6.1 Historical Context

The general bathymetry of the North Atlantic mid-ocean ridge was mapped by the early 1960s and studies of oceanic circulation across the ridge and deep water flow through the Charlie Gibbs Fracture Zone (CGFZ) were well advanced by the start of MAR-ECO field work (see, for example, Krauss 1986; Rossby 1999; Bower et al.2002). Gradually improved bathymetric data revealed the axial valley, numerous hills and valleys, and major fracture zones reaching abyssal depths. Circulation features are shown in Figure 6.1, including the Sub-Polar Front (SPF), which crosses the ridge in the vicinity of the CGFZ at around 52° N and may be significant to biogeography.

The SPF separates the Cold Temperate Waters Province (CTWP), and the Warm Temperate Waters Province (WTWP), defined by The Oslo–Paris Commission (OSPAR) based on extensive reviews of the regional biogeography data (Dinter 2001). Provinces defined by Longhurst ( 1998) were mainly based on surface features, one of them being an east–west asymmetry in the diversity patterns of zooplankton in the central North Atlantic (Beaugrandet al. 2000, 2002). Biogeography of the bathyal benthic fauna at the northern MAR was addressed in recent studies of the Reykjanes Ridge and seamounts south of the Azores (Mironov et al. 2006), but almost no data were available from the CGFZ-to-Azores section of the MAR. On the ocean-basin scale, Mironov ( 1994) proposed the concept of “meridional asymmetry”: specifically, that some western Atlantic species are widely distributed in the Azorean-Madeiran waters whereas the eastern Atlantic benthic invertebrates are confined (with very rare exceptions) to the East Atlantic.

Pelagic and demersal nekton of the northern MAR were investigated by various historical expeditions that crossed the North Atlantic (see, for example, Murray & Hjort 1912; Schmidt 1931; Tåning 1944), and later by the Atlantic Zoogeography Program (Backus et al. 1977), and German expeditions to the mid-ocean and seamounts (see, for example, Post 1987; Fock et al. 2004). Information existed on the distribution of cephalopods at various specific locations in the Atlantic (Vecchione et al. 2010), revealing general latitudinal patterns and information from isolated seamounts, but none were focused on the MAR. Although the fish fauna and general distribution patterns of deepwater fishes of the northern Atlantic Ocean had been described (see, for example, Whitehead et al. 1986; Haedrich & Merrett 1988; Merrett & Haedrich 1997), surprisingly few previous studies have focused specifically on the role of the mid-oceanic ridges in the distribution and ecology of either pelagic or demersal fishes. Studies from the Azores have shown very low endemism, and that most species have distributional affinities with the eastern Atlantic and the Mediterranean (Santos et al.1997; Menezes et al. 2006). Considerable knowledge of fishes associated with ridge systems has been gained from fisheries-related research (Bergstad et al. 2008b, 2008c), but most reports focused strongly on target species and usually on only the shallower parts of the ridge and specific seamounts. Only in exceptional cases have full species lists of the catches been published (see, for example, Hareide & Garnes 2001; Kukuev 2004). Areas of the northern MAR have been, and still are, exploited for fish species such as redfish (Sebastes spp.) (Clark et al.2007). Pelagic fisheries of the open ocean have targeted tuna, swordfish, and sharks that tend to be found near fronts, eddies, and islands. Whales also occur in such areas (Sigurjónsson et al. 1991) and, like the epipelagic fishes, they migrate extensively, perhaps associated with the MAR.

Life-history strategies had not been studied for any species on the MAR, but information was available for some species on adjacent seamounts or continental slopes. These data constituted valuable comparative sources for new studies of the diversity of life-history strategies characterizing ridge-associated species.

Knowledge of large-scale distributions across and along the MAR was lacking for most pelagic and demersal macro- and megafaunal groups. Basin-wide population connections were also unknown. It was uncertain whether the MAR fauna was unique or composed of elements from the adjacent continental slopes.

MAR food webs were unknown, except for a few studies along the Reykjanes Ridge, and life-history information was only available for a very limited number of zooplankton taxa (copepods, mainly Calanus spp.), but lacking for most other species. The general trophic positions of some common zooplankton species, primarily copepods, amphipods, and euphausiids, inhabiting the epi- and upper mesopelagic layers above the Reykjanes Ridge have been described (Magnusson & Magnusson 1995; Petursdottir et al. 2008). Also, the spawning aggregations of redfish confined to the western slopes of the Reykjanes Ridge suggest that this is a productive area (Pedchenko & Dolgov 2005). However, no information existed on how the MAR affects productivity or abundance of mesopelagic organisms.