Developing nitrogen budgets using an Integrated Biophysical Model to
investigate current and future phytoplankton dynamics in a rapidly
changing subtropical estuary, Barataria Basin
Abstract
Louisiana is undergoing rapid change from natural and anthropogenic
forces, such as sea level rise, subsidence, and eutrophication. Sediment
diversions on the lower Mississippi River are proposed as a large-scale
restoration strategy to create new wetlands and sustain existing wetland
areas in Barataria Basin, Louisiana. This will introduce a large volume
of sediment and nutrient rich freshwater from the Mississippi River to
the receiving basins. This will result in, at least, short term changes
in light and nutrient dynamics and has potential to alter phytoplankton
composition. In order to understand nitrogen dynamics in Barataria Basin
due to large scale coastal restoration practices, the nitrogen budgets
(including particulate and dissolved forms) were calculated from outputs
of the Integrated Biophysical Model, which is based on the existing
Delft3D model coupled with a water quality model (D-WAQ). Creating
nitrogen budgets in estuarine systems allows for better understanding of
the major sources, sinks, inputs and exports across the system,
increasing understanding of the amount of nitrogen available to drive
estuarine primary production. Quantification of nitrogen inputs, outputs
and processes is essential because it is the limiting nutrient for most
estuarine primary producers (e.g., phytoplankton and emergent
macrophytes). Preliminary model results for the existing conditions
suggest that the dissolved inorganic nitrogen in the estuarine waters is
mainly derived from diffusional sediment fluxes and mineralization of
particulate organic nitrogen. Most of the dissolved inorganic nitrogen
was assimilated for phytoplankton growth. A relatively small portion of
dissolved inorganic nitrogen was removed from the system through
denitrification in the water column. More particulate organic nitrogen
originated from emergent macrophytes than from phytoplankton primary
production. These model results will help better understand how proposed
sediment diversions on the lower Mississippi River may change the future
ecological conditions of estuarine open water in coastal Louisiana.