Extreme Flooding and Nitrogen Dynamics of a Blackwater River

Extreme floods, including those expected to become more frequent in a warming world, may impact nutrient metabolism in streams. However, flood impacts on spatial and temporal variability of nutrient dynamics on large rivers (e.g., fourth order and higher) have been understudied. In 2016, Hurricane Matthew provided a unique opportunity to evaluate nitrate retention and processing on the Lumbee River, a blackwater stream in southeastern North Carolina. The 3,000+ km2 watershed received as much as 400 mm of rain in 48 hr as the storm moved across the Atlantic Coastal Plain. Resulting floods in the watershed were the largest on record, based on more than 80 years of continuous streamflow measurements at the watershed outlet. We used a modified Lagrangian sampling method to collect water samples and supporting water quality data at multiple points along three reaches of the Lumbee River for several months before and after Hurricane Matthew. Samples were analyzed for nitrate-nitrogen and used to estimate retention and areal uptake rates for multiple subsections within each reach. Although nitrate-nitrogen concentrations did not change significantly after the flood, we found that the spatial variability of within-reach retention and areal uptake increased substantially following the flood, evidenced by changes to within-reach interquartile ranges. The spatial variability of areal uptake returned to pre-flood levels approximately eight months after Hurricane Matthew, but retention variability remained elevated at the end of our field study. These results highlight the potential for extreme flooding to impact biogeochemical processes in large rivers long after flood waters subside.

Duke Scholars

Author Ryan Emanuel Environmental Sciences and Policy