The site below Casino Creek and the site below Lewistown had 

 somewhat less than 60% of their diatom floras in common, 

 indicating minor perturbations in this reach. 



The immediate impact of the oil spill appeared to be limited 

 to the physical coating of objects in the stream, including 

 substrates and macroalgae . The oil did not appear to exert a 

 toxic effect or to alter the composition and structure of the 

 periphyton community. 



There are several possible reasons why the oil spill did not 

 have a measurable impact on the composition and structure of the 

 periphyton community in Casino Creek and Big Spring Creek: 



• The used motor oil may have contained only small amounts of 

 volatile or soluble organic compounds; 



• Dilution and flushing may have been sufficient to negate the 

 effects of any soluble or volatile organics in the oil ; 



• Cold temperatures at the time of the spill may have kept the 

 oil viscous and prevented it from mixing readily with stream 

 water; and 



• The short time elapsed between the spill and periphyton 

 sampling may not have been long enough for the oil to have 

 worked its full effect on the algae. 



The full biological impact of the oil may have occurred much 

 later when water temperatures were higher. The persistence of 

 unweathered oil within sediments can have a long-term effect on 

 the structure of benthic communities and cause the demise of 

 sensitive aquatic species (USEPA 1986) . 



The diatom associations at the three Big Spring Creek sites 



11 



