Tidal fronts are sharp horizontal gradients of density created by the turbulent mixing generated by tidal currents over shallow topography. They form in certain regions on continental shelves, including the areas around Great Britain and France, and Georges Bank on the east coast of the United States.
Georges Bank has had one of the most important fisheries on the east coast. The unusually high biomass of fish is thought to be related to the presence of strong tidal fronts surrounding Georges Bank. These fronts control the nutrient supply to the phytoplankton, and thus the production of biomass on the bank. To investigate how the tidal fronts control primary production, Dr. Changsheng Chen, of the University of Massachusetts-Dartmouth, and I developed a turbulence-closure/mixed-layer/primitive-equation/ecosystem model of the tidal forcing and planktonic biologiy on the bank.
This model showed that the summertime primary production on the bank is strongly controlled by the tidal forcing. Over a tidal cycle, nutrients are mixed into the surface waters in the tidal fronts, supporting the formation of phytoplankton patches in the fronts. The well-mixed waters on the top of the bank support high primary production fueled by an excess of regenerated and "new" nutrients.
Planktonic dynamics
averaged over the 25th tidal cycle.
Surface patterns of phytoplankton biomass from the 3-D model compared to 3 satellite images from the OCTS sensor.
Animation of the
temperature over two tidal cycles on the bank.
Animation of
the phytoplankton over two tidal cycles on the bank.
Animation of
the temperature over two tidal cycles in the northern front.
Animation of
the phytoplankton over two tidal cycles in the northern front.This work has been funded by U.S. GLOBEC
Franks, P.J.S. and C. Chen. 1996. Plankton production in tidal
fronts:
A model of Georges Bank
in summer. Journal of
Marine Research
54:631-651.
Click here to download a Postscript version of the manuscript.
Franks, P.J.S. and C. Chen. 2001. A 3-D prognostic numerical model study of
the Georges Bank ecosystem. Part II: biological-physical model.
Deep-Sea Research II 48:457-482.
Click here to download a pdf version of the manuscript.
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