Great South Channel Diner: When Swarms of Tiny Organisms Aggregate Every Spring, They Create a Hotspot Where Whales Come to Feast

Article excerpt

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When people are hungry, they go to place where they know they can find their favorite food. Right whales do much the same thing.

In the Great South Channel, a deepwater passage between Nantucket and Georges Bank, thick swarms of tiny marine organisms known as copepods appear every spring. Basking sharks, cod, haddock, and endangered North Atlantic right whales all feast upon these dense patches. In a way, the Great South Channel is like the right whales' favorite restaurant, and they keep coming back every year.

Nobody knows how these patches form. But understanding why they do would certainly help policymakers design marine protected areas and fisheries regulations to help conserve marine animals. This is why we are at sea aboard the research vessel Tioga at the crack of dawn. Our team from the Autonomous Systems Laboratory at Woods Hole Oceanographic Institution (WHOI) is helping to untangle the combination of physical and biological processes that form these dense copepod patches.

Above the steady drone of the engines, a loud "beep" penetrates the air on the bridge. The tone indicates that we are close to an underwater glider that has been surveying the area for the past two weeks. It's 8 a.m. on a Saturday morning in mid-May, and everyone is in position to recover the glider.

Back ashore in the lab, Dave Fratantoni, a WHOI physical oceanographer and my Ph.D. advisor, checks the glider's most recent position and relays the information to the rest of us in the field. Captain Ken Houlter, at the helm, guides the ship toward the waypoint. Ian Hanley, the first mate, rigs the deck for recovery. I huddle over a computer with Ben Hodges, our lab's research assistant, reading the data stream that accompanies the tone, trying to discern the range and bearing to the vehicle. We relay the information to Houlter, who alters course accordingly. Then we begin to scan the horizon for the glider's little yellow tail, a flag smaller than a sheet of paper marking not only the location of the vehicle but also a new piece to the puzzle of plankton patchiness.

At the mercy of currents

Scientists have known for many years that right whales and other predators feed on copepods in the Great South Channel, but what makes this place so special remains a mystery. In fact, biologists don't know how whales even find these dense patches. Past research has suggested that the physical environment--the currents, tides, and possibly the bathymetry of the ocean floor--might play roles in creating these important feeding areas. However, no one has yet identified the specific mechanisms--physical, biological, or a combination--by which dense copepod patches form.

The waters of the Great South Channel are extremely dynamic. A fairly steady coastal current flows south along the east coast of Cape Cod's forearm and into the Great South Channel. When it reaches the channel, some of this coastal current flows south around Nantucket Shoals, while another part turns toward the east and joins the northeastward flow along the northern flank of Georges Bank.

To the south, part of another current, flowing southwestward along the southern flank of Georges Bank, turns north into the Great South Channel.

On top of all this, strong currents driven by winds and tides are also flowing. At any given moment, these may be twice as fast as the steady currents. The copepods are at the whim of these complex flow patterns. To determine where the copepods will go, we must understand the flows that move them around.

A multipronged approach

The complex nature of the currents in the Great South Channel demands that we employ a variety of different techniques and tools to study the ocean circulation. Each is better suited for studying some aspects of the physical oceanography than others; combining methods allows us to understand more of the picture than any one method alone. …