, called sapropels, were deposited (e.g. at 10 500 and 6100 years ago) (Krom et al. 2005). The climate around the basin was less arid; there was increased rainfall in central and eastern Africa and a higher river flow in the Nile. It is not clear whether the circulation remained anti-estuarine or became estuarine. Such climate changes often translated to an impoverishment in taxa as there was no latitudinal range to track the isothermals. Hence, extinction and recolonization have played a major role in the Mediterranean Sea (Cunningham & Collins 1998).Currently a Mediterranean climate dominates the region, that is wet cool winters and dry warm summers, with a total precipitation of 420 mm. a−1. River discharge into the Mediterranean Sea is dominated by rivers from the northern temperate zone, such as the Ebro, Rhône, Po and Seyhan, and especially from the East and Central European rivers flowing into the Black Sea (Danube, Dnjestr, Dnjepr and Don). Runoff from the arid southern (sub)tropical zone is very reduced (e.g. Nile), such that there is a 1:5 shortfall in precipitation and runoff over evaporation.
Sea level changes are related to tectonics, solar modulation of climate (and ice sheets), geoid and deformation of ocean basins (Lambeck et al. 2002). They have globally and locally had a major effect on the oceans and especially the continental shelves. After the dramatic events of the MSC, sea levels have changed regularly throughout the Pliocene and Pleistocene. The lowest sea levels were observed 140 000 years ago (–130 m) and 30 000 years ago (–120 m) (Fig. 1d). The Mediterranean Sea with its sill determined topography has directly felt the impact of changing sea levels through changing flow regimes between the Black Sea, the Eastern Mediterranean, the Western Mediterranean and the Atlantic Ocean. The Black Sea was separated on and off from the Mediterranean Sea through exposure of the shallow sills at the Straits of Dardanelles and Bosporus (Aksu et al. 2002).
Physical oceanography of the Mediterranean Sea
Mediterranean circulation is forced by topography (water exchange through the various straits), wind stress, and buoyancy flux at the surface due to fresh water and heat fluxes (Robinson et al. 2001). It is highly variable at the basin, sub-basin and mesoscale spatiotemporal scales. As evaporation (most intense in the Eastern Basin) dominates over precipitation, an anti-estuarine (reverse thermohaline) circulation and locally deep convection characterize the Mediterranean Sea (Malanotte-Rizzoli et al. 1996; Malanotte-Rizzoli et al. 1997) (Fig. 2). Less dense Atlantic water enters at the surface through the Straits of Gibraltar with an average salinity of 36.15 ppt (Millot 1999). Salinity tends to increase eastward generating a west-east gradient with a maximum in the eastern Mediterranean Sea (38.0–38.5 ppt) because of the intense water evaporation. The inflowing Atlantic water describes in the Alboran Sea, a quasi-permanent anticyclonic gyre in the west and a more variable one in the east. The particular water circulation in the Alboran Sea generates an oceanographic front located from Almeria (Spain) to Oran (Morocco), called the Almeria-Oran Front (AOF). This is an oceanographic front exhibiting a pronounced step temperature (1.4 ˚C) and salinity (2 ppt) gradient over a distance of 2 km with average water current speed of 40 cm/s flowing southeastward from the Spanish coast to the coast of North Africa (Tintore et al. 1988). The flow of surface water to the east follows the North African coast (Algerian Current) with a strong jet near the Libyan coast (Alhammoud et al. 2005). Surface circulation in the Eastern Mediterranean is counterclockwise (Hamad et al. 2005). It is complicated by the many mesoscale eddies and jets that are characteristic of the Mediterranean in general. The Eastern Mediterranean is starved of phosphorus and hence ultra-oligotrophic. It is reflected in the high water transparency, the low depth-integrated chlorophyll values, the high contribution of pico- and nanophytoplankton and the deeper position of the deep chlorophyll maximum (DCM) (Krom et al. 2005). Deep convection is observed in the South Adriatic Sea with cascading through the Strait of Otranto, the South Aegean Sea and between Rhodes and Cyprus.
- Issue online: 1 OCT 2007
- Version of Record online: 1 OCT 2007
- Received 1 March 2007; revision accepted 26 June 2007
Table S1 Information on the species data sets analysed (reference, sample size and location), genetic parameters (number of nucleotides, polymorphic sites and haplotypes) main genetic partition results (genetic pool, genetic variation and genetic partitioning) and demographic parameters (values of mismatch observed mean, tau &tgr; , theta, q0 and q1) (A), Significant gene pools: 0, no panmixia, but structure not associated with Atlantic?Mediterranean differentiation; 1, one gene pool; 2, Atlantic and Mediterranean; 3, Atlantic, Western Mediterranean and Eastern Mediterranean. (B), Percentage of genetic variation (from AMOVA): if gene pool < 1, source of variation, among populations; if gene pool = 2, source of variation, among groups. (C), Partition of samples: 0, one gene pool; 1, Atlantic; 2, Mediterranean; 2′, Western Mediterranean; 2″, Eastern Mediterranean. (D), No samples from Eastern Atlantic, so only two gene pool arrangement could be tested. (E), Samples from Venice not included as there is the suspicion of another taxon being involved. (F), Samples from Black Sea not included
Table S2 Species and geographical partitions used in the historical demography analysis. Data sets (#) refer to the last column of Table 1, partitions refer to Table 1 of the supplementary material, graph type relates to Figure 3. In bold significant results at the 0.05 level, except Fu’s Fs test, which was tested at the 0.02 level
Table S3 Mitochondrial gene fragment, mutation rates (see text for references), number of nucleotides, generation time (from FISHBASE, http://www.fishbase.org), &tgr;(tau) value and expansion times expressed in units of mutation rate (&tgr;/2ut) and in t = 1000 years
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