50 



the placement of nearly 24,000 m^ of dredged material during the 1998-99 or 1999-2000 

 disposal seasons (Appendix A). The recently deposited silt overlays a horizon of medium 

 sand at depth, which is likely a component of the 1997-98 sediments. 



A final objective of the August 2000 monitoring survey was to determine the benthic 

 recolonization status of the NL-91 and D/S Mound Complex, through comparisons with 

 previous surveys and results from nearby reference areas. Overall, the benthic habitat 

 conditions over the NL-91 and D/S Mound Complex in August 2000 were found to be 

 relatively healthy or undisturbed, with OSI values ranging from +6 to +10. The overall 

 average OSI value for the disposal mound stations (+8.0) was slightly higher than the 

 reference area average of +7.0, indicating that benthic habitat quality over the mound was 

 comparable to that on the ambient seafloor at the time of the survey. The August 2000 OSI 

 average of +8.0 is also comparable to the averages for the 1997 and 1998 surveys (+8.5 and 

 +7.5, respectively; Table 4-1). This suggests that benthic habitat quality in general has been 

 consistently healthy at this mound since the previous surveys in 1997 and 1998. 



The bulk of the supplemental cap material was placed during the period 1997 to 1999 

 (Appendix A), therefore, the August 2000 survey occurred after this material had been in 

 place on the seafloor for over one year. It was predicted that the recolonization status of the 

 NL-9 1 and D/S Mound Complex more than one year following cap material placement 

 would be advanced, with a community comprised of Stage II and Stage III organisms. The 

 August 2000 results confirmed this prediction: both Stage II and III organisms appeared to be 

 abundant in the sediment profile images obtained at stations across the mound (Figure 3-7). 

 Stage II on III has been observed consistently at this mound since 1997 (Table 4-1). As in 

 previous surveys, the Stage II community in August 2000 was comprised predominantly of 

 the amphipod Ampelisca sp, which formed dense tube mats at the sediment surface. At the 

 time of the survey, these tube mats appeared to be in various stages of decay and re- 

 generation, consistent with the cyclic nature of Ampeliscid amphipod populations (Figure 3- 

 8). The widespread presence of decayed amphipod tubes and detritus at the sediment surface 

 suggests that conditions in and around the NLDS were relatively quiescent in the weeks 

 leading up to the August 2000 survey, allowing the organic debris to accumulate on the 

 bottom. It is concluded that the supplemental CDM placed intermittantly over the NL-91 

 and D/S Mound Complex since 1 997 had been recolonized to an advanced degree by both 

 Stage II and III organisms in August 2000. 



Although not directly affected by the placement of supplemental cap material, several 

 stations on the periphery of the REMOTS® survey grid have shown significant improvement 

 in benthic habitat quality, relative to previous surveys. Stations 300E and 400E are located 

 over an area of seafloor that received CDM in 1 992 from the Dow Chemical project. In 

 1995, the data collected from these stations indicated benthic habitat recovery was 

 proceeding as anticipated (SAIC 2001a). Again in 1997, Stations 300E and 400E displayed 

 healthy benthic conditions with deep RPD depths, evidence of Stage III activity, and 



Monitoring Cruise at the New London Disposal Site, August 2000 



