approximately every 3 months at a depth of 7.6 meters in another Cali- 

 fornia sand-bottom community (Diener and Parr, 1977) . In this study, 

 densities fluctuated greatly with no consistent pattern that could be 

 related to added beach sediments, seasonality or measured sediment 

 parameters. 



(2) Soolelepis squamata . This polychaete ranked 10th in 

 abundance intertidally, 4th at the 3.7-meter stations and was rare at 

 the 6.1-meter stations [Table 11). This species not only had a strong 

 preference for shallow water, but also was highly seasonal in its re- 

 cruitment to the nearshore habitat (Fig. 38). Abundance was high at all 

 3.7-meter stations, survey II (6 April 1977), and estimated densities 

 were 1,600 per square meter (Table 14). Abundance decreased signifi- 

 cantly at all stations for the next survey and subsequently abundance 

 was either very low or the species was absent. The decrease observed at 

 station B, 3.7 meters, during survey III (2 June 1977) may be attribu- 

 table to beach replenishment or to normal population fluctuations. 

 However, since the control station was not sampled in survey III, no 

 decision is possible. It is significant that on survey IV (12 August 

 1977), densities were very low at station C at 3.7 meters. This species 

 also recruited in high numbers to an area obviously physically impacted 

 by beach replenishment (station A, 3.7 meters, 6 April; Figs. 8 and 9). 



(3) Magetona piteVkai . This polychaete was found only once 

 in the intertidal zone but it ranked sixth and seventh in abundance 

 for the 3.7- and 6.1-meter stations, respectively (Table 11). It 

 appears to have responded with increased settlement (densities to 1,300 

 per square meter) at impacted stations A and B during beach replenish- 

 ment (Fig. 38). This increase in density was not found at control 

 station C which implies that this dense settlement may not have been a 

 seasonal effect but a response to the changed physical conditions pro- 

 duced by beach replenishment. 



(4) Saoloplos armiger . This polychaete worm ranked 9th in 

 abundance intertidally, 11th at 3.7 meters, and 9th at 6.1 meters (Table 

 11). This species was found at all depths at all surveys. Maximum 

 density was 910 per square meter (Fig. 38, Table 15) found at station A, 

 6.1 meters (9 March 1977) before beach replenishment began. Changes in 

 population densities do not appear related to beach replenishment or any 

 of the sediment parameters. 



(5) Owenia fusiformis collaris . This cosmopolitan species 

 builds tubes and lives in the subtidal bottom off Imperial Beach. These 

 sand-bottom, tube-dwelling organisms increase the stabilization of 

 sediments (Rhoads, 1974). Occurring in small clumps or larger patches, 

 the tubes are able to stabilize marine substrates much the way plants do 

 to soil in terrestrial ecosystems. 



Owenia sorts out and concentrates the mineral hornblende 

 in the process of tube building and repair (Fager, 1964) . Since horn- 

 blende comprises about 50 percent of the sediment at Imperial Beach, it 



96 



