36 



BIOLOGICAL REPORT 31 



currents resulting from this "pumping," however, are 

 relatively small compared with those created by lo- 

 cal winds (Signell 1987). Winds along the axis of 

 the bay are most significant in influencing circulation 

 and are important to mixing, transport, and exchange 

 for the bay (Fig. 3.7). Because of the more com- 

 plex bathymetry at the mouth of the bay (Fig. 2.4), 

 tidally induced "residual currents," or currents 

 caused by the channeling of water as it moves across 

 irregular surfaces, are of greater importance to 

 subtidal circulation. Tidally induced eddies formed 

 near the mouth of the bay (Signell 1 987) can affect 

 the fate of transported material. 



The effects of local winds on circulation are most 

 pronounced in the smaller, shallower fringing har- 

 bors and embayments. The circulation of New 

 Bedford Outer Harbor, for example, is controlled 

 by its enclosed nature. Although a weak pattern of 

 "out on top, in on bottom" exists, it can be 



dominated by wind patterns such as a light south- 

 erly wind, which may stall surface movement 

 (Camp. Dresser, and McKee. Inc. 1990). At its 

 boundary' with Buzzards Bay, circulation in New 

 Bedford Harbor is more tidally driven. Flushing of 

 New Bedford Harbor and many of the other har- 

 bors and smaller embayments results from a com- 

 bination of tidal influences, winds, runoff, and 

 warming of the shallower waters and can be vari- 

 able depending on the dominance of any one or 

 more of these parameters. Probably more impor- 

 tant than the effect of tidal and wind-driven flows 

 on water exchange is their effect on vertical mix- 

 ing. Although stratification is generally weak the 

 tidal currents near the head of the bay are also 

 small. It appears that wind-driven mixing plays a 

 major role in vertical mixing, hence affecting oxy- 

 gen balance and biotic communities within this 

 system. 



