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A three-dimensional coupled hydrodynamic–biogeochemical model with multiple nutrient and plankton functional groups was developed and adapted to the Gulf of Mexico to investigate the role of nutrients and the complexity of plankton community in dissolved oxygen (DO) dynamics. A 15-year hindcast was achieved covering the period of 2006–2020. Extensive model validation against in situ data demonstrates that the model was capable of reproducing vertical distributions of DO and spatial distributions of bottom DO concentration, as well as their interannual variations. The study demonstrates that bottom DO dynamics and hypoxia evolution are significantly influenced by both physical processes and local biochemistry, with sedimentary oxygen consumption and vertical diffusion identified as key contributors. Summer hydrodynamics play a critical role in nutrient distribution and limitation: a notable expansion of Si limitation was simulated when coastal currents shifted eastward or northward. This effect, especially pronounced on the western part of the Louisiana–Texas shelf, underscores the importance of nutrient limitation in shaping DO dynamics. The model identifies a bi-peak primary production pattern in spring and early summer, aligned with satellite chlorophyll a variations, attributed to the complexity of the plankton community and interactions among different plankton groups. Our findings emphasize the necessity of integrating sophisticated plankton community dynamics into biogeochemical models to understand primary production variability and its impact on bottom hypoxia. [ABSTRACT FROM AUTHOR]

We introduced a sediment-induced light attenuation algorithm into a biogeochemical model of the Coupled Ocean–Atmosphere–Wave–Sediment Transport (COAWST) modeling system. A fully coupled ocean–atmospheric–sediment–biogeochemical simulation was carried out to assess the impact of sediment-induced light attenuation on primary production in the northern Gulf of Mexico during the passage of Hurricane Gustav in 2008. When compared with model results without sediment-induced light attenuation, our new model showed a better agreement with satellite data on both the magnitude of nearshore chlorophyll concentration and the spatial distribution of offshore bloom. When Hurricane Gustav approached, resuspended sediment shifted the inner shelf ecosystem from a nutrient-limited one to a light-limited one. Only 1 week after Hurricane Gustav's landfall, accumulated nutrients and a favorable optical environment induced a posthurricane algal bloom in the top 20 m of the water column, while the productivity in the lower water column was still light-limited due to slow-settling sediment. Corresponding with the elevated offshore NO3 flux (38.71 mmol N m -1 s -1) and decreased chlorophyll flux (43.10 mg m -1 s -1), the outer shelf posthurricane bloom should have resulted from the cross-shelf nutrient supply instead of the lateral dispersed chlorophyll. Sensitivity tests indicated that sediment light attenuation efficiency affected primary production when sediment concentration was moderately high. Model uncertainties due to colored dissolved organic matter and parameterization of sediment-induced light attenuation are also discussed. [ABSTRACT FROM AUTHOR]

Wave‐supported fluid mud (WSFM) plays an important role in sediment downslope transport on the continental shelves. In this study, we incorporated WSFM processes in the wave boundary layer (WBL) into the Community Sediment Transport Modeling System (CSTMS) on the platform of the Coupled Ocean‐Atmosphere‐Wave‐and‐Sediment Transport modeling system (COAWST). The WSFM module was introduced between the bottommost water layer and top sediment layer, which accounted for the key sediment exchange processes (e.g., resuspension, vertical settling, diffusion, and horizontal advection) at the water‐WBL and WBL‐sediment bed boundaries. To test its robustness, we adapted the updated model (CSTMS + WBL) to the Atchafalaya shelf in the northern Gulf of Mexico and successfully reproduced the sediment dynamics in March 2008, when active WSFM processes were reported. Compared with original CSTMS results, including WSFM module weakened the overall intensity of sediment resuspension, and the CSTMS + WBL model simulated a lutocline between the WBL and overlying water due to the formation of WSFM. Downslope WSFM transport resulted in offshore deposition (>4 cm), which greatly changed the net erosion/deposition pattern on the inner shelf off the Chenier Plain. WSFM flux was comparable with suspended sediment flux (SSF) off the Atchafalaya Bay, and it peaked along the Chenier Plain coast where wave activities were strong and the bathymetric slope was steep. The influence of fluvial sediment supply on sediment dynamics was limited in the Atchafalaya Bay. Sensitivity tests of free settling, flocculation, and hindered settling effects suggested that sediments were transported further offshore due to reduced settling velocity in the WBL once fluid mud was formed. Although sediment concentration in the WBL was sensitive to surface sediment critical shear stress, cohesive bed behavior was less important in WSFM dynamics when compared with strong hydrodynamic during cold fronts. Plain Language Summary: Fluid mud is an intermediate stage between a consolidated seafloor and dilute fine sediment suspension. Its high density (>10 g/L) and downslope movement are important in sediment transport over the inner continental shelf (water depths < 15 m). In this study we adopted a numerical model to simulate fluid mud transport on the Atchafalaya continental shelf in coastal Louisiana. We found that fluid mud formed during the passages of cold fronts, and its transport contributed to offshore sediment deposition. This downslope sediment transport process was more affected by the settling velocity of sediment particles rather than fluvial supply from the Atchafalaya River or sediment consolidation on the seabed. Key Points: Model results well reproduce hydrodynamics and WSFM dynamics during cold frontsWSFM transport results in onshore erosion and offshore deposition over the Atchafalaya inner shelfBed consolidation might influence WSFM concentration after several rounds of erosion [ABSTRACT FROM AUTHOR]

We introduce a sediment-induced light attenuation algorithm into the biogeochemical model of the Regional Ocean Modeling System (ROMS). A fully coupled ocean-atmospheric-sediment-biogeochemical simulation is carried out to assess the impact of sediment-induced light attenuation on primary production in the northern Gulf of Mexico during Hurricane Gustav in 2008. The new model shows a better agreement with satellite data on both the magnitude of nearshore chlorophyll concentration and the distribution of offshore bloom. When Gustav approaches, resuspended sediments shift the inner shelf ecosystem from a nutrient-limited one to light-limited. One week after Gustav's landfall, accumulated nutrient and favorable optical environment induces a post-hurricane algal bloom in the top 20 m of water column, while the productivity in the lower column is still light-limited due to unsettled sediment. Corresponding with the elevated offshore NO3 flux (38.71 mmol N/m/s) and decreased chlorophyll flux (43.10 mg/m/s), the post-hurricane bloom in the outer shelf is resulted from the cross-shelf nutrient supply instead of the lateral dispersed chlorophyll. Sensitivity tests indicate sediment light attenuation efficiency affects primary production when sediment concentration is moderately high. [ABSTRACT FROM AUTHOR]

A three-dimensional numerical model was applied to the Barataria Estuary in the Northern Gulf of Mexico to study its salinity variations as well as the impacts from the Mississippi River discharges and proposed river diversions. Model-observation comparison showed that the model was able to reproduce the hydrodynamic fields on subtidal to seasonal time scales. Salinity in the Barataria Estuary was high in fall and low in summer, with a greater variability in the lower estuary than the upper estuary. While salinity in the upper estuary was controlled by discharges from a local freshwater diversion, salinity in the lower estuary was mostly affected by the mixed Mississippi River water transported via the tidal inlets in the south. The correlation between Mississippi River discharge and estuarine salinity indicated that low salinity Mississippi River water could intrude into the estuary through the middle and east tidal inlets. Sensitivity tests were performed to assess the impacts from the Mississippi River discharges and proposed mid-Barataria Estuary sediment diversion. Model results illustrated that salinity in the estuary was more sensitive to an increase of Mississippi River discharge than a decrease. The proposed mid-Barataria sediment diversion was likely to induce a dramatic decrease of salinity in the lower estuary. The ecosystem consequences of the fluctuation of Mississippi River discharge as well as that of the proposed river diversions need further investigations. • Estuary salinity is highly impacted by mixed Mississippi River water from the tidal inlets. • Estuary salinity is more sensitive to a increased Mississippi discharge than an decreased one. • Proposed river diversion is likely to introduce a dramatic decrease of salinity in the lower estuary. [ABSTRACT FROM AUTHOR]

Although many studies have been performed in the northern Gulf of Mexico, there have been a paucity of high-resolution sediment data collected from seabed. New multicores were collected repeatedly at 4 fixed stations in the inner Louisiana shelf on March 12, 2021, July 7, 2021, November 20, 2021, and March 5, 2022, respectively, to study detailed spatial and temporal variations of surficial sediment. Being a very high sediment accumulation zone, Raccoon Island dredge pit and its nearby shelf were selected as our study area and additional six vibracores were collected inside and outside the pit for comparison. This study area is about 140 km west of the modern Mississippi Delta and 70 km southeast of the Atchafalaya Delta. A total of 624 samples from both vibracores and multicores were analyzed for this study. Spatially, our results show that sediment near Ship Shoal was the coarsest and sediment deposited in the pit was the finest. The organic matter was highest inside the pit and lowest near Ship Shoal. The carbonate content was the lowest near Ship Shoal. Temporally, surficial sediments were generally coarser in July and November of 2021 and finer in March of 2021 and March of 2022. Although the Mississippi and Atchafalaya Rivers supply a large amount of sediment during spring season, it takes time for the river-derived sediment to be delivered into the study area. Hurricanes Delta, Zeta and Ida passed this area and greatly impacted sediment transport in the years 2021 and 2022. The comparatively coarser sediment in July and November of 2021 were likely related to the resuspension of some coarse sediment by these hurricane events. Besides the river supply, Ship Shoal and barrier islands are nearby localized sandy sources and inner Louisiana shelf is a widespread mud source. This spatial heterogeneity of surficial sediment interplays with the timing of Mississippi and Atchafalaya River flood season and hurricanes and together dictates the sediment texture. This research on a filled-up dredge pit can benefit future evaluation of the safety of existing oil infrastructure and the feasibility of building new pipelines and platforms after dredging activities. Our results also shed light on the study of river- and hurricane-impacted sediment transport in muddy and mixed-texture passive continental margins around the world. • Pit sediment hasn't reached an equilibrium with ambient environment yet. • Surficial sediments were coarser in July and November and finer in March. • Sediment grain size spatial variations were categorized into four groups. • This research will benefit future coastal restoration and sediment management. [ABSTRACT FROM AUTHOR]