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The Continental Shelf-Edge
An Oceanographic Primer for Pelagic Birders
Mike Gooley

gooley@chem.bu.edu
Updated July, 2008, by Emmalee Tarry





Pelagic Birding Directory

Seabird Areas New England

The Continental Shelf Edge

Nor'easter Birding

 

Birders have been venturing to the canyons at the edge of the continental shelf south of New England since at least the mid 1980's. Places like the Hydrographer Canyon are well known for sightings of warm-water specialties - like Audubon's Shearwater and White-tailed Tropicbird - and deep-water wanderers - like White-faced Storm-Petrel. Unfortunately, there is little in the popular literature explaining the underlying oceanographic processes that make this area so interesting. The purpose of this primer is to introduce the physical and biological changes that occur in this transition zone between the shallow shelf and the deep ocean basin.

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.

Norfolk Canyon Under sea feature

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.

mgooleyfig3

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.
http://fermi.jhuapl.edu/avhrr/SW/averages/01aug/SW_01aug27_2325_multi.gif
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.

So, how does tropical blue water end up in the northeast canyons when the true Gulf Stream is located much further offshore? The answer involves a fascinating interaction between a mightly ocean current and an ancient chain of extinct volcanoes.

When the Gulf Stream reaches the waters south and east of Georges Bank it encounters a 600 mile-long chain of extinct volcanoes known as the New England Seamount Chain. These flat-topped pinnacles reach to within 5,000 feet of the surface from waters that are as much as 16,000 feet deep. Theory suggests that the seamounts act like rocks in a stream diverting the current into steep north/south undulations called meanders. For a good look at this phenomenon see the following satellite image from 16 April 2001: http://srbdata.jhuapl.edu/d0043/avhrr/gs/averages/01apr/gs_01apr16_2206_multi.gif

Gulf Stream rings form when meanders become so elongated that they detach from the true current. Northward meanders pinch off a parcel of tropical water from the other side of the stream and enclose it in a clockwise spinning ring of Gulf Stream water. This kind of ring is called a warm-core ring. Southward meanders pinch off slugs of shelf-water inside a counter-clockwise spinning ring of Gulf Stream water and eject them into the open ocean south of the stream. These are known as cold-core rings.

For an awesome overview of the entire Gulf Stream system off eastern North American go to the following http://kingfish.coastal.edu/gulfstream/page2.html. Be sure to select next at the bottom of each page.
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.

In many ways, warm-core rings are similar to "cut-off" low pressure systems in the upper atmosphere with the exception being that their circulation is opposite (clockwise vs. couterclockwise). Like upper level lows they create a lot of instability in the "ocean weather." Perhaps hardest hit is the bottom community in the canyons. The strong ring currents can create swirling sediment storms that last for days either burying or exposing bottom creatures, depending upon location.

Nearer the surface, long streamers of shelf-water can be drawn out (in a process technically known as entrainment) along the eastern edge of a ring, where the current turns offshore, and deposited way out over the deep ocean basin. This process can lead to an exception to the rule that a color and temperature change is located somewhere near the shelf-edge. If you happen to go to a canyon that is covered by an entrainment streamer you could conceivably continue way out over the slope and still be in green shelf-water. To the plankton and larval fish of Georges Bank, this is the equivalent of a "natural disaster" resulting in certain death in the barren open ocean water where there is not enough food to sustain them.

Rings sometimes wobble southward to interact with the north wall of the Gulf Stream. When they do, entrainment streamers (this time from the Gulf Stream) can be shot up the western side of the ring, injecting the slope and shelf-edge with hot tropical water. Rings can also interact with nearby meanders leading to the formation of additional rings. For an excellent series of satellite images showing warm-rings and entrainment streamers go to: http://fermi.jhuapl.edu/avhrr/gallery/sst/eddy_97jun11/eddy.html

The result of all this complicated physics is an extremely dynamic environment at the shelf-edge and over the slope off eastern North America. Tremendous amounts of heat and salt as well as entire communities of marine life are transferred back and forth across the Gulf Stream. Significant changes can occur in this environment on time scales of days to weeks. This is why conditions can be so different in any given canyon each time you visit it on a pelagic. One year a Hydrographer trip might find cobalt blue, 77 degree water and the next year the same trip might find the entire canyon covered by 65 degree green-water. It all depends on where the warm-core rings are located.

Warm-core Rings Bring Tropical Marine Life to the Northeast Canyons

When a warm-core ring enters the northeast canyons it brings with it an entire community of tropical marine life - everything from predatory fish down to tiny plankton. A knowledge of typical "Gulf Stream" species can help the pelagic naturalist know when he or she has entered the ring water.

One sure-fire clue that you are in tropical water is the presence of a jellyfish-like creature known as the Portuguese Man-of-War. It is actually not a true jellyfish, but a member of a closely-related group called the siphonophora. While it looks like a single organism, in reality it is a colony of highly modified individuals, each
adapted to specific tasks. A gas-filled bladder (resembling a purple inflated
zip-lock bag!!) allows it to sail before the wind and troll for small fish with
stinging tentacles that can be up to thirty feet long. If you get close to one
you might even notice small fish swimming fearlessly among the tentacles. These
are the Man-of-War fish - a species that is immune to the stinging cells and
takes refuge among them.

Photograph taken by Jeff Slovin.

Portuguese Man O'War Jeff Slovin

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 surface.

Clumps of Sargassum act like floating reefs in the deep blue water. Bring aboard a few good-sized clumps and you've got an instant invertebrate zoology laboratory exercise. There are representatives of many animal phyla adapted to carry out their entire life cyles on the weed. They all share the common characteristic of "Sargassum-colored" camoflage. Most common are small Gulfweed Shrimps that hop actively out of the weed when you handle it. There are also at least two species of crabs - including the large, beautiful Gulfweed Swimming Crab - a relative of the Blue Crab. There are flatworms and mollusks known as nudibranchs. Even the fronds are covered with small, plant-like creatures called hydroids and encrusting animals called bryozoans.

The weed also provides cover for small fish. Almost every good-sized clump will have a school of tiny, Sargassum-colored filefish beneath it. In large patches the bizarre, frog-like Sargassum fish - exquisitely shaped and colored to resemble a piece of the weed- often turns up. And, finally, don't be surprised if you see what looks like a tiny fragment of Sargassum get up and "fly" across the sea surface. Flying fish lay their eggs in the weed and the juveniles take advantage of Sargassum camoflage to escape detection by predators. For a nice introduction to the floating Sargassum community with photos go to: http://people.clemson.edu/~jwfoltz/WFB416/subj/Oceans1/sargasso/sargass.htm

Adult flying fish are another reliable indicator of tropical water. They can be up to ten inches in length with long silvery "wings" (modified pectoral flippers). It's always a pleasant surprise to see one launch out of the water and glide for as much as 100 yards before reentering with a splash. Where there are flying fish there are usually larger pelagic fish that prey on them. A gorgeous tropical species known as Dorado (or Mahi- Mahi) can often be seen hanging out under the weed and around the orange floats of the offshore lobster gear in the canyons.

Even sharks and their relatives can act as tropical indicators. Blue Sharks are common in the Gulf of Maine and especially in the cooler waters on the southern flank of Georges Bank. Out in the blue water, however, they are less common and other, warmer-water species take their place. The one that has emerged as a tropical indicator in my experience is the Hammerhead. It is even possible to encounter huge Manta Rays.

What does all this mean for pelagic birders?

The purpose of this primer has been to increase awareness of the physical and biological environment at the southern New England shelf-edge among the pelagic birding community. I hope that it will help promote a better understanding of water types and and how they influence seabird distribution. The key to a successful canyon pelagic is knowing where to look for the right kind of water, and being able to recognize it when you have reached it.

A good example of this is an experience I had as co-leader of a canyon whale/bird trip in June 2001. Late in the afternoon of our second day at sea we entered the 1000 fathom depths just southeast of the Hydrographer where our temperature charts (see figure 3) showed the northern edge of a warm-core ring. Before long there was a noticeable increase in the temperature of the air flowing over the boat. At the same time the sea temperature gauge shot up to 73.9 degrees. With very blue water, Sargassum weed, and large flying fish in the area it was clear that we were in the tropical water of the ring. I leaned over the rail on the bridge and shouted down to the birders in the bow the following message:

"If we're going to see any Audubon's Shearwaters, this is where it will be."

As if on cue, a couple of minutes later two Audubon's took flight and crossed the bow! There was no ESP involved, just an awareness of the oceanography and the habitat preferences of the birds.

We are still largely ignorant of the patterns of seabird distribution and abundance in the northeast canyons. Even the list of species the occur in the area is probably not complete. Black-capped Petrels and Bridled Terns - species closely associated in people's minds with Gulf Stream trips off Cape Hatteras - have been seen in the Hudson Canyon off northern New Jersey. A Fea's Petrel was even seen (and documented unmistakeably with photographs - see Fea's Petrel) by whale researchers in a large canyon called "The Gully" east of Sable Island, Nova Scotia in July 1997. I'm convinced that warm-core rings and Gulf Stream meanders probably bring these tropical birds into our canyons from time to time. We are just not out there enough to see them. Every canyon trip is important and the more birders who get out the better will be our understanding of this fascinating ocean wilderness area.

References:

Backus, R.H. and D.W. Bourne (eds.) Georges Bank. Cambridge, MA. MIT Press. c 1987.
Emery, K.O. and E. Uchupi. Western North Atlantic Ocean: topography, rocks, structure, water, life, and sediment. Tulsa, OK. American Association of Petroleum Geologists. c. 1972.

Teal, J.& M. The Sargasso Sea. Boston, MA. Little, Brown. c.1975.

Valentine, P.C. The shelf-slope transition-canyon and upper slope sedimentary processes on the southern margin of Georges Bank. Washington, DC. U.S. Geological Survey. c.1987.

¹ Updated July 2008 - Removed dead links

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