the ebb. Since the tidal current is weak, currents 

 of 1 knot or more occur only with strong winds. The 

 largest velocity likely to occur is about 1.5 knots. 



Eastward of Mount Desert Island (region 5) the tidal 

 currents along the coast are stronger and more 

 regular than those farther west. Between Mount 

 Desert Island and Portland there is a westward 

 resultant drift along the coast. 



In Grand Manan Channel (region 6) the average 

 velocity at strength (i.e. maximum flood or ebb) of 

 the current is about 2.5 knots. The current sets 

 approximately parallel to the channel, the flood 

 setting northeastward and the ebb southwestward. 



The speed and direction of tidal currents changes constantly during the tidal 

 cycle. In the offshore regions, tidal currents flow in rotary movement, with 

 no period of slack water. Along the coast, the boundary compresses the rotary 

 flow into an elongated ellipse with an apparent period of slack water. 



The interaction between tidal currents and bottom friction results in 

 turbulent mixing that tends to suppress the formation of a therraocline by 

 keeping the water column well mixed. The rougher the bottom and the faster 

 the currents, the more turbulence will be created. Turbulent energy declines 

 more or less exponentially upwards through the water column. Only when the 

 thermocline is within 33 to 66 feet (10 to 20 m) of the bottom will this 

 process result in mixing across the thermocline. Mixing of the water column 

 is important in supplying nutrients to surface waters thus increasing 

 plankton production (see "Nutrient Cycle" and "Phytoplankton" below). 

 Increased mixing also distributes pollutants in the water column more quickly. 

 Increased tidal currents in eastern Maine (regions 5 and 6) cause increased 

 turbulent mixing resulting in high surface salinities and lower surface 

 temperatures from west to east. 



Information on the nontidal drift in the Gulf of Maine has been obtained 

 primarily by using surface drift bottles and seabed drifters (Bigelow 1927; 

 Chevier 1959; Rumpus and Lauzier 1965; Graham 1970b; and Bumpus 1973). Some 

 additional information has been obtained through current meters (Forrester 

 1959 and Vermersch et al. 1979) and geostrophic calculations (Bigelow 1927; 

 Watson 1936; and Hopkins and Garfield 1979). 



Whenever sufficient river discharge is entering a coastal area, a weak current 

 system develops, with a flow parallel to the coast due to the relatively low 

 density of the inshore waters (Iselin 1959). In Maine the flow is in a 

 southerly direction along the coast. This flow is not steady but fluctuates 

 with changes in river discharge and onshore movement of oceanic water. In 

 addition, wind-induced currents can completely mask this current. 



A schematic representation of the summer nontidal surface currents in the Gulf 

 of Maine (deduced from returns of drift bottles) is illustrated in figure 4-9. 

 Bumpus and Lauzier (1965) were able to show that the circulation pattern was 

 most pronounced during spring and summer and least defined in fall and 

 winter. 



4-23 



10-80 



