Going Off the Edge
Any canyon pelagic begins with a transit of the continental shelf. The shelf is simply a submerged coastal plain composed of sediments laid down upon the crystalline bedrock foundation at the seaward margin of the continent. South of Cape Cod and the Islands the shelf platform is a smooth, very gradual slope and is about 80 miles wide. In the Gulf of Maine the shelf is very rugged - due to glaciation during the last Ice Age - and extends some 200 miles out from the coast of Maine to the southern flank of a huge offshore rim known as Georges Bank.
The seaward margin of this shelf platform is known as the continental shelf-edge or shelf-break. It is located along the 100 fathom (600 foot) depth contour and marks the sharp transition between two fundamentally different geological provinces - the shelf and the deep ocean basin. We often refer to crossing this line as "going off the edge" and the realm beyond it as "the deep water." Starting at the 100 fathom line, the sea floor plunges into the deep, with depth increasing from 600 to 6000 feet (or essentially one mile deep) over a horizontal distance of only around 10 miles. This steep escarpment is known as the continental slope and is the area explored by most canyon trips.
The base of the slope is located along the 1000 fathom (6000 foot) depth contour. Beyond this the sea floor continues to grade downward, but at a more gradual rate, over what is called the continental rise. Land-based sediments from the shelf sporadically spilling over the edge in the underwater equivalent of avalanches have formed this feature, but, unlike the sediments on the shelf, these lie on top of a foundation of oceanic rock, formed nearly 200 million years ago at the Mid-Ocean Ridge.
The waters at the base of the slope are -at 6,000 feet - still only a little less than half as deep as the average depth of the Atlantic basin. Pelagic trips seldom venture far beyond the 1000 fathom line, but, if they did, one would have to travel nearly 250 miles south to find a flat bottom. At that point one would be over the Hatteras Abyssal Plain half way to Bermuda and the depth would be about 16,000 feet.
Figure 1. A 3-D view of the seafloor structure at the edge of the continental shelf, slope, and rise off the Middle Atlantic States. The big gorge on the right is the Washington Canyon and the one on the left is the Norfolk. Bottom structure is very similar to that found in the northeast canyons. Note all the minor gullies along the slope between the major canyons. Also note how the major canyons and minor gullies empty out onto the continental rise at the base of the image. [Image courtesy of the NOAA Photo Library www.photolib.noaa.gov.
Submarine Canyons: Hot spots for Offshore Biodiversity
The shelf edge south of New England and the Gulf of Maine is cut by numerous steep-sided gorges called submarine canyons. There are 15 major and minor ones along the southern flank of Georges Bank alone. Like their terrestrial counterparts, they were cut by swift flowing rivers, probably during a period of intense erosion about 50 million years ago that also excavated the ancient landscape that is now the Gulf of Maine. They have been kept free of sediment in more recent time by deep sea mud avalanches (called turbidity currents) that occur on time scales of hundreds to thousands of years and by the burrowing activities of bottom creatures.
Hydrographer Canyon is probably the most familiar to New England pelagic birders and is typical of most of the larger canyons in the area. It takes its name from one of the US Coast & Geodetic Survey vessels that mapped the Georges Bank canyons back in the 1930's. The northern terminus -or canyon head - is cut 11 miles back into the shelf from the 100 fathom contour. At the canyon-mouth it is 3.3 miles wide and its walls descend at a slope of 25 to 35 degrees to a depth of 3000 feet along the mid-point or canyon axis.
The steepness of the canyon walls has protected them from trawl and dredge-based fishing activity and preserved bottom communities in a near pristine state. In fact, the canyons are hotspots of offshore biodiversity with a much greater species richness than the adjacent shelf. Down in the narrows of the canyon floor strong tidal currents nourish lush "gardens" of burrowing sea anemones. The steep canyon walls are riddled with burrows excavated by large Tilefish, prompting those who first observed them from submersibles to call them "Pueblo Villages." The Pueblo Village Community supports a diversity of bottom life, including large Lobsters abundant enough to support a commerical fishery. Along the edge of the canyon walls orange floats and aluminum poles called "hi-fliers" mark the anchored ends of long trawls of lobster pots set down into the deeps. It is sometimes even possible to gauge your position relative to the canyon walls by keeping an eye on the lobster gear floats.
In addition to all the bottom life, there is a whole new habitat up in the water column that is not found over the shelf. Between 600 feet and the bottom there is a cold, dimly lit region known as the "midwater" or mesopelagic zone. Much of the life in this zone exhibits vertical migration, remaining below 600 feet during the day and venturing up to the surface at night to feed under the cover of darkness. Abundant shrimp-like krill, small bioluminescent lanternfish, and medium-sized Shortfin Squid are common inhabitants of this layer. They, in turn, provide food for the predatory fish, whales, dolphins, and birds that forage in the canyons.
For a fascinating acount of a research trips to study the midwater community in our own Oceanographer Canyon go to the Harbor Branch Oceanographic Institute's web page and http://at-sea.org/missions/deepscope3/
The Slope Water: Dynamic zone with Boreal and Tropical Influences
In addition to geological changes there is also a transition in the physical properties of the seawater that occurs at the shelf-edge. A sharp boundary line (called an oceanic front) is usually found somewhere between the 100 and 1000 fathom contours. To the north of this line lies the cooler, less saline, high-nutrient shelf water. To the south lies warmer, higher salinity, low-nutrient deep ocean water. This water mass is known as the slope water and is wedged between the shelf-edge and the average position of the Gulf Stream's north wall around 38.5 degrees north latitude.
An excellent indicator of the position of this frontal boundary is a color change in the water. The shelf water is usually a bright, "bottle" green color and very turbid. On the shallow shelf platform nutrients are within reach of mixing by winds and tide resulting in high levels of phytoplankton (microscopic plant) production. The chlorophyll in these tiny plants absorbs most of the blue from sunlight entering the water and reflects strongly in the green wavelengths. The vast numbers of plankton organisms also create turbidity and limit visibility into the water. Fishermen will often refer to this shelf-water as the "green water" and refer to its turbidity by describing it as "dirty."
Out in the deep water nutrients tend to sink below the reach of mixing by winds and limit the level of phytoplankton production. In general, the slope water supports only one-third to one-fifth as much phytoplankton as the adjacent shelf water. With fewer plants and low chlorophyll concentrations, the light scattered back from the water is mostly blue. It is also much less turbid (or "cleaner") due to fewer organisms and detritus in suspension. This is called the "blue water" and can vary from greenish blue to deep, cobalt blue, depending upon local conditions. Figure 2 (from the University of Maine School of Marine Sciences) illustrates these differences nicely.
Figure 2. Chlorophyll concentrations over the NE U.S. coastal waters for period 6/26 to 7/3/01. Note high chlorophyll (1-2mg/cu.m) east of Cape Cod over Georges Bank, low chl (<0.5mg/cu.m) over slope south of Georges Bank, and pink areas of very low (<0.1mg/cu.m) in tropical water over the slope and rise.
The color change usually marks a sharp increase in water temperature. If there isn't a lot of wind you can usually feel this temperature break as a sudden change to warmer air flowing over the bow. During the warmer half of the year the increase across this break averages around six degrees - from the mid-sixties to mid-seventies fahrenheit. At times, however, it can be much more dramatic - as much as twelve degrees between the 100 and 1000 fathom lines south of Georges Bank. Crossing through a gradient like this can be like going from Maine to North Carolina over a horizontal distance of only a few miles.
Figure 3. Northeast Canyons sea temperature chart for June 25, 2001 courtesy of Offshore Satellite Services, Inc. (www.offshore-seatemp.com). Note temperature break from 65 degree shelfwater to 74 degree warm-core ring along the 500 fathom line just east of Hydrographer. This change occurs over a horizontal distance of only six miles. Also, note the spatial complexity of temperature over the canyons.
There is a common misconception that this warm, blue water is the true Gulf Stream current brushing up against the shelf-edge south of New England on its way eastward towards northern Europe. A look at any good satellite image of ocean temperature for the Western North Atlantic, however, will dispel this myth quickly. See http://fermi.jhuapl.edu/avhrr/gallery/sst/stream.html. The average position of the north wall of the stream is located around 38 to 38.5 north putting it about 90 to 120 miles south of places like Hydrographer Canyon.. See the following web site for real time pictures of sea surface temperatures. http://fermi.jhuapl.edu/
While I'm at it, another common misconception is that the cold water on Georges Bank and in the Gulf of Maine is the Labrador Current. This myth is also easily dispelled by a good satellite image like those found on the John Hopkins web page.
The true Labrador Current is visible in this image as a blue filament of cold
water flowing down the east side of Newfoundland and terminating over the slope
off the southwest tip of the Grand Banks. This is as close as it gets to New
England waters. Cold water in the Gulf of Maine is just a chilled subsurface
layer -produced at the surface the previous winter season -being mixed to the
surface by strong tidal currents.
As many as three warm-core rings form each year in this region
between Cape Hatteras and the Grand Banks. They are large features, reaching up
to 100 nautical miles in diameter and extending from the surface to depths of
several thousand feet. At their edges currents of up to one knot extend from
surface to bottom. Separated from the main current, rings wobble up against the
shelf-edge and then drift slowly westward at speeds of one to four nautical
miles a day. Eventually, after a lifetime of a few months, they coalesce with
the Gulf Stream again near Cape Hatteras.
Another excellent indicator of tropical water is a pelagic seaweed
called Sargassum or Gulfweed. It is not unusual to find scattered clumps of
this yellowish weed as far inshore as the 40 fathom line along the outer shelf.
Significant amounts in blue water, however, are an excellent indicator of
tropical conditions. This weed is capable of carrying out its entire life cycle
(reproducing by asexual buds) drifting in bottomless blue water. It's name is
derived from the Portuguese salgaza which translates to "grapes" - a reference
to the small bladders on the fronds that keep the plant afloat. The vast
central quiet zone of the North Atlantic Gyre to the southeast of Bermuda
derives its name - Sargasso Sea - from the huge amounts of it that collect
there under calm winds. It has been estimated that a typical square mile of
this ocean area could have two to five tons of this weed floating at the
¹ Updated July 2008 - Removed dead links