EFFECTS OF UN-IONIZED AMMONIA ON JUVENILE UNIONIDS IN SEDIMENT TOXICITY TESTS

Teresa Newton1, Jon O'Donnell2, Michelle Bartsch1, LeeAnne Thorson2, and Bill 
Richardson1, 1U.S. Geological Survey, Upper Midwest Environmental Sciences 
Center, La Crosse, WI and 2University of Wisconsin-La Crosse, River Studies 
Center, La Crosse, WI.

The National Park Service has identified ammonia as one of the primary threats 
to biota in the St. Croix Riverway.  Ammonia is a relatively toxic compound 
generated in water and sediments by heterotrophic bacteria as a by-product of 
organic matter decomposition.  Ammonia and other contaminants preferentially 
accumulate in sediments and porewater.  Recent data suggests that unionids are 
sensitive to un-ionized ammonia (NH3), relative to other organisms.  We 
conducted two 96-hour and two 10-day sediment toxicity tests.  Ammonium chloride 
was delivered to each of 36 experimental units (6 replicates of 6 
concentrations) by peristaltic pump and diffused from an airstone beneath test 
(reference) sediments into the overlying water.  Twenty Lampsilis cardium 
juveniles were placed in cages that were buried ?2.5 cm into test sediments to 
facilitate porewater exposure and juvenile recovery.  Survival, growth, the 
stressed:alive ratio (stressed defined as no evidence of foot movement but 
ciliary activity present), and NH3 concentrations in porewater were measured at 
the end of each test.  In all tests, survival exceeded 95% in the controls.  The 
LC50s were 127 and 165 ug/L in the 96-hour tests and 99 and 137 ug/L in the 10-
day tests.  The EC50s (based on the stressed:alive ratio) were 73 and 119 ug/L 
in the 96-hour tests and 77 and 98 ug/L in the 10-day tests.  Growth was 
substantially reduced, relative to controls, between 32 and 91 ug/L in all 
tests.  A companion study measuring concentrations of NH3 in porewater over a 
150 km reach of the Riverway, found concentrations ranging from 0.1-141 ug/L.  
These data suggest that in some locations and under certain conditions (low flow 
and high temperature), sedimentary ammonia concentrations in the Riverway 
approach, and sometimes exceed, concentrations shown to cause lethal and 
sublethal effects in laboratory tests.
	
Keywords:  mussels, ammonia, sediment porewater, toxicity, St. Croix