Science Corner - Crossing the Shelfbreak
Joint Program* students investigate the connection between a density-driven current and enhanced productivity
Co-Authors Lisa Graziano, SEA, Chief Scientist Glen Gawarkiewicz, Woods Hole Oceanographic Institution, Physical Oceanography
Joint Program student Emily Roland points out the location of a Gulf Stream Ring in her analysis of CTD data, during the poster session in the Cramer's main saloon.
When most people think of the Atlantic Ocean south of New England, the Gulf Stream comes to mind as the major circulation feature, of importance to mariners and oceanographers alike. But on a recent cruise with the Joint Program's incoming class of graduate students, the Corwith Cramer focused on a smaller, lesser-known phenomenon closer to home.
Sixty nautical miles south of Martha's Vineyard, at the edge of the continental shelf, lies a major oceanographic feature called the shelfbreak front. This front runs along the edge of the continental shelf from Greenland to Cape Hatteras, although its surface outcrop may extend tens of miles further offshore, over the continental slope. The front is a water mass boundary between waters of the shallow continental shelf, in this case Nantucket Shoals, which become cold and well-mixed in winter, and the deep continental slope, where the influence of the Gulf Stream prevents mixing to the ocean bottom in winter.
Figure 1: The shelfbreak south of Cape Cod, showing the Corwith Cramer sampling grid on cruise 199A, 2005. The western and lower transect stations were CTD casts only; the eastern transect was sampled with Hydrocasts.
The front can actually be seen by observable changes in the water characteristics, including continuous underway surface measurements made by the Cramer (temperature, salinity, and fluorescence), and visible changes in water color. Marine life is concentrated at the front, including seabirds such as the Greater Shearwater and Storm Petrel, many kinds of fish, fishing boats, and marine mammals. Sperm whales, Pilot whales, and several species of dolphin were sited on cruise C-199A. The front is associated with high productivity of phytoplankton and zooplankton as well.
The high productivity at the shelfbreak front is due to persistent upwelling which brings shelf water from the frigid depths to density layers near the surface. This upwelled water brings with it a high concentration of nutrients such as nitrate and phosphate which drive primary production. When these nutrients are brought close to the surface where light is available for photosynthesis, a peak in phytoplankton productivity occurs which enables a complex food chain to thrive throughout the year. It is the shelfbreak front, and its associated jet, that drives the persistent upwelling.
Figure 2. Contoured data from the eastern transect, shown from north to south. A) Temperature, showing the surface warm layer over cold shelf water, and the transition from shelf to slope water at about the 100 meter depth contour. B) Density, calculated from temperature and salinity. The position of the 26.0 kg/m3 isopycnal (line of equal density) is shown by the dashed line. The shelfbreak jet, moving to the west at 1 knot, is indicated by the X. C) Salinity, showing the sharp boundary between fresher shelf and saltier slope water. D) Nitrate concentration (µM), showing the very high values near the bottom and following the 26.0 isopycnal up to 20 meters depth. E) Chlorophyll-a fluorescence, a measure of phytoplankton abundance. (Enlarge image)
Cruise C-199A (June-July, 2005) crossed the shelfbreak with a series of CTD and Hydrocasts (Figure 1) to resolve the front and sample for the upwelling of nutrients along isopycnals (constant density surfaces), based on previous numerical model results and analysis of pre-existing data. Using the ADCP, a shelfbreak jet moving west at about 1 knot was identified over the 125 meter depth contour. The position of the jet is indicated by the X in Figure 2b.
Hydrocasts with a CTD and in situ Fluorometer were done at seven stations on the eastern line of the sampling grid. Nutrient sampling was centered on the 26.0 kg/m3 density line (Figure 2b). In the summer, this density change is primarily due to the sharp salinity gradient as seen in Figure 2c. A plume of high nitrate was measured along the 26.0 kg/m3 density surface, indicating upwelling of highnutrient water from the bottom, at about the 100 meter depth contour (Figure 2d). This high-nitrate plume reached to within 20 meters of the surface, well within the lighted region inhabited by the phytoplankton. The direct result of this injection of nitrate can be seen in Figure 2e: a peak in chlorophyll-a fluorescence, which indicates phytoplankton abundance, centered at 30 meters depth right over the 100-m depth contour.
The data collected here is important as coastal oceanographers begin to plan coastal observatories for the future where the shelfbreak front can be measured continually. Beyond the science, the cruise allowed entering Joint Program students to feel the wind in the sails, see whales and dolphins, learn about a challenging and dynamic oceanographic feature, and live and work closely with other new students. An enthusiastic poster session near the end of the cruise underlined how excited the students were to be aboard the ship and it will undoubtedly be a cruise they will remember for a long time.
* Every year SEA operates a joint program with Woods Hole Oceanographic Institution and Massachusetts Institute of Technology