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 dissolved organic matter (DOC, TDN, TDP) in four sites within a 119 km long reach of the Ohio River, near Louisville, KY. In this study we test whether specific geomorphological and biological factors influenced variations in dissolved organic matter. Concentrations of DOC in the river averaged ≈1200 μmol/L, and varied by nearly two orders of magnitude seasonally (mean DOC during base flow ≈620 μmol/L). Peak periods for DOC at all sites were during April-May. The site nearest a navigation dam (deeper, lower current velocities) had significantly lower concentrations of TDN and greater C:N ratios than 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 levels of DOC during most months of the year. Concentrations of DOC, TDN, and TDP were not significantly different in littoral and pelagic habitats 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 exceeded or equaled Redfield ratios in summer and fall (6 to 10), but were below Redfield (1.8 to 3.0) during winter and spring. Regression models suggest that total phytoplankton densities and flow conditions are the two most important factors regulating DOM in this very large river.