Intrusions of Scotian Shelf and Slope Water on to Georges Bank and some of the biological consequences

Jim Manning

While we have known for years that Georges Bank is visited by a variety of source waters, our recent MER/GLOBEC observations (1992-present) provide us with a greater appreciation for the importance of the advective intrusions in the form of both Scotian Shelf (SS) washovers into the NE peak surface waters and ring-induced impingement of slope water at depth on the southern flank. The washover has been evident each year (except perhaps 1994) as indicated by satellite imagery and hydrographic observations and, as previous summary talks addressed, is evidently linked to a multi-year cycle of variability far to the north. The slope water intrusions on the other hand are not always evident from the satellite imagery and have time scales similar to the passage of rings (days-weeks). The two phenomenon may occur simultaneously (1995 and 1997 in particular) such that the Scotian Shelf lens is sub-surface and just bankward and a condensed shelf-slope front and is carried along-bank by an intensified shelf-break jet. After the ring has passed, much of the lens is detrained away from the bank. In 1992, the only year that did not have rings nearby, the SS lens extended all the way along the shelf edge to the southwestern corner of the bank. In all the washover years (1997 in particular), the remnant SS water apparently bifurcates such that the on-bank portion is sometimes carried along the tidal-front jet. Hence, the combination of these two "foreign" water masses interspersed with the resident Georges Bank water result in a subtle but complicated hydrographic cross-bank structure which varies year-to-year.

As more and more biological samples are processed we are beginning to make some inferences about the effect of these intrusions. Some examples are the clear delineation of Hydroid polyps (Sullivan) on the shelf water side of the May 1994 and May 1995 slope intrusion, the orders-of-magnitude greater volume of Salps (Bollens) on the slope water side, the much higher feeding and reproductive rate of calanus in the 1995 SS lens (Gallager), and the greater volume (Lough) and better health (Buckley) of larval fish on the shelf water side of the intrusions. The attempt to relate biological signals to hydrographic conditions is particularly challenging in the case of larval fish since the populations are often located in the vicinity of strong fronts (both vertical and horizontal). While we had set out to investigate the effect of stratification via solar heating, we have found intrusions of advective water masses the primary influence on the water column structure.