61_ 



Bottom dissolved oxygen concentrations are measured by the Connecticut 

 Department of Environmental Protection (CTDEP) Bureau of Water Management 

 (Figure 4-2). The data were collected as part of the LISS Summer Hypoxia Monitoring 

 Program and consisted of surface and bottom water DO values for eighteen primary 

 stations throughout 1997 and 1998, and a number of secondary summer/fall stations (July 

 to October). Annual stations C2 and D3 and seasonal monitoring stations 5,8, and 9 were 

 selected for their proximity to WLIS (Figure 4-2). Both surface and bottom waters were 

 tested at each station. Because the bottom water concentrations were measured 

 approximately 5 meters above the acmal substrate, the dissolved oxygen in the water near 

 the sediment surface is most likely lower than the observed values. 



In 1997, the dissolved oxygen conditions did not drop as low as reported in 

 previous years during the late summer months. Although the oxygen concentrations 

 decreased sharply from June 4 to June 27, the levels did not fall below 3.6 mg 1"^ during 

 the summer or fall. However, recovery was slower in September 1997 than in 1996 due to 

 warmer temperatures. During the September survey, the overlying bottom waters at 

 stations 5 and 8 had DO levels of 4.6 mg 1"'. The dissolved oxygen increased from 7.8 to 

 10.8 mg r^ over the winter months and peaked in March at 11.1 mg 1"^ during the second 

 survey time (Figure 4-2). Aldiough the overlying bottom waters did not become hypoxic, 

 the water near the sediment interface may have been so in certain regions. 



4.1.3 Hypoxia and Seasonal Effects on Benthic Community Parameters 



Seasonal changes affect the environmental conditions of the seafloor observed in the 

 REMOTS® surveys. We observed shallower RPD and lower OSI values at SW-REF in 

 September 1997 relative to July 1996, indicating that the reference area was affected by the 

 late summer hypoxia. Although the stations at reference area SOUTH in late September 

 1997 had OSI values and RPD depths similar to the July 1996 stations, the numbers were 

 comparable to SW-REF (Table 3-3). 



Figures 4-3 and 4-4 show replicates from South and SW-REF, respectively, that are 

 representative of the variability among replicates in each area. Two of the eighteen 

 replicates at South did have deeper RPDs than South 2A which contributed to deeper average 

 RPD depth. As the temperamre of the air and water decreases in the fall and into the 

 winter, mixing of the stratified layers of the Sound occurs and dissolved oxygen 

 concentrations increase (Figure 4-2). Surveys over the I and H mound indicated greater 

 RPD depths and improved conditions in March relative to September. 



Water temperamre is directly correlated with the metabolic activity of benthic 

 organisms. Seasonal changes in bottom sediment temperature were recorded at a central 



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



