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+ km2watershed 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.
Human activities have increased nitrate export from rivers, degrading coastal water quality. At deltaic river mouths, the flow of water through wetlands increases nitrate removal, and the spatial organization of removal rates influences coastal water quality. To understand the spatial distribution of nitrate removal in a river‐dominated delta, we deployed 23 benthic chambers across ecogeomorphic zones with varying elevation, vegetation, and sediment properties in Wax Lake Delta (Louisiana, USA) in June 2018. Regression analyses indicate that normalized difference vegetation index is a useful predictor of summertime nitrate removal. Mass transfer velocity were approximately three times greater on a vegetated submerged levee (13 mm hr−1), where normalized difference vegetation index was greatest, compared to other locations (4.6 mm hr−1). Two methods were developed to upscale nitrate removal across the delta. The flooded‐delta method integrates spatially explicit potential removal rates across submerged portions of the delta and suggests that intermediate elevations on the delta—including submerged levees—are responsible for 70% of potential nitrate removal despite covering only 33% of the flooded area. The channel network method treats the delta as a network of river channels and suggests that although secondary channels are more efficient than primary channels at removing received nitrate, primary channels collectively contribute more to overall removal because they convey more of the total nitrate load. The two upscaling methods predict similar rates of nitrate removal, equivalent to less than 4% of nitrate entering the delta. To protect coastal waters against high nitrate loads, management policies should aim to reduce upstream nutrient loads.
more » « less- Award ID(s):
- 1812019
- NSF-PAR ID:
- 10449656
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Water Resources Research
- Volume:
- 56
- Issue:
- 8
- ISSN:
- 0043-1397
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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