Cross-Frontal Exchange and Scotian Shelf Cross-over Workshops

10-12 October, 2000

Holiday Inn, Falmouth, Massachusetts



Cross-Frontal Exchange

Presentations Discussion Topics Upcoming North Sea Study (LIFECO) (St. John)

Scotian Shelf Cross-over

Presentations Discussion Questions Synthesis Topics Appendix


Biological/Physical Interaction Studies

Biological and Physical Processes during the Cross-frontal Transport of Zooplankton at the Northern Front and Southern Flank Tidal Front of Georges Bank

Karen Wishner and Dawn Outram (URI)

The purpose of this work was to examine the cross-frontal distributions and exchange of zooplankton, and their predators and prey, relative to the tidal cycle, frontal features, and the day-night cycle. In conjunction with Gifford et al., we studied two regions in 1999: the Northeast Front from 28 March - 13 April (EN321) and the South Flank Tidal Front from 13 June - 1 July (EN325). Work was coordinated with the Barth / Hebert / Ullman Seasoar and drifter cruises on the Oceanus at the same times and places. Sampling included hydrographic and Scanfish surveys (Belkin, Boyer, Ullman), vertically-stratified zooplankton distributions (1 m2 MOCNESS) (Wishner), naupliar growth rate and feeding studies (Gifford), and CTD-rosette casts for determining the prey field and phytoplankton (Gifford, Sieracki). The sampling strategy consisted of two modes: an Eulerian mode during which CTD and MOCNESS samples were collected every 4 hours at a single station for 24 hrs (done at 3 stations spanning the tidal front) and a tidally-coordinated mode during which transects of these 3 stations were made day and night at two different tidal phases (maximum on-Bank slack and maximum off-Bank slack). CTD and MOCNESS tows were done in quick succession at each station.

At both fronts, well-mixed water extended farther off-Bank during the maximum off-Bank slack tide, while stratifed water extended up onto the Bank during the opposite phase. Calanus distributions usually peaked in the surface layer when the water was stratified. Abundances were highest in strongly stratified situations and lowest in the on-Bank well-mixed water. Thus the Calanus distributions at any particular location and along the transects were strongly associated with the tidal phase. Calanus did not vertically migrate, so day and night tows were similar for this species. However, at the Northern Front, a vertically migrating species showed strong day - night differences in addition to tidal phase effects.

We are working with the Barth / Hebert / Ullman group to align the tows within the tidal cycle and ultimately to quantify cross-frontal exchange of the zooplankton. Additionalprojects using these samples include Bucklin / McGillicuddy's studies of Pseudocalanus cross-frontal distributions and Sullivan's studies of the predator distributions.