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4.0 DISCUSSION 



4.1 Seasonal Hypoxia 



As predicted, comparisons between the July 1996 and September 1995 REMOTS® 

 data sets for the CLIS disposal mounds and reference areas indicate a marked improvement 

 in benthic conditions. With no distinct change in successional stage stams. the OSI values 

 calculated for the July 1996 REMOTS® stations were considerably higher. This 

 improvement was primarily due to the incorporation of more molecular oxygen (O.) in the 

 surficial sediment layers, resulting in deeper RPD depths. The level of oxygenation at the 

 sediment-water interface is controlled by the extent of biomrbation. as well as the 

 concentration of dissolved oxygen (DO) in the bottom waters to support biological 

 (respiration) and chemical (oxidation) consumption requirements. 



During the September 1995 REMOTS® sediment-profile photography surveys over 

 NHAV 93. CLIS 94, F\T, and the CLIS reference areas, a trend of shallower than 

 expected RPD depths and indications of low DO concentrations was observed despite the 

 presence of mamre benthic assemblages (Morris 1997). In addition, water quality' data 

 obtained from the Connecticut Department of En\ironmental Protection (CTDEP) 

 documented the occurrence of a seasonal hypoxic event within the central Long Island 

 Sound region two weeks prior to the September 1995 monitoring cruise at CLIS (Figures 

 4-1 and 4-2: Morris 1997). 



The 1996 CTDEP water quality data indicate the July 1996 monitoring cruise was 

 completed before the seasonal reduction of available oxygen reached critical levels within 

 the central Long Island Sound region (Figures 4-1 and 4-2). In early July, bottom water 

 DO concentrations at the primary (H2 and H4) and secondary (23, 26, and 27) water 

 quality monitoring stations ranged from 5.0 mgT"^ to 6.5 mg-l"'. Oxygen concentrations of 

 > 5.0 mg-r^ are thought to be protective of most Long Island Sound marine life (LISS 

 1990). Warm bottom waters and a consistent supply of molecular oxygen (O.) promote 

 increased bioturbational activity' within the infaunal populations of the disposal mounds and 

 reference areas. The feeding and foraging efforts of errant polychaete worms composing 

 the Stage III assemblage incorporate oxygen-rich bottom waters into the surficial 

 sediments, resulting in deeper RPD depths and elevated OSI values. 



As expected, the CTDEP data recorded the occurrence of a seasonal hypoxic event 

 in the bonom waters of the central Long Island Sound region approximately four weeks 

 after the 1996 survey activity (Julian Day 233; Figures 4-1 and 4-2). Bottom water DO 

 concentrations reached a seasonal low at five of six water quality monitoring stations 



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



