VARIATION IN SOURCES AND QUALITY OF STREAM WATER DISSOLVED ORGANIC MATTER ACROSS A GRADIENT OF WATERSHED AGRICULTURE LAND USES

Drainages with high percentages of agriculture land use typically show differences in stream water dissolved organic matter (DOM) compared to streams with forested watersheds, and these changes may contribute to the widespread environmental and ecological changes observed in these streams. However, few studies have unambiguously linking watershed land use to the source, amount, composition, and reactivity of DOM in lotic systems limiting the use of DOM characteristics in monitoring and regulatory decisions, despite the central role of DOM in stream ecosystem function. In the present study, we collected water samples monthly from six streams (first to third order) from April to October, 2014, under base flow conditions within the Bear Creek Watershed, northwestern Alabama. These streams drain watersheds that varied from 15% to 61% along a gradient of percent agricultural land use. The samples were analyzed for the amount and quality of DOM, including DOC and DON concentrations, DOM optical properties, and the amount and quality of bioreactive DOM over the course of 28 d laboratory incubations. Results indicate that both DOC concentrations and percent biodegradable DOC were positively correlated with watershed percent agricultural land use. However, DOM molecular weight and aromaticity, indicated by slope ratio (S_275-295nm/S_350-400nm) and SUVA₂₅₄, had a negative correlation with watershed percent agricultural land use. Four fluorescence components (C1 – C4) were identified by Excitation Emission Matrix - Parallel Factor Analysis. C1, C2 and C3 were assigned as humic-like components derived from terrestrial plant detritus, and the relative abundances of C1 and C3 to total fluorescence were positively correlated with watershed percent agricultural land use. By comparison, the relative abundance of C4 representing protein-like DOM decreased with increasing watershed percent agricultural land use, in contrast to many previous studies reporting increasing amounts of proteinous DOM in streams and rivers impacted by human activities. Collectively, these data suggest that agricultural lands can alter amount, composition, reactivity, and fate of DOM in aquatic systems.