Surface Water Hydrology and Nitrate Dynamics in Delta Islands of Prograding Wax Lake Delta, Louisiana

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The Wax Lake Delta is an ideal ecosystem to study the effects of a large-scale river diversion on the biogeochemistry of coastal wetlands, and the capacity of these wetlands to assimilate nutrients delivered by these diversions. USGS works to develop a better understanding of surface water hydrology and nitrate dynamics in this area. 

Wax Lake Delta, Louisiana
Wax Lake Delta, Louisiana

Science Issue and Relevance: The Wax Lake Delta (WLD) is an area of rapidly prograding deltaic wetlands that is building as a result of water and sediment diverted from the Mississippi River (MR) via the Atchafalaya River and WLD outlet. The quantity of freshwater and sediment delivered to the WLD is comparable to many large diversions that are currently planned under Louisiana’s Comprehensive Master Plan for a Sustainable Coast. As such, the WLD is an ideal ecosystem to study the effects of a large-scale river diversion on the biogeochemistry of coastal wetlands, and the capacity of these wetlands to assimilate nutrients delivered by river diversions. There are strong feedbacks among vegetation, morphology, and hydrodynamics because nitrate uptake on the marsh surface is a function of water residence time, water depth, soil organic content, and temperature, all of which are regulated by morphology and vegetation growth. The goal is to develop a better understanding of nitrate uptake and nitrate-vegetation-morphologic feedback because nitrate is the key factor in Gulf of Mexico hypoxia and is also crucial to the delta-top ecosystem. Adding nitrate to the models is crucial if they are to be effective tools for practical prediction and management of river diversions as they relate to eutrophication issues.

Methodology for Addressing the Issue: Mike Island (29.4948°N, 91.4471°W) in the WLD has been instrumented with six hydro-nutrient platforms (HNPs). Four HNPs are arranged around the perimeter/edge of the island-top marsh and two are situated within the deeper island interior. The HNPs provide continuous water velocity, water depth, and concentrations of suspended sediment and nutrients. These data are supplemented by monthly manual sampling of these parameters at 30 stations on the island to provide increased spatial coverage. Time-series models will be used to examine the effect of important drivers (river discharge, wind, tides, temperature) on constituent fluxes and mass balance on the island.

Future Steps: Continued time-series data collection at the 6 HNPs through 2016. After data collection is complete, we will develop a mechanistic model for nitrate uptake and nitrate-vegetation-morphology feedbacks.

Mike Island in the Wax Lake Delta
Mike Island in the Wax Lake Delta

Location of the Study: Throughout the Louisiana coastal zone, Latitude: 29.678°, Longitude: -91.552°