OLIVER ET AL.: RELATIONSHIPS BETWEEN WAVE DISTURBANCE ANDZONATION 



race after the fall storms, and the down-canyon 

 movements of station markers along this part of 

 the ridge. The change in the terrace wall position 

 was reflected in a 1 or 2 m increase in the depth of a 

 midwall reference station and in the shifting of the 

 wall base closer to the permanent stations on the 

 ridge. The station markers (rods) at station C and 

 especially D were also moved and slanted in a 

 down-canyon direction. Furthermore, a large ship 

 anchor, dropped in slightly deeper water below 

 station D, moved about 10 m in the same down- 

 canyon direction (Arnal et al. 1973). In contrast, 

 the station stakes at stations A and B remained in 

 a vertical position and did not move. The slumping 

 of the terrace wall during the fall and winter was 

 observed in other years and in other parts of the 

 canyon head as well (Arnal et al. 1973; pers. obs.). 



The four ridge stations (A-D) were established 

 along a gradient of substrate motion. Station D 

 was nearest to the slump zone and station A was 

 farthest from the terrace wall, where there was no 

 evidence of sediment slumping. The animals were 

 sampled once along the transect in December 

 1972. Although there were no significant differ- 

 ences in the median diameter of the sediment and 

 the sorting coefficients among the four stations 

 (P>0.1; Kruskal-Walhs test, Conover 1971), the 

 surface sedimentary structures indicated the per- 

 sistence of the substrate disturbance gradient 

 from October to December. During this entire 

 period, the sandy sediment was well consolidated 

 and formed parallel ripple marks at stations A and 

 B, but was poorly consolidated and had no ripple 

 marks at stations C and especially D (the area of 

 creeping). 



This change in substrate consolidation and rip- 

 ple mark patterns was not caused by changes in 

 tidal or oscillatory-wave currents. Tidal and 

 longshore currents did not vary along the 40 m 

 transect. Moreover, since oscillatory bottom cur- 

 rents primarily depend on the grain size, water 

 depth, wave height, and wave period (Komar 

 1976), which were all constant along the transect, 

 the bottom threshold of sediment movement by 

 these currents did not differ among the ridge sta- 

 tions. Therefore, the slumping gradient cannot be 

 quantified in terms of resuspended material or 

 bottom current velocity. On the other hand, poorly 

 consolidated sandy sediments characterized all 

 the slump areas we observed in the canyon head 

 and were also documented in a large slump near 

 the Scripps Canyon in southern California (Van- 

 Blaricom 1978; pers. obs.). These slumping events 



are extremely difficult to quantify because they 

 have never been witnessed (see Shepard and Dill 

 1966). Nevertheless, the observations and indirect 

 measurements indicate a unidirectional gradient 

 of substrate motion related to the distance from 

 slumping canyon walls and channeling topog- 

 raphy. Stations A (low movement) and D (high 

 movement) represent the ends of this gradient. 



OFFSHORE ZONATION PATTERNS 

 ALONG THE SOUTHERN SANDFLAT 



The gradational nature of the invertebrate as- 

 semblages was apparent from the changes in 

 species abundance along the offshore transect 

 (Table 2). The crustaceans were the numerically 

 dominant group at the 6 and 9 m stations (M-1 and 

 M-2). The density of crustacean species and indi- 

 viduals was highest at 9 m (Figures 7, 8). Six of the 

 seven most abundant species at this station were 

 crustaceans (Table 2). The number of crustaceans 

 decreased steadily seaward of 9 m. The 14 m sta- 

 tion was a distinct transition zone between a shal- 

 low offshore crustacean assemblage and a deeper 

 polychaete assemblage (Table 2; Figure 7). 



The 6 m station was located at the seaward edge 

 of a large bed of Dendraster excentricus . The width 

 of the sand dollar bed increased with distance from 

 the canyon head. It was approximately 40 m at the 

 southern sandflat transect (Figure 1) and >75 m 

 wide 2 km to the south. Merrill and Hobson ( 1970) 

 described a protected open coast/), excentricus bed 

 which was similar to the local situation. 



Crustacean Zone 

 Crustaceans 



The abundant animals of the shallow-water 

 zone were small, actively burrowing, deposit- 

 feeding amphipods and ostracods (Table 2). The 

 amphipods belonged to three typically sand- 

 dwelling families, Oedicerotidae, Phoxocephali- 

 dae, and Haustoriidae, and the ostracods to the 

 Philomedidae. Each family was represented 

 mainly or exclusively by one genus of several 

 species and each species was often found within 

 distinct depth limits. 



The genus Eohaustorius contained the only rep- 

 resentatives of the subfamily Haustoriinae on this 

 coast (Bousfield 1970). Temporal variations in 

 Eohaustorius sencillus and E. sawyeri were rela- 

 tively complementary at the 6 m station and may 



443 



