e. Largest topographic changes also occurred along shallow sand 

 terraces and in canyon axes . 



f. Sediment probably moves by "creeping" down slopes, especially 

 in canyon or axes . 



III. FAUNAL DISTRIBUTION AND SEDIMENT MOVEMENT 



Barnard (1966) reported a distinct relationship between dominants and 

 sediment grain size in two adjacent shallow-water communities, and Masse 

 (1972) described the effect of exposure to wave energy on faunal distribu- 

 tion. Substrate consolidation is mostly a function of grain size which 

 is determined by waves moving sediment. Since no attempt was made to 

 separate these phenomena, sediment movement here refers to the general 

 instability of a moving substrate. 



This section discusses the physical aspects of sediment movement 

 described previously, and shows the significance of sediment movement in 

 determining animal distributions. 



In addition to the observations and measurements of sediment movement 

 in the canyon and the routine biological sampling, a sampling line 

 (transect) was located along the outer end of the Kaiser Company's con- 

 struction trestle (Fig. 2, bents 26 to 29) . The transect followed a 

 gradient of substrate stability on the 18-meter isobath parallel to the 

 submarine terrace north of the canyon. The seaward end of the transect 

 (station R19) was on the stable part of the northern submarine ridge. 

 Shoreward of station R19, stations were adjacent to terrace slopes of 

 increasing incline and decreasing stability as they approached the area 

 of large topographic change and sediment activity near bents 18 and 19 

 (Fig. 2). Because all stations were located at equal depths (18 meters), 

 the effects of sediment movement could be isolated from other factors 

 associated with changes in water depth. 



One sample (five replicate cores) was taken from each of the four 

 ridge stations (stations R26 to R29) in December 1972 and abundance 

 changes of the major groups of animals with increased substrate stability 

 were noted (Table 1). In a comparison of fauna at control stations (C6, 

 CIO, and C20) with fauna from less stable canyon stations (P6, PIO, and 

 P20) , stability was a greater influence on abundance than depth although 

 there was a gradual but distinct change in the fauna with increasing water 

 depth (Table 2). Resuspension and migration of sediment caused by wave 

 action was of major importance in determining animal distributions. 



In the canyon head the benthic assemblages changed abruptly in re- 

 sponse to sediment movement or substrate stability. Many animals charac- 

 teristic of the control stations were also found on the relatively stable 

 submarine ridges but few inhabited the unstable terrace slopes; e.g., the 

 fauna on the flat, stable part of the north ridge (Fig. 2) was more similar 

 to the assemblage at a comparable depth on the control transect (Fig. 1) 

 than the fauna on the deeper south ridge (Fig. 2, P-3) , which had a 



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