6-33 



depth and are at seasonal minima in the inner harbor during summer 

 months. Three of the NAI inner harbor stations are located in shallow 

 areas where the decreasing oxygen concentration with depth is not as 

 important a factor. The other two NAI stations are located in channels, 

 where periodic ship traffic may contribute to the mixing and aeration of 

 bottom water and where saline and comparatively oxygen-rich water from 

 Long Island Sound moves up-harbor along the bottom. 



An additional factor in the August population decline appears 

 to be associated substratum type, which is a function of local hydro- 

 graphic conditions and dredging history. Coarse-grained substrata, in 

 addition to containing lower levels of heavy metal ions, are indicative 

 of bottom scour by water movement which also tends to produce vertical 

 mixing and locally elevated dissolved oxygen values. The effect of 

 substratum in limiting the August mortalities may be seen at Station 8. 

 Prior to August 1977 this station consisted of organic-rich mud and 

 showed an August effect. During 1978, after the station siibstrate had 

 been changed to gravel probably by current changes resulting from dredging 

 activity, August mortalities did not occur. In addition. Station 4A, a 

 sand-gravel substratum station located in shallow water on a pile of 

 dredged material in the inner harbor reached some of its highest den- 

 sities in August. 



The August mortalities were most pronounced when the three 

 factors (location in the inner harbor, deep-water, and muddy substrata), 

 occur together. These factors characterize most of the R & M inner 

 harbor stations. Non-coincidence of the factors results in an unclear 

 pattern of seasonality as seen in the shallow water Morris Cove sta- 

 tions. A combination of deep-water and muddy substrata seems to have 

 produced a degree of stress during August which was sufficient to pro- 

 duce the characteristic seasonal pattern but without the catastrophic 

 population decreases observed in the inner harbor (Figure 6-6) . 



One additional feature of Figure 6-6 is that Morris Cove sta- 

 tions generally exhibited greater faunal densities than the inner harbor 

 stations and were only rarely devoid of benthic macrofauna. The deeper 

 Morris Cove stations showed considerable persistence and predictibility 



