Water sources for Georges Bank

Abstract

The origins of Georges Bank source waters can be quantified by the oxygen isotope — salinity tracer. Regionally, water is “tagged” by freshwater of different H 2 18O/H 2 16O ratios, and these waters can subsequently be identified after mixing downstream. This conservative Lagrangian tracer of source waters is useful for tracking passive plankton and larvae that may be important to sustain the Georges Bank ecosystem. The origin of Georges Bank freshwater sources is primarily in the Gulf of St. Lawrence consisting of Labrador Shelf Water, which enters via the Strait of Belle Isle, and St. Lawrence River Water. These two mix with the underlying Slope Water in the Gulf of St. Lawrence and along the Scotian Shelf. At the Northeast Channel, the input to the Gulf of Maine, there is a single freshwater endmember consisting of 5.6% St. Lawrence River Water and 94.6% Labrador Shelf Water by volume. These proportions remained constant for the years 1995–1997. The water on central Georges Bank consists of a mixture of water entering the Northeast Channel with Gulf of Maine River outflow. There is considerable interannual variation in the Maine River Water proportion, ranging from 3.3% in 1982 to 0.23% by volume in 1996. During 1995–1998 there was no correlation between Georges Bank salinity, which decreased monotonically, and the proportion of Maine River Water on the Bank. The interannual fluctuations of salinity on Georges Bank are driven by variations in upstream Scotian Shelf sources and its mixing onto the Bank and not by changes in local river outflow.