25_ 



The mapping of successional stages is based on the theory that organism-sediment 

 interactions follow a predictable sequence after a major seafloor disturbance (Rhoads and 

 Germano 1982). This sequence is defined by end-member assemblages of benthic 

 organisms. Stage I is made up of pioneering assemblages usually consisting of dense 

 aggregations of near-surface, tube-dwelling polychaetes. If left undisturbed, Stage II 

 infaunal deposit feeders such as shallow-dwelling bivalves or tubicolous amphipods then 

 colonize the recovering seafloor. Stage III organisms are generally head-down deposit- 

 feeding invertebrates whose presence results in distinctive subsurface feeding voids. Stage 

 III taxa are associated with relatively low-disturbance regimes (Rhoads and Germano 

 1986). 



Organism-sediment index values are calculated by summarizing the apparent RPD 

 depth, successional status, and indicators of methane or low oxygen. OSIs can range from 

 -10 (azoic with methane gas present in sediment) to 1 1 (aerobic bottom with deep apparent 

 RPD, evidence of mature macrofaunal assemblage, and no apparent methane). OSI values 

 are useful in mapping disturbances and quantifying ecosystem recovery (Rhoads and 

 Germano 1982). 



Eleven stations within the thirteen-station survey grid showed evidence of Stage III 

 organisms (Figure 3-6). The most common stages noted in the replicate photographs were 

 Stage I and Stage I on III. Replicate median OSIs range from 2.5 at 600E (low RPD, no 

 Stage III due to recent CDM deposition) to 11 at 400E (Figure 3-6; Appendix A: Table 3- 

 la). Low OSIs (<6) are concentrated at the center (CTR, 200E, 200S), and at the 

 extremes of the southern and eastern legs of the grid (600S, 600E). Overall, the mean OSI 

 value for the NHAV 93 mound was 6.4, a substantial improvement over the July 1994 

 value of 3.5. 



The results of the July 1994 REMOTS® survey indicated the presence of three areas 

 of concern (CTR, 200N, and 400S; Morris and Tufts 1997). All three stations exhibited 

 shallow to diffusional RPD depths, limited recolonization, and lower OSI values than 

 anticipated. As part of the DAMOS tiered monitoring protocols, sediment from stations 

 CTR, 200N, and 400S was collected and subjected to Ampelisca bioassay testing for 

 toxicity. No significant differences in mortality were found between the sediment samples 

 originating from NHAV 93 stations CTR, 200N, and 400S and the sediments collected 

 from CLIS-REF (Mueller 1994). As a result, no action was taken at NHAV 93 (i.e., cap 

 supplementation) and the stations were closely monitored for changes in benthic conditions 

 (Morris and Tufts 1997). 



The September 1995 REMOTS® results indicate that, in general, the NHAV 93 

 mound is recovering from the impact of dredged material disposal as predicted (Germano 

 et al. 1994). The three stations that exhibited poor benthic conditions with low RPDs in 



Monitoring Cruise at the Central Long Island Sound Disposal Site, September 1995 



