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LIS location from July 1974 to January 1976, with the highest temperatures (about 22° C) 

 in late August and lowest (about 3° C) in March (Rhoads and Boyer 1982). Therefore, 

 metabolic activities, such as bioturbation and decomposition, were expected to be noticeably 

 greater m September than March. Greater biomrbation would suggest greater oxygenation 

 of the sediment and redox layer. However, the decomposition of organic matter, which 

 increases both the chemical and biological oxygen demands, also occurs at a faster rate in 

 warmer waters. The cause and effect relationships of RPDs, metabolic rates, and dissolved 

 oxygen conditions of the waters are complex (Rhoads and Germano 1982). Stage I species, 

 which are known to die off in the cooler waters in the winter and then rapidly reproduce 

 and recolonize in the spring, were less common in March than in September. 



Redox rebounds observed at the disposal mounds represent retrograde conditions 

 caused by decreases in pore water irrigation by benthos and/or higher rates of oxygen 

 consumption at depth. The depth of the RPD decreases and rebounds upwards. The 

 rebound tends to be a result of seasonal changes in water temperature, metabolic rates, and 

 chemical reaction rates. For instance, when organic matter settles on the seafloor and 

 begins decomposing, the oxygen demand of the sediments increases, decreasing the RPD. 

 The organic matter attracts pioneering Stage I organisms which are filter feeders that are 

 not effective at exchanging pore water with surface water and are associated with shallow 

 RPD depths (Rhoads and Germano 1982). 



Redox rebounds were visible in both September and March at the H mound, and to 

 a lesser extent at the I mound and the potential reference area, SE-REF (Figures 4-5 

 and 4-6). Replicate HIOONA shows the spatial variability of redox rebound. Because 

 redox rebounds were not visible in photographs of ambient sediments from SOUTH or 

 SW-REF in September, the higher organic matter concentration of the dredged sediments 

 most likely has contributed to the redox rebounds at the disposal mounds. In March, 

 decreased metabolic rates primarily account for the redox rebounds at the disposal mounds 

 and at SE-REF 



Redox rebounds at times appeared similar to relic RPDs. For a few replicates, the 

 measured redox rebound acmally represents the depth of the relic RPD. Both redox 

 rebounds and relic RPDs are characterized by a lighter band of sediment, with a higher 

 reflectance than the surrounding sediments, in the middle or lower section of the image 

 (Rhoads and Germano 1990). A relic RPD as observed in I 150EA (Figure 4-6), occurs 

 when a relatively thin layer of dredged material is placed over the ambient sediments and 

 represents the former ambient RPD. A new RPD will be formed at the sediment surface. 

 The thickness of the recently deposited dredged material can be measured from the surface 

 to the top of the relic RPD. The appearance of layered dredged material also results from 

 multiple barge load disposals during the same season such as detected in H50NA in 



Monitoring Cruise at the WLIS Disposal Site, September 1997 and March 1998 



