Mississippi River Delta Research Articles

Plant and soil responses to sediment deposition and nutrient‐enrichment in healthy, deteriorating, and newly created coastal marshes of the Mississippi River Delta

Coastal marshes vulnerable to sea‐level rise may benefit from sediment amendments to increase elevation. However, nutrient‐loading to estuaries may counter elevation gain through reduced root production and/or increased decomposition. Here, we test if belowground plant productivity, root decomposition, and marsh accretion response to nutrient‐loading differs with sediment addition in three marsh types: a healthy intermediate salinity marsh, a deteriorating brackish marsh, and a created marsh in Barataria Basin, Louisiana, United States. In this region, wetland loss is rapid, and a major restoration project is underway to introduce Mississippi River water, sediment, and nutrients to offset marsh loss. Porewater nutrient concentrations, vegetation structure, belowground productivity and decomposition, and accretion rates were measured over 450 days. Experimental nitrate additions yielded high porewater ammonium‐N concentrations in the Deteriorating marsh but had a smaller effect in Healthy and Created marshes. Belowground productivity in the Deteriorating marsh was neutral to negatively affected by nutrient and sediment treatments, whereas positive effects occurred in Healthy and Created marshes. Despite elevation increase from sediment treatments, and substantial effects of nutrient treatments on porewater nutrient concentrations, belowground decomposition and surface accretion rates were not affected by treatments. Sediment deposition increased species richness in the Healthy marsh and added sediment and added nutrients with sediment increased plant height in the Created and Deteriorating marshes, respectively. Over the timescale measured, experimental sedimentation and pulsed nutrient input had few consistent effects on belowground ingrowth, decomposition, or surface accretion. Our findings highlight that response to nutrient pulses are complex and depend on baseline marsh conditions.

Temporal Scales of Mass Wasting Sedimentation across the Mississippi River Delta Front Delineated by 210Pb/137Cs Geochronology

Temporal Scales of Mass Wasting Sedimentation across the Mississippi River Delta Front Delineated by 210Pb/137Cs Geochronology

Return of the Phrag-i: evaluating sexual reproduction mechanisms amenable to dieback recovery and potential invasiveness across Phragmites australis haplotypes

Phragmites australis is one of the most invasive wetland plants on the planet with both native and invasive haplotypes occurring in the United States. Three Phragmites haplotypes (Delta-, EU- and Gulf-types) co-occur in marshes of the Mississippi River Delta (MRD), where a recent dieback of Phragmites has prompted investigations about the potential for recolonization by seed. In other areas of the US, the invasive EU-type has been shown to spread by seed, yet little is known about reproduction modes of the Delta- and Gulf-types. We conducted a survey at 35 sites along the Mississippi River Delta region in southeast Louisiana to examine the potential for sexual reproduction across haplotypes as well as the potential for hybridization. Seed and pollen samples were collected from Phragmites populations to examine flowering phenology and determine pollen viability of the three lineages. We also conducted a seedbank assay in stands of three haplotypes to test the potential for recruitment by seed. Despite the observed potential for sexual reproduction in Delta- and EU- types, no Phragmites seedlings germinated from the seedbank. EU was the only haplotype to exhibit germination from seeds collected from seed heads. Both spatial separation and temporal isolation in flowering times indicate that hybridization between Phragmites haplotypes in the lower MRD is unlikely. High pollen production, increased pollen production following dieback, and viable seeds in the EU-type suggest that this invasive haplotype has a greater potential to invade new areas and adapt to stressors through sexual reproduction compared to than Delta-or Gulf haplotypes.

Rising seas could cross thresholds for initiating coastal wetland drowning within decades across much of the United States

Accelerated sea-level rise is an existential threat to coastal wetlands, but the timing and extent of wetland drowning are debated. Recent data syntheses have clarified future relative sea-level rise exposure and sensitivity thresholds for drowning. Here, we integrate these advances to estimate when and where rising sea levels could cross thresholds for initiating wetland drowning across the conterminous United States. Our results show that there is much spatial variation in relative sea-level rise rates, which impacts the potential timing and extent of wetlands crossing thresholds. High rates of relative sea-level rise along wetland-rich parts of the Gulf of Mexico and Atlantic coasts highlight areas where wetlands are already drowning or could begin to drown within decades, including large wetland landscapes within the Mississippi River delta, Greater Everglades, Chesapeake Bay, Texas, Georgia, and the Carolinas. Collectively, our results underscore the need to prepare for transformative coastal change.

Ned Randolph. Muddy thinking in the Mississippi River Delta: a call for reclamation

Ned Randolph. Muddy thinking in the Mississippi River Delta: a call for reclamation

Vegetation dieback in the Mississippi River Delta triggered by acute drought and chronic relative sea-level rise

Vegetation dieback and recovery may be dependent on the interplay between infrequent acute disturbances and underlying chronic stresses. Coastal wetlands are vulnerable to the chronic stress of sea-level rise, which may affect their susceptibility to acute disturbance events. Here, we show that a large-scale vegetation dieback in the Mississippi River Delta was precipitated by salt-water incursion during an extreme drought in the summer of 2012 and was most severe in areas exposed to greater flooding. Using 16 years of data (2007–2022) from a coastwide network of monitoring stations, we show that the impacts of the dieback lasted five years and that recovery was only partial in areas exposed to greater inundation. Dieback marshes experienced an increase in percent time flooded from 43% in 2007 to 75% in 2022 and a decline in vegetation cover and species richness over the same period. Thus, while drought-induced high salinities and soil saturation triggered a significant dieback event, the chronic increase in inundation is causing a longer-term decline in cover, more widespread losses, and reduced capacity to recover from acute stressors. Overall, our findings point to the importance of mitigating the underlying stresses to foster resilience to both acute and persistent causes of vegetation loss.

Spatial distribution and movement of Atlantic tarpon (Megalops atlanticus) in the northern Gulf of Mexico.

Atlantic tarpon (Megalops atlanticus) are capable of long-distance migrations (hundreds of kilometers) but also exhibit resident behaviors in estuarine and coastal habitats. The aim of this study was to characterize the spatial distribution of juvenile tarpon and identify migration pathways of adult tarpon in the northern Gulf of Mexico. Spatial distribution of juvenile tarpon was investigated using gillnet data collected by Texas Parks and Wildlife Department (TPWD) over the past four decades. Generalized additive models (GAMs) indicated that salinity and water temperature played a significant role in tarpon presence, with tarpon occurrences peaking in the fall and increasing over the past four decades in this region. Adult tarpon caught off Texas (n = 40) and Louisiana (n = 4) were tagged with acoustic transmitters to characterize spatial and temporal trends in their movements and migrations. Of the 44 acoustic transmitters deployed, 18 of the individuals were detected (n = 16 west of the Mississippi River Delta and n = 2 east of the Mississippi River Delta). Tarpon tagged west of the Mississippi River Delta off Texas migrated south in the fall and winter into areas of south Texas and potentially into Mexico, while individuals tagged east of the delta migrated into Florida during the same time period, suggesting the presence of two unique migratory contingents or subpopulations in this region. An improved understanding of the habitat requirements and migratory patterns of tarpon inhabiting the Gulf of Mexico is critically needed by resource managers to assess the vulnerability of each contingent to fishing pressure, and this information will guide multi-state and multi-national conservation efforts to rebuild and sustain tarpon populations.

Remembering and Forgetting the Past on the Mississippi River: Toward an Environmental History of Catholicism

Abstract: After the flood of 1927, the US government decided to master the Mississippi River. Nowhere was this state-sponsored determination more evident than in the lower Mississippi River delta. An historical examination of a cluster of Catholic churches in Pointe Coupée Parish, Louisiana, permits some understanding of how physical and material alterations to landscapes are not easily traced, especially when we consider the temporal and spatial scales of change to the Mississippi over the last one hundred years. In that time, Catholics of Pointe Coupée Parish managed to retain, erase, and revise aspects of their bonds to a river once central to the lives of their ancestors. This is not at all surprising, as the fallibility of memory is made apparent every time we try to tell stories about our pasts. Pointe Coupée Parish is one site where we can explore how alterations to a landscape have an impact not only on the trajectory of a Catholic community's future, but also on access to its past. Absences of memory in the living, combined with the erasure of material remains in modified landscapes, challenge us to consider how we might account for the relationship between Catholicism and the built environment throughout American history.

Hydrodynamically‐Driven Deposition of Mud in River Systems

AbstractThe riverine transport and deposition of mud is the primary agent of landscape construction and evolution in many fluvial and coastal environments. Previous efforts exploring this process have raised uncertainty regarding the effects of hydrodynamic and chemical controls on the transport and deposition of mud, and thus the constructions of muddy coastal and upstream environments. As such, direct measurements are necessary to constrain the deposition of mud by river systems. Here, we combine laboratory evidence and a field investigation in the Mississippi River delta to explore the controls on the riverine transport and deposition of mud. We show that the flocculation of mud, with floc diameters greater than 10 μm, in freshwater is a ubiquitous phenomenon, causing the sedimentation of mud to be driven by changes in local hydrodynamics, and thus providing an explanation for how river systems construct landscapes through the deposition of mud in both coastal and upstream environments.

Short-Term Sediment Dispersal on a Large Retreating Coastal River Delta via 234Th and 7Be Sediment Geochronology: The Mississippi River Delta Front

Many Mississippi River Delta studies have shown recent declines in fluvial sediment load from the river and associated land loss. In contrast, recent sedimentary processes on the subaqueous delta are less documented. To help address this knowledge gap, multicores were collected offshore from the three main river outlets at water depths of 25–280 m in June 2017 just after the peak river discharge period, with locations selected based on 2017 U.S. Geological Survey seabed mapping. The coring locations included the undisturbed upper foreset, mudflow lobes, gullies, and the undisturbed prodelta. Nine multicores were analyzed for Beryllium-7 activity, and four cores were analyzed for excess Thorium-234 activity via gamma spectrometry, granulometry and X-radiography. Our results indicate a general trend of declining 7Be and 234Th activities and inventories with increasing distance from sources and in deeper water. The core X-radiographs are graded from the predominantly physically stratified nearshore to the more bioturbated offshore, consistent with the sedimentation patterns. Sediment focusing assessed via the 7Be and 234Th sediment inventories shows preferential sedimentation in gully and lobe environments, whereas the upper foreset and prodelta focusing factors are relatively depleted. Overall, short-term sediment deposition from the main fluvial source remains active offshore from all three major river outlets, despite the overall declining river load.

Sedimentary Processes and Instability on the Mississippi River Delta Front near the Shipwreck of the SS Virginia

Sediment cores were collected from a mudflow lobe (80 m water depth) offshore of the Mississippi River’s Southwest Pass in 2017 to better understand the sedimentology near the lobe entraining the SS Virginia shipwreck (sunk by a German U-boat in 1942) and surrounding Mississippi River delta front. Core analyses included 210Pb/137Cs geochronology, granulometry, and X-radiography. Sediment accumulation rates (SAR) calculated from excess 210Pb activity in multicores are 0.22–0.29 cm/y at seabed depths less than 20 cm and 0.29–0.51 cm/y at depths greater than 20 cm. Accumulation rates for 137Cs have been ~0.15 to ~0.37 cm/y since 1954 and 1963, respectively. Sediment accumulation rates from 210Pb, 137Cs geochronology and indicators of relative sedimentation and bioturbation from X-radiographs suggest that rates of sediment accumulation near the Virginia have declined since the mid-20th century. This may be explained by the multi-decade downslope mass transport of the mudflow lobe in which the shipwreck is embedded and decreases in sediment supply delivered offshore from the Mississippi river. Mass transport calculations of the Virginia lobe derived from core properties and published lobe advection rates suggest downslope mass transport is far higher than sediment resupply from the Mississippi river, consistent with recent studies of delta retreat.

Dieback and dredge soils of Phragmites australis in the Mississippi River Delta negatively impact plant biomass

Phragmites australis is exhibiting extensive dieback in the Lower Mississippi River Delta (MRD). We explored the potential for restoration of these marshes by (1) characterizing the chemical profiles of soils collected from healthy and dieback stands of P. australis and from sites recently created from dredge-disposal soils that were expected to be colonized by P. australis and (2) experimentally testing the effects of these soil types on the growth of three common P. australis lineages, Delta, Gulf and European. Soil chemical properties included Al, Ca, Cu, Fe, K, Mg, Mn, Na, P, S, Zn, % organic matter, % carbon, % nitrogen, and pH. Dieback soils were characterized by higher % organic matter, % carbon, % nitrogen, and higher S and Fe concentrations, whereas healthy soils had higher Cu, Al, P and Zn. In comparison, dredge sites were low in nutrients and organic matter compared to healthy soils. Rhizomes of each P. australis lineage were planted in each soil type in a common garden and greenhouse and allowed to grow for five months. Aboveground biomass was 16% lower in dieback and 44% lower in dredge soils than in healthy soils. However, we could detect no significant differences in response to soil types among lineages. Although dredge and dieback sites are not optimal for P. australis growth, plants can thrive on these soils, and we recommend restorative measures be initiated as soon as possible to minimize soil erosion.

Facies-neoichnological variability and sedimentation rates of modern continental shelves

Current ichnofacies models for shelf environments are largely based on analysis of ancient sedimentary deposits and have rarely been applied to studies of modern siliciclastic shelves. Siliciclastic shelf margins are key components of source rocks and unconventional shale plays that can be used as modern analogs to ancient systems. This paper examines the sedimentology and ichnology of shelf margin cores of the Mississippi River Delta containing strata formed during the 2004 (Ivan) and 2005 (Katrina and Rita) hurricane events in the Gulf of Mexico, together with those from the Kikori-Fly-Purari rivers in the Gulf of Papua, New Guinea and the Tūranganui-a-Kiwa/Poverty Bay shelf deposits of the Waipaoa River basin, offshore New Zealand. The cores provide examples of modern-day traces created by marine organisms on muddy shelves and are used to evaluate ichnological models that have been applied to determine sedimentation rates and depositional processes in ancient settings.In total, six facies were recognised based on lithology and ichnological characteristics: (1) laminated very fine sand, produced by waxing and waning currents, displaying scarce to moderate bioturbation; (2) laminated silty mud, formed by waxing and waning currents, exhibiting scarce, very low to moderate bioturbation; (3) normally graded sand and silt, created by suspension settling following a wave-enhanced sediment-gravity flow (WESGF), which reveals low to moderate bioturbation; (4) fluid mudflow/suspension settling deposits, characterised by structureless mud with low to high bioturbation; (5) biogenically mottled silty mud deposited by suspension settling which show high bioturbation intensity; and, (6) storm-generated facies of laminated sand formed by rapid sediment accumulation on the shelf.The different study areas show remarkably similar traces with variable intensity, diversity, abundance and distribution. The Mississippi River Delta cores are typified by Helminthopsis, Chondrites, Schaubcylindrichnus, Skolithos, Siphonichnus, navichnia, fugichnia and abundant bivalve shells. The deposits from the Gulf of Papua consist of Phycosiphon, Chondrites, Schaubcylindrichnus, Skolithos, Helminthopsis, Arenicolites, worm-like traces and abundant bivalve shell fragments. In addition to the traces and bivalves, Tūranganui-a-Kiwa/Poverty Bay shelf includes Asterosoma, Thalassinoides and gastropods. These traces are consistent with the newly defined Phycosiphon Ichnofacies (MacEachern and Bann, 2020), which characterise muddy prodelta settings. This research demonstrates that the neoichnology of modern shelf sediments match models developed from studies of ancient systems, and provides a robust tool for analysing and interpreting ancient shelf strata.

A hyperspectral view of the nearshore Mississippi River Delta: Characterizing suspended particles in coastal wetlands using imaging spectroscopy

Suspended particle concentration (Total Suspended Solids or TSS) determines sediment availability for deposition, whereas particle physico-chemical properties (e.g. composition and size) can influence sediment transport and accretion. These variables are critical to modeling and understanding nearshore sediment dynamics and coastal soil accretion and erosion. Upcoming satellite imaging spectrometers (e.g., GLIMR, SBG) have the potential to improve the detailed mapping and characterization of suspended particles in nearshore areas. Here, we conducted a detailed feasibility study assessing the capacity of imaging spectroscopy to accurately map suspended particle concentrations and physico-chemical properties in the Mississippi River Deltaic Plain (Louisiana, USA), a nearshore system with extreme coastline vulnerability. To that end, a large dataset of in situ TSS, Particulate Organic Carbon (POC) concentration, particle size, inherent optical properties (IOPs), and hyperspectral remote sensing reflectance (Rrs) was collected during the 2021 NASA Delta-X field campaigns in coastal Louisiana. Analyses of the data revealed that the enhanced spectral information provided by imaging spectroscopy (hyperspectral Rrs) improved the retrieval of TSS and POC/TSS (% organic carbon content), but highlighted the challenge of retrieving particle size in this system. Partial-least-squares-regression (PLSR) algorithms for TSS performed better than existing state-of-the-art algorithms and confirmed the importance of using local measurements and near-infrared Rrs. Implementation on Airborne Visible Infrared Imaging Spectrometer-Next Generation (AVIRIS-NG) imagery generated high-resolution (< 5 m), high-quality maps of TSS and POC/TSS in contrasted areas with a wide range of TSS concentrations from (0.1 to 100 mg L−1), and demonstrated the enhanced potential of upcoming missions such as SBG and GLIMR to inform studies of nearshore sediment dynamics and coastal vulnerability/resilience.

Coastal wetland restoration through the lens of Odum's theory of ecosystem development

Advancing ecological restoration assessments requires a more detailed consideration of species interactions and ecosystem processes. Most restoration projects rely on a few metrics not always directly linked with ecological theory. Here, we used Odum's theory of ecosystem development to assess and compare the ecosystem structure and services of created marshes (4–6 years old) with preexisting, reference marshes in a brackish water region of the Mississippi River Delta. We built ecosystem models for created and reference marshes that integrated large datasets of stomach contents, stable isotopes, and taxa abundances. Despite strong resemblance in community structure, created marshes were at an earlier succession stage compared to the reference marshes, having lower biomass (including exploited species), higher biomass turnover and production, less dependence on detritus, lower material cycling, and less energy flowing through specialist pathways. Although preserving preexisting marshes should be a priority, created marshes may still be an important tool for the restoration of coastal areas and their ecosystem services. In addition, our results show that comparisons of species biodiversity alone may fail to capture essential differences in ecosystem processes between habitats, which reinforces the importance of ecosystem modeling approaches to assess restoration projects.

Morphodynamic modeling of Fourleague Bay in Mississippi River Delta: Sediment fluxes across river-estuary-wetland boundaries

Morphodynamic modeling of Fourleague Bay in Mississippi River Delta: Sediment fluxes across river-estuary-wetland boundaries

Vegetation restoration following dieback of Phragmites australis in the Mississippi River Delta, USA

Vegetation restoration following dieback of Phragmites australis in the Mississippi River Delta, USA

Twenty-Eight Years of Plant Community Development and Dynamics in the Balize Mississippi River Delta, Louisiana, USA

Deltaic landscapes go through cycles of birth, growth, decline, and death governed by intertwined geological, biological, and ecological processes. In this study, we tracked deltaic lobes in the Balize Mississippi River Delta, Louisiana, USA, over 28 years (1984–2012). Hydrologic and geomorphic patterns as well as sustained patterns of wetland plant richness, diversity, and biomass are described. Plant diversity and biomass were modeled by nMDS ordination. Taxa (53) were harvested and dried (116,706 g) from 965 (0.25 m2) plots and divided into three groups: I. four foundation species, corresponding to 78.9% of the total harvest; II. nine pioneer species, corresponding to 13.6% of the total harvest; and III. all other taxa, corresponding to 7.5% of the total harvest (eight miscellaneous grasses, eight miscellaneous sedges, and twenty-four miscellaneous herbs). Autogenic/allogenic processes (sedimentation, subsidence, plant colonization, and succession events) affected composition and biomass. Eleven important species were identified. Taxon richness increased on mudflats during primary succession (fifteen to twenty-five taxa per site), then declined to fewer than five taxa per site. The niche space theory explained patterns of community change, with a similar total biomass/yr (~500 g/m2/yr) at all study sites. Quantile regression analyses showed that the water quality and quantity of the Mississippi River influenced biomass, especially in springtime waters. Stochastic events (storms, herbivory, salt burn, and flood pulses) impacted biomass. Long-term studies like this are required in a future of climate unknowns.

Hurricane Effects on Benthic Nitrogen Cycling in an Emerging Coastal Deltaic Floodplain Within the Mississippi River Delta Plain

Hurricane Effects on Benthic Nitrogen Cycling in an Emerging Coastal Deltaic Floodplain Within the Mississippi River Delta Plain

LONG TERM MORPHOLOGIC MODELLING OF DELTA DEVELOPMENT IN BRETON SOUND RESULTING FROM A PROPOSED DIVERSION STRUCTURE

Sediment deprivation, hydrologic alteration, subsidence, sea level rise, and saltwater intrusion have been causing significant land loss in coastal Louisiana. Breton Sound combined with the nearby Barataria Bay, and Mississippi River Delta have lost approximately 1800 square kilometers (or 447,000 acres) of land, representing one of the highest land loss rates in the world since the 1930s when the Mississippi River was leveed. To address this problem, the Coastal Protection and Restoration Authority (CPRA) initiated several sediment diversion projects in the Lower Mississippi River (LMR). The Mid- Breton Sediment Diversion (MBrSD) project is one of the costal restoration projects proposed to restore natural processes in Breton Sound, which can strategically reestablish hydrologic flows, carry land-building sediments, nourish marshes and sustain land.