Inland river networks regulate the export of nutrients from the terrestrial landscape, making them critical for mitigating eutrophication of downstream ecosystems. Yet the ability of rivers to process and retain nutrients has been understudied as previous research has focused mainly on small headwater streams. It is critical to understand how entire river ecosystems, not just sections of streams, influence regional and continental patterns of nutrient export to protect water resources. This research will use a novel field approach to gather empirical measurements of nutrient uptake in multiple rivers across the west and midwest and integrate the data into a dynamic network scale model to evaluate controls on nutrient uptake, thereby integrating aquatic ecosystem ecology and hydrological modeling. This approach will generate critical predictive relationships regarding the capacity of rivers spanning a range of nutrient and sediment conditions to mitigate downstream nutrient export, which is an essential step towards effective water quality management at the river network scale. The intellectual merit of the research includes the transformation of ecological theory regarding nutrient cycling in rivers and improved understanding of the ecosystem services that rivers provide. In turn, the broader impacts of the work will result in unparalleled educational opportunities for graduate students to collaborate on cutting edge river research, watershed modeling tools immediately useful to water resource managers, and data to improve emerging technologies for real-time nutrient monitoring to be used by national observatory programs.
The capacity for large streams and rivers to retain nutrients is understudied relative to headwater streams. Nutrient uptake in rivers has often been inferred/modeled from empirical studies of smaller systems where benthic processes drive nutrient uptake. Using the pulse addition technique, we quantified nutrient spiraling metrics in 15 rivers throughout the US across gradients of background nutrient concentrations and turbidity. As part of this collaborative project, we measured a suite of ancillary variables to explore potential drivers of nutrient uptake. At the Cary Institute, we measured the composition and concentrations of suspended particulate matter across the 15 rivers. In addition, we examined how these suspended particles in rivers may be fueling invertebrate food webs. In general, our findings demonstrate that large rivers have combined benthic and pelagic nutrient removal processes that result in their capacity for nutrient removal. Comparing nutrient spiraling metrics from the 15 rivers highlighted that turbidity strongly influenced phosphate dynamics, and potentially confounded the signature of biological demand. Nevertheless, in 15 rivers with variable nutrient and sediment loads, nutrient uptake was measurable 87% of the time for ammonium, 67% for nitrate, and 60% for phosphate. Although controls on nutrient removal vary, rivers do retain nutrients and demand is not saturated, even with high ambient concentrations. In addition, suspended particles contribute to both nutrient removal in large rivers and support aquatic benthic macroinvertebrate consumers. Our data also demonstrate that although macroinvertebrate diets track the availability of food resources, their feeding is selective and they preferentially consume higher quality food resources like animal parts and diatoms. Our collaborative research highlights the importance of large river processes in whole-watershed nutrient dynamics and reveals that the activity in the pelagic zone of rivers is likely to drive nutrient cycling and food web dynamics.
