137_ 



ampeliscid amphipods at the surface sediments. At the time of the surveys, many of the 

 tubes were empty and the mats were susceptible to disturbance. In the surveys conducted 

 later in the summer (1995 and 1997) the sediments underneath the mats contained evidence 

 of restricted circulation in the bottom waters or organic enrichment of the sediments 

 (shallow RPD thicknesses, methane bubbles, senescent tube mats, e.g. at USCGA and NL- 

 91). The presence of these indicators at the reference areas suggests that a regional 

 environmental stress or disturbance may have affected the eastern LIS region in 1995 and 

 1997. 



It is generally assumed that eastern LIS does not experience hypoxic conditions 

 (defined by the EPA's Long Island Sound Study as 3.0 mg-1" 1 or less). CT DEP data for 

 this region show a seasonal decrease in DO values from May to December with a low 

 period from late July to August or September (Kaputa and Olsen 2000). Stations further 

 away from NLDS (K2, J2, and N3) also follow the same pattern of lower DO values in the 

 surface and bottom waters from May to December. Dissolved oxygen levels normally 

 decrease to 6 or 7 mgT 1 for all of these stations. The lowest values recorded since 1991 

 for these stations approached 5.9 mg-1" 1 in the summer of 1991. Based on the CT DEP time 

 series data there is no evidence that 1995 or 1997 experienced conditions markedly 

 different from 1992 or 1998. However the surveys in 1995 and 1997 were conducted later 

 in the summer when environmental stress may have been sustained for a longer period. 



While these measured values do not seem sufficient for true hypoxia, they may 

 contribute to the conditions observed. The most likely explanation for the presence of 

 patches of reduced sediments and methane is that seasonal senescence of dense mats of 

 tube building amphipods may create temporary reducing conditions near the sediment 

 surface. Once these mats are eroded, the reduced sediments will be exposed to overlying 

 waters and rapidly oxidized through bioturbation and diffusion. Sediment profile surveys 

 conducted during this period (late August - early September) might contain all three of 

 these conditions (senescent tube mats and reduced sediments, reduced surface sediments, 

 thin RPD at surface). DAMOS surveys in western and central Long Island Sound have 

 noted that survey data during late summer may be complicated by the sediment disturbance 

 induced by hypoxic conditions and have recommended that surveys should be conducted in 

 early summer or early fall after recovery (Morris 1998, Murray and Saffert 1999). While 

 the eastern Long Island Sound may not experience hypoxia, it seems clear that seasonal 

 biological processes may affect survey results. Future surveys at NLDS could optimally be 

 scheduled after recolonization has begun (early June) but before mid- August when tube 

 mats appear to senesce. 



Monitoring Cruise at the New London Disposal Site, 1992 - 1998 



