OLIVER ET AL.: RELATIONSfflPS BETWEEN WAVE DISTURBANCE AND ZONATION 



small (< 1 cm long) Prionospio cirri fera was found 

 as deep as 30-40 cm in the sediment (Figure 4). It 

 was the most abundant polychaete from the long 

 cores at the 18 m station (Figure 4) and yet ranked 

 only seventh in abundance, when the top 10 cm of 

 the sediment was considered separately. Spionids 

 have ciliated feeding palps and dorsal gill fila- 

 ments. They generally live in mucus-lined bur- 

 rows or sand tubes and feed on, or just above, the 

 bottom surface (Hartman 1941 ). Perhaps P. cirrif- 

 era inhabits the burrows of thalassinid shrimps in 

 a manner similar to the phoronid worm Phoronis 

 pallida. In Bodega Bay, Calif., P. pallida is found 

 deep in the sediment in association with Upogebia 

 pugettensis (Thompson 1972). The lower portion of 

 the phoronid tube is constructed of relatively 

 coarse sand and the distal end is composed of fine 

 black sediment where the tube passes through the 

 burrow wall of U. pugettensis . The tube is oriented 

 normal to the shrimp burrow such that the 

 lophophore can be extended into the burrow for 

 feeding and respiration (Zimmer^). Coinciden- 

 tally, the highest concentration of P. cirrifera per 

 vertical stratum (175) was found deep (30-40 cm) 

 in the only core that contained a large thalassinid, 

 Callianassa sp. 



It is important to note that these vertical pat- 

 terns were not artifacts resulting from migration 

 down the core or from physical disturbance. Corers 

 were oriented horizontally while out of water and 

 quickly cut by extruding the core from the corer 

 bottom. Moreover, animals that were known to 

 live very near to the sediment-water interface 

 were not displaced (e.g., small bivalves, crusta- 

 ceans, and Nephtys cornuta). 



Other Animal Groups 



Although the density of crustaceans was very 

 low in the polychaete zone (Figure 7), the number 

 of tube-dwelling forms was greater (e.g., Am- 

 pelisca and Photis species). Variations in the 

 number of individuals and species of bivalve mol- 

 luscs (Figures 7, 8) were due to periodical heavy 

 settlement along the entire transect. Juvenile 

 mortality was almost complete and very few 

 specimens were observed that were larger than a 

 few millimeters. The same observation was made 

 by Muss (1973) in the 0resund in Denmark. The 



most abundant local bivalves were Tellina mod- 

 esta and Mysella aleutica (Table 2). The distribu- 

 tion of T. modesta was correlated with the percent- 

 age of silt in shallow- water sediments by Barnard 

 (1963). Large individuals were observed in the 

 northern bay and in Moss Landing Harbor, where 

 the fine sediment fraction was greater than that 

 along the transects. There were also high numbers 

 of small (1 mm) juvenile bivalves present in the 

 surface strata of the long cores (Figure 5); how- 

 ever, most of the biomass of molluscs in the lower, 

 hydraulically dredged strata was due to a few 

 large individuals of Solen sicarius and Macoma 

 spp. (Figure 6). 



Ophiuroid communities are generally found at 

 depths ranging from 45 to 90 m in southern 

 California (Barnard and Ziesenhenne 1961). 

 Large individuals of Amphiura acrystata were 

 found deep in the substrate at the 24 m station. 

 The animal's oral disc was usually 10-15 cm below 

 the surface and its arms extended through the 

 sediment into the water column. All observed in- 

 dividuals appeared to be suspension feeding. The 

 density of A. acrystata was <llm^ at the 24 m 

 station, but increased to 1 or 2lvciP- south of the 

 study area. 



NORTHERN SANDFLAT 



The northern sandflat received larger waves 

 and the bottom surge was consistently greater 

 than that along the southern sandflat. The 

 theoretical difference in relative wave energy 

 reaching both beaches was estimated from a de- 

 tailed wave refraction diagram.^ Assuming wave 

 arrival from the northwest and wave period of 14 s, 

 the total energy reaching the southern transect is 

 only three-quarters of that arriving at a compara- 

 ble segment of the northern beach. The southern 

 stations also had finer sediment compared with 

 the same northern depths (Table 1). During winter 

 storms, wave heights in the northern area were 

 often more than a meter higher than those at the 

 southern study site. These differences only oc- 

 curred close to the canyon in the vicinity of the 

 sampling transects and were corroborated by 

 hundreds of scuba diving observations of substrate 

 motion and wave swell on the two sandflats. 



If the primary control of the offshore zonation 

 pattern is due to wave-induced substrate motion, 



^R. Zimmer, Professor, Biology Department, University of 

 Southern California, Los Angeles, CA 90007, pers. commun. 

 June 1973. 



^U.S. Army Corps of Engineers wave refraction diagrams 14- 

 51-1, 1948. 



447 



