Instruments to measure current, temperature, salinity, pressure, light transmission, and suspended matter, and to photograph the sea floor were deployed at LT-A from 1989 to 2006. Instruments to measure current, temperature, salinity, and suspended matter were deployed at LT-B from 1997 to 2004. At both sites, the instruments were suspended in the water column from a surface buoy or on a subsurface mooring (fig. 3).  Bottom measurements were made with instruments attached to a tripod frame that rests on the sea floor (fig. 4a, 4b).  Instrumentation was recovered and replaced at each site three times each year, typically in February, June, and September. About 160 separate mooring or tripod deployments were made on about 90 research cruises to collect these long-term observations.

In addition to the time-series physical observations, sediment traps were deployed at both LT-A and LT-B.  The data obtained from these traps are used to estimate the relative amount of suspended sediment in the bottom water, to link variations in the trapping rate with changes in oceanographic conditions, and to determine the chemical and physical properties of the trapped sediment. Traps were used to collect samples during storm and non-storm periods and during seasonal cycles in primary productivity, providing information on the nature and amounts of material in transport under different oceanographic conditions. The sample suite was also designed to monitor changes in contaminant concentrations of trapped material since 1989 and especially since the start of operation of the Massachusetts Bay outfall. Outfall-related changes in chemistry are expected to be detected earlier and more intensively in trapped suspended sediment than in the surficial bottom sediments because trapped sediments are immediately isolated by the sampling device. In contrast, particles falling on the sea floor can be mixed downward by benthic organisms and diluted by older particles from below the water-sediment interface.

The moorings and tripods were deployed and recovered using the U.S. Coast Guard Cutter White Heath (fig. 5A) from 1989 to 1998, and the U.S. Coast Guard Cutter Marcus Hanna  (fig. 5B) from 1998 to 2006. Supplemental instrument deployments, recoveries, and gear testing were accomplished using the fishing vessel Christopher Andrew (fig. 5C).

LT-A

LT-A (42° 22.6' N., 70° 47.0' W.) is located about 13 km east of the entrance to Boston Harbor at a water depth of about 32 m (fig. 1). Because of marine traffic entering and leaving the Port of Boston, the instrumented moorings were deployed near the USCG Boston Approach B Buoy; this buoy marked the site and provided some protection from accidental disruption.  Moorings were clustered within about 500 m of the B Buoy (fig. 6).  LT-A is located on the south side of a submerged ridge that is about 25 m deep.  The sea floor at LT-A is covered with gravel and cobbles (fig. 7A, 7B).  The MWRA outfall is north of the ridge in water about 35 m deep.

Four mooring configurations were used during the 16-year period that instruments were deployed at LT-A.  The objective was to obtain measurements of currents and water properties at three nominal depths:  near-surface ( about 5 m below the surface, above the thermocline in summer); near-bottom (about 22 m, 10 m above the bottom, below the thermocline in summer); and bottom (1-2 m above bottom) (table 1).  Maintaining instruments near the surface was technically challenging because of the numerous large and small ships that pass close to the B buoy, the pickup buoy for Boston Harbor.  Surface floats in this area were vulnerable to accidental collision, as were subsurface floats moored within about 5 m of the surface.  The length of the mooring components positioned the instruments at fixed heights above the sea floor; the depths of the instruments below the surface vary by a few meters depending on the water depth where the subsurface mooring was deployed, which ranged from 30 to 34 m at LT-A.

Table 1. Summary of instruments deployed at LT-A, 1989-2006, to obtain measurements at nominal depths of 5, 22, and 31 m (meters). The nominal water depth at LT-A was 32 m. The measurements at the nominal depth of 22 m were made from instruments located 10 m above bottom on a subsurface mooring. The measurements at the nominal depth of 31 m were made from instruments mounted 1 to 2 m above bottom on a bottom tripod. The actual depths of these measurements varied slightly depending on the actual water depth at LT-A where the mooring and tripod were deployed.

[~, about; ADCP, acoustic Doppler current profiler; LNB, Large Navigation buoy; MicroCAT, temperature and salinity sensor; SEACAT, temperature, salinity and light transmission sensor; VMCM, vector-measuring current meter. See Instrumentation for a description of each instrument.]

Mooring configuration from 1989 through 1994 (fig. 8A, 8B, 8C)

From 1989 to 1994, the Boston Approach B buoy was a 10-m Large Navigational Buoy (LNB). A vector-measuring current meter (VMCM) and a SEACAT (measuring temperature, salinity, and light transmission) were suspended at a depth of 5 m from the LNB. A subsurface mooring was used to obtain current, temperature, salinity, and light-transmission measurements at about 22 m (10 m above bottom). A tripod system on the sea floor measured current, temperature, salinity, light transmission, and pressure. A camera mounted on the tripod photographed the sea floor.  A funnel sediment trap at about 16 m and a time-series sediment trap at 4 m above bottom (about 28 m from the surface) on the subsurface mooring, and tube traps on the tripod and subsurface mooring, collected suspended sediments.

Mooring configuration from 1995 through 1996 (fig. 9A, 9B)

In 1994, LNB was discontinued by the USCG and replaced with a smaller buoy that would not support near-surface instrumentation. A small surface mooring was used to obtain current measurements at 5 m below the surface; both slack-moored and taut-moored with elastic tethers configurations were used.  A subsurface mooring was used to obtain current, temperature, salinity, and light-transmission measurements at about 22 m (10 m above bottom).  A tripod system on the sea floor measured current, temperature, salinity, light transmission, and pressure. A camera mounted on the tripod photographed the sea floor. In addition, acoustic Doppler current profiler (ADCP) measurements were initiated; the ADCP measured current from about 6 m above bottom to about 2 m below surface in 2-m bins. The ADCP was deployed on a small separate tripod frame (fig. 4B) from 1994 to 1996.  A funnel sediment trap at about 16 m (16 m above bottom) and a time-series sediment trap at about 28 m bottom (4 m above bottom) on the subsurface mooring, and tube traps on the tripod and subsurface mooring, collected suspended sediments.

Mooring configuration from 1997 through 2002 (fig. 10A, 10B)

In 1997, the surface mooring used to obtain near-surface measurements was discontinued because it was difficult to maintain in this area of frequent ship traffic; no near-surface measurements were made in 1997.  A subsurface mooring was used to obtain current, temperature, salinity, and light-transmission measurements at about 22 m (10 m above bottom). A tripod system on the sea floor measured current, temperature, salinity, light transmission, and pressure. A camera mounted on the tripod photographed the sea floor. In 1998, temperature and salinity measurements at about 5 m were obtained from an instrument mounted on the float of the subsurface mooring. The ADCP was transferred to the large tripod in 1997.  A funnel sediment trap at 16 m (16 m above bottom) and a time-series sediment trap at about 28 m (4 m above bottom) on the subsurface mooring, and tube traps on the tripod and subsurface mooring, collected suspended sediments.

Mooring configuration from 2003 to 2006 (fig. 11A, 11B)

In 2003, a VMCM was added to the subsurface mooring at about 7 m.  Temperature and salinity measurements at about 5 m were obtained from an instrument mounted on the float of the subsurface mooring, and current, temperature, salinity, and light-transmission measurements were obtained at about 22 m (10 m above bottom). A tripod system on the sea floor measured current, temperature, salinity, light transmission, and pressure. A camera mounted on the tripod photographed the sea floor.  A funnel sediment trap at about 16 m (16 m above bottom) and a time-series sediment trap at about 28 m (4 m above bottom) on the subsurface mooring, and tube traps on the tripod and subsurface mooring, collected suspended sediments.

LT- B

Oceanographic observations were initiated at LT-B (42° 9.8' N., 70° 38.4' W.) (fig. 1, 12A, 12B) in 1997. This location was selected to provide observations along the western shore of Massachusetts Bay in the area of moderate residual flow (fig. 2) and storm-driven southeastward flow caused by winds from the northeast (Butman and others, 2007; Warner and others, 2008). The location is about 28 km south-southeast from the Massachusetts Bay outfall. The sea floor at LT-B is gravel (fig. 13).  Observations at LT-B included temperature and conductivity at about 12 m (10 m above bottom) and 21 m (1 m above bottom), and time-series sediment trap at about 18 m (4 m above bottom) (fig. 14A, 14B). Currents were measured throughout the water column with an ADCP mounted on several versions of a small tripod frame (fig. 4B). The subsurface and tripod moorings at LT-B were deployed in about a 200-m north-south trending band (fig. 12B).

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