Ecology of the Northeast US Continental Shelf
Physical Setting and Habitat
The Northeast U.S. Continental Shelf Large Marine Ecosystem (NES LME) encompasses an area of approximately 260,000 km2 from Cape Hatteras in the south to the Gulf of Maine in the north. The shelf is wide off northern New England, extending over 200 km from shore, and relatively narrow off Cape Hatteras where the shelf break is approximately 30 km from shore (Figure 1). The physiography of the Gulf of Maine and its complex shoreline was strongly shaped by glacial activity during the last ice age which ended approximately 12,000 years ago. Similarly, Georges Bank was created with the retreat of the Laurentide ice sheet during the Wisconsinan glaciation event. To the south, in the Middle-Atlantic Bight, the topography is more uniform and the shelf gently slopes to the edge of the continental shelf.
The Gulf of Maine, a semi-enclosed continental shelf sea, is characterized by an extremely complex physiographic structure. Three major deep basins occur in the Gulf (Figure 1). Georges Basin is the smallest but deepest of the three, covering an area of 4100 km2 with a mean depth of approximately 300m. Wilkinson and Jordan Basins are similar in average depth (approximately 225 m) with Wilkinson the larger of the two (~7100 km2 vs ~6700 km2). There are over 20 smaller basins located with the Gulf of Maine. Two relatively large ledge-bank systems (Stellwagen and Jeffries Ledges) occur within the Gulf of Maine proper. Four major river systems feed into the Gulf of Maine (the Androscoggin, Penobscot, Merrimack, and Kennebec Rivers), playing an important role in the oceanography of the coastal Gulf of Maine.
Georges Bank, a broad shallow submarine plateau forming the seaward boundary of the Gulf of Maine, is delineated to the north and east by the Northeast Channel and to the south and west by the Great South Channel (Figure 1). The bank encompasses approximately 42,000 km2 within the 100 m isobath. When water levels were substantially lower at the end of the last ice age, Georges Bank was part of a larger cape, and later with rising sea levels (approximately 14,000 years ago), an island. It was completely submerged approximately 11,500 year ago. The average depth of Georges Bank is now 75 m but is just 30 m at its shallowest on Georges Shoals.
The seaward margin of Georges Bank on the continental slope is incised with 11 major submarine canyons. Submarine canyons are typically deep, V-shaped valleys cut into the sediments of the continental slope and shelf approximately perpendicular to the depth contours of those structures, (Figure 2). A large number of smaller, unnamed canyons, also called gullies, cross the slope in between the large ones, but do not impinge on the shelf.
Canyons converge at their lower ends into a smaller number of collection valleys, called canyon channels, that continue down across the continental rise toward the abyssal plain. Like terrestrial canyons, these submarine megafeatures were created by erosion. However, while several of the large canyons are aligned with the now-submerged valleys of rivers that flowed across the shelf during the last ice age when sea level was nearly 100 m lower, none of the submarine canyons were actually exposed; they were not carved by rivers like terrestrial canyons. Rather, they are thought to have formed initially by slumping of accumulated sediments beginning on the slope, which propagated progressively shelfward with time, and were further excavated by sediment flows. Hence the largest canyons are thought to be the oldest. Submarine canyons in the NES LME are not restricted to the Georges Bank region. Major canyons further to the south include Block Canyon off southern New England, Hudson Canyon (part of the Hudson River drainage system, and Norfolk Canyon off the Virginia capes.
Major influences in the Middle-Atlantic Bight region include several large estuaries including the Hudson, Delaware Bay, and Chesapeake Bay.
Although our major focus is on the continental shelf and adjacent slope regions, it is worth noting the importance of a major seamount chain located off the New England shelf. Seamounts are focal points of biological activity in the deep sea. They are the subject of considerable recent conservation interest because of the unique fauna of these systems. The profile, topography, and the associated complex currents associated with seamounts make them unique habitats, but ones that are difficult to sample and study. Only a small fraction of the estimated 30,000 Pacific and 800+ Atlantic seamounts have been extensively studied. Fundamental questions regarding the diversity of organisms, their abundance, how they colonized the seamounts, and how the seamount community is structured and functions remain unanswered.
The New England Seamounts (NES; Figure 3) comprise the longest seamount chain in the North Atlantic, encompassing more than 30 major volcanic peaks extending from Georges Bank southeast for about 1200 km to the eastern end of the Bermuda Rise. Bear Seamount (39° 55’N 67° 30’W) is the most inshore of the New England Seamounts, located inside the U.S. Exclusive Economic Zone (EEZ) south of Georges Bank. It rises from the continental slope at depths of 2000-3000 m to a generally flat summit at 1100 m depth.
Sediments are the bottom materials deposited by water, wind or glaciers, as opposed to the more permanent bedrock. Sediments are by far the dominant type of surficial substrate in the NES LME and slope. They are important in an ecosystem context due to their abundance and for other reasons including: 1) some or all life stages of many plant and animal species are closely tied to certain sediment types, so their distribution and abundance are partly determined by sediments; 2) sediment-dwelling organisms from microbes through benthic macrofauna are important in food webs and other ecosystem functions; 3) sediments are a significant site for deposition and uptake of organic carbon and contaminants, and nutrient regeneration, and they sometimes contribute to bottom water hypoxia and release of toxic compounds such as hydrogen sulfide and ammonia; and 4) sediments are relatively amenable to monitoring to determine trends over space and time in contamination and other ecosystem indicators.
Geologists typically divide sediments into several size classes. The largest is gravels, which are 2 mm or more in diameter; with increasing size, these sediments are termed pebbles, cobbles, and then boulders. Sands are between 2 mm and 62.5 microns. Silts are from 62.5 down to 4 microns, and clays are 4 microns or less. Both silts and clays are also called “muds”. The finer sediments are more easily moved by bottom currents, which gives rise to the familiar pattern of sands and gravels being found in inshore and other high-energy (“erosional”) areas, and silts and clays in deeper and less energetic (“depositional”) areas.
In the Middle Atlantic Bight, the pattern of sediment distribution is relatively simple (Figure 4). Most of the surficial sediments on the continental shelf are sands and gravels. Silts and clays predominate at and beyond the shelf edge, with most of the slope being 70-100% mud. Fine sediments are also common in the shelf valleys leading to the submarine canyons, as well as in areas such as the “Mud Patch” south of Rhode Island. There are some larger materials, left by retreating glaciers, along the coast of Long Island and to the north and east.
North and east of Cape Cod, sediment distributions are more complex (Figure 4). This is partly due to the area’s rugged bottom topography, which features many basins, swells, knolls, banks, and submarine canyons. Glacier-transported materials are much more common in this region. Bottom currents are also complex, and have a large influence on the area’s sediment types. The shallower parts of Georges Bank are predominantly sandy, and areas with relatively stable sands (which are moved only by storms) can be distinguished from areas where the sands are often in motion - this has important implications for faunal distributions. On the southern flank of the bank, sand waves over 15 m in height occur. The bank also has large areas of gravel pavement, especially at its northern edge, which are considered valuable habitat for species such as cod and scallop.
North of Georges Bank, in the Gulf of Maine proper, the topographic highs have sands and larger materials including glacial erratics (boulders), while the basins are floored with muds interspersed with boulders and rocky outcrops. The sedimentary characteristics of the Gulf of Maine are the most complex in the region, with an intricate mosaic of bottom types in the nearshore Gulf of Maine, expanses of clay and silty sands in the deeper portions of the central and western gulf and a mix of sand/silt/clay in the deepest reaches. Areas of exposed bedrock are also found throughout the gulf (Figure 4).
Complex Physical Habitats
Hard, immobile substrates (including the larger of the sediment types discussed above) provide a distinct, important habitat for biota to attach to or live within or near. Besides providing stable attachment sites and shelter, the added surface area of complexly-structured hard substrates often increases food supply. Some or all life stages of many species are dependent on complex hard substrate, while other species use this structure although they are not as strictly tied to it. Man-made structures such as bulkheads, piers, bridges, shipwrecks and artificial reefs provide many of the same functions as do the natural hard substrates.
Rocky coastal areas are rare in the southern Middle Atlantic Bight, but become more common north and east of New Jersey and Long Island. Offshore (as noted in the Sediments section), bottom substrates in the Middle Atlantic include relatively little natural rock. However, the amount of complex hard substrate has been substantially augmented by man, especially via shipwrecks and construction of artificial reefs. It has been estimated that there is now more man-made than natural habitat of this type in the Middle Atlantic. The increase in amount of this habitat has probably affected distribution and abundance of harvested stocks including lobster, cod, red hake, ocean pout, scup, black sea bass and tautog, as well as the many other species associated with the habitat (Figure 5). There is a long-standing scientific debate over the extent to which artificial reefs increase overall production of fishery species, versus simply concentrating these resources, which in turn could increase the risk of overfishing them.
In northern New England, rocky substrates are the rule along exposed coastlines and in shallow waters. Bedrock and boulders left by glaciers are also very common at greater depths. There are several large submarine ledges (e. g., Jeffreys, Cashes) rising above the surrounding bottom.
Complex Biogenic Habitats
Seabed habitats comprise a complex blend of bottom features and associated animal communities. Often, habitats are “biogenic”; that is, formed by the animals themselves (Figure 6). These may also provide shelter for other species, including fish. Areas that are structurally complex as a result of geological features or biogenic structures often support highly diverse biological communities. Some of these habitats are also particularly vulnerable to disturbance by natural forces and human activities. It is for this latter reason that habitat protection has assumed an important role in current fishery management.
The types of habitat described above are centered on physiographic features associated with the sea bed. However, many marine animals spend their lives in the water column itself with some taking excursions to the sea floor for feeding and other purposes. The physical geography of the sea is defined not only by bottom characteristics but by a complex array of oceanographic features including currents and frontal zones (see Oceanography). Animals principally associated with the water column are considered to inhabit the pelagic ecosystem. Many types of schooling fish, marine mammals, sea turtles and top predators such as sharks, tunas, and billfish are important components of the pelagic ecosystem. Other important members of these pelagic communities include small (in some cases microscopic) animals that are important links in the food web (see Zooplankton). These zooplankton species drift in the ocean currents and are often concentrated in frontal zones and other oceanographic features. Frontal zones can be generated by tidal forces or by the confluence of water masses characterized by different temperature and other features. Fronts can often be recognized at the surface by concentrations of sea foam, debris, or other materials. In areas such as Georges Bank, fronts or convergence zones separate areas that are well mixed by tidal forces and winds from areas that are seasonally stratified (or layered, with warmer and/or fresher water on top) and these are important pelagic habitat areas for many species (Figure 7).
Many species forage in oceanographic structures such as fronts where their prey are concentrated. For example, large shoals of small pelagic fish such as herring and mackerel are often found at tidal mixing fronts where high densities of their planktonic prey are found. In turn, fishing activities directed at pelagic species are often concentrated in these areas to capitalize on these natural associations between predators and their prey.
Essential Fish Habitat
Habitat protection is a cornerstone in the development of ecosystem based fisheries management. Ecosystem based fisheries management is inherently geographically specific, and therefore naturally linked to considerations of habitat and local seascapes. The specification of “habitat areas of particular concern” under current management measures shows how fine-scale information on habitat and associated biological communities can be used to protect critical areas.
Under our current fishery management law – The Sustainable Fisheries Act - we have provisions for the identification and protection of ‘Essential Fish Habitat” (see accompanying text box). The interest in protecting vital habitat centers on the role it plays in the productivity of living marine resources. Habitats provide food and shelter for many species and therefore directly affect their productivity. If we lose critical habitat, the ability to support these organisms is diminished. The amount of sea life that an ecosystem can sustain – its carrying capacity – depends on the availability of appropriate habitat, among other factors. For species that live on or near the seabed, the types of physical habitat we have described is critical. For other species that spend their lives in the water column, oceanographic features such as frontal zones may be critical habitats (see above).