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


Discussion Question #4

#4.    What are the impacts of Scotian Shelf Cross-overs on the target species in the Georges Bank ecosystem?

Facilitator: Lew Incze
Rapporteur: Peter Smith

Most of the sampling program in GLOBEC did not focus on cross-overs per se. As a result, we need to consider all possible fortuitous samples (where we were in the right place at the right time) and the results of process cruises where we might use knowledge about vertical distributions, rates and species interactions to estimate impacts. Peter Smith had a cruise which sampled specifically in crossover water, but he did not cover much of the other areas of the bank and there were relatively few biological samples. However, Peter's data could be sandwiched between two of the broadscale cruises that would supply some of the missing data for the rest of the bank, allowing us to piece together a more complete picture. Likewise, two of the larval fish process cruises in April (EL9904) and May (EL9905), though they focussed on the tidal front on the southern flank, also conducted bongo/CTD grids far enough offshore that they may have sampled into cross-over waters. By combining all these sources, there may be enough material to address (at least partially) the impacts of Scotian Shelf (SS) cross-overs on the Georges Bank (GB) ecosystem. These opportunities need to be looked for in the cruise/data time tables that everyone agreed should be assembled as a product of this workshop.

Several suggestions were made that deserved further attention after the meeting.

1) Larval feeding selectivity: do the larvae prefer or do better with one species of prey vs. another? For instance, Pseudocalanus spp. vs. Calanus spp., or P. newmani vs. P. moultoni? This could result from prey catchability or predator preferences (both behavioral), or even differences in the degree of aggregation/vertical distribution of the different taxa (presumably, increased concentrations lead to increased feeding rates until prey reach satiating levels). Is there a difference in the abundance of the important taxa (for larval feeding) between SS water and GB water in spring?
2) Similar questions could be asked about the vertical distribution and abundance of important predators. Are the communities of predators the same, and/or are they at the same stage of development in SS and GB waters in spring? Are the vertical distributions such that you could be causing the overlap of otherwise non-overlapped taxa? (The group estimated from available data that most cross-overs are in the layer above 50 m, and some may be much shallower.) This question is not just limited to larval fish, but may apply to other plankton as well.
3) Dave Townsend showed data from the interior of the Gulf of Maine that suggested that the spring bloom occurred earlier in SS water compared to the eastern Gulf, and that SS water in early spring appeared to introduce water of lower nutrients but more advanced bloom conditions. Does this difference apply to GB as well, that is, is the SS bloom earlier? If SS cross-overs onto GB carry water of reduced nutrients and more advanced spring bloom conditions, what are the consequences for the other holoplankton and meroplankton on the bank? What about benthic invertebrates located beneath the layer of particle production?
4) Does SS water introduce important meroplankton or ichthyoplankton to GB?
5) As another possibility, it was suggested that Mellisa Wagner be contacted about possible differences in condition of
C. finmarchicus that might be attributable (or expected) with SS water.

Ann Bucklin commented on the exceptional abundance of Pseudocalanus spp. on GB during 1999. She suggested that the prevalence of SS cross-overs in 1999 supported the unusually high abundance of P. newmani in that year. By comparison, P. moultoni abundance is comparatively robust and steady from year-to-year, and she suggested that it was the addition of P. newmani that lead to the exceptional over-all abundance of the genus. By extension of this example, the "innocular" events represented by SS cross-overs may have large impacts on the bank if they occur at the right time(s). Have we considered this possibility for all the important taxa? A comparison could be made with 1997, a comparatively "low cross-over" year.

In general, participants in the discussion agreed that even if SS water were not that different from GB water when viewed as systems, cross-over events certainly could have impacts that result from their timing relative to timing of the local processes. Thus, some consideration should be given to differences in timing of planktonic production cycles in the two systems, and also documenting the history and estimating the volumetric exchanges involved in cross-over events and the sum of these events in different years. The feeling was that the estimates derived preliminarily for this meeting sounded high and should be reexamined.

Secondary circulation effects on local biology and distribution were discussed briefly (e.g., baroclinic flow around an intrusion), but it was felt generally that these effects were small (because of small horizontal density gradients) compared to other factors that move plankton around.

Finally, workshop biologists were almost all planktologists. Fish and invertebrate specialists should be consulted for their input.