Diurnal variations in dissolved organic matter and ammonium uptake in six open-canopy streams

Daily fluctuations of primary production can result in diurnal variations in labile dissolved organic C (DOC) and N (DON) concentrations, but we know little about how biotic uptake of labile DOC and DON changes from day to night. We compared daytime vs nighttime uptake of ammonium (NH4+), acetate (labile DOC), and glycine (labile DON) using short-term nutrient releases conducted in 6 open-canopy streams in northwestern Wyoming, USA. NH4+ uptake was primarily a function of autotrophic demand because it was often higher during the day, and daytime uptake was driven by gross primary production when placed in context of prior data from the region (r(2) = 0.60, p < 0.001). However, acetate uptake showed no significant diurnal change and the quality of fine benthic organic matter explained diurnal variation in uptake across streams, indicating control via heterotrophic demand (r(2) = 0.94, p = 0.006). Patterns in glycine uptake were distinct from both NH4+ and acetate uptake. Glycine uptake decreased as the ratio of total dissolved N to soluble reactive P increased (r(2) = 0.72, p = 0.002), indicating that DON uptake might be P-limited. Uptake did not differ significantly by solute type, suggesting labile forms of DOC and DON can be cycled as rapidly as NH4+, a solute traditionally considered to be extremely bioreactive in headwater streams. Thus, demand for labile DOC and DON was high despite the fact that bulk DOC and DON concentrations were high relative to inorganic N. This result suggests that ambient availability of labile organic nutrients might not satisfy demand. Last, diurnal variation in NH4+, DOC, and DON demand was as high as annual variation in some forested streams; therefore, future stream nutrient uptake studies should be consider how and when uptake is measured, particularly in streams with open canopies.