Circulation and Water Properties in Massachusetts Bay

1. Circulation and Water Properties in Massachusetts Bay Rocky Geyer Woods Hole Oceanographic Institution September 22, 1999

2. Concerns Inflow Outflow Human Health Contaminants Bacteria Viruses Bioaccumulation Ecological Nutrients Contaminants Organic Material Food Chain Community Structure Living Resources ATMOSPHERE Light GasExchange WATER COLUMN N2,|O2, CO2 Mammals Planktivorous Fish Sources Rivers Boundary Nonpoint Effluents Piscivorous Fish Phytoplankton Dissolved Zooplankton Demersal Fish Microbes Particulate Detritus Regeneration Epibenthos Microbes N, P, Si, O2, CO2 SEDIMENT Infauna

3. Physical EnvironmentGulf of Maine Circulation • There is a general counterclockwise circulation in the Gulf of Maine, with inflow from the Scotian shelf, flow to the southwest along the coast of Maine towards Massachusetts Bay. Some of the water sweeping past Cape Ann enters Massachusetts Bay and contributes to a counter-clockwise circulation in Massachusetts Bay. • The blue arrows are near-surface currents and the red arrows are deep currents. The general tendency is for a counter-clockwise circulation, sweeping south past the mouth of Mass Bay.

4. Physical Environment • The circulation of Massachusetts Bay is also counter-clockwise • Mean flows and variability based on moored data from the Massachusetts Bays Program

5. Physical EnvironmentSurface Drifter Trajectories

6. Physical EnvironmentAlexandrium abundance May 1993

7. Physical Environment Seasonal Variation of Temperature • Temperature across Massachusetts Bay, from Boston Harbor to Stellwagen Bank varies seasonally (data from the Mass Bays Program) • Note the warmest bottom water occurs in October

8. Physical Environment Seasonal Temperature Cycle Baseline Period 1992 - 1998 • Near-surface and near-bottom temperature seasonal cycle at the outfall site is relatively uniform both in timing and magnitude • Wintertime surface and bottom temperatures are close to freezing • Temperatures begin to warm up in the beginning of April • Surface water warms much faster than the bottom water • Temperature differential between the near-surface and near-bottom temperature greater more than 10° C by August • Surface water cools in the fall, but the bottom water continues to warm slowly until the water becomes well mixed, usually in mid- to late October

9. Physical EnvironmentAnnual Temperature Cycle Baseline Period 1992 - 1998 • Surface water temperature shows nearly the same pattern every year • Variations in the bottom water occur, particularly in the maximum temperature reached • In 1994, 1995, and 1996, the bottom water was considerably warmer than in the other years – around 12 C • During 1992, 1993 and 1998 the temperatures reached about 8 C • These differences appear to be explained by the wind forcing • Years with more southerly winds tend to have colder bottom water temperatures due to upwelling phenomena

10. Physical Environment Upwelling • Satellite image of upwelling during southwesterly winds

11. Physical EnvironmentUpwelling Index

12. Physical EnvironmentFreshwater Inputs • Distant sources • Rivers entering the Gulf of Maine • Scotian Shelf, which carries the input from the Gulf of St. Lawrence • More northerly sources • Local inputs • Charles River • MWRA Deer Island outfall.

13. Physical EnvironmentSeasonal Salinity Cycle Baseline Period 1992 - 1998 • Seasonal pattern of salinity is not as regular as that of temperature, • Tendency is for the salinity to decrease during the spring, usually reaching a minimum in May • Both surface and bottom salinity vary seasonally, • Surface salinity is usually lower than the bottom salinity. • Local impact of the freshwater inflow on Massachusetts Bay

14. Physical EnvironmentAnnual Salinity Cycle Baseline Period 1992 - 1998 • Surface and bottom salinity at the outfall site show a distinct drop every year • Large interannual differences in the magnitude of the decrease are evident • Lowest salinities (as low as 28 psu) were observed in the spring of 1998 • 1998 unusual in that the low salinity water persisted longer into the summer than other years • 1998 was the wettest year of the monitoring program • Differences between surface and bottom salinity • maximum during the spring, and • tends to vanish during the winter, when the water column becomes well mixed

15. Physical Environment Seasonal Stratification Baseline Period 1992 - 1998 Stratification at N-21 4 3.5 3 2.5 sigma-t 2 1.5 1 0.5 0 jan feb mar apr may jun jul aug sep oct nov dec • Density stratification shows a pronounced seasonal cycle due to the combined effects of temperature and salinity • Maximum stratification usually occurs in July and August, due to the seasonal warming of the surface layer. • Magnitude of the density difference is typical of mid-latitude coastal waters that are too deep to be mixed by the tides. • Stratification usually vanishes during the • some instances when enough low-salinity water was near the surface to produce stratification.

16. Physical Environment Annual Stratification Baseline Period 1992 - 1998 • Variation of stratification (density) shows a regular seasonal pattern • Only slight interannual variation observed • Stratification is usually initiated in the spring by salinity effects • By the middle of the summer it is dominated by thermal stratification • Most of the interannual density variability comes from the variations in salinity stratification • 1998 is notable due to the large amount of freshwater inflow to the system The blue line is the total stratification; green line is the contribution of temperature to the stratification

17. Physical Environment Annual and Inter-annual Variation of Dissolved Oxygen • Dissolved oxygen in the bottom water at the outfall site is inversely correlated with the variation of near-bottom temperature. • 1994 and 1995 had warmest near-bottom waters and the lowest DO values during baseline • Possible causes • Upwelling that replenishes the deep dissolved oxygen values. • More Gulf of Maine water is advected in during years with weak upwelling(deep GOM water appears to be lower in DO than waters of the same depth in Massachusetts Bay) • Biological processes

18. Physical EnvironmentHydrodynamic Modeling USGS Model Grid

19. Physical EnvironmentHydrodynamic Modeling Model-Data Comparison

20. Physical Environment Hydrodynamic Modeling Effluent Plume Dilution Winter Conditions A hydrographic section from Boston Harbor outfall to Cape Cod Bay for the present discharge location (upper panel)and the new outfall (lower panel) shows • Higher concentrations are associated with the old outfall, mainly in Boston Harbor • The farfield dilution is projected to be virtually the same when the outfall is online • Effluent will extend to the surface in dilute concentrations from the new outfall during the winter, due to the absence of stratification

21. Physical Environment Hydrodynamic Modeling Effluent Plume Dilution Summer Conditions A hydrographic section from Boston Harbor outfall to Cape Cod Bay for the present discharge location (upper panel)and the new outfall (lower panel) shows • Trapping of the effluent in surface layer during the summer at the Harbor outfall location • Effluent from the new outfall is trapped below the thermocline in the summer • Difference is likely to reduce the impact of nutrient loading from the outfall on the ecosystem

22. Physical EnvironmentHydrodynamic Modeling Effluent Plume Dilution Plan view, Surface

23. Physical EnvironmentHydrodynamic Modeling Effluent Plume Dilution Plan view, mid-depth

24. Physical Environment Effect of outfall on near-field

25. Physical EnvironmentSummary • Massachusetts Bay is part of the larger circulation regime of the Gulf of Maine • Currents, water properties, and biology are strongly influenced by the conditions in the Gulf of Maine • Interconnection applies to the outfall site as well as Massachusetts Bay as a whole • Large seasonal variation in stratification is the dominant characteristic of the water properties of Massachusetts Bay • Well-mixed conditions in the winter • Strong stratification in the summer

26. Physical EnvironmentSummary • Interannual variations in bottom water temperature and dissolved oxygen at the outfall site appear to be related to wind forcing • Persistent southerly winds during the summer lead to colder bottom temperatures and higher DO • Weaker southerly winds lead to warmer bottom waters and lower DO • USGS circulation model results • main influence of the new outfall will be a reduction of the impacts of the effluent in Boston Harbor • Farfield will not be significantly altered