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this time, historical dredged material and reference areas experienced very limited direct 

 physical disturbance, whereas areas that received fresh dredged material experienced a 

 short period of physical disturbance followed by recovery. In some areas, dredged 

 material was placed two or three times during the six years. All of the monitoring surveys 

 were conducted in late summer (July 30-September 6), a period with elevated water 

 temperatures and the potential for ecological stress or seasonal senescence of settling 

 organisms (see below). 



The most consistent biological characteristic observed over the monitoring period 

 was the widespread presence of tube building amphipods in surface sediments. These 

 organisms collect fine-grained sediments to construct their tubes, and the presence of the 

 tubes enhances trapping and deposition of fine sediments (Mills 1967). The mats can 

 become very dense and restrict bioturbation and circulation in sediments below the tubes 

 (the result is a relatively thin redox potential discontinuity or RPD). In both disposal areas 

 and reference areas, a mixed layer of fine sand and coarse shells was present beneath the 

 tubes, but this layer is often difficult to see. Clumps of mussels also were seen and widely 

 reported from the area within and around the disposal site. In areas with shells or pebbles 

 on the surface, hydro ids and mussels were seen attached to the hard substrate. 



When the amphipod tubes are physically disturbed or abandoned (due to natural 

 seasonal decline, senescence or environmental stress), they are easily eroded, and the sand 

 or shell surface is again exposed to bottom currents. As a result, summer periods (when 

 the tube mats are present and widespread in and around the NLDS) may represent active 

 deposition of fine sediment, with subsequent die-off or thinning of the tubes and sediment 

 reworking in the winter. 



The surface sediment characteristics are a combination of the material deposited and 

 processes of physical and biological reworking. The DAMOS monitoring results reported 

 here serve to demonstrate that the surface sediment characteristics throughout the disposal 

 site and reference areas became similar over time (with the exception of areas mantled with 

 coarse sand or pebbles). The NLDS is subject to relatively strong tidal currents but is also 

 sheltered from wave disturbance (Waddell et al. 1999). When tidal currents are sufficient 

 to transport fine sand as bedload, some fine materials may be winnowed leaving a lag 

 deposit of sands and shells too large for transport. Semi-diurnal tidal currents at the NLDS 

 appear to be strong enough to rework unconsolidated surface sediments through this 

 process until surface sediments have a lag deposit of sand or shells. However fine surface 

 sediments are also bound by biological activity and may be remarkably resistant to erosion 

 while the organisms are alive. Tidal currents are likely to be slightly weaker in the 

 depression where the NL-91 and D/S mound complex is located compared to the tops of 

 nearby, shallower mounds. 



Monitoring Cruise at the New London Disposal Site, 1992 - 1998 



