CONCENTRATIONS AND CONTROLS OF DISSOLVED ORGANIC-MATTER IN A CONSTRICTED-CHANNEL REGION OF THE OHIO RIVER

A 12-month study was conducted to measure the concentrations of dissol ved organic matter (DOG, TDN, TDP) in four sites within a 119 km long reach of the Ohio River, near Louisville, KY. In this study we test wh ether specific geomorphological and biological factors influenced vari ations in dissolved organic matter. Concentrations of DOC in the river averaged approximate to 1200 mu mol/L, and varied by nearly two order s of magnitude seasonally (mean DOC during base flow approximate to 62 0 mu mol/L). Peak periods for DOC at all sites were during April-May. The site nearest a navigation dam (deeper, lower current velocities) h ad significantly lower concentrations of TDN and greater C:N ratios th an upstream sites. The largest tributary entering this reach (Kentucky River) had no significant effect on levels of DOM in the main river, despite having significantly greater concentrations of TDN and lower l evels of DOC during most months of the year. Concentrations of DOC, TD N, and TDP were not significantly different in littoral and pelagic ha bitats at all sites studied, suggesting little floodplain influence on DOM in this constricted-channel section of the Ohio River. C:N ratios of DOM in the Ohio were significantly different among seasons; C:N ex ceeded or equaled Redfield ratios in summer and fall (6 to 10), but we re below Redfield (1.8 to 3.0) during winter and spring. Regression mo dels suggest that total phytoplankton densities and flow conditions ar e the two most important factors regulating DOM in this very large riv er.