OMSAP Public MeetingSeptember 1999

Circulation and Water Properties in Massachusetts Bay

Rocky GeyerWoods Hole Oceanographic Institution

September 22, 1999

InflowOutflow

Light

ConcernsEcologicalEcological Nutrients Contaminants Organic Material Food Chain Community Structure Living Resources

Human Health Contaminants Bacteria Viruses Bioaccumulation

SEDIMENT

Mammals

Infauna

Piscivorous Fish

Zooplankton

Phytoplankton

Planktivorous Fish

Epibenthos

Demersal Fish

Regeneration

DetritusParticulate

Microbes

Dissolved

WATER COLUMN

Sources RiversRivers BoundaryBoundary Nonpoint EffluentsEffluents

Gas ExchangeExchangeN2, | O2, CO2

ATMOSPHERE

N, P, Si, ON, P, Si, O22, CO, CO22 Microbes

OMSAP Public MeetingSeptember 1999

Physical Environment Gulf 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.

OMSAP Public MeetingSeptember 1999

Physical Environment

The circulation of Massachusetts Bay is also counter-clockwise

Mean flows and variability based on moored data from the Massachusetts Bays Program

OMSAP Public MeetingSeptember 1999

Physical Environment Surface Drifter Trajectories

71° 0' 70°50' 70°40' 70°30' 70°20' 70°10'

41°50'

42° 0'

42°10'

42°20'

42°30'

42°40'

April 91

July 90 April 90

Oct 90 Jan 91

OMSAP Public MeetingSeptember 1999

Physical Environment Alexandrium abundance May 1993

OMSAP Public MeetingSeptember 1999

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

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Boston Stellwagen

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-70.9 -70.85 -70.8 -70.75 -70.7 -70.65 -70.6 -70.55 -70.5 -70.45

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Feb 5 1991

longitude

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OMSAP Public MeetingSeptember 1999 jan feb mar apr may jun jul aug sep oct nov dec

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Temperature at N-21

near-surface near-bottom

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

OMSAP Public MeetingSeptember 1999

Physical Environment Annual Temperature Cycle Baseline Period 1992 - 1998

1992 1993 1994 1995 1996 1997 1998 19990

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Near-surface and near-bottom temperature, N-21

near-surface near-bottom

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

OMSAP Public MeetingSeptember 1999

Physical Environment Upwelling

Satellite image of upwelling during southwesterly winds

OMSAP Public MeetingSeptember 1999

Physical Environment Upwelling Index

1992 1993 1994 1995 1996 1997 1998

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1.4Upwelling Index: summer N-S wind stress

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Near-surface and near-bottom temperature, N-21

near-surface near-bottom

OMSAP Public MeetingSeptember 1999

Physical Environment Freshwater 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.

OMSAP Public MeetingSeptember 1999

Physical Environment Seasonal 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

jan feb mar apr may jun jul aug sep oct nov dec27

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psu

Salinity at N-21

near-surface near-bottom

OMSAP Public MeetingSeptember 1999

Physical Environment Annual 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

1992 1993 1994 1995 1996 1997 1998 199928

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psu

Near-surface and near-bottom salinity, N-21

near-surface near-bottom

OMSAP Public MeetingSeptember 1999

Physical Environment Seasonal Stratification Baseline Period 1992 - 1998

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.

jan feb mar apr may jun jul aug sep oct nov dec0

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4Stratification at N-21

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OMSAP Public MeetingSeptember 1999

1992 1993 1994 1995 1996 1997 1998 19990

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Stratification at N-21

total stratifiction

temperature-stratification

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

OMSAP Public MeetingSeptember 1999

1992 1993 1994 1995 1996 1997 1998 1999

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Bottom Temperature and Dissolved Oxygen, N-21

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

OMSAP Public MeetingSeptember 1999

Physical Environment Hydrodynamic Modeling USGS Model Grid

OMSAP Public MeetingSeptember 1999

Physical Environment Hydrodynamic Modeling Model-Data Comparison

OMSAP Public MeetingSeptember 1999

Physical Environment Hydrodynamic Modeling Effluent Plume Dilution Winter Conditions

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

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

OMSAP Public MeetingSeptember 1999

Physical Environment Hydrodynamic Modeling Effluent Plume Dilution Summer Conditions

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

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

OMSAP Public MeetingSeptember 1999

Physical Environment Hydrodynamic Modeling Effluent Plume Dilution Plan view, Surface

OMSAP Public MeetingSeptember 1999

Physical Environment Hydrodynamic Modeling Effluent Plume Dilution Plan view, mid-depth

OMSAP Public MeetingSeptember 1999

Physical Environment Effect of outfall on near-field

OMSAP Public MeetingSeptember 1999

Physical Environment Summary

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

OMSAP Public MeetingSeptember 1999

Physical Environment Summary

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