from the deeper sediments to surficial sediments, and perhaps even to 

 the overlying water, especially if sediments are resuspended by currents 

 or boat- or wind-driven waves. The fingernail clam, Musculium 

 transversum, the organism of primary interest in this study, makes 

 shallow burrows in the sediment and may be exposed to much higher levels 

 of ammonia than organisms living in the water column, at the mud-water 

 interface, or on plants, rocks, and woody debris. 



Ammonia toxicity is due to the un-ionized (NH3) form (USEPA 1985). 

 The proportion of total ammonia existing in the un-ionized form is 

 controlled primarily by pH and temperature (Emerson et al . 1975). The 

 pH of sediments can fluctuate dramatically on a seasonal basis, and the 

 pH of the overlying water can fluctuate daily, so that episodes of 

 toxicity may occur even if the total ammonia concentration remains 

 relatively constant. Ammonia loading of rivers tends to increase during 

 winter because the microorganism-mediated conversion of ammonia to 

 nitrate stops at cold temperatures. Also, aquatic vegetation does not 

 remove ammonia (a plant nutrient) during winter dormancy. Water quality 

 standards frequently allow higher levels of ammonia in the winter be- 

 cause the proportion of total ammonia existing in the toxic, un-ionized 

 form is less at cold temperatures. However, the sensitivity of fish to 

 ammonia increases at cold temperatures, so even though there may be less 

 un-ionized ammonia, acute toxicity may still occur (Reinbold and Pesci- 

 telli 1990). Research is needed to determine the effect of cold temper- 

 atures on the sensitivity of invertebrates, as well as fish, to ammonia. 



Musculium transversum is sensitive to ammonia. Anderson, Sparks 

 and Paparo (1978) found that un-ionized ammonia concentrations of 0.08- 

 0.09 mg/1 (expressed as un-ionized ammonia nitrogen, NH3-N, in mg/1) 



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