PNNL

Abstract

Shifts in the frequency and intensity of storms due to climate change may have important consequences for the hydrology and biogeochemistry of rivers. However, our understanding of event-related biogeochemical dynamics in large rivers lags that of small streams. To fill this gap, we used high-frequency sensor data collected during four consecutive summers from a main channel and backwater site of the Upper Mississippi River. We identified high discharge events and calculated event concentration-discharge responses for both physical-chemical (nitrate, turbidity and fluorescent dissolved organic matter) and biological (chlorophyll-a and cyanobacteria) parameters using metrics of hysteresis and slope. We found a range of responses across events, particularly for nitrate, which lacked general patterns. Although fluorescent dissolved organic matter (FDOM) and turbidity exhibited more consistent responses across events, contrasting hysteresis metrics indicated that FDOM was flushed to the river from more distant sources than turbidity. Lastly, we found that the event characteristics best explaining concentration responses differed between sites, with event magnitude more frequently related to responses in the main channel, and antecedent wetness conditions associated with variation in responses in the backwater. Our results indicate that event responses in large rivers are distinct across the diverse habitats and biogeochemical components of a large floodplain river, which will have implications for local and downstream ecological communities as the climate shifts.