Cross-Frontal Exchange and Scotian Shelf Cross-over Workshops
10-12 October, 2000
Holiday Inn, Falmouth, Massachusetts
Discussion Question #1
#1. What is the biological definition of a Scotian Shelf Cross-over?
Facilitator: Dave Townsend
Karen Wishner showed a plot of Calanus counts vs salinity from an April '97 (EN298) process cruise which indicated a strong increase in Calanus as salinity decreased with a peak in the ~32 psu salinity range (see Presentation I). A plot from a May cruise (EN301) showed a similar but not quite as dramatic dependence. Ann Bucklin added that Pseudocalanus newmani appear to originate in the Browns Bank region. Karen suggested that we should see if the salinity vs Calanus dependence holds for the broad scale cruises as well. However, Carin Ashjian cautioned that we must watch out for the temperature and salinity data from the Mocness hauls; she's experienced difficulties in the past. Ann said we also mustn't forget that biological indicators are not conserved quantities and that there will be changes in numbers and relative species composition as assemblages are carried across the Channel.
Bob Beardsley asked what is the typical vertical distribution of P. newmani? Are they bottom dwellers? If so that would have an effect on their water mass marking ability. Ann said that Newmani are not always on the bottom but they do tend to be found lower in the water column.
Dave Townsend then suggested that icthyoplankton can act as tracers of cross-over events. Bob observed that there would be differences between the behavior of passive drifters, which are tied to the surface by buoyancy, and those various organisms that can change their depth and behavior. Peter Smith noted that there is not much of a density difference between the cross-over waters and the waters of the Northeast Peak, so that the vertical structure of the passive, buoyant particles would not change significantly if they were exchanged. Lew Incze asked whether anyone knew of any unique biological indicators of Scotian Shelf waters. Peter Wiebe said icthyoplankton have growth rates with significant temperature dependencies and thus cross-over water might be indicated by anomalous developmental conditions of fish larvae.
The discussion then turned to the physical aspects of the cross-overs. How much water is brought across in events -- how deep -- and could it act as a barrier to cross-bank movement of plankton, particularly along the southern flank? Jim Bisagni and others have estimated the cross-channel transport of one well-resolved event from 1992 satellite data to be of order 0.2 Sv over a period of a couple of months. Charlie Flagg commented that this estimate is VERY large compared to the alongshelf estimates further west (~0.4 Sv) and would imply a change in water mass storage and/or greater water mass losses than usual.
Returning to the biological signatures of cross-overs, Ann B. noted that random events could innoculate the Bank with species that are not spawned on the Bank. It was also noted that that the cross-over events could bring over boluses containing predators as well as plankton and other food sources for the Bank's larval fish. Peter W. added that there are warm water species that extend northward only as far as Georges Bank and perhaps their absence in NE Peak samples could indicate the presence cross-over waters. Charlie F. observed that we are presently using salinity as our primary cross-over signal. Any new indicator, such as a biological one, would have to be validated by its salinity signature, so the question remains as to the utility of such an indicator.
Questions were then raised about benthic invertebrate distributions. Are there bottom dwellers whose distribution might be effected by cross-over waters and biota? No immediate answer was given. The scallop regulations have distorted their distributions.
Finally, Bob B. asked whether nutrients could be used as indicators of cross-overs. Dave T. said that the inorganic nutrients appear to be pretty well used up in the cross-over waters. Charlie F. stated that new nutrients on the NE Peak probably originate in the Maine Surface and Intermediate waters, getting on the Peak over the northern flank.