Dye/drifter/video studies of transport on Georges Bank: prelminary results of R/V Endeavor Cruise 323/324, May-June 1999

J. R. Ledwell, J. H. Churchill, S. M. Gallager, D. J. McGillicuddy, Jr.,
C. S. Davis, and C. J. Sellers

A series of water-tracking experiments were performed on the flanks of Georges Bank from R/V Endeavor to study processes governing the exchange of plankton across the tidal mixing front in May and early June 1999. Rhodmine-WT and drogued drifters were used to track patches of water in a surface experiment on the South Flank and in two pycnocline experiments, one on the South Flank and one on the North Flank. Plankton distributions in these patches were measured with the towed Video Plankton Recorder (VPR), which was also used to measure dye, chlorophyl, and hydrographic parameters. Tracking was aided by the shipboard ADCP and by forecasting with the Dartmouth College numerical model (QUODDY) on board the ship. Plankton densities and swimming behavior were observed with a VPR system mounted on an ROV during times of light winds near the tidal mixing front.

The combination of techniques proved effective at following water patches and plankton populations within them for periods of up to five days as they circulated and dispersed along the flanks of the bank. The surface layer experiment on the South Flank, showed an example of wind-driven transport across the front and onto the crest under easterly winds. The surface waters were rich in hydroids and very poor in Calanus, while the pycnocline was rich in Calanus.

The pycnocline experiment on the South Flank included some surface drifters, which again showed transport toward the crest. The dye patch and drifters in the pycnocline showed mostly along-isobath flow, but with eddy dispersion toward the front. The dye release was performed in a strong patch of Calanus, which, however, nearly disappeared at the end of the experiment upon the arrival of large schools of herring, mackerel and whales.

In the experiment on the North Flank, the drifters gave various results on flow toward the tidal mixing front. Again, the dye patch moved primarily along isobaths. Just below this patch was an abundant population of the colonial diatom Chaetoceros socialis, concentrated in a thin layer at the base of the pycnocline and extending into the deep mixed layer.

The experiments promise to improve our understanding of the transport of plankton toward and across the tidal mixing front, both in the surface layer and in the pycnocline. The drogue and dye data shold give quantitative estimates of mixing and transport parameters, and the VPR observations will help to assess the extent to which such activities as swimming and predation add to fluid motions in determining plankton populations and fluxes. These results will ultimately be of use in coupled bio-physical models of the ecology of coastal banks.