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

Reported reduced sportfish densities in the main channel of the Ouachita River prompted an investigation, beginning in 1990, into potential causes of ongoing impairment to aquatic biota. In-stream monitoring that incorporated toxicity testing of sediments and water was conducted to discern potential sources of contaminants that might be related to the suboptimal fishery 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 tributaries of the Ouachita River. Larval survival and growth tests were conducted using the fathead minnow, while survival and reproduction were assessed for 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 rarely observed in the mainstem of the River and, moreover, represented intermittent 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 significant growth reductions in C. tentans exposed to sediments collected from West and East Two bayous, Smackover and Coffee creeks. These results suggest there is intermittent impairment in tributaries of the Ouachita River due to ambient water and sediment conditions that are aside from current concerns for mercury contamination