URI oceanographers find gulf stream migration affects biological productivity in unexpected way

Situated between the continental shelf of the eastern United States and the north wall of the Gulf Stream flowing eastward from Cape Hatteras, the Slope Sea is a transition region between the productively rich coastal waters and the productively static open ocean.

In the current SeaWiFS special issue of Deep Sea Research II, University of Rhode Island oceanographers Stephanie E. Schollaert, Thomas Rossby, and James Yoder describe their four-year, NASA-funded study of the Slope Sea along the Gulf Stream in order to understand the processes that control the yearly variability of surface concentrations of chlorophyll, the pigment found in plants and algae.

Annually, cold, fresh Labrador waters “spill” into the Slope Sea, influencing the path of the Gulf Stream, pushing it south in the spring. Since the advent of ocean color remote sensing in 1978 and particularly since the 1997 launch of the dedicated ocean color sensor SeaWiFS, the surface chlorophyll concentration of waters off the U.S. east coast have been found to be highest in the north (e.g., Gulf of Maine, Georges Bank, Labrador shelf) and during the winter when the Gulf Stream is farthest south and more Labrador water is present.

The scientists expected that during the years when the Slope Sea expanded due to a greater transport of Labrador water, primary productivity, or the production of plankton that forms the basis of the food chain, would be increased. However, their results showed the opposite effect.

The Gulf Stream location determines the area of the Slope Sea and the extent to which nutrients are present. Years when the Gulf Stream is farther south the average chlorophyll concentrations are smaller and vice-versa when the Gulf Stream is offset to the north. An increased number of Gulf Stream rings may also play an important role in supplying additional nutrients to the surface waters. While the north-south movement of the Gulf Stream is the primary determinant of Slope Sea chlorophyll concentrations, the current’s movement may also generate other effects that may influence biological productivity. Understanding how the Gulf Stream’s migration affects biological productivity in the Slope Sea will help scientists make inferences about large-scale, low frequency climatic effects upon the carbon cycle of ocean margin waters.

The URI Graduate School of Oceanography is one of the country’s largest marine science education programs, and one of the world’s foremost marine research institutions. Founded in 1961 in Narragansett, RI, GSO serves a community of scientists who are researching the causes of and solutions to such problems as acid rain, harmful algal blooms, global warming, air and water pollution, oil spills, overfishing, and coastal erosion. GSO is home to the Coastal Institute, the Coastal Resources Center, Rhode Island Sea Grant, the Institute for Archaeological Oceanography, the Pell Marine Science Library, and the National Sea Grant Library.