920 Middle Shelf
| Geology & Seabed Morphology
| Sediments | Oceanography Plants | Animals | Cultural Environment | Topics & Habitats |
The Middle Shelf District has been divided into three Units:
921 Middle Shelf Banks
922 Middle Shelf Basins
923 Valley and Plains
Geology and Seabed Morphology
The Middle Shelf extends roughly from a depth of 110 m near shore to the inner edges of the major offshore banks (Baccaro, LaHave, Emerald, Western, Sable Island, Banquereau, and Georges). Definition of the Middle Shelf District is based on the physiography of the Scotian Shelf but has been extended to include the Gulf of Maine within the same depth ranges. It belongs to the submerged Atlantic Coastal Plain, and its topography was determined by
erosional and tectonic processes while it was above sea level. On the Scotian Shelf (the continental shelf south of Nova Scotia), the inner margin of the Middle Shelf is a trough from 145-180 m deep and 40-50 km wide. Outside the trough is a zone that has three broad basins (Roseway, LaHave, and Emerald) and contains isolated deep banks (Roseway and Sambro) on the western end (of 80-100 m depth), and on the east several banks (Middle, Canso, Misaine, and Western
Banquereau) separated by valleys and intervening ridges. The banks have a cover of Quaternary and Cretaceous material over a Meguma basement (see Figure 31).
In the Gulf of Maine, the Middle Shelf includes a series of basins that extend from Northeast Channel (which separates Georges Bank from Browns Bank) through a series of smaller basins to Grand Manan Basin. This section falls into two geological provinces. The northern part is in the Fundian Lowlands, of which the Bay of Fundy is a part; this series of basins may reflect a former drainage pattern which started in the Bay of Fundy and went out over the shelf. The southern part (Georges Basin and Northeast Channel) is part of the Atlantic Coastal Plain (which is similar geologically to the Scotian Shelf).
Sediments
Middle Shelf areas have sediments derived from glacial activity, and subsequent reworking by an advancing sea level at the end of the glacial period. Bank areas to depths of 110-120 m have coarse sediments of sand, gravel, and occasional boulders. On the bank edges, the bottom is made of sands that have clay and silt. Basins have clay and localized occurrences of silt. The shoreward flanks of the basins have glacial till deposits, and a string of moraine deposits extends from off Country Harbour into the Gulf of Maine.
Oceanography
The Middle Shelf may be subdivided on the basis of water masses into a Scotian Shelf portion and a Fundy/Gulf of Maine portion. Both have the general seasonal pattern described elsewhere for continental shelf areas. Temperatures and salinities in the Middle Shelf tend to be intermediate between those for the Inner Shelf and the Outer Shelf. The influence of the Nova Scotia Current is felt all along the Middle Shelf into the Gulf of Maine. In the Gulf of Maine, there are frontal zones at the northern edge of Georges Bank and a front is induced by tides at the outflow of the Bay of Fundy just offshore. These zones have led to high biological productivity and have an important effect on the distribution and movement patterns of several fish species.
Plants
Phytoplankton are the main plants in the Region and are found chiefly in the upper or "mixed" layer of the ocean where turbulence keeps them suspended. Coralline algae form pale to pinkish crusts on rock and gravel surfaces on the banks. Productivity is generally not as great as nearer to shore or closer to the edge of the continental shelf, but several locations have significant production (e.g., the northern edge of Georges Bank and the eastern side of the Gulf of Maine off southwestern Nova Scotia). Much of the productivity of the Middle Shelf takes place in a "bloom" in the spring from late March to May. The abundance of plants stabilizes at a lower concentration during the summer and usually peaks in the fall. Over the winter, concentrations drop as the mixed layer dissipates. The Middle Shelf also contains occasional drifting seaweeds derived from interactions with slope water and the Gulf Stream much further offshore.
Animals
The Middle Shelf has fewer organisms than the Inner Shelf and a relative absence of filter feeders (except on banks and shallow areas) and animals that graze on seaweeds (e.g., sea urchins). Bottom sediments are also generally finer in the Middle Shelf, especially in the basins, and soft-bottom animals are relatively more important than on the Inner Shelf. Animal communities are diverse and contain a broad range of species and types typical of continental shelf oceanic environments of the Northwest Atlantic.
Animals on the Middle Shelf receive their energy either directly or indirectly from sunlight captured by phytoplankton in the surface waters. Zooplankton are adapted almost exclusively to feeding on phytoplankton and are represented principally by copepods such as Calanus, Metridia, and Oithona. Several kinds of zooplankton, including hyperiid amphipods, krill, arrow worms, and jellyfish feed on copepods and other zooplankton. Several of these groups have representatives which feed on the seabed, but they are a minor component.
Most bottom-dwelling invertebrates spend their early life stages in the water column, where they feed on phytoplankton and in many cases shift to a diet of zooplankton before they return to the seabed. The shift to the warmer temperatures of the surface waters enhances growth. Some lobster stocks occur on the Middle Shelf. Lobster commonly move into the basins of the Middle Shelf and to the Outer Shelf banks and continental slope.
Many of the fish species feed directly on zooplankton during their early life stages. The eggs of species such as cod and haddock float and are in surface waters when they hatch. The young fish stay near the ocean surface for a time before they shift to living near the bottom. The young of many of the so-called groundfish are often distributed over or near banks, suggesting that spawning takes place there.
Various fish and whale species may pass through the Middle Shelf en route to areas just beyond the boundaries of the Region, and octopuses may occur in the deep basins.
Cultural Environment
Fishing banks in this District (e.g., Sambro Bank and Roseway Bank) are important bottom fishing grounds for summer inshore fishing. Some banks at the western end are included in the "offshore" lob-ster fisheries. The area contains submarine cables. Some localities have been used for the dumping of munitions.
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