Water masses . Coastal waters of Maine are not a uniform water mass 

 experiencing gradual changes in temperature and salinity. Instead, the 

 nearshore waters are composed of relatively discrete masses of water moving 

 along the coast. The boundaries between these different water masses are 

 often marked by precipitous changes in temperature and salinity. Species 

 distribution may be linked to these masses. 



Based on the distribution of temperature and salinity, Gulf of Maine water can 

 be characterized as being composed of three water masses: Maine surface water 

 (MSW) , Maine intermediate water (MIW) , and Maine bottom water (MBW; Hopkins 

 and Garfield 1979). MBW commonly has salinities between 31 and 33 ppt and in 

 the nearshore regions salinities may be <30 ppt. The temperature range of 

 approximately 33 to 63°F (1 to 17°C) reflects the seasonal heating and cooling 

 cycle. The surface temperature-salinity properties along the coast of Maine 

 are influenced by the eastward increase in tidal mixing. Such tidal mixing 

 tends to cause lower surface temperatures as well as higher salinities toward 

 the east. 



MIW is a low-salinity, minimum-temperature water. The salinities are too low 

 to be of offshore origin and the water mass volume shows seasonal fluctuation; 

 thus, local coastal formation is suggested. It may be that water from the 

 Nova Scotian Shelf, in which salinity is lowered by discharge from the St. 

 Lawrence River (Sutcliffe et al. 1976), or local coast of Maine water which is 

 cooled during winter, or both comprise MIW (Bigelow 1927; Hopkins and Garfield 

 1979) . MIW characteristics converge with MSW during the winter and the two 

 water masses diverge during the spring as MSW is freshened and warmed. MIW is 

 the remnant water mass from the previous winter. It is not destroyed during 

 sunmier heating, because the mixing processes associated with cooling extend 

 deeper than do those associated with warming. The MIW core is found between 

 160 and 400 feet (50 to 120 m) , with the lower limit and volume dependent upon 

 seasonal production. MIW is an envelope of water with temperature ranging 

 from 34 to 43°F (1 to 6°C) and salinity from 32 to 33 ppt. These ranges will 

 vary depending on the season, the year, and the location. These aspects are 

 discussed in greater detail by Hopkins and Garfield (1979). 



MBW is a warmer, higher salinity water than MIW. It occupies the depths 

 between MIW and the bottom. Its water type is unaffected by air-sea 

 interaction (because the direct effects are absorbed by MIW). The only 

 off shelf exposure (below 250 feet; 75 m) occurs through the Northeast Channel, 

 where the sill is about 800 feet (240 m) . This permits entry of slope water 

 (water from the continental slope) into the Gulf of Maine. The position of 

 MBW, between the MIW and the slope water, implies mixing between these three 

 water masses. 



Using historical data Hopkins and Garfield (1979) have provided strong 

 evidence for local water mass production during the winter along the coast of 

 Maine. The production zone is a coastal zone that extends offshore, the area 

 and depth of which vary with the winter cooling processes (i.e., heat loss to 

 the atmosphere). During colder years (as in the mid-1960s) the zone may 

 extend to Georges Bank and to depths of about 500 feet (150 m) ; and during 

 warmer years (as in the early 1970s) the zone may extend 30 miles (50 km) 

 offshore and to depths of 400 feet (120 ra) . The depth penetration depends on 

 the heat content of the underlying waters. The water mass produced during the 

 winter in this zone becomes a major constituent of MIW. It is assumed that 



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