63 



The CTDEP water quality data indicate DO concentrations steadily declined from 

 12.2 mg-r' in mid-March (Julian Day 75) to approximately 2.5 mg 1' in early September 

 (Julian Day 250). The July 1996 monitoring cruise (Julian Day 198) was completed before 

 the expected seasonal reduction in available oxygen within the western Long Island Sound 

 region (Figure 4-6). In mid- July, bottom water DO concentrations at the primary (C2 and 

 D3) and secondary (5, 8, and 9) water quality monitoring stations ranged from 4.8 mg-l' 

 to 6.75 mg-r\ 



Oxygen concentrations of >5.0 mg 1"' are thought to be protective of most Long 

 Island Sound marine life (LISS 1990). Warm bottom waters and a consistent supply of 

 molecular oxygen (O2) 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 a Stage III assemblage incorporate oxygen-rich 

 bottom waters into the surficial sediments, resulting in deeper RPD depths and elevated 

 OSI values. As DO concentrations decrease through the spring and summer months, the 

 level of oxygenation within the surface sediments also decreases, resulting in shallower 

 RPDs and the appearance of redox rebound intervals. Environmental stress and mortality 

 within the infaunal populations and resident macrofauna result in a reduction in habitat 

 quality, decreased biological productivity, and lower OSI values. 



As expected, the CTDEP data recorded the start of the seasonal hypoxia event in 

 the bottom waters of the western Long Island Sound region approximately four weeks after 

 the 1996 survey activity. DO concentrations dropped below 3.0 mgl' (Julian Day 225) 

 and remained at hypoxic levels for an additional four weeks, reaching a seasonal low on 

 Julian Day 250. Bottom water DO concentrations in early September ranged from 

 2.2 mg-r' at C2 to 2.8 mg-l' at Station 9. Near anoxic conditions (0.7 mg 1') were found 

 over the seafloor at Stations A4 and B3 in extreme western Long Island Sound during the 

 same time period. By the third week in September, bottom water DO concentrations in the 

 region had remmed to levels greater than 6.0 mg 1', favorable for reestablishing a solid 

 benthic community. 



In the past, annual monitoring surveys at the Long Island Sound disposal sites were 

 performed in mid-summer, allowing four or more weeks between the end of the disposal 

 season (31 May) and any benthic community assessment operations. In addition, the 

 summer months provide warmer bottom water temperatures (17 to 21°C), which increase 

 the metabolic rates and bioturbation activity of the benthic infaunal populations. However, 

 the occurrence of seasonal hypoxia in the western Long Island Sound region in mid- 

 summer has been identified as an obstacle to benthic recolonization at WLIS since 1985 

 (SAIC 1988). 



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



