ECOLOGY OF BUZZARDS BAY: An Estuarine Profile 



33 



24 



-S- 20 



a 16 



2 12 



Fig. 3.4. Composite seasonal water column 

 temperature in Buzzards Bay (Station 1) and New 

 Bedford Outer Harbor (Stations 2-5) Data from 

 Howes and Taylor (1990) 



had water column anoxia and fish kills (from mid 

 1 980 "s to present) related to periodic summer strati- 

 fication (Costa et al. 1 992; Howes and Goehringer 

 1 992). For most of the year higher winds produce 

 a well-mixed water column. It is unclear whether 

 the low watershed-to-bay surface area ratio that 

 results in the relatively low freshwater input also 

 produces a lower frequency and/or weaker stratifi- 

 cation of bay waters or if these processes in part 

 maintain the stable benthic communities in the cen- 

 tral basin of the bay. However, given the high oxy- 

 gen demand of central basin sediments (Howes and 

 Taylor 1 989; Banta et al. 1 990), prolonged stratifi- 

 cation is likely to lead to low oxygen bottom wa- 

 ters. It appears then that the physical structure and 

 the mixing processes of the Buzzards Bay system 

 may be providing a potential buffer to biotic 

 communities inhabiting the open bay. 



3.3. Circulation/Currents 



and the Tidal and Wind 



Regime 



Buzzards Bay is a relatively shallow estuary, with 

 mean low water depths ranging from 5 to 1 m at 

 the head to slightly over 20 m at the mouth. Depth 

 profiles in transects across the bay show a 



relatively smooth asymmetric bottom near the head, 

 gradually becoming more irregular and convoluted 

 near the mouth. The circulation patterns within Buz- 

 zards Bay are predominately tidal and wind-driven 

 flows acting on a large-scale estuarine density driven 

 flow of about 1 cm/s(Signell 1987). 



The location and semienclosed nature of Buz- 

 zards Bay result in tidal parameters significantly dif- 

 ferent from those found in the nearby waters of Vine- 

 yard Sound and Cape Cod Bay. To understand 

 these differences, it is necessary to look at the New 

 England Bight as a whole, from Long Island Sound 

 to Buzzards Bay (Fig. 3.5). Tides in Buzzards Bay 

 are predominately semidiurnal and dominated by 

 the lunar cycle. The southern New England shelf 

 tidal wave first reaches Rhode Island Sound in the 

 "gap" between Block Island and Martha's Vine- 

 yard and then moves into the shallower basins of 

 Vineyard Sound, Narragansett Bay, and Buzzards 

 Bay. Due to the configuration of Buzzards Bay, the 

 tidal signal is amplified by the shoaling and narrow- 

 ing of the embayment toward the head, while the 

 wave moving through Vineyard Sound is diminished 

 due to interference with the progressing wave en- 

 tering Vineyard Sound from the Gulf of Maine 

 (Redfield 1953). The interaction of incident waves 

 from the southern New England shelf and their re- 

 flection from the head of the bay dominate tidal 

 parameters in Buzzards Bay. 



The tide range is approximately 1 m with little or 

 no temporal lag throughout the bay, the headwa- 

 ters lagging only 20 min behind the mouth (Signell 

 1987). In contrast. Vineyard Sound operates more 

 like a strait with tidal influence from two sources: 

 the Gulf of Maine wave from the east and the south- 

 ern New England shelf wave from the southwest. 

 The effect is a decreased tidal amplitude and a sig- 

 nificant temporal lag of roughly 2-4 h behind Buz- 

 zards Bay (Redfield 1 953). The contrasting occur- 

 rence of the tidal wave within these two adjacent 

 water bodies causes large phase and amplitude dif- 

 ferences between the bay and sound and generates 

 extremely swift currents between Buzzards Bay and 

 Vineyard Sound (averaging 1 20- 1 50 cm/s in Woods 

 Hole and Robinsons Hole). These exchange 



