Understanding Drivers of Salinity and Temperature Dynamics in Barataria Estuary, Louisiana.

Barataria Estuary is an economically and ecologically important estuary in coastal Louisiana, USA. Due to rapid wetland loss, extreme Mississippi River flood and drought events (e.g., the flood of 2019 and the drought of 2022), devastating storm events (e.g., Hurricane Ida in 2021), eustatic sea‐level rise (SLR), high subsidence rates, and human activities, temporal and spatial variability of salinity and temperature in the estuary is highly complex. This study comprehensively investigates environmental drivers that govern salinity and temperature dynamics, as well as the effects of a proposed large‐scale, land‐building diversion of Mississippi River water. A three‐dimensional (3D), process‐based hydrodynamic, salinity and temperature transport model system is formulated and implemented. Three different modeling domains are set up for nested computations. The model system is validated for the year 2018 against measurements of water level, salinity, and temperature. A series of numerical experiments are then carried out to quantitatively examine impacts of various environmental drivers, as well as nearshore density stratification and local baroclinic forcing. The drivers include wind, rainfall, freshwater point‐source diversion, SLR, and Mississippi River discharge. Interestingly, the analysis shows that the proposed Mid‐Barataria diversion and rainfall can cause a reduction of annual salinity up to 14 and 10 ppt, respectively. Neglecting the effect of nearshore density stratification could underestimate salinity by up to 9 ppt. The well‐mixed estuary can be adequately modeled using depth‐averaged models. However, to adequately capture proper salinity and temperature stratification, it is necessary to use 3D models for the coastal regional domain. Plain Language Summary: Salinity and temperature are essential factors for coastal ecosystems. Barataria Estuary in coastal Louisiana is a semi‐enclosed shallow estuary influenced by natural processes and human activities, which we call drivers. To investigate how various drivers affect salinity and temperature in the estuary, we developed a numerical model. Drivers considered in this study include wind, rainfall, river discharge, sea‐level rise, and diversion projects that introduce freshwater from the Mississippi River. To be sure the model performed well, we compared the model output to observed measurements of water level, salinity, and temperature in Barataria Estuary. Next, we looked at the model results with and without each driver to see how it impacted the results. We found that the most significant drivers are the proposed Mid‐Barataria diversion and rainfall. Both can cause huge salinity reductions in the middle and lower regions of the estuary. This research also showed the importance of using a three‐dimensional model, rather than a horizontal two‐dimensional model in which no changes are allowed in the water column, to capture salinity and temperature behavior in deep areas outside of barrier islands of Barataria Estuary. Key Points: Barataria salinity can be heavily influenced by the Mid‐Barataria diversion and rainfall, reduced up to 14 and 10 ppt, respectivelyNeglecting the effect of nearshore density stratification underestimates salinity in Barataria Estuary by up to 9 pptThe well‐mixed Barataria Estuary can be adequately modeled using a 2D local model, but offshore conditions need to be provided by a 3D model [ABSTRACT FROM AUTHOR]