The Scotian Slope District is a very large area extending from the outer limit of the Outer Shelf (District 930), at approximately 200 m deep, to the political and resource management boundaries at depths of 4,000-5,000 m. This is a fully oceanic environment.
Geology and Seabed Morphology
The District includes the continental slope and rise, but as the boundary between them is not distinct, no attempt has been made to separate them as Units. The slope is indented by canyons and channels, including The Gully and the Laurentian Channel, both of which originate in District 930 (see Figure 32).
The area is underlain by thick post-Atlantic Rift sediments which accumulated continuously in the Scotian Basin since the Mesozoic. The Jurassic and Cretaceous rocks are mildly folded and faulted along the continental margin. Both the Shelburne sub-basin in the southwest and the Sable sub-basin in the northeast have extensive salt deposits. Late Tertiary and Quaternary deposits are horizontally bedded, and some outcrop in canyons and scarp features on the continental slope. The area is subject to high seismic activity with main stress in a southwest-northeast direction with earthquakes of up to magnitude 6.0. The Newfoundland earthquake of 1929 registered 7.2 on the Richter Scale.
Recent sediments accumulating on the continental shelf are slumped along the shelf break and travel down the slope, often in the canyons, as turbidity currents. Thick accumulations of these slumped sediments are found on the slope between 200 and 2,000 m in depth. This talus material includes sand (Sable Island Sand and Gravel), marine silty clay (LaHave Clay), glaciomarine silty clay (Emerald Clay), and diamicton Till. The surfaces of these deposits are marked with pockmarks, paleo-iceberg scour marks, and sand ridges (Laurentian Fan). From the base of the slope towards the deep water is a gradation of surficial sediments: discontinuous, stratified mud series; muds alternating with silt and sand; and finally a sand sheet of Late Pleistocene or Holocene age. These deposits are cut by erosion channels of the same age. The deepwater sediments are covered with a thin layer of pelagic or hemipelagic sediment which includes fine mineral particles and the shells and spicules of marine organisms, for example, Radiolaria.
The District is oceanic in character. Surface waters are generally characterized as slope water derived from the Gulf Stream as warm-core rings diluted about 20 per cent with coastal water. In the area of the shelf break, this water mixes with coastal water of the continental shelf and, with mixing caused by tidally induced "shelf-break fronts," maintains nutrient-rich water at the surface. This effect diminishes with distance, so that nutrients are lower in the Gulf Stream and central Atlantic waters. Deep water at the foot of the slope is clear water derived from the Labrador Current moving along the bottom in a west-southwest direction.
Phytoplankton in the surface water is responsible for the primary productivity that occurs in the District. However, this is only significant in the area of the shelf break because the level of nutrients available diminishes rapidly towards deep water. Some floating patches of Sargassum weed occur; these are of relatively little ecological significance, even though they support a distinct community of animals and may rarely reach the Nova Scotia coast.
The deepwater and oceanic conditions of District 940 support communities of animals not normally encountered in continental shelf waters. The two habitats of the Offshore, open-water and benthic, will be treated separately.
Open-water animals depend upon the primary productivity of the surface waters. Phytoplankton is grazed by herbivorous zooplankton - copepods, cladocerans, euphausiids, and a wide range of larval forms. There are also many carnivorous species, including crustaceans, medusae, and the larvae and juveniles of fish.
The nekton, or free-swimming animals, range in scale from jellyfish to whales, but the predominant forms are crustaceans, cephalopods, and fish. In the deep water these animals are grouped into vertically zoned communities: epipelagic (top), mesopelagic (middle), and bathypelagic (bottom). The mesopelagic community is characterized by a diurnal vertical migration - rising to the surface at night and descending to the depths at day. This migration of several hundred metres allows the deep-water species to take advantage of surface productivity. The epipelagic community includes surface-swimming molluscs (Janthina and Argonauta), cnidarians (Valella and Physalia), and fish (swordfish and flying fish). A number of species of invertebrates and fish are associated with Sargassum seaweed, and goose barnacles (Lepas) are associated with floating objects. The mesopelagic community is composed of crustaceans (shrimps and amphipods), cephalopods (squid and pelagic octopus and fish, particularly the distinctive Lanternfish, Viperfish, and Hatchetfish). These species are all predatory carnivores, are often darkly coloured, and may have reflective plates and photophores (light-producing organs). Lanternfish are found at a depth of 700-1,200 m during the day but rise to within 100 m of the surface to feed at night. The bathypelagic community lives in close association with the bottom and includes economically important types such as Grenadier that occur down to 2,500 m in depth. Many of the species that occur in the bathypelagic zone, such as the Giant Squid, which appears on a 30-year cycle, are poorly known.
The benthic habitat includes communities that live in or on the ocean bottom. In District 940 this is an environment without light. The generally soft sediments support an infauna of worms: Pogonophora and Polychaeta, cnidarians, sea pens, whip corals, solitary corals
(Flabellum), a wide variety of scaphopod, pelecypod, and gastropod molluscs, and echinoderms. The crinoid (sea lily) Rhizocrinus lofotensis has been found on the slope at a depth of 1,700 m. Epifauna includes any animal that roams around the sea bottom or attaches itself to a solid object. Old ice-rafted boulders are colonized by sponges, cnidarians, bryozoa, and brachiopods, while crustaceans, sea spiders, and brittle starfish are vagrants. A variety of bottom-feeding fish occur, including the Atlantic Batfish, Deep-sea Anglerfish, and chimaeras. Blue
Hake occurs at a 1,300-2,500 m depth.
Though the deep water off Nova Scotia is distant from most common human experience, there are some cultural aspects of note. The open sea is fished for pelagic species such as swordfish and sharks which occur along the edges of the Gulf Stream. There is high potential for oil and gas development when technology, economic climate, and environmental considerations make this feasible. Deep water in The Gully has been voluntarily closed to tanker traffic by oil companies to protect the Northern Bottlenose Whale population. Many shipwrecks lie on the bottom, including some vessels that were intentionally disposed of. The freighter Suerte, which came ashore at Three Fathom Harbour, Halifax County, was towed out to be sunk off the continental slope in 1962. Munitions have been dumped south of Georges Bank in 2,000 m of water. The area is traversed by several submarine cables, some of which were damaged by the slumping of sediments caused by the Newfoundland earthquake. Cables recovered for repair or replacement are usually covered with growths of deep-sea animals. It is reasonable to presume that wrecks or any other objects of human origin that are placed on the bottom will become colonized by epibenthic animals.
|T3.5 Offshore Bottom Characteristics|
|T6.1 Ocean Currents|
|T6.2 Oceanic Environments|
|T11.14 Marine Fishes|
|T11.17 Marine Invertebrates|
|T12.6 The Ocean and Resources||
|H1.1 Offshore Open Water|
|H1.2 Offshore Benthic|