Nitrate Removal Across Ecogeomorphic Zones in Wax Lake Delta, Louisiana (USA)

Abstract

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.

Stable ≠ Sustainable: Delta Dynamics Versus the Human Need for Stability

“…2). Work in other systems has shown that network structure can also be used to estimate flux partitioning and the delivery of fluxes at the shore (Tejedor et al, 2015) and that estimation of nutrient and sediment fluxes can be addressed in similar ways (Dong et al, 2020;Knights et al, 2020).…”

Influences on Discharge Partitioning on a Large River Delta: Case Study of the Mississippi‐Atchafalaya Diversion, 1926–1950

“…10.1029/2020WR028090 2 of 14 and anthropogenic processes influence water, sediment, and nutrient transport pathways (Knights et al, 2020;Sanks et al, 2020). The USACE analyses (Fisk, 1952;Latimer & Schweitzer, 1951) were based on empirical analyses of extensive datasets.…”

RivGraph: Automatic extraction and analysis of river and delta channel network topology

“…The development of the flow directions algorithms itself provided insights into the nature of river channel network structure in braided rivers and deltas (Schwenk et al, 2020). For deltas specifically, RivGr aph-extracted networks have been used to study how water and sediment are partitioned at bifurcations (Dong et al, 2020), to determine how distance to the channel network plays a controlling role on Arctic delta lake dynamics (Vulis et al, 2020), and to construct a networkbased model of nitrate removal across the Wax Lake Delta (Knights et al, 2020). For braided rivers, RivGraph was used to extract channel networks from hydrodynamic simulations in order to develop the novel "entropic braiding index" (eBI, Tejedor et al, 2019), and a function for computing the eBI (as well as the classic braiding index) for braided rivers is provided in RivGraph.…”