28 



active feeding. In addition, several of the voids contained coarse-grained particles that 

 were presumably discarded during particle-selective feeding habits (Figure 3-7). 



Despite the healthy benthic community at BLDS, the RPD values are relatively 

 shallow. Normally, the combined activity of the subsurface burrowing Stage III 

 organisms, the sedentary tube-dwelling polychaetes, and surface-dwelling Stage II infauna 

 served to transport oxygen into the sediments resulting in relatively deep RPD depths. 

 While irrigating their tubes, sedentary tube-dwelling polychaetes pump high dissolved 

 oxygen concentration seawater out of the tube bottoms directly into the sediments, forcing 

 a unidirectional upward displacement of interstitial fluid. However, the net diffusion of 

 dissolved oxygen from seawater injection and subsequent advective porewater transport 

 will be less where there is high sediment oxygen demand (SOD). With high SOD, oxygen 

 that is transported across the sediment-water interface is rapidly consumed by microbial 

 respiration and oxidation reactions. The observed RPD depths of only a few centimeters at 

 BLDS, combined with a well-established infaunal deposit- feeding community (Figure 3-5), 

 suggest that BLDS sediments have relatively high SOD. 



Monitoring Cruise at the Historic Boston Lightship Disposal Site, August 1994 



