Water Sources for Georges Bank

Bob Houghton and Rick Fairbanks

As part of a project to identify the climate change sensitive upstream water sources responsible for the hydrographic environment and flux of passive organisms and dissolved constituents into the Georges Bank region, analysis of oxygen isotope properties, a unique freshwater tracer along the NE continental margin, is used to identify water sources on Georges Bank (GB). In the Gulf of St. Lawrence and on the Scotian Shelf, St. Lawrence River water (SLRW) and Labrador Shelf water (LShW) mix with Labrador Slope water (LSW) to produce the water entering the Gulf of Maine via Northeast Channel and Cape Sable Shelf (NEC/CS). The oxygen-salinity properties of this water are distributed along a mixing curve (Figure 1) between LSW and a freshwater end-member consisting of 5% SLRW and 95% LShW. The SLRW provides 45% of the freshwater content (relative to 34.8 psu) of this end-member. Two realizations of this curve in 1995 were virtually identical. Future analysis will look for interannual variations in this freshwater end-member which would be affected by changes of the relative fluxes of LShW and SLRW.

Water on Georges Bank (GBW) is derived primarily from the NEC/CS inflow either mixing with Maine Coastal water (MCW) within the Gulf or from flow shunted directly onto the Bank. There have been significant variations over the past decade (Figure 2). In 1996 and 1997, when Georges Bank salinities have been low, there was no MCW component so presumably the shunt circulation predominated and the seasonal salinity variation on the Bank is derived from changes in volumes and/or mixing along this curve. In 1995 GBW appears to be distributed roughly along a mixing line with end-members consisting of the intersection of the Maine Coastal Water - Maine Intermediate Water mixing curve with the NEC/CS mixing curve and a mixture of 50% MCW and 50% freshwater from NEC/CS. Along this curve Maine River Water contributes 1.2% of the water by volume and 12% of the freshwater (relative to 34.8 psu) to GB. In 1994 the coastal influence was even greater with contributions of 63%, 2.2%, and 38% respectively. Greater still is the MCW influence on the Bank in 1981-1982.

It is interesting to note that the influence of MCW is least when salinities on the Bank are lowest which is counter intuitive since MCW is the closest source of freshwater. Our results suggest that fluctuations of the MCW component on the Bank is more than just variations in dilution and that perhaps there is exclusion of coastal water due to changes in circulation and mixing patterns induced by changes of upstream fluxes into the Gulf of Maine.