Flood disturbances are a critical driver of river structure and function, and the frequency and timing of these extreme events are likely to increase and shift with climate change. In 2019, historic flooding occurred in the US Midwest and Great Plains, after a “bomb cyclone” weather system affected the region. This flood event likely mobilized nutrients from agricultural watersheds and reshaped river structure in ways that could affect downstream water quality, and river ecosystem functions, such as primary production (GPP) and ecosystem respiration (ER). Here, we asked whether we could detect and document flood effects on nutrient export within a Great Plains river network, and whether the flood event affected river ecosystem metabolism. We used open-access flow, nitrate concentrations, and dissolved oxygen datasets from three rivers in eastern Nebraska, USA (Maple Creek, Elkhorn River, Platte River) that represent an increasing hydrologic gradient. We modeled daily nitrate fluxes pre- and post-flood using interpolation, regression, and composite models with the R package ‘loadflex’, and then calculated the proportion of annual nitrate export, and the proportion of annual discharge represented by the peak flood event. We modeled ecosystem metabolism (GPP, ER) with the R package ‘streamMetabolizer’ and compared median monthly estimates pre- and post-flood.
Results/Conclusions
Our findings indicate flood-driven changes to nitrate fluxes and ecosystem metabolism in Great Plains rivers. Highest nitrate fluxes occurred during highest flows, where maximum nitrate fluxes (kg/d) coincided with the 2019 flood (Maple = 1.66*105 kg/d, Elkhorn = 2.19 106 kg/d; Platte = 7.75*105 kg/d). The fraction of annual nitrate exported during the event differed with stream size. In Maple Creek, the flood exported ~11.8% of the annual nitrate load (11.5% of annual discharge), whereas the flood exported a smaller fraction of the annual nitrate load (Elkhorn = 4.2%; Platte = 2.4%), also matching the fraction of annual discharge represented by the flood (~3% for both rivers). Patterns of GPP and ER in the Elkhorn River were also affected by flooding. Median monthly GPP was 2.8-16.1× lower in 2019 than median monthly GPP from 2008-2016, whereas ER was 1.2-6.1× higher. The largest increases and decreases in GPP and ER were observed directly after the flood (March). Our analyses suggest significant nutrient export during flood events, especially from smaller streams. Lower GPP and higher ER implies that transport and processing of organic carbon could be modified by more frequent floods in the Great Plains, with implications for riverine carbon budgets.