Plankton dynamics on the outer southeastern U.S. continental shelf. Part I: Lagrangian particle tracing experiments

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Abstract
The residence time and flow patterns of plankton populations on the outer southeastern U.S. continental shelf were studied with Lagrangian particle tracing experiments. Flow and temperature fields used for these experiments were constructed by applying optimal interpolation methods to current meter data obtained during the Georgia Bight Experiment I and II which took place 25 February to 18 June 1980 and 10 June to 24 September 1981, respectively. The interpolated fields reproduced the flow and temperature structures associated with Gulf Stream frontal eddies and bottom intrusions, which are the upwelling mechanisms of interest in this region. The general particle tracing results showed that plankton residence time and flow trajectory are controlled primarily by the Gulf Stream location and wind direction. During times when the Gulf Stream is located near the shelf break, plankton are transported rapidly to the north with little onshore flow. Residence times are short, being on the order of three to four days. When the Gulf Stream is located offshore of the shelf break and wind patterns are variable, particle transport shows no preferred direction and residence times on the outer southeastern U.S. shelf are long; sometimes in excess of thirty days. Tracing of particles in waters upwelled in frontal eddies and bottom intrusions showed considerable differences in the fate of plankton associated with these features. Residence times of waters and particles upwelled in frontal eddies are short, four to six days, and transport is northward with the Gulf Stream. Bottom intrusion waters, by contrast, remain on the continental shelf for more than twenty days and transport of these waters and of associated particles is across the shelf to the inshore regions. The particle tracing experiments showed that the different upwelling regimes and changing physical environment greatly affect the transport of material from and across the outer southeastern U.S. continental shelf. This in turn implies that these physical processes are a major component influencing the structure of plankton communities of this region.