10 



comparison with the June 1991 survey at MQR, where replicates were dominated by 

 indeterminate causes of boundary roughness. 



3.1.3 Apparent Redox Potential Discontinuity 



The Apparent Redox Potential Discontinuity (RPD) depth is the depth of the upper 

 oxygenated sediment layer. This value is an important indication of dissolved oxygen 

 conditions within sediment pore waters and is a function of the supply of molecular oxygen 

 from the overlying sea water, and the consumption of oxygen in the sediment. Since the 

 actual oxygen status in the sediment is not measured, the apparent RPD is estimated by 

 measuring the thickness of the layer of high reflectance; reduced sediments are usually grey 

 to black and contrast with the lighter oxygenated sediments. 



Replicate-averaged RPD values ranged from 0.54 to 2.00 during the 1992 survey 

 compared to the CLIS reference area in June 1991 (1.49 to 2.71). The range of RPDs in the 

 replicates has increased since the June 1991 sampling (Figure 3-2). The range of replicate- 

 averaged apparent RPD values in June 1991 was 0.21 to 1,96. 



3.2 Habitat Quality at MQR 



3.2.1 Infaunal Successional Stages 



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

 interactions follow a predictable sequence after a major seafloor perturbation (e.g., passage 

 of a storm, dredged material deposition; Rhoads and Germano 1982). This sequence is 

 defined by end-member assemblages of benthic organisms. Pioneering assemblages (Stage I) 

 usually consist of dense aggregations of near-surface, tube-dwelling polychaetes. Early Stage 

 I assemblages are then replaced (barring any disturbance) by infaunal deposit feeders (Stage 

 II) like shallow dwelling bivalves or, as is common in New England waters, tubicolous 

 amphipods. Stage III taxa are associated with relatively low-disturbance regimes. These 

 infaunal invertebrates generally are head-down deposit feeders whose presence results in 

 distinctive subsurface feeding void features. There are several divisions among these end- 

 member groups (Figure 3-3). 



Most of the replicate photographs from the August 1992 MQR survey contained either 

 only Stage I taxa (60%) or Stage I taxa overlying Stage III (35%; Figure 3-3). No stations 

 were classified as azoic. Successional stage results from the CLIS reference area (1991) 

 indicated an inverse relationship, with 68% of the replicate stations displaying Stage I taxa 

 overlying Stage III, 16% of the stations displaying only Stage I, and 16% displaying only 

 Stage III. However, the habitat succession has progressed since the 1991 survey, when 16% 

 of the stations were azoic, and less than 10% of the stations were classified as Stage III or 

 Stage I overlying Stage III (Figure 3-3). 



Recolonization of the Mill-Quinnipiac River Disposal Mound (MQR): Results of a REM OTS® Survey, August 1992 



