the control station through survey IV but were noticeably lowered at all 

 stations by survey V (Fig. 14). This uniform decrease of organic 

 carbon correlated with the onset of winter surf (Fig. 7) and the 

 return of the offshore sediments (3.7 and 6.1 meters) to values com- 

 parable at the initiation of this study (Figs. 8 and 9). A regression 

 analysis of combined stations and surveys showed a significant (p<.05) 

 positive correlation between organic carbon and silt. 



Organic carbon values at station C, about 1.1 kilometers north from 

 the Tijuana River, appeared not to be influenced by its proximity to 

 this potential source of organic detritus. 



4. Temperature . 



Temperature is considered a major factor in controlling the distri- 

 bution of organisms. Temperatures recorded during sampling at Imperial 

 Beach varied little between stations (Table 5). Offshore at the 3.7- 

 and 6.1-meter depths, temperatures showed more seasonal variation but 

 differed little between stations during each survey. 



5. Wave Data and Seasonal Storms . 



Wave patterns and storms are forces which determine the littoral 

 zone profile and influence the diversity and abundance of organisms 

 found near the shore. Local storms affecting Imperial Beach generally 

 cause short-period waves of 10 seconds or less, and although they may 

 occur at any time, they tend to occur more often during the winter. 

 North Pacific storms generally occur during late fall, winter, and early 

 spring and cause longer waves with periods between 12 and 20 seconds. 

 Southern tropical storms produce waves from the south with periods 

 between 12 and 20 seconds in late spring to early fall (Muslin, 1978). 



The wave data for this study are shown in Figure 7. The data are 

 presented in terms of the <n^> cm^ , which is directly related to the 

 wave energy as described in Section III, 1. 



Wave energies ranged from 50 to 650 square centimeters and the wave 

 period ranged from 6 to 18 seconds. These wave energies and frequencies 

 can be converted to horizontal velocities at a given depth (for more 

 detail see U.S. Army, Corps of Engineers, Coastal Engineering Research 

 Center, 1977). Comparison of these calculated bottom velocities with 

 threshold of motion studies (Menard, 1950; Manohar 1955; Inman, 1957) 

 indicates whether sand suspension or sand transport might occur. B*- jm 

 velocities of about 0.2 meter per second are needed to resuspend '^ a 

 in the size range found at Imperial Beach and about 0.3 meter pe 

 second is necessary to initiate ripple formation (Fig. 15). I', these 

 values are compared with some typical wave frequencies and periods for 

 Imperial Beach (Table 6), it is evident that at the 3.7-meter stations 



44 



