SUMMARY 



Four composite periphyton samples were collected in March 

 1999 from Casino Creek and Big Spring Creek in central Montana 

 after several hundred gallons of used motor oil were released 

 into Casino Creek. Samples were collected following DEQ standard 

 operating procedures, processed and analyzed using standard 

 methods for periphyton, and evaluated following modified USEPA 

 rapid bioassessment protocols for wadeable streams. 



Aside from coating substrates and macroalgae, the oil did 

 not appear to have an immediate effect on the species composition 

 and structure of the periphyton community. The lack of a toxic 

 or chemical effect may be due to cold water temperatures at the 

 time of the spill, dilution and flushing of toxic chemicals, low 

 concentrations of volatile and soluble compounds in the oil, and 

 the relatively short time that had elapsed between the spill and 

 periphyton sampling. The spilled oil may not have had its full 

 biological effect until much later when water temperatures were 

 higher . 



Diatom association metrics appeared to indicate problematic 

 levels of siltation at all of the sampling sites. Siltation 

 levels were highest in Casino Creek and in Big Spring Creek below 

 Casino Creek. Siltation index values in Big Spring Creek were 

 higher than those recorded in August 1998 for the same reach of 

 stream. It's possible that the spilled oil served as a substrate 

 for motile diatoms and had the same effect on the siltation index 

 as do fine particles of inorganic sediment. It's also possible 

 that the elevated siltation index levels were due entirely or in 

 part to the recorded abundance of motile, cold season diatoms in 

 the genera Navicula and Nitzschia. 



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