21 



The apparent RPD depth is a measure of the level of oxygenation in the upper 

 sediment layers. This value indicates dissolved oxygen conditions within sediment pore 

 water as well as the availability and consumption of molecular oxygen (O2) in the surface 

 sediments. Since actual oxygen status in the sediment is not measured, the apparent RPD 

 is estimated by measuring the thickness of the layer of high reflectance oxidized sediments 

 in contrast to the usually gray to black reduced material at depth (Rhoads and Germano 

 1982). 



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, mbe-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 stams, and indicators of methane or low oxygen. OSIs can range from 

 -10 (azoic with methane gas present in sediment) to 11 (aerobic bottom with deep apparent 

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

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

 Germano 1982). 



The replicate-averaged mean redox potential discontinuity (RPD) depths over the 

 CLIS 95 mound ranged from 0.94 cm at lOOS to 3. 18 cm at 200N (Figure 3-6). There 

 was no distinct pattern in the RPD values within the REMOTS® grid; however, the range 

 was relatively high for a new dredged material deposit. No methane was noted at any 

 station over the CLIS 95 mound, but low dissolved oxygen (DO) was detected in one 

 replicate of Station lOOS, effecting the OSI value for that station. 



With the exception of lOOS, median OSI values were higher than expected for a 

 sediment mound at five months postdisposal, ranging from 3.0 to 10.0 (Figure 3-6). Deep 

 RPD depths and a mamre benthic assemblage were die reasons for the elevated OSI values. 

 The successional stage status of CLIS 95 was quite advanced for an area recovermg from a 

 recent benthic dismrbance. Stage III activity was detected at every station over the CLIS 

 95 mound with most replicates being classified as Stage I on III. One replicate over 

 Station 300W failed to show evidence of Stage III organisms in the surface or subsurface 



Monitoring Cruise at the Central Long Island Sound Disposal Site, July 1996 



