OLIVER ET AL.: RELATIONSHIPS BETWEEN WAVE DISTURBANCE AND ZONATION 



settlement of M. sacculata. The correlation of the 

 abundance of M. sacculata with the total number 

 of polychaetes at M-4 was highly significant (r = 

 0.71; P<0.001). Magelona sacculata generally ac- 

 counted for about one-quarter of the polychaete 

 numbers at M-4. As a result, polychaete abun- 

 dance patterns in deeper water were dominated by 

 seasonal changes in larval availability (Figure 14) 

 and recruitment, and were not clearly related to 

 substrate motions. This correlation between the 

 abundance of M. sacculata and the total 

 polychaete numbers decreased in shallower water 

 (M-2, 9 m: r = 0.34, P>0.1; M-1, 6 m: r = 0.39, 

 P>0.1). 



In summary, seasonal abundance patterns were 

 probably affected by a number of environmental 

 factors including seasonal reproductive cycles and 

 substrate motions. The recruitment or survival of 

 polychaetes was lowest at the time of year and 

 water depth of maximum substrate motions. 

 Hence, the zonation of polychaetes in the crusta- 

 cean zone was apparently influenced by seasonal 

 changes in wave-induced substrate movements. 

 On the other hand, seasonal variations in crusta- 

 cean abundance and in deeper living polychaete 

 populations could not be related to sediment mo- 

 tion in a simple manner. 



DISCUSSION 



The general zonation of benthic invertebrate 

 communities observed in central Monterey Bay is 

 common along much of the temperate open coast of 

 western North America. Carey (1965, 1972), Lie 

 (1969), and Lie and Kisker ( 1970) observed a high 

 abundance of crustaceans at their shallowest 

 sampling stations and the numerical dominance of 

 polychaetes in deeper areas off Oregon and 

 Washington. Barnard (1963) and VanBlaricom 

 (1978) found the same two zones in southern 

 California and Hodgson and Nybakken ( 1973) de- 

 scribed a comparable pattern in the northern part 

 of Monterey Bay. A similar change from a crusta- 

 cean- to a polychaete-dominated assemblage was 

 also related to wave exposure by Masse (1972) 

 in the Mediterranean. On the other hand. Day 

 et al. (1971) and Field (1971) did not find a rich 

 crustacean fauna in the "turbulent" zone they 

 described in 3-20 m on the continental shelves 

 of North Carolina and False Bay, South Africa, 

 respectively. 



The composition of the fauna along the sandflats 

 was similar to that found at comparable depths in 



southern California, but the animal density in 

 Monterey Bay was almost a power of 10 greater 

 (compare Table 2 with Barnard 1963). This dispar- 

 ity was greatest in the crustacean zone. The differ- 

 ences are probably related to different sampling 

 methods: diver corers contrasted to the orange- 

 peel grab used in the earlier studies. 



None of the previous studies provide convincing 

 evidence for a relationship between community 

 zonation and wave-induced substrate motions. Al- 

 though the evidence from the present study is de- 

 scriptive and correlative, it is consistent with 

 many observations. The general hypothesis is that 

 wave-induced sediment movement has a strong 

 influence on community zonation along the sub- 

 tidal high-energy beach. 



Some of the strongest evidence supporting this 

 hypothesis comes from the natural history pat- 

 terns of the fauna. There was a significant positive 

 correlation (r = 0.92,P<0.05) between the water 

 depth of a station and the numbers of tube build- 

 ers, burrow dwellers, and commensal animals. A 

 similar trend emerges when the animals are 

 grouped into mobile and sedentary forms (Figure 

 15). Apparently, biogenic structures were difficult 

 or impossible to establish and maintain in areas of 

 more intense physical sediment movement. Al- 

 though wave disturbance might destroy burrows 

 and tubes and dislodge their inhabitants, some 

 adults were tolerant of heavy sediment accumula- 

 tions and capable of vertical substrate migrations 

 (e.g., Nothria elegans). The zonation of poly- 

 chaetes may be largely determined by the habitat 

 selection of settling larvae (Oliver 1979). 



100 -I 



M-l M-2 M-3 M-4 M-5 



6m 9m 14m 18m 24m 



STATIONS 



Figure 15. — Variations in animal motility patterns along 

 southern sandflat in Monterey Bay, Calif, (percentage of total 

 individuals). 



451 



