ECOLOGY OF BUZZARDS BAY: An Estuarine Profile 



31 



3.2. Salinity, Temperature, 

 and Density 



The salinity of Buzzards Bay waters is the result 

 of mixing of oceanic water with freshwater inflow 

 (and rain). The distribution of freshwater input is 

 consistent with the geology and watershed distri- 

 bution and suggests that more than two-thirds of 

 the inflow is along the western shore with the most 

 concentrated flows near the head of the bay (Table 

 3.1, Fig. 3.1). The distribution of freshwater flow 

 and the circulation pattern of the bay result in a gra- 

 dient of decreasing salinity with increasing distance 

 from the mouth of the bay (Fig. 3.3). The gradient 

 is found in each season, but the greatest dilution is 

 found in the April transect with surface waters at 

 the head of the bay dipping to almost 28 ppt. con- 

 sistent with the period of maximum freshwater dis- 

 charge (Fig. 3.2A). The greater dilution of surface 

 versus bottom water (Fig. 3.3 ) is typical of estuar- 

 ies where the less dense fresh water enters near the 

 surface over denser, cold saline bay waters. 



As is the case for most of the New England 

 coastal region. Buzzards Bay experiences great 

 extremes in seawater temperature. Cape Cod is situ- 

 ated at the transition between the cold waters of 

 the Gulf of Maine and the warmer waters of the 

 Mid- Atlantic Bight; however, because exchanges 

 are with Rhode Island and Vineyard sounds, they 

 are primarily with warmer water lying south of Cape 

 Cod. Buzzards Bay is included in the American At- 

 lantic Temperate Region, which extends from Cape 

 Cod south to Texas and is largely influenced by the 

 warm waters of the Gulf Stream generated by the 

 westward flow of the North Equatorial Current 

 through the West Indies and Mexico and northward 

 along the east coast of the United States. At Cape 

 Cod, the current turns east and becomes the North 

 Atlantic Drift, ultimately flowing to the British Isles 

 and Europe. In contrast. Cape Cod and Massa- 

 chusetts Bay are influenced by the Maine Current, 

 a branch of the Labrador Current flowing south from 

 Greenland. The temperature differences between 

 Cape Cod Bay and Buzzards Bay can be as much 

 as 5.5° C. Buzzards Bay water temperatures range 



over an annual cycle from to 22° C in the bottom 

 waters (Fig. 3.4) with greater extremes near the 

 surface. The central bay typically remains ice free 

 during the winter; however, occasionally the entire 

 upper bay ices over. 



Water column stratification occurs when less 

 dense (warmer or fresher) surface water overlies 

 more dense (colder or more saline) bottom waters. 

 Periodic stratification occurs in Buzzards Bay (Fig. 

 3.3). The causes and level of stratification are not 

 the same throughout the year. Vertical temperature 

 gradients (Fig. 3.3), when they occur, are typically 

 generated by radiative heating of the surface wa- 

 ters and are the dominant cause of stratification in 

 the lower bay during summer (e.g., Fig. 3.3 top). 

 Thermal stratification generally has a diurnal com- 

 ponent and is readily broken down; however, be- 

 cause it occurs during the wannest months, its ef- 

 fects on dissolved oxygen balance below the ther- 

 mocline may be generally more significant than the 

 typical salinity stratification. Salinity stratification, 

 while it can occur year-round in response to short- 

 term meteorological events, is strongest in Buzzards 

 Bay in spring when freshwater inflow is greatest (Fig. 

 3.3). Fortunately, spring water temperatures are low 

 (Fig. 3.4), resulting in low oxygen demand and dis- 

 solved oxygen levels that remain high even during 

 stratification. 



For the most part water column stratification in 

 the central region of Buzzards Bay periodically ex- 

 ists during summer months predominately because 

 of thermal density differences but occasionally due 

 to pulses of fresh water, causing salinity effects as 

 well. Oxygen conditions in bottom waters of the 

 central bay generally remain over 80% saturation 

 (Howes and Taylor 1990; Howes 1993), and there- 

 fore the periodic stratification does not appear to sig- 

 nificantly affect benthic communities. This condition 

 is in strong contrast with the smaller embayments of 

 the bay where freshwater inputs are most concen- 

 trated. Even in the shallow waters of the 

 embayments, pulses of fresh water following sum- 

 mer storms add to thermal stratification, and short- 

 term hypoxia can occur. Similar embayments (1-2 

 m deep) on the southern shore of Falmouth have 



