706 



(over 6 years old) all supported a healthy mature benthic community. All reference areas 

 experienced some limited patches of disturbance (presence of recolonizing Stage I 

 organisms, eroded tube mats, shallow RPDs) at various times within the survey period. 

 None of the individual reference stations exhibited consistent disturbance, that is, the patches 

 were in different places each year. Overall, the reference areas supported a healthy benthic 

 assemblage and displayed typical features of seasonal settlement and disturbance (see 

 below). 



Assessment of the health of the benthic community at NLDS requires the ability to 

 separate site-specific characteristics from regional environmental characteristics. During 

 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, hydroids 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 

 both here and in Volume U serve to demonstrate that the surface sediment characteristics 

 throughout NLDS and reference areas became similar over time (with the exception of areas 

 mantled with coarse sand or pebbles). The disposal site is subject to relatively strong tidal 

 currents, but the landmasses surrounding NLDS shelter the seafloor from wave disturbance 



Monitoring Cruise at the New London Disposal Site, Seawolf Mound 1995 - 1998 



