914 Northumberland Strait
Geology and Seabed Morphology
This Unit consists of the Northumberland Strait and the southeastern Gulf of St. Lawrence between Prince Edward Island and Cape Breton. The coastlines of the western portion are low and featureless, and the steep terrains of the Antigonish and Cape Breton highlands border the area to the east. Underlying Northumberland Strait is a system of troughs and depressions, including a continuous trough along the axis. At the eastern end, water depth gradually increases in the direction of the Laurentian Channel. Between Wood Islands and Caribou this trough is 3.5 km wide and has a series of depressions to 90 m. The bridge to Prince Edward Island at Cape Tormentine crosses a maximum depth of about 30 m. The trough extends along the Cape Breton coast and reaches a depth of 90 m in the north, adjacent to the Laurentian Channel.
Western parts of the Unit are underlain by horizontal or gently folded sedimentary rock of Upper Carboniferous and Permian age, while folded and faulted Ordovician to Middle Carboniferous rocks of the Antigonish and Cape Breton highlands extend under St. Georges Bay and to about 16 km offshore at the eastern end. Prince Edward Island was separated from the mainland as the result of the drowning of a pre-glacial river valley system that was later enlarged by glacial erosion. The low coastlines of the Unit consist of sandstone cliffs, intertidal platforms with sand and mud deposits, barrier islands, and beaches. This area has a generally sheltered wave environment, small tides (0.5-2 m), and a relative abundance of sediments. The sea bottom in the area is sculpted into a variety of features, including sand waves, megaripples, and sand ridges.
The Gulf of St. Lawrence, including Northumberland Strait, has patches of reworked sand and gravel, glacially derived marine silt, and clays at greater depths. Sediments originated largely during the post-glacial period as the sea transgressed the area and reworked material deposited during the glaciation, but sediments originate locally through erosion of coastal bedrock. Local sand deposits in the littoral zone arise
in part from sandstone deposits along the coast. Little sediment is supplied to the coast by rivers owing to the small size of the drainage systems.
The major driving forces for circulation are freshwater runoff, winds, and tides. The surface currents flow from west to east (from Gaspé to the Cabot Strait). Currents are generally weak and shallow depths prevail. The ocean environment exhibits a large temperature range from winter to summer. Ice forms in winter to thicknesses of as much as 120 cm, and water temperatures fall to approximately -1°C. In
summer, mixing is constrained by shallow depths and stratification, and open-water temperatures reach 20°C. Erratic tides are diurnal or semi-diurnal, ranging from 1.1-2.9 m.
The Northumberland Strait has warmer water temperatures in summer than other continental shelf regions of Nova Scotia. Temperatures in winter are comparatively colder due to extensive winter ice and low surface salinities in spring. The west- and north-facing coasts of Northumberland Strait are exposed to higher wave energy levels than east-facing shorelines because wave heights increase from west to east. Beaches and intertidal zones are affected by ice from mid-December to April or May of each year.
Seaweeds are diverse and moderately abundant on exposed rock surfaces but not as abundant as in other Inner Shelf areas because of winter ice scouring. However, the red seaweeds Furcellaria and Irish Moss are abundant enough to be commercially exploited. Irish Moss occurs throughout Northumberland Strait from the sublittoral to 12 m in depth. The warm coastal waters allow several warm-water species to survive, including Gracilaria tikvahiae, Stilophora spp., Dazilea spp., Chondria spp., Griffithsia globulifera, and Lomentaria baileyana.
Phytoplankton in Northumberland Strait include diatoms (most abundant) and dinoflagellates. Diatoms tend to dominate blooms (typically in spring and fall), and the other groups dominate at other times of the spring-to-fall period.
Eelgrass beds develop subtidally adjacent to shore and in coastal bays and estuaries in areas having fine substrate. In many areas the Eelgrass is scoured during winter by sea ice. In some embayments, salt marshes occur with other halophytic vascular plants.
The Northumberland Strait is shallow enough in areas to support species of plants and animals associated with warm, shallow water. Among the nearshore invertebrate fauna is a shallow warm-water fauna which includes a group of benthic invertebrates associated with American Oyster.
The Unit has relatively high levels of primary and secondary productivity and supports a range of species of invertebrates, fish, and algae.
Key fish species include American Plaice, herring, mackerel, cod, Winter Flounder, White Hake, alewife, silversides, smelts, and Atlantic Salmon. Atlantic Salmon occur in many of the rivers, and Capelin spawn in the intertidal-to-immediate subtidal waters in localized areas near Chéticamp and in the St. Georges Bay area. Localized winter fisheries for smelt occur in bays and estuaries along Northumberland Strait.
Other species include redfish, Witch Flounder, Yellowtail Flounder, American Eel, and Thorny Skate. Witch Flounder are plentiful on smooth muddy bottoms northeast of Prince Edward Island and just west of Cape Breton Island. Bluefin Tuna can occur in the fall in the southeastern Gulf of St. Lawrence, particularly St. Georges Bay.
Sea Scallop are distributed throughout Northumberland Strait and major concentrations occur in the Caribou/Wood Islands area. The American Oyster is native to coastal areas of Northumberland Strait where it is restricted to the shallow inshore waters of protected bays and estuaries. Oysters can tolerate low winter temperatures and summer temperatures up to 32°C but require warm summer temperatures to
reproduce and grow. Lobster are also abundant in the area. Productive soft-shelled clam flats are found in Northumberland Strait, particularly in protected bays and estuaries. Adults live in sediments which range from clean, medium fine sand and anaerobic mud to mud and gravel. Snow Crabs occur on muddy bottoms between Prince Edward Island and Cape Breton and extend through Cabot Strait into Sydney Bight (Unit 915). Other invertebrates include Rock Crab, Ocean Quahog, and Softshell Clam.
Grey and Harbour seals and Harbour Porpoise are found year-round in nearshore areas. Harp and Hooded seals can be found on ice in the Gulf of St. Lawrence and may drift through the Unit. Grey Seals are known to whelp in the vicinity of Pictou Island, and many can be found on ice in the Northumberland Strait.
Whales and porpoises enter the area in spring, feed through the summer, and leave in winter.
A range of species occurs, including Killer Whale, Pilot Whale, Blue Whale, Minke Whale, Fin Whale, Atlantic White-sided Dolphin, and White-beaked Dolphin.
The warm summer water temperatures of the Northumberland Strait support a warm-water fauna which includes shellfish of commercial importance: lobsters, Snow Crab, Softshell Clam, quahogs, and oysters harvested from natural populations. Offshore are scallop and ground fisheries, seasonal pelagic fisheries (e.g., tuna), and the harvested seaweeds Furcellaria and Irish Moss. These are all small boat fisheries from many small harbours (e.g., Toney River and Ballantines Cove). Fisheries are managed as part of Fishing Zone 1. Aquaculture for Blue Mussels is becoming important in the warm bays. Marine life and bird- and whale-watching, particularly based from the Cape Breton Highlands National Park, are important for tourism. There is considerable recreational and commercial vessel traffic in the summer.
Sites of Special Interest
- Eastern Prince Edward Island and St. Georges Bay - Bluefin Tuna migrate into the southern Gulf of St. Lawrence and may be caught here in the fall
- Pictou Causeway between Trenton and Pictou - colony of cormorants on old bridge pilings
|T2.7 Offshore Geology |
|T3.5 Offshore Bottom Characteristics|
|T6.1 Ocean Currents|
|T6.2 Oceanic Environments|
|T11.7 Seabirds and Birds of
|T11.12 Marine Mammals|
|T11.14 Marine Fishes|
|T11.17 Marine Invertebrates|
|T12.10 Plants and Resources|
|T12.11 Animals and Resources||
|Associated Coastal Districts and Units|
|210 Plateau-Fir Forest|
|220 Steep Slopes|
|312 Pictou-Antigonish Highlands|
|314 Mabou Highlands|
|521 Northumberland Plain|
|522 Judique Coastal Lowland|
|551 Inverness Coastal Plain|
|582 Pictou Valleys|
|583 Antigonish Uplands|
|592 St. Lawrence Slopes|