single transect will deviate from this "true" value by 13 percent half 

 of the time and the average error is 20.6 percent. Considering the 

 homogeneous appearance of the beach this finding was at first un- 

 expected. Yet, most of the beach organisms are motile crustaceans 

 which can form aggregations creating small-scale patchiness. 



c. Species Acquisition . The number of species occuring at a 

 given site is important to ecologists. How effectively can the true 

 population be estimated? Species acquisition curves provide one approach 

 to answering this question. An area is considered to be well sampled in 

 terms of species composition if increased sampling only rarely adds 

 additional species; i.e. the species acquisition curve approaches an 

 asymptote in relation to sampling effort. Species acquisition relation- 

 ships of Imperial Beach samples are presented in Figure 16. At inter- 

 tidal stations when sample size was 90, approximately 70 percent of the 

 total species were found in the first 30 samples. At subtidal stations, 

 when sample size was 15, 70 percent of the species were found in the 

 first six samples. Therefore, the sampling method used effectively 

 depicts species composition at a given time and locality. Acquisition 

 curves through time were also constructed (Fig. 17). These were not 

 asymptotic indicating somewhat continuous seasonal introductions of 

 new species with time. Introduction rates were higher subtidally in 

 association with higher diversity at these depths. These data indi- 

 cate that sites cannot be fully typified with respect to species com- 

 position by a single, or even a few, surveys. If knowledge of species 

 composition is an important design criterion, such information as 

 presented above is useful in optimizing the allocation of program time 

 and cost resources. 



2. Biological Impacts of Dredge Disposal . 



It is assumed that most organisms residing within beach sediments 

 which were deposited upon probably perished due to burial. Some lique- 

 faction of indigenous sediments was evident during deposition and 

 possibly motile species (amphiods, decapods, etc. ] escaped burial. 

 The assumption that introduced sediments from San Diego Bay were 

 defaunated following pumping at high pressures through mechanical 

 impeller booster pumps along variable lengths of pipe (up to 7,600 

 meters) was verified by inspecting core samples from the surface of 

 newly deposited sediments. They were devoid of live organisms. 



A list of all species collected at different times and depths is 

 presented in Appendix B. 



a. Intertidal Abundance . Total abundance of organisms from the 

 intertidal stations ranged from 100 to 2,100 per square meter and 

 averaged 882 per square meter during the study. This is high compared 

 to densities of 200 per square meter reported for the area in February 



