Effects of solar radiation on the utilization of dissolved organic matter (DOM) from two headwater streams

Abstract

The effect of solar radiation on dissolved organic matter (DOM) utilization was studied in 2 contrasting streams in Arkansas, USA, from June 2002 through July 2003. Moores Creek is an agricultural stream with elevated nutrient and dissolved organic carbon (DOC) concentrations. Huey Hollow is a forest stream with low nutrient and DOC concentrations. A series of in situ experiments were conducted seasonally in both streams to assess how exposure of DOM to solar radiation impacted its utilization, measured as bacterial production following exposure and subsequent inocu- lation with whole stream water. Exposure of DOM to solar radiation significantly decreased its uti- lization during most seasons in both streams. Both streams experienced one period when exposure of DOM significantly increased bacterial production; during these periods, DOM appeared to be the least bioavailable and most photochemically reactive. In spring, ultraviolet absorption coefficients, normalized to DOC concentration, suggested a greater proportion of photoreactive DOM in Huey Hollow. Additionally the δ 13 C signature of DOM (δ 13 CDOM) suggested the largest terrestrial input occurred in spring when exposure of DOM significantly increased bacterial production. Both the proportion of photoreactive DOM and δ 13 CDOM exhibited little seasonal variation in Moores Creek; however, the relative bioavailability of DOM did vary, suggesting some change in DOM composition. In spring the relative bioavailability of DOM was lowest in Moores Creek, and DOM exposure appeared to have further reduced its bioavailability. Elevated ammonium concentrations at this time suggest photochemically enhanced humification may be an important mechanism influencing DOM cycling, and warrants study in streams impacted by agricultural land use.