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GULF OF MAINE WATER MASSES
The Gulf of Maine is a semi-enclosed coastal sea bounded by New England on the west and north, by New Brunswick and Nova Scotia on the east and north, and by Georges Bank at the shelf break. The complex current patterns in the Gulf of Maine (Fig. 10) have long interested researchers. In the 1920's, Bigelow (1927) attempted to understand these current gyres through the extensive use of drift bottles. Scientists today, even with all of their high technology instruments, are still amazed at the accuracy of his early observations (see Wiggin and Mooers 1992).
Recent temperature and salinity data are mainly in the form of profiles (Fig. 11). These have the advantage of revealing separate water masses by their temperature and salinity characteristics. Shown here is a section from the Maine coast (on the left) to Georges Bank (on the right) which clearly outlines a mass of cold intermediate water which develops in the spring and summer. This water mass sets up a pronounced thermocline (sudden temperature drop) which would be unlikely to be crossed by lobster postlarvae (see Boudreau et al. 1992).
Temperature
Although physical oceanography has traditionally depended upon relatively few temperature measurements over a brief period from research vessels, this situation is rapidly changing with the addition of satellite data. Over the last two decades, satellite remote sensing has proven to be a method by which large ocean areas could be sampled almost simultaneously at least every few days.
Satellite-derived Sea Surface Temperature (SST) data has recently been used to trace the path of warm Gulf Stream warm core rings (WCR) as they invade the southern flank of the Georges Bank (Fig. 12). The area shown in red (Fig. 13) is as much as 80C (approx. 140F) warmer than the surrounding waters (Conkling 1995).
Current Patterns
Part of the oil companies' plans for drilling on the Georges Bank involved an analysis of Gulf of Maine current patterns on a monthly basis. These maps (Fig. 14 A, B, C, D, E, F, G, H, I, J, K, L), which are unfortunately out of print, were part of the Environmental Impact Statement for Lease Sale 42. They reveal the complex interaction of coastal winds and Gulf Stream waters. The counterclockwise current pattern appears to be most pronounced during the spring and then weakens towards late summer. Recently updated maps based on the latest technology have been modeled at the University of Maine.
Salinityís
Little has been published on the overall salinity regime of the Gulf of Maine. Perhaps one of the best maps (Fig. 15) is by Brooks showing data for June 1983 (see Wiggin and Mooers 1992). His map shows the influence of fresher cooler waters extending down the coast and across the northern flank of Georges Bank. Details of Georges Bank salinity at a depth of 20 m or about 66 ft (Fig. 16 A and 16B) reveal a tongue of fresher water crossing the central bank during the winter. Note the rapid increase in salinity along the shelf margin during both seasons (Backus 1987).
Dissolved Oxygen
Dissolved oxygen levels on Georges Bank (Fig. 17A and 17B) vary greatly with the seasons from a high around 8.6 mL/L in winter to around 6.0 mL/L in summer. Lowest dissolved oxygen levels are found in the deep slope waters and locally could form anoxic conditions in the deeper canyons (Backus 1987).
Data on bottom temperature is sketchy and comes from a variety of sources. The only comprehensive bottom temperature map available goes back to the early 1960's (Fritz 1965). This map gives an average of bottom temperatures (Fig. 18A and 18B) over a 6-year period between the months of September to November based on 761 stations in the Gulf of Maine and adjacent waters (Fig. 19A and 19B). It showed bottom temperatures ranging from 60 to 70C (430-450F) along the coasts of Maine and Massachusetts reaching a maximum of 110C (520F) in Cape Cod Bay and 140C (570F) near Martha's Vineyard. Across Georges Bank the bottom temperature reached 150C (590F) and along the southern edge of the bank the temperature varied from 110-130C (520-550F).
Large scale, generalized, bottom temperature maps of the same area were included in a study of fish distribution in the New York Bight (Grosslein and Azarovitz 1982). These allow us to compare bottom temperatures between autumn and spring for the years 1973 and 1974 (Fig. 20 A, B, C, D). Unfortunately, there is no precise information provided on the number of stations sampled or the frequency of sampling. These maps show spring temperatures as low as 40 to 60C (40-430F) along the coasts of Maine and Massachusetts averaging about 60C across Georges Bank. They show minimum temperatures south of Georges Bank of 80C (460F) out to the 1000 m (about 3200 ft) depth contour. In autumn, bottom temperatures rose as high as 160C (610F) on Georges Bank, but remained in the 80-100C (460-500F) range on the southern edge.
Bottom Temperature Changes
Some work has also been done (Fig. 21) on the changes and trend in bottom temperatures over an 8 year period (Davis 1978). This work indicates that during autumn the mean bottom temperature in the Gulf of Maine increased rather steadily from a low of 5.4 0C (41.7 0F) in 1966 to a high of 8.4 0C (47.1 0F) in 1974. A similar pattern was observed in the fall temperatures. Davis assumed that fluctuations in the volume of slope water entering the Northeast Channel were mainly responsible for the general temperature trend.