62 



The data collected at primary monitoring stations H2 and H4 also suggest a seasonal 

 hypoxic episode was occurring in the central Long Island Sound region during the summer 

 months. A steady decrease in detectable oxygen was observed from Julian Day 100 (7 

 April 1995) through Julian Day 226 (mid- August; Figure 4-2). As the water temperature 

 began to increase in the spring and summer months, water quality in Long Island Sound 

 was slowly depressed by falling DO concentrations and lack of significant fresh water input 

 from the surrounding tributaries due to drought. 



Station H4 was located in the center of Long Island Sound, approximately 6 km 

 southeast of CLIS, in water 30 m deep (Figure 4-1). Bottom DO concentrations at Station 

 H4 dropped to 4.2 mg T 1 in early August and remained at those levels through the 

 September 1995 survey. Station H2 showed a major decrease in DO in early and mid- 

 August, with values falling to 2.4 mgT 1 then slowly increasing to 3.8 mgT 1 by late August. 

 This station was located 6.5 km northwest of the center of CLIS in water 15 m deep 

 (Figure 4-1). 



In September, dissolved oxygen concentrations began to climb towards 6.0 mgT 

 and continued to increase as the autumn of 1995 progressed. The primary and secondary 

 station data both indicated a decrease in dissolved oxygen concentrations within the central 

 Long Island Sound region immediately preceding the September 1995 field activity. The 

 REMOTS® sediment-profile photographic survey over the project mounds and reference 

 areas at CLIS observed the aftermath of the hypoxic event within the benthic community. 

 Although DO concentrations seemed to be increasing at the time of the REMOTS® survey, 

 complete recovery within the benthic community (OSI values >6, deep RPD, presence of 

 Stage II and Stage III assemblages) would not be seen for several weeks. 



The degree and effects of the seasonal hypoxia varied with the sampling location at 

 CLIS during the 1995 monitoring cruise. In general, the CLIS reference areas showed a 

 decline in benthic habitat quality with lower RPD depths than expected and no Stage II 

 organisms present. The NHAV 93 mound showed improvement relative to the July 1994 

 survey; Stage I organisms occupied the surface sediments, and Stage III individuals were 

 present at depth. The CLIS 94 mound recovered better than expected with a Stage I on III 

 recolonization status, and several deep RPD measurements, but displayed indications of a 

 low DO event. The FVP mound continued to exhibit difficulty in fully establishing and 

 maintaining a stable benthic community with low RPD and OSI values near the mound 

 center. 



The REMOTS® data from the FVP mound show a steady decline in the apparent 

 RPD and OSI since the 1991 CLIS survey, with the exception of Station CTR, where the 

 OSI has ranged from 4 to 6 since the 1987 CLIS survey (Figures 4-3 and 4-4). This trend 

 was also noted in the data collected during the CLIS survey in November 1993. The 



Monitoring Cruise at the Central Long Island Sound Disposal Site, September 1995 



