Multiple stressors effects on nitrate uptake vary across benthic and hyporheic compartments

Abstract

Agricultural land use strongly alters nitrate (NO3) dynamics in headwater streams, but the specific mechanisms linking agricultural stressors to benthic and hyporheic NO3 uptake remain unclear. Using stream-side mesocosms and 15N-NO3 additions, we examined the individual and combined effects of fine sediment and eutrophication (i.e., increased phosphorus and light levels) on NO3 uptake in the benthic and hyporheic compartment. Eutrophication increased benthic uptake rates by 12-fold compared to the control, as phosphorus and light additions stimulated biofilm growth. Eutrophication increased hyporheic NO3 uptake by 7-fold relative to the control, this was likely due to enhanced heterotrophic uptake, which benefited from phosphorus and dissolved organic material exudated by benthic algae. The fine sediment treatment did not change benthic uptake relative to the control but increased hyporheic uptake by 14-fold. This was due to anoxic conditions, which may have stimulated hyporheic denitrification. In the combined treatment, eutrophication exerted dominance effects in the benthic compartment, while we found antagonistic stressor interactions in the hyporheic compartment. Our findings indicate that the significant effects of agriculture on NO3 uptake observed previously in field conditions may be primarily attributed to eutrophication and only marginally to other stressors, such as fine sediment. Moreover, our compartment-specific results imply that results obtained in the benthic compartment can not be transferred to the hyporheic compartment. We advocate a compartment-specific approach when quantifying stressor effects on NO3 uptake. Such approaches will help to increase the accuracy of effect size estimates, which are essential for managing functional attributes of streams subjected to agricultural land use.