stress for Long Island Sound sediments between winter and summer months. 

 When the sediments are brought back to a laboratory, however, growth rate 

 of bacteria may change because of the absence of the benthic organisms, 

 thus resulting in a more resistable sediment structure. Before each of 

 our flume runs, these sediments are sieved carefully to ensure that the 

 binding effect of bacteria is reduced. However, the bacteria growth rate 

 may increase again after the sediments are allowed to settle in the 

 flume. Consequently, the longer the settling time is, the higher the 

 bacteria content is and the less erodable the sediments become. In 

 addition, the settling times of our flume runs (1 to 10 days) are 

 comparable to the time scale of bacteria growth. The shape of the curve 

 in Figure 6.12 also tends to support this possibility. 



Being quite probable, this idea should be and can be tested by further 



experiments. Sterilized sediments can be added to the flume and 



bacterial growth monitored at the same time when entrainment of sediments 



is measured. In addition, a method for assaying microbial polysaccharide 

 production could be used. 



(4) Diffusion of chemical species from pure water and reaction at the 



surface . It could be possible for the chemical reaction to occur in 

 waters of different salinity and ionic strength, and despite flushing of 

 pore waters after each flume run. It needs to be tested by monitoring 

 pore water concentration and by electromicroscope and microprobe analyses 

 of the surface sediments. 



Effect of salinity . 



Typical effect of salinity on the erodability of Mississippi Sound 



sediments can be illustrated by comparing the entrainment results of Site 1 



sediments in ppt salinity water (Figure 6.8(a)) and in 30 ppt salinity water 

 (Figure 6.13). 



The critical shear stress in 30 ppt salinity water is much higher than 

 that in the ppt salinity water. In addition, the equilibrium concentration 

 in the high salinity water is more than an order of magnitude smaller than 

 that in the zero salinity water. 



157 



