Instruments to measure current, temperature, salinity, bottom pressure, light transmission, suspended matter, and to photograph the sea floor were deployed at Site A from 1989 to 2002. Instruments to measure current, temperature, and suspended matter were deployed at Site B from 1997 to 2002.

In addition to the time-series physical observations, sediment traps were deployed at both Sites A and B. The data they collected are used to estimate the relative amount of suspended sediment falling through 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 collected 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 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.

Instrumentation was recovered and replaced at each site 3 times each year (typically in February, June, and September). The moorings at Site A and Site B were deployed and recovered using the U.S. Coast Guard Cutter White Heath (figure 3A) between 1989 and 1998, and the U.S. Coast Guard Cutter Marcus Hannah between 1998 and 2002 ( figure 3B). Additional instrument deployments and recoveries in emergencies and gear testing were accomplished using the fishing vessel Christopher Andrew (figure 3C).

Site A

The location of the long-term Site A (42° 22.6' N. 70° 47.0' W.) is 13 km east of the entrance to Boston Harbor, at a water depth of about 33 m ( figures 1, 4). Because this is an area of heavy marine activity, the instruments were deployed adjacent to the USCG Boston Approach B Buoy to mark the site and to provide some protection from accidental disruption. Instruments were deployed to measure near surface, near bottom, and bottom currents and water properties. During the 13-year period that instruments have been deployed at this site, three configurations of instruments have been used. Deployments of the surface, subsurface, and bottom moorings at Site A were clustered within about a 500-m circle to the south of the USCG buoy B (figure 4). The sea floor at Site A is gravel (figure 5A, B)

1989 - 1994

During this period, the Boston Approach B buoy was a large 40-foot discus buoy capable of supporting oceanographic instrumentation. A vector measuring current meter (VMCM), a transmissometer, and a SEACAT temperature and salinity sensor were suspended at a depth of 5 m below the surface from the Large Navigational Buoy. A second VMCM, transmissometer, and SEACAT were deployed 10 m above bottom (or about 23 m from the surface) on a taut subsurface current mooring. A bottom tripod system that measured current, temperature, salinity, light transmission, and pressure was deployed on the sea floor. Instruments mounted on the tripod also measured dissolved oxygen, photographed the sea floor, and sampled suspended sediment during selected events. A time series sediment trap at 5 m above bottom (about 28 m from the surface) collected sediments for successive 9-day intervals.

1994 - 1996

In 1994, the 10-m Large Navigation Buoy (LNB) was discontinued and replaced with a smaller buoy that would not support deployment of the near-surface VMCM. A small surface mooring was maintained to allow current measurements 5 m below the surface, and a subsurface mooring was used to obtain measurements at 10 m above the bottom. In addition, Acoustic Doppler Current Profiler (ADCP) measurements were initiated at Site A. The ADCP provided measurements of current from about 6 mab (meters above bottom) to about 5 mbs (meters below surface) in 2-m bins. The ADCP was initially deployed on a small tripod frame (figure 13C) between 1994 and 1996, then on the large tripod frame beginning in 1997.

1997 - 2002

In 1997, the surface mooring was discontinued, and near-surface temperature and salinity measurements were obtained from instruments mounted on the top of the subsurface float about 7 m below the surface. Acoustic Doppler Current Profiler (ADCP) measurements were transferred to the large tripod maintained at Site A. The subsurface mooring collected temperature and conductivity about 7 mbs, measured currents (VMCM), temperature, and conductivity at 10 mab, and held a time-series sediment trap 4 mab.

The following figures are in PDF format.

Figure 1

Figure 3A

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Figure 13C

Figure 13D