In-Stream Monitoring of Sediments and Water in the Lower Ouachita River for Site Impact to Aquatic Biota

Reportedreduced sportfish densitiesin the main channel of the Ouachita River prompted an investigation, beginning in 1990, into potentialcauses of ongoing impairment to aquatic biota. In-stream monitoring that incorporated toxicity testing of sedimentsand water was conducted to discernpotentialsources of contaminants that might be related to the suboptimal fish- ery populations. Organisms selected to evaluate chronic impairment included larval fish, clams, midges and water fleas. The fathead minnow {Pimephales promelas) and cladoceran (Ceriodaphnia dubid)were used to estimate patterns of toxicity associated with water from seven designated reaches and selected tributariesof the OuachitaRiver. Larval survivaland growth tests were conducted using the fathead minnow, while survival and reproduction were assessedfor the cladoceran. An enzyme assay using the Asian clam (Corbicula fluminea), and growth and survival tests with Chironomus tentans were used to evaluate ambient sediment toxicity within these same reaches and tributaries.Ambient toxicity was rarelyobserved in the mainstem of the River and, moreover, representedintermittent events. However, impaired growth in larval fish, poor reproduction in cladocera, and reduced enzyme activity in clams were evident for several tributaries.Results of 10-day whole sediment tests showed signifi- cant growth reductions in C. tentans exposed to sedimentscollected from Westand East Twobayous, Smackoverand Coffee creeks. These results suggest there is intermittent impairment in tributaries of the Ouachita River due to ambient water and sedimentconditions that are aside from current concerns for mercury contamination.