Abstract | Introduction | Presentation | Diversity | Main marine currents | The different oceanic zones : Summary of the species - Marine currents and other maps | Cosmopolitanism and endemism |
Species indicative of continental drift Species whose localization is difficult to explain | Anthropic mechanisms | Conclusion
Main marine currents :
Strongly simplified sketch of the global overturning circulation system.
In the Atlantic, warm and saline waters flow northward all the way from the Southern Ocean into the Labrador and Nordic Seas. By contrast, there is no deepwater formation in the North Pacific, and its surface waters are fresher. Deep waters formed in the Southern Ocean become denser and thus spread in deeper levels than those from the North Atlantic. Note the small, localized deepwater formation areas in comparison with the
widespread zones of mixing-driven upwelling. Wind-driven upwelling occurs along the Antarctic Circumpolar Current (ACC). After Rahmstorf .
Issued from : Kuhlbrodt T., Griesel A., Montoya M., Levermann A., Hofmann M. & Rahmstorf S.in Reviews of Geophysics, 2007, 45 (RG2001). [p.2, Fig.1].
Idealized meridional section representing a zonally averaged picture of the Atlantic Ocean.
Straight arrows sketch the MOC (Meridional Overturning Circulation). The color shading depicts a zonally averaged density profile derived from observational data [Levitus, 1982]. The thermocline, the region where the temperature gradient is large, separates the light and warm upper waters from the denser and cooler deep waters. The two main upwelling mechanisms, wind-driven and mixing-driven, are displayed. Wind-driven upwelling is a consequence of a northward flow of the surface waters in the Southern Ocean, the Ekman transport, that is driven by strong westerly winds. Since the Ekman transport is divergent, waters upwell from depth. Mixing along the density gradient, called diapycnal mixing, causes mixing-driven upwelling; this is partly due to internal waves triggered at the ocean's boundaries. Deepwater formation (DWF) occurs in the high northern and southern latitudes, creating North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW), respectively. The locations of DWF are tightly linked with the distribution of surface fluxes of heat and fresh water; since these influence the buoyancy of the water, they are subsumed as buoyancy fluxes. The freshly formed NADW has to flow over the shallow sill between Greenland, Iceland, and Scotland. Close to the zone of wind-driven upwelling in the Southern Ocean is the Deacon cell recirculation, visible in the zonally integrated meridional velocity in ocean models. Note that in the real ocean the ratio of the meridional extent to the typical depth is about 5000 to 1.
Issued from : Kuhlbrodt T., Griesel A., Montoya M., Levermann A., Hofmann M. & Rahmstorf S.in Reviews of Geophysics, 2007, 45 (RG2001). [p.3, Fig.2].
1) Surface currents ( fig. C2)
By definition, planktonic organisms cannot free themselves of ocean currents. Very early on the authors suggested that there was probably a correlation between the main surface- (best-known at the time) or deep-water currents and the geographical distribution of certain genera and species. Sewell (1948, p.387) classified copepods into two groups: epipelagic subjected to surface currents and meso- and bathypelagic subjected to intermediate and deep currents (Sewell, 1948, maps 1 & 2).
Knowledge of bathymetric distribution is often imprecise because of sampling techniques and further complicated by circadian or ontogenic vertical migration and upwellings, which makes it very difficult to establish causality.
This problem may be approached using a subset of the matrix associating the presence of species in the various zones with the route followed by certain currents (fig. C2). Most of the fairly cosmopolitan species probably fall under this schema. The geographical data covers little over a century but is the product of several millennia, or rather longer if we include the aspects of historical geology mentioned earlier
The Agulhas Current (Boden & Parker, 1986; Hutchings et al., 1995) constitutes an essential exchange route between the Indo-Pacific and the Atlantic. The only barrier is a thermal one relative to surface-dwelling tropical species (Van der Spoel & Heyman, 1983, figs. 18, 19, 20) e.g. for Pontellina morii (Fleminger & Hulsemann, 1974, fig. 12) and an equatorial form such as Copilia lata. If we impose as the selection criteria a species' presence in zones 16, 5, 7, 8 and absence from the Arctic zone, ignoring all other zones, 240 meet these criteria. Sander & Moore (1979) explain the localisation of Dioithona oculata by this mode of transport, and this must also be the case for most of the species included above. Jones (1966 b), however, demonstrates that the southern tip of South Africa isolates the Indo-Pacific and Atlantic tropical populations of Candacia pachydactyla. The Atlantic and Pacific forms reveal morphological differences reflecting an imbalance in the genetic pool. Fleminger & Hulsemann (1987) demonstrate similar genetic gradients for populations of Calanus helgolandicus from the Atlantic to the Mediterranean and through to the Black Sea, as do Bucklin et al. (1996) for Calanus finmarchicus in the North Atlantic.
The south equatorial Atlantic current splits near equatorial Brazil into one branch heading north towards the West Indies (zone 7) and another flowing south off the south coast of Brazil (Le Pichon et al., 1978; Angel, 1979; Boltovskoy, 1981). 177 species among the 240 in the preceding list are also found in this zone (zone 13).
Most of the species carried by the Gulf Stream are found in the Sargasso Sea and the temperate zones of the eastern Atlantic (zone 8). The North-Atlantic Drift and the Irminger Current carry some of these species as far as southern Iceland and Greenland (Aksnes & Blindheim, 1996).
The warm Kuroshio Current which flows along the Pacific part of the Philippines, the edge of the China Seas and the southern and central fringe of Japan continues towards the eastern Pacific and the Californian coast (North Pacific Drift). The percentage of species common to zones 22 and 25 reflects the faunal community's relationship to this means of transport. 332 species meet these conditions. Eucalanus californicus and Pareucalanus parki illustrate this distribution mode (Fleminger & Hulsemann, 1973) as do Pontellopsis yamadae and Mimocalanus heronae. The current generated by the northern subtropical anticyclonic gyre (North Equatorial Current) maintains this community of species (Beklemishev et al., 1972; Voronina, 1978; van der Spoel & Pierrot-Bults, 1979; Beklemishev, 1981). 41 species can enter the Gulf of Alaska sensu lato in the Alaska Current including: Acartia liljborgi, Aetideus bradyi, Euchirella bitumida, messinensis, pulchra, Gaetanus armiger, minor, pungens, Pachyptilus abbreviatus, Cosmocalanus darwini, Clausocalanus mastigophorus, Paraeuchaeta hanseni, tonsa, Lucicutia magna, Megacalanus princeps, Gaussia princeps, Metridia princeps, Acrocalanus gibber, gracilis, Archescolecithrix auropecten, Lophothrix frontalis, Scaphocalanus echinatus, Scolecithricella abyssalis, S. dentata, Scolecithrix danae, Scottocalanus helenae, infrequens, Spinocalanus brevicaudatus, Oithona setigera, Corycaeus affinis, C. catus, C. erythraeus, Conaea rapax, Oncaea media, Monothula subtilis, Sapphirina intestinata, Oculosetella gracilis.
2) Deep-water Currents ( fig. C3)
Intermediate deep-water currents (Sewell, 1948, chart 2; van der Spoel & Heyman, 1983, fig. 24) give an insight into the localisation of certain species and in particular the bipolar forms sensu lato.
The following Arctic and sub-Arctic forms are observed in temperate and tropical parts of the Atlantic, or in the Pacific: Chiridius obtusifrons (in zones 27-10-24-9-11), Xanthocalanus profundus (27-8), propinquus (27-9-11-6-16-5), Euaugaptilus hyperboreus (27-22-8-17), Spinocalanus polaris (27-23-9-7), Mimocalanus crassus (27-7-16), Paraeuchaeta norvegica (27-10-9-11-7-8-14-5), Bradyidius similis (27-10-23-24-9-11-8), Centropages abdominalis (27-23-24-22-25-21-14-19-26), Homeognathia brevis (27-9-8-14-17-26), Epicalymma schmitti (27-24-9-8-4), umbonata (27-24-9-8-4), Oncaea lacinia (27-23-24-9-14-4).
Conversely, Antarctic and sub-Antarctic forms may be observed further North: Aetideopsis minor (4-3-13-27), Aetideus arcuatus (3-26-17-7-8-24), australis (4-3-26-12-18-25), Chiridiella subaequalis (4-8-23), Euchirella latirostris (3-12-18), rostromagna (4-3-26-13-18), similis (3-26-5-12-18-8), Gaetanus antarcticus (4-3-26-13-12-16-19-8-22-24), Pseudochirella hirsuta (4-3-26-12-16), mawsoni (4-3-26-18-16), spinosa (3-12-8), Augaptilus cornutus (4-3-16-8-11-23-24), Euaugaptilus aliquantus (4-3-13-16), maxillaris (4-3-16-6-17-8-22), perasetosus (4-3-26-13), placitus (4-26-17-8-22), Haloptilus ocellatus (4-3-13), Pontoptilus ovalis (4-16-6-8), Pseudaugaptilus longiremis (4-16-7-8-10), Calanus australis (3-26-5-13-18-16-20), Candacia cheirura (3-26-5-13-12-18-16-20), maxima (4-3-5-13-16), Farrania frigida (4-26-12-16-17-21-7-22), Rhincalanus gigas (4-3-26-13-18-16-20), Subeucalanus longiceps (4-3-26-5-13-18-16-6), Paraeuchaeta abbreviata (4-3-18-20-9), aequatorialis (4-3-26-12-16-20-19-17-21-8-22), antarctica (4-3-26-13-18-16), biloba (4-3-26-5-13-18-16-21), calva (3-12-16-17-21-7-22-11), comosa (3-26-13-12-18-16-19-17-21-7-22), confusa (3-26-21-7-22), dactylifera (4-3-13-12-18-19), exigua (4-3-5-12-18-16), kurilensis (4-3-12-18-16-20-19-22-23-9), malayensis (3-26-5-16-20-17-21-7-22), parvula (4-3-12-16), rasa (4-3-26-13-12-16), regalis (4-3-26-12-20-19), tumidula (4-3-20-17-21-7-8-22-23), Heterorhabdus austrinus (4-3-26-5-13-18-16-8), Lucicutia bradyana (3-26-5-12-18-16-19), Bathycalanus bradyi (4-3-26-5-12-16-6-19-17-25-22-23-24-9), Bradycalanus gigas (4-16), sarsi (3-26-16-8-11-10), typicus (4-3-26-5-16-20-19-17-7-8-25-22-9), Cephalophanes frigidus (4-3-26-5-13-18-16-20-6-19-17-15-21-7-8-14-25-22-11-23-9-10-27), Cornucalanus robustus (4-3-26-6), simplex (4-26-16-6-17-8), Onchocalanus hirtipes (4-7-8-22-9-10), magnus (4-3-26-16-7-22-23-24), Pseudoamallothrix obtusifrons (4-3-26-16-20-19-17-7-8-11), Amallothrix dentipes (4-3-26-13-12-18-16), A. robusta (4-3-26-20-6-17-8-9-10), Landrumius gigas (3-26-19-17), Mixtocalanus alter (4-3-21-8-22), Racovitzanus antarcticus (4-3-26-5-13-18-8-25-22-23-24-9-27), Scaphocalanus impar (4-13), S. major (4-3-18-16-6-17-21-7-8-22-23-24-9), Spinocalanus horridus (4-3-12-21-7-22-23-24-10-27), S. terranovae (4-3-14), Teneriforma naso (4-18-16-19-7-8), Homeognathia flemingeri (4-3-16-24), Atrophia minuta (3-26-21-7-8-14-24-9), wilsonae (4-24), Oncaea curvata (4-3-26-5-13).
Sewell (1948, p.524, 528 & foll.) emphasised the importance of the Intermediate Antarctic Current in bringing species as far as the Gulf of Oman, for example Paraeuchaeta hanseni, Valdiviella insignis and Bathycalanus bradyi.
The Intermediate Mediterranean Current from which stems the Lusitanian Current (Fraser, 1961 c; R. G. Johnson, 1997) allows Undinella stirni, described as Mediterranean where it is cited several times (Grice, 1971 b; Scotto di carlo et al., 1984; 1991), to be observed in the Sargasso Sea (Deevey & Brooks, 1977). This species is mesopelagic (1000 m et plus). Similarly Labidocera brunescens noted by Sars (1925) North of the Azores may also be an indicator. However, the Mediterranean origins of these species cannot be claimed with certainty.