Direct and Indirect Effects of Extreme Climate Events on Nitrate Processing by Streams

Abstract:

Previous studies have shown that Extreme Climate Events (ECEs) can have profound effects on regional groundwater, surface water hydrology, and biogeochemistry. Because ECEs can impact surface water-ground water exchange (aka hyporheic exchange) and thus nutrient processing, they have the capacity to regulate nutrient fate and transport across the aquatic continuum (e.g., headwaters to oceans). In this work we focus on two hydrological changes that can be brought about by ECEs, namely change in stream discharge and change in the magnitude and direction (losing vs. gaining) of ambient groundwater flux. We also consider two changes that are indirect consequences of ECEs, namely altered stream morphological features (e.g., fluvial ripples and riffle-pool sequences), and stream biogeochemistry (e.g., creation of favorable/disfavorable conditions for nitrate generation or removal). To this end, we utilize a simple process-based model to evaluate in-stream N-cycling at two scales of hyporheic exchange (fluvial ripples and riffle-pool sequences), ten ambient flow scenarios (five gaining and losing conditions and two stream discharges), and three biogeochemical settings (low-respiration, high-nitrate, and high-ammonium). We demonstrate that increasing groundwater flux (gaining or losing) and stream discharge respectively attenuates and improves N-cycling across all bedform scales and biogeochemical settings. However, the source/sink behavior of streams is scale- and environment- dependent. For instance, hyporheic exchange across ripples dominates nitrate removal in high-ammonium scenario, while riffle-pool sequences dominate nitrate production in low-respiration scenario and, to a lesser extent, high-nitrate scenario. These results suggest that ECEs that impact some combination of stream discharge, groundwater flux, bedform morphology, or biogeochemistry have the capacity to control key fluvial ecosystem services like nitrate removal.