Fig. 11. Map showing the topography of the Charlie-Gibbs Fracture Zone.
At around 52oN, a major topographical feature known as the Charlie-Gibbs Fracture zone divides the MAR into a northern and southern section. The CGFZ is a system of two main parallel deep rift valleys running perpendicular to the main Ridge axis.
Previous studies by current meter moorings and deep drifters have shown major flow of deep-water between the western and eastern deep-sea basins of the North Atlantic through these deep channels.
The topography is truly spectacular with depths ranging from 4500 m in the deepest channel to only 700-800m on top of adjacent seamounts. Near the CGFZ is also the near-surface frontal zone between cold water to the north and warm saline water to the south, known as the Sub-polar Front.
The ecological processes in this topographically and hydrographically interesting area are not well understood, but observations made during the MAR-ECO cruises in 2003 and 2004 should stimulate further studies in future years.
Fig. 12. Geographical distribution of crustacean plankton catches. (Photo: David Shale)
The abundance of large mid-water crustaceans; mainly decapods (shrimps) and amphipods, increased from the north towards CGFZ where there was a maximum. Further south, the abundance declined significantly. This indicates that the standing stocks of crustaceans are particularly high in the frontal zone near CGFZ. This was not only seen in catches, but also from optical recording of zooplankton by the Underwater Video Profiler (UVP) operated in the upper 1000m. The crustacean fauna is characterised by large beautiful red shrimps and krill. Further analyses are needed to clarify species compositions and biogeographical patterns.
Another indication of the significance of this area came from near-surface copepod egg production studies. Nowhere along the MAR were the egg production rate higher than in the CGFZ and Sub-polar front. Copepods are important grazers of phytoplankton at the surface, and themselves major prey of vertically migrating mid-water predators such as small fish, large crustaceans and gelatinous zooplankton.
An example of biogeographical pattern was offered by the occurrence of two different squid species, Gonatus sp. and Heteroteuthis dispar north and south of the frontal zone, respectively. It is anticipated that many similar patterns will be revealed by future detailed analyses of the data obtained on both Leg 1 and 2.
Mammals occurred along the entire section of the MAR studied. On Leg 1, the mammal team observed more than 1400 whales and dolphins, and baleen whales (particularly sei whale, and fin whale) where especially abundant near the CGFZ in association with steep topography. Seabirds were observed systematically on Leg 1, and more than 1900 birds were counted and identified along the survey tracks.
Scientists to be credited: MAR-ECO zooplankton and pelagic nekton teams. PIs: Tone Falkenhaug, Astthor Gislason, Uwe Piatkowski, Leif Nøttestad, Henrik Skov.
Fig. 13. Geographical distribution of copepod egg production. (Photo: Astthor Gislason)