subtidal densities at Imperial Beach which are an order of magnitude, 

 lower (190 to 510 per square meter). Barnard (1963) reported a density 

 of 3,600 organisms per square meter from nearshore sands in southern 

 California. Screen sizes used in were similar to those used in this 

 study; however, collection methods differed. Estimated densities for 

 abundant species and rank order of abundance for the 3.7- and 6.1- 

 meter depths are presented in Tables 11, 14, and 15. The high numbers 

 obtained in the present study by use of core samples may represent 

 unusually high population densities, but more likely are typical of such 

 habitats and were recorded due to greater effectiveness in sampling. 

 Parr and Diener (1978) reported relatively elevated numbers elsewhere in 

 southern California using replicate diver-operated core samplers. 



Total population densities at Imperial Beach were highly 

 variable at 3.7- and 6.1-meter depths. Certain species were capable of 

 considerable population increases and decreases and spatial patchiness 

 along this offshore zone. For example, significant (p<.05) sixfold 

 differences in abundance existed between 3.7-meter stations in survey 

 I before beach replenishment; these were primarily due to differences in 

 density of the ostracod, Euphilomedes sp., which exceeded 8,000 per 

 square meter at station B and was not present in samples from station C 

 located only 950 meters south. Clearly, much patchiness is evident in 

 this seemingly homogeneous environment. However, some patterns in 

 relation to sediment deposition were evident. Sediment decreased in 

 average grain size as the fine sediment fractions increased at the 3.7- 

 and 6.1 -meter depths offshore at disposal stations A and B, and these 

 changes in fine sediments had diminished 7 weeks after termination of 

 beach replenishment (Figs. 8 and 9). No such fluctuations were evident 

 at the control station C. During this period, fine sediments washed 

 from the dredge disposal were transported offshore, covering and mixing 

 with the existing bottom sediments. 



The 6-meter depths were sampled, just before beach replenishment, 

 while 3.7-meter depths were sampled 2 weeks after the operation started 

 (Table 1) . Abundance and numbers of species per station are given in 

 Figures 26 to 29, and in Table 12. The patterns are complex. Signifi- 

 cant differences between stations existed before and during the disposal 

 operation. High numbers at 6.1-meter stations A and B when fine sedi- 

 ments were prevalent and at the 3.7-meter station A following deposition 

 were possibly due to the change in particle size and increased organic 

 matter. For example, at 3.7-meter stations following deposition survey 

 IV, three polychaete species comprised most of the abundance; at the 3.7- 

 meter station B, an amphipod and a cumacean were also prevalent (Table 

 14). Large numbers of another polychaete species settled at the 6.1- 

 meter station A several weeks after disposal, but not at stations 

 B or C. 



Populations were influenced significantly by physical factors. 

 For example, subtidal abundances were relatively high during calmer 



68 



