642 Marine Microbiology 



from tidal currents along the break in slope at the 50-fathom con- 

 tour. 



The most significant discharge to the bay is the effluent from 

 the City of Los Angeles' sewage treatment plant at Hyperion. 

 During 1956-59, about 1 x 10^ m^/day of secondary effluent were 

 disposed of through a submarine outfall about 1.5 km from shore 

 at a depth of 15 m. The activated-sludge treatment included 

 screening, grit removal, primary settling, aeration, final settling, 

 chlorination, separate sludge digestion, and fertilizer production. 



Oceanographic investigations of Santa Monica Bay were 

 made by the Scripps Institution of Oceanography of the Univer- 

 sity of California, the Allan Hancock Foundation of the Univer- 

 sity of Southern California, and the City of Los Angeles during 

 the period 1954-59. The sanitary, gross biological, and physical 

 aspects of the discharge of Hyperion effluent have been pre- 

 viously reported (3, 7). Receiving water conditions in the bay 

 have been subsequently changed by expansion and modification 

 of the sewage treatment and disposal regimen; nevertheless, some 

 of the results of earlier investigations are pertinent to the present 

 study. 



As the treatment plant effluent rose through the 15 m water 

 depth, it was initially diluted with about 20 parts sea water per 

 part effluent, and appeared on the surface as a "boil". Stirring pro- 

 cesses continued for about four hours at which time the dilution 

 was 100:1. Subsequent mixing took place slowly by means of 

 lateral eddy diffusion and, after twenty-four hours in the water, 

 the dilution was about 300:1. A restricted visible field and a lar- 

 ger in-shore water mass were formed. 



The in-shore water mass was a typically bell-shaped, len- 

 ticular mass extending over about 50 km^, which remained within 

 the upper 10 m because of its high vertical stability. The boun- 

 dary of this mass was characteristically established by the 300:1 

 dilution contour (the highest ratio which could be determined 

 with any assurance), gradients in plankton, oxygen and turbidity 

 concentrations, color changes at the surface, and convergences. 



Coliform l^acteria in surface waters were reduced by an or- 

 der of magnitude every 6.5 hours during the first day after dis- 

 charge in accordance with the relationships: 



