69_ 



indeterminate in 1997 (Figure 3-14). Consistent with 1997 results, Stage III organisms were 

 present at many stations of the 1998 survey (21 of 29 stations). One replicate at 150N, 

 located on the apex of the mound, was classified as azoic. Many of the Stage II amphipod 

 tube mats appeared to be disturbed. However, large chaetopterid tubes with hydroids were 

 visible in several replicates, suggesting advanced recolonization over the Seawolf Mound 

 dredged material with limited winnowing (Figure 3-19). These replicates also showed the 

 widespread distribution of gray clay over the Seawolf Mound. 



The presence of gray clay may affect the successional status and the measured RPD, 

 both of which are used to calculate the OSI values (Volume I, SAIC 2001). Gray clay was 

 detected only on the apex and plateau region of the mound (Figure 3-20). Sulfidic, organic- 

 rich sediments may also affect recolonization rates and dissolved oxygen levels. Patchy 

 sulfidic sediments were observed in sediment-profile photographs collected over the apex 

 and plateau of the mound. Some sulfidic sediment was also seen on the apron of the mound 

 and was similar to sediments seen in some of the replicates from NE REF. 



The median of replicate OSI values ranged from +3 to +9, with an overall average of 

 +6.1 (Table 3-1, Figure 3-21). The Seawolf Mound median OSI values were slightly below 

 those of the reference area, which varied between +5.0 to +10.0 (+6.7 average), and the 

 1997 Seawolf average of +7.5. 



3.1.3 Benthic Community Analysis 



Analysis of benthic grab samples collected in September 1997 indicated that the 

 Seawolf Mound was in the intermediate stages of recolonization, with abundances of 

 organisms increasing with distance from the center of the mound. The total number of 

 individuals sorted from the six Seawolf benthic grab samples was 2,600, of which 100 taxa 

 were identified (Blake and Williams 1997; Appendix C). Of the species used for all analyses, 

 nearly half were polychaetes (39 species). Additional taxa included, (in order of number of 

 species present): amphipods, bivalves, gastropods, decapods, isopods, one mysid, a small 

 number of nemerteans, oligochaetes, phoronids, echinoderms, hemichordates, and chordates 

 (treated as one taxon each). 



The center (CTR) station (Figure 2-5) had the lowest faunal abundance, with only 50 

 individuals belonging to 17 taxa. Station 75E had 200 individuals belonging to 26 taxa. 

 Two stations were sampled 150 m from the center station, and both had nearly twice the 

 number of species as measured at 75E. At 150N, 50 taxa were counted, with twice as many 

 animals, notably Nucula annulata (301 individuals), relative to 150W (46 taxa). Faunal 

 abundances were greatest at the two stations sampled 300 m from the center: at 300WSW, 

 518 animals belonging to 54 taxa were counted, and at 300SE, 1118 animals belonging to 

 66 taxa were counted. The trend of increasing faunal abundance with distance from the 



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



