Louisiana Bight Research Articles

Understanding the Fates and Chemical Compositions of Weathered Oil in Coastal Marshes since the 2010 Deepwater Horizon Oil Spill

Abstract Coastal marshes were heavily impacted by the Deepwater Horizon (DWH) oil spill in 2010, with approximately 90% of shoreline impacts occurring in Louisiana's coastal wetlands. Spilled crude oils impact an environment through four major mechanisms: ecosystem exposure to reactive and toxic aromatic compounds; covering and smothering that hinders normal plant and animal physiology; depletion of dissolved oxygen; and disruption of the aquatic food web. Crude oil's ability to cause environmental harm depends upon its composition, which is a very complex mixture of many thousands of reduced carbon compounds made from the degradation of plant material deposited deep underground. This study reviews the results from the chemical characterization of petroleum hydrocarbons, at various weathering stages, in >2000 marsh surface sediments and select sediment cores samples collected from various sampling locations in Terrebonne Bay, Grand isle, and northern Barataria Bay from 2010 to 2018. The sediment samples were analyzed for target saturated alkanes, polycyclic aromatic compounds, and the forensic biomarker (hopane and sterane) compounds. The chemical characterization of the compositional changes of target compounds in DWH oil, from its pre-stranding stage just offshore in the Louisiana Bight, through stranding on marshy shorelines and through its degradation and weathering over eight years has given insights into the complexity of oil residues and potential for impacts in these varying environmental conditions. Stranded oil initially had two prominent fates: settling on surface sediment/soils of the marshes, and subsurface deposition primarily by means of settling into fiddler crab burrows. Both initial fates affected shorelines and 10–20 meters inward. Over time, surface oil residues were spread beyond initially impacted areas by Tropical Storm Isaac in 2012 and other weather events, and oil residues were quickly degraded. Subsurface stranded oil was degraded much more slowly under anaerobic conditions and some was re-released as fairly fresh oil during the coastal erosions caused by DWH surface oiling damage to the marsh plants. However, these re-releases were relatively slow and were quickly aerobically degraded once the stranded oil reached marsh surfaces. There was also evidence of anaerobic degradation of heavily weathered surface oil residues during the 2015 to 2018 timeframe. This eight-year study establishes a very complex narrative between the physical and chemical properties of stranded oil and its interactions with coastal marsh environments.

Observations of Shoaling Density Current Regime Changes in Internal Wave Interactions

AbstractWe present in situ and remote observations of a Mississippi plume front in the Louisiana Bight. The plume propagated freely across the bight, rather than as a coastal current. The observed cross-front circulation pattern is typical of density currents, as are the small width (≈100 m) of the plume front and the presence of surface frontal convergence. A comparison of observations with stratified density current theory is conducted. Additionally, subcritical to supercritical transitions of frontal propagation speed relative to internal gravity wave (IGW) speed are demonstrated to occur. That is in part due to IGW speed reduction with decrease in seabed depth during the frontal propagation toward the shore. Theoretical steady-state density current propagation speed is in good agreement with the observations in the critical and supercritical regimes but not in the inherently unsteady subcritical regime. The latter may be due to interaction of IGW with the front, an effect previously demonstrated only in laboratory and numerical experiments. In the critical regime, finite-amplitude IGWs form and remain locked to the front. A critical to supercritical transition eventually occurs as the ambient conditions change during frontal propagation, after which IGWs are not supported at the front. The subcritical (critical) to critical (supercritical) transition is related to Froude number ahead (under) the front, consistently with theory. Finally, we find that the front-locked IGW (critical) regime is itself dependent on significant nonlinear speed enhancement of the IGW by their growth to finite amplitude at the front.

Marine X‐Band Radar Currents and Bathymetry: An Argument for a Wave Number‐Dependent Retrieval Method

AbstractThis study evaluates shipboard marine X‐band radar (MR) near‐surface current and bathymetry measurements under shallow water conditions. The retrieval algorithm is based on the surface wave signal within three‐dimensional wave number frequency MR backscatter intensity variance spectra. The MR data were collected during a research cruise that investigated submesoscale processes and their impact on oil spill transport in the Louisiana Bight. The MR currents and bathymetry are validated using measurements from 500 GPS‐equipped surface drifters, a shipboard acoustic Doppler current profiler, and a shipboard single‐beam echo sounder. Earlier results from the same experiment but using a different set of sensors indicate strong upper ocean vertical current shear over a 3.5 hr period, despite only mild wind forcing. Here, the MR currents are derived as a function of wave number, providing a measure of vertical shear for the full duration of the cruise. Strong vertical shear is frequently observed, with a maximum difference of 0.42 m/s between the MR high (effective depth of 0.9 m) and low ( 2.4 m) wave number bins. Treating the drifter and echo sounder measurements as truth, the accuracies of the MR near‐surface currents and bathymetry are 0.04–0.07 m/s and 1.2 m (or 7% of the mean water depth). However, it is shown that 50% of the MR high wave number and drifter current differences is due to vertical shear. The shallow water MR near‐surface current accuracy thus matches findings from a previous deep water validation where vertical shear was much weaker.

Wave dynamics near Barataria Bay tidal inlets during spring–summer time

The wave characteristics and wave–current interaction in the six tidal inlets connecting the Barataria Basin and the northern Gulf of Mexico are investigated during spring–summer time period when both cold front passage and persistent southeast wind events exist. A fully-coupled unstructured-grid, three-dimensional, FVCOM-SWAVE model is employed and validated against observational data during April–June 2010. Model simulation results indicate that waves in the shelf side of the inlets are mainly wind seas and swells from the Gulf of Mexico that are modified by ocean currents in the Louisiana Bight. These currents include Mississippi River plume, buoyancy- and wind-driven alongshore current, as well as the tidal jet in the Barataria Pass. The bay side of the inlets, similar to the interior of the Barataria Basin, is dominated by locally generated waves. In the middle of the inlets, both wind seas and swells are presented at times, though different inlets behave differently in terms of contribution from each component. The contribution of local wind seas enhances during cold front passage. Contrastingly, the influence of swells from continental shelf is more pronounced during persistent southeast wind event. FVCOM-SWAVE predicts a decrease in significant wave height inside the Louisiana Bight due to current-induced refraction and wave stretching effect caused by clockwise gyre from Mississippi plume. The modulation of significant wave height at Barataria Pass due to partial wave blocking for the opposing current, and wave stretching for the following current can be up to 20%. The effect of depth-induced breaking and bottom friction overshadows the one from wave–current interaction at other shallower inlets.

DOES THE PEB INDEX RESPOND ONLY TO HYPOXIA IN THE MISSISSIPPI DELTA, GULF OF MEXICO?

The frequency of anthropogenic seasonal hypoxia on the continental shelf west of the Mississippi Delta (the Louisiana Bight) has increased since the middle of the 20 th century. This study applies the PEB index, a proxy for hypoxia, to four ~2 m kasten cores taken southwest of Southwest Pass of the Mississippi Delta. The PEB index is defined as the cumulative percent of Protononion atlanticum (= Nonionella atlantica of this study), Nonionella opima, Epistominella vitrea and Buliminella morgani . The PEB index varies little in the shallowest core, KC4 (59 m water depth). Assemblages were strongly dominated by PEB taxa (average 95%), due mainly to the dominance of E. vitrea (average 76%). In core KC3 (75 m), an average PEB value of 84% is also due mainly to E. vitrea (61%). The PEB index of KC3 was consistently higher above 90 cm, a level dated at 1955–1960 ( 210 Pb age estimate), and could reflect an increased influence of hypoxia or an increased rate of sediment accumulation associated with delta progradation. A similar trend characterizes core KC2 (87 m). The PEB index is 18% from 240 cm to 140 cm, increasing up core as the abundance of E. vitrea increases, likely due to delta progradation. Other PEB taxa, B. morgani and N. opima , increase in the top 30 cm of the core, probably due to increased hypoxia between 1952 and 1968 ( 210 Pb age estimate). Core KC1 (473 m) differed from shallower cores in its higher species diversity. Bolivina lowmani, Cassidulina neocarinata , and Bolivina ordinaria each comprise ~15% of assemblages and PEB taxa are rare (average 5%) but increase to 19% in a 30 cm interval attributed to off-shelf transport. We conclude that the PEB index is a useful indicator of anthropogenic hypoxia but dominance of E. vitrea may also be attributable to sedimentological factors related to delta progradation.

Current–wave interaction in the Mississippi–Atchafalaya river plume on the Texas–Louisiana shelf

Wave–current interaction over the Texas–Louisiana shelf, and its effects on the dispersal and mixing of the Mississippi–Atchafalaya river plume, have been investigated using the Coupled Ocean–Atmosphere–Wave–Sediment Transport (COAWST) Modeling System. The modeling system is driven by realistic wave and current conditions at the open boundaries and high frequency1-D wind measured from a nearby meteorological station. Skill analysis demonstrates that the model reproduces the wave and salinity fields reasonably well. Waves over the Texas–Louisiana shelf are dominated by locally forced wind seas, and generally propagate in the same direction as the winds. Investigation into the spatial differences in the effect of waves reveals two distinct dynamical regions: the Chenier shelf, the shelf region extending roughly offshore from Sabine Lake to Vermilion Bay, and the Louisiana Bight, the region between the Mississippi Delta and Terrebonne Bay. A variety of model runs are performed, where specific wave processes are either included or excluded, in order to isolate the processes acting in different regions. The Chenier shelf is mainly affected by wave enhanced bottom stress, whereas the Louisiana Bight is mostly affected by the surface wave induced mixing and 3-D wave forces. The wave enhanced bottom stress suppresses cross-shore exchange, and acts to trap more freshwater in the nearshore regions shallower than 50m over the Chenier shelf. Wave enhanced bottom stress plays only a minor role in the Louisiana Bight, where the surface-trapped Mississippi plume rarely feels the bottom. The surface intensified wave mixing and 3-D wave forces reduce the surface salinity and weaken the stratification in the region associated with the thin recirculating Mississippi plume in the Louisiana Bight. Model results indicate that the surface wave mixing, the 3-D wave forces, and the wave bottom stress exhibit little interaction over the Texas–Louisiana shelf. Finally, we have demonstrated that the one-way coupling is capable of resolving the majority of wave effects over the entire shelf if the seasonal scale is of interest.

Sediment-preserved diatom assemblages can distinguish a petroleum activity signal separately from the nutrient signal of the Mississippi River in coastal Louisiana

We analyzed the preserved diatom assemblages in dated sediment cores collected from five locations in the Louisiana Bight to test if there was a signature of petroleum extraction activities (hopanes and barium) distinct from the well-documented effects of nutrient loading. The results of a multi-dimensional scaling analysis indicate that the diatom assemblage changes documented throughout the 40year record could be explained by three variables: barium and hopanes concentrations, and Mississippi River nitrogen loading. The results of a canonical correspondence analysis demonstrated that these signals could be distinguished through correlations with specific diatom species. The abundance of Actinoptychus senarius, for example, was negatively correlated with barium and the Pseudo-nitzschia delicatissima complex was positively correlated with nitrogen loading. These results provide a “proof-of-concept” demonstration that diatom assemblages preserved in the sediments can be used to study the effects of petroleum extraction activities, and that these ‘petroleum signals’ may be distinguished from other significant influences such as nutrient loading.

Overview of Research into the Coastal Effects of the Macondo Blowout from the Coastal Waters Consortium: A GoMRI Consortium

ABSTRACT The Coastal Waters Consortium (CWC) led by Louisiana Universities Marine Consortium is one of eight Gulf of Mexico Research Initiative research consortia. The CWC focuses on: oil transport and fate, chemical evolution and biological degradation, and environmental effects.The following is an overview of a portion of the research conducted within the consortium. The consortium works in a system that was impacted by the Deepwater Horizon oil disaster and additionally impacted by freshwater diversions resulting in changes in salinity, tropical storms, and hurricanes. First, we conducted model simulations assessing oil transport into the Barataria Bay estuary, which indicate that easterly winds and feeding of the anticyclonic gyre in the Louisiana Bight pushed the oil into Barataria Bay. In subtidal sediments adjacent to oiled marshes, marsh detritus from eroding marsh edges eventually became entrained in the sediment column. Biotic impacts vary. The above-ground plant biomass appears healthy at the individual sampling sites; overall the most seaward (i.e., likely oil-impacted) areas of Terrebonne and Barataria Bay have shown, via satellite data, a distinct decline in marsh vegetation coverage since 2010. Oysters appear to be affected by predation and salinity variation. Microbial diversity from marsh-edge sediments is distinct from before and after the spill, and between unoiled and oiled marshes, with lower diversity in oiled marshes; but the greatest community composition shifts are in marshes affected by the freshwater diversions. Changes in microbial diversity in the water column at the stream-side edge of oiled marshes are extensive and are related to marsh edge erosion. In contrast, oiling of marshes had no impact on ammonia oxidizer or denitrifier abundances and on soil biogeochemical process rates 2+ years post-spill. Analysis of long-term offshore phytoplankton community and hypoxia data indicate some signal of the Macondo oil, but these components of the ecosystem remain mostly influenced by the fresh water and nutrients delivered by the Mississippi River. The consortium continues to work to tease apart oil impacts, effects of salinity, natural variation, and disturbance from tropical storms and hurricanes to determine the trajectory for health of shelf waters and Louisiana's marshes.

Effect of Changes in Dissolved Oxygen Concentrations on the Spatial Dynamics of the Gulf Menhaden Fishery in the Northern Gulf of Mexico

AbstractDeclines in dissolved oxygen (DO) concentrations in aquatic environments can lead to conditions of hypoxia (DO ≤ 2 mg/L), which can directly and indirectly affect aquatic organisms. Direct effects include changes in growth and mortality; indirect effects include changes in distribution, movement, and interactions with other species. For mobile species, such as the pelagic filter‐feeding Gulf Menhaden Brevoortia patronus, indirect effects are more prevalent than direct effects. The northern Gulf of Mexico experiences one of the largest areas of seasonal hypoxia in the world; this area overlaps spatially and temporally with the Gulf Menhaden commercial purse‐seine fishery, which is among the largest fisheries by weight in the United States. Harvest records from the Gulf Menhaden fishery in 2006–2009 and fine‐scale spatial and temporal predictions from a physical–biogeochemical model were used with spatially varying regression models to examine the effects of bottom DO concentration, spatial location, depth, week, and year on four response variables: probability of fishing, total Gulf Menhaden catch, total fishery effort, and CPUE. We found nearshore shifts in the probability of fishing as DO concentration declined, and we detected a general westward shift in all response variables. We also found increases in CPUE as DO concentration declined in the Louisiana Bight, an area that experiences chronic, severe hypoxia. The overall effects of environmental conditions on fishing response variables appeared to be moderate. Nevertheless, movement of either Gulf Menhaden or the purse‐seine fishery in response to environmental conditions could potentially affect the susceptibility of Gulf Menhaden to harvest and could therefore influence assessment of the stock and associated stock status indicators.Received March 4, 2014; accepted June 26, 2014

A modeling study of the physical processes affecting the development of seasonal hypoxia over the inner Louisiana-Texas shelf: Circulation and stratification

The physical processes affecting the development of seasonal hypoxia over the Louisiana-Texas shelf were examined using a high-resolution, three-dimensional, unstructured-grid, Finite Volume Coastal Ocean Model (FVCOM). The model was forced with the observed freshwater fluxes from the Mississippi and Atchafalaya Rivers, surface winds, heat fluxes, tides and offshore conditions. The simulations were carried out over a six-month period, from April to September 2002, and the model performance was evaluated against several independent series of observations that included tidal gauge data, Acoustic Doppler Current Profiler (ADCP) data, shipboard measurements of temperature and salinity, vertical salinity and sigma-t profiles, and satellite imagery. The model accurately described the offshore circulation mode generated over the Louisiana-Texas shelf by the westerly winds during summer months, as well as the prevalent westward flow along the coast caused by the easterly winds during the rest of the study period. The seasonal cycle of stratification also was well represented by the model. During 2002, the stratification was initiated in early spring and subsequently enhanced by the intensity and phasing of riverine freshwater discharges. Strong stratification persisted throughout the summer and was finally broken down in September by tropical storms. The model simulations also revealed a quasi-permanent anticyclonic gyre in the Louisiana Bight region formed by the rotational transformation of the Mississippi River plume, whose existence during 2002 was supported by the satellite imagery and ADCP current measurements. Model simulations support the conclusion that local wind forcing and buoyancy flux resulting from riverine freshwater discharges were the dominant mechanisms affecting the circulation and stratification over the inner Louisiana-Texas shelf.

How does the character of oxygen demand control the structure of hypoxia on the Texas–Louisiana continental shelf?

A realistic hydrodynamic model of the Texas–Louisiana shelf is configured with various simple oxygen respiration models to isolate the effects of stratification and circulation on the formation and maintenance of hypoxia. Biological activity is parameterized through various forms of respiration rather than using a complex biogeochemical model. The model domain covers the region that has historically been observed to be affected by seasonal hypoxia, and is forced with observed fresh water fluxes from the Mississippi and Atchafalaya Rivers and winds. Three simple parameterizations of biological respiration are compared. Results of the numerical simulations indicate that water column respiration (dependent on the position of the two regional fresh water plumes) reproduces observed temporal and spatial structures of seasonal hypoxia in Louisiana Bight, whereas benthic respiration (dependent on local temperature and oxygen concentrations) reproduces the temporal and spatial structure of hypoxia west of Terrebonne Bay. The differences in the structure of hypoxia are related to the differences in vertical stratification east and west of Terrebonne Bay, which are controlled by the outflow characteristics from the two major river plumes. The model shows two dynamically distinct plumes. The Mississippi River plume enters the shelf near the shelf edge, forms a recirculating gyre in Louisiana Bight, and typically does not interact directly with topography. Conversely, the Atchafalaya River plume enters a broad shelf at the coast, is more diffuse, and interacts more with the shallow coastal topography. Both plumes are strongly affected by winds, and tend offshore during the mean summer conditions of upwelling winds. The principle conclusion of this study is that the biological processes responsible for producing hypoxia change from east to west, with the shelf region south of Terrebone Bay being the approximate dividing line between water column respiration (predominantly causing hypoxia to the east) and benthic respiration (causing hypoxia to the west). Also, the formation and destruction of hypoxia is a primarily vertical process; hypoxia formed in one region is not advected laterally to different shelf regions.

Chromophoric dissolved organic matter (CDOM) source characterization in the Louisiana Bight

Chromophoric dissolved organic matter (CDOM) in the Mississippi plume region may have several distinct sources: riverine (terrestrial soils), wetland (terrestrial plants), biological production (phytoplankton, zooplankton, microbial), and sediments. Complex mixing, photodegradation, and biological processes make differentiation of the specific sources of CDOM difficult. Using a combination of high resolution in situ observations on an undulating vehicle, the ECOShuttle, a pumping system mounted on the vehicle, and detailed chemical and biological analyses of discrete samples allowed us to characterize two specific sources of CDOM in the Louisiana Bight: the river water constrained in the upper 12 m of the Mississippi River plume and several subsurface layers of CDOM below the plume. The subsurface CDOM maxima were coincident with steep pycnoclines and sometimes with maxima in chlorophyll a fluorescence. Both sources were actively supplying CDOM to the same location by entirely different processes. The subsurface CDOM was more biologically labile and photochemically refractory than the surface CDOM. Optical properties were also different with a relatively higher protein fluorescence and a lower spectral slope coefficient in the subsurface CDOM. The geographical extent of the two sources was determined by three-dimensional mapping of the area, and due to the relatively calm conditions in the summer of 2000, thin layers of CDOM produced in the subsurface were observed throughout the region. While riverine inputs dominated the distribution of CDOM in surface waters <12 m in depth, biological production of CDOM, probably due to the bacterial degradation of phytoplankton produced DOM dominated the subsurface waters.

A Lagrangian view of fluorescent chromophoric dissolved organic matter distributions in the Mississippi River plume

In April 2001, three instrumented surface drifters were deployed in the Mississippi River plume near the mouth of Southwest Pass. The plume was characterized by strong surface gradients of salinity, temperature, and chromophoric dissolved organic matter fluorescence ( F CDOM). The drifters initially headed west and attained peak speeds of 1 m s −1 within 5 h after release. Thereafter, velocity decreased as the triad headed north in the Louisiana Bight. Linear relationships between F CDOM and salinity identified two subsurface sources of high salinity water (salinity >35) underlying the surface plume. The platforms stalled in a surface front about 40 h after deployment, and then slowly drifted south along the eastern perimeter of the plume. Shoreward of the plume front there were patches of low-salinity, high F CDOM ‘patches’ of surface waters that likely originated in the marshes, bayous, and other smaller distributaries of the delta.

Nutrient Enrichments and Phytoplankton Growth in the Surface Waters of the Louisiana Bight

Nitrate concentrations have increased twofold in the Mississippi River during the past three decades. The increased nitrogen loading to the Louisiana shelf has been postulated as a factor leading to eutrophication and the subsequent development of hypoxia west of the Mississippi River delta. While ratios of nitrogen:phosphorus and nitrogen:silica are relatively high in surface waters on the western Louisiana shelf, nitrogen has been posed as the ‘limiting’ nutrient in this region. Bioassays were performed with nutrient additions to surface waters collected from the Louisiana shelf to examine the potential for specific nutrient limitation. Experiments were conducted in March and September 1991, and May 1992. The growth responses of natural and cultured phytoplankton populations were determined by measuring the time course of in vivo and 3-(3,4 dichlorophenyl)-1, 1-dimethylurea (DCMU)-induced fluorescence, as well as initial and final chlorophylla concentrations. The results suggest that phosphate and silicate potentially limit phytoplankton growth during the winter-spring, particularly at low salinities. In late summer, in contrast, nitrogen limitation may be prominent at higher salinities.

Circulation observations in the Louisiana bight using LANDSAT imagery

A method for quantifying the turbidity of offshore water masses using LANDSAT imagery is discussed and the results of a laboratory experiment correlating radiance with concentrations of suspended Mississippi River sediment are presented. The results of the experiment are used to plot suspended sediment contours on eight LANDSAT images of the Louisiana Bight. These contours are observed to depend on the speed and direction of the wind as well as the amount of fresh water discharged by the Mississippi River. The presence of a clockwise circulation in the bight is also indicated by the contours.