Mississippi River Deltaic Plain Research Articles

Contrasting organic carbon respiration pathways in coastal wetlands undergoing accelerated sea level changes

Carbon cycling in coastal wetland soil is controlled by a complex interplay between microbial processes and porewater chemistry that are often influenced by various external forcings like wind, river discharge, and sea-level changes, where most of the organic carbon is mineralized to its inorganic form by various aerobic and anaerobic respiration pathways. The export of this inorganic carbon (DIC) from coastal wetlands has long been recognized as a significant component of the global carbon cycle. The major objective of this work is to determine the relative contribution of various respiration pathways to seasonal DIC production in two contrasting marshes (brackish and salt). The DIC fluxes estimates for the brackish and salt marshes were found to range between 36.52 ± 5.81 and 33.98 ± 2.21 mmol m−2 d−1 in winter and 133.10 ± 102.60 and 82.37 ± 30.87 mmol m−2 d−1 during summer of 2020. For the brackish marsh, aerobic respiration and iron reduction were found to be the primary contributors to DIC production representing 17.91–35.21 % and 61.13–81.97 % of total measured organic matter (OM) respiration respectively. On the other hand, aerobic respiration and sulfate reduction were the primary contributors to DIC production in the salt marsh, accounting for 37.91–83.93 % and 15.87–62.04 % of the total measured OM respiration respectively. The Mississippi River Deltaic Plain experiences high relative sea level rise and expected to undergo rapid change in salinity regime in near future from additional changes in river discharge, proposed sediment diversion plans, and storm surge intensities. The current study represents the first attempt to concurrently estimate various respiration pathways in this region and more studies are needed to understand the trajectories of soil OM respiration pathways and its impact on coastal carbon cycling.

Machine Learning Predictions of Vertical Accretion in the Mississippi River Deltaic Plain

AbstractDeltaic landscapes consist of vast wetland systems that rely on sedimentation to maintain their elevation and ecological communities against relative sea‐level rise. In the Mississippi River Deltaic plain, rising relative sea level and anthropogenic activities are causing land loss that will continue unless vertical accretion of sediment on the wetland surface is enough to fill the accommodation space. Even though the fate of the Mississippi Deltaic plain is tied directly to vertical accretion, there is not yet a clear understanding of the system‐wide controls on this process. Here, we investigate vertical accretion in coastal Louisiana using a data set of 266 stations from the Coastwide Reference Monitoring System (CRMS). Using linear regression models, we analyze vertical accretion in freshwater‐intermediate, brackish, and saline marsh communities. Integrating results from these models into a Gaussian Process regression model, we predict controls on vertical accretion rates across the deltaic plain. Consistent with previous studies, our results suggest that tidal amplitude and flood depth are critical controls on vertical accretion. These effects are additive and marshes with high tidal amplitudes and flood depths experience the most vertical accretion. Interestingly, the normalized difference vegetation index is found to be important for predicting vertical accretion, but not because of an increase in biomass production, but because it records unique marsh communities and flooding regimes. This study emphasizes the importance of incorporating marsh specific information into predictive models for the vertical accretion of coastal wetlands and that better predictions of wetland accretion probably require denser observational data.

Correction: Accelerating Elevation Gain Indicates Land Loss Associated with Erosion in Mississippi River Deltaic Plain Tidal Wetlands

Correction: Accelerating Elevation Gain Indicates Land Loss Associated with Erosion in Mississippi River Deltaic Plain Tidal Wetlands

Accelerating Elevation Gain Indicates Land Loss Associated with Erosion in Mississippi River Deltaic Plain Tidal Wetlands

In recent years, the Mississippi River Deltaic Plain (MRDP) has experienced the highest rates of wetland loss in the USA. Although the process of vertical drowning has been heavily studied in coastal wetlands, less is known about the relationship between elevation change and land loss in wetlands that are experiencing lateral erosion and the contribution of erosion to land loss in the MRDP. We quantified relationships of elevation change and land change in ten submerging tidal wetlands and found that, despite significant land loss, elevation trajectories in seven of the land loss study sites were positive. Furthermore, we observed an acceleration in elevation gain preceding the conversion from vegetated marsh to open water.To identify regional contributions of lateral erosion to land loss, we quantified the relationship of elevation change and land change in 159 tidal marsh sites in the MRDP. Approximately half the sites were persistently losing land, and 82% of these sites were vulnerable to erosion, identifying erosion as a dominant mechanism of coastal wetland loss in this region. Notably, the sites that were vulnerable to erosion were experiencing land loss while also gaining elevation, and sites with the highest land loss exhibited accelerating elevation gain. Together, these data illustrate that (1) erosion is a dominant mechanism of wetland loss in the MRDP, (2) accelerated elevation gain is an indicator of erosion, and (3) consideration of elevation change trajectories within the context of land change is critical for providing accurate coastal wetland vulnerability assessments.

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.

ENSO and NAO linkages to interannual salinity variability in north central Gulf of Mexico estuaries through teleconnections with precipitation

Though the importance of Earth's internal climate modes such as the El Niño-Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO) to regional-scale climate variability is well recognized, the degree to which these oscillations are reflected by spatio-temporal salinity variability over interannual timescales in estuaries is less understood. Here an 11-year continuous salinity monitoring dataset spanning 223 stations across Louisiana's coastal wetlands along the northern Gulf of Mexico is examined with empirical orthogonal function (EOF) analysis to identify dominant modes of interannual variability in the salinity field. The first EOF mode accounts for 72% of the variance in the salinity field and captures a domain-wide pattern where salinities vary in-phase through space in response to local precipitation anomalies occurring in the vicinity of the study area. This local precipitation anomaly is positively correlated with ENSO (Nino3.4 index), consistent with the El Niño – wet (La Niña – dry) precipitation teleconnection that is prevalent throughout the northern Gulf of Mexico coast. The second EOF mode, which accounts for 13% of the variance in the salinity field, is expressed primarily in the marshes across the lower reaches of the Mississippi River deltaic plain (MRDP). EOF2 is anticorrelated with annual Mississippi River discharge anomaly such that salinities in the lower MRDP decrease as discharge increases, pointing to enhanced advection of fresh river plume waters over the shelf into the estuary via estuary-ocean exchange during years of anomalously high river discharge. Mississippi River discharge anomaly is positively correlated with the NAO at a one-year time lag, through a teleconnection with precipitation throughout much of the central region of the Mississippi River drainage basin. Together, these findings indicate that most of the interannual salinity variability across Louisiana's coastal wetlands can be linked to climate variability through teleconnections with precipitation. Incorporating these dynamics into restoration planning, monitoring, and adaptive management efforts may help constrain background environmental variation and better isolate restoration effects.

Aboveground wetland vegetation growth responses to simulated sediment diversion inundation scenarios

Coastal Louisiana is experiencing extremely high rates of wetland loss, particularly within the Mississippi River Deltaic Plain. Although multiple stressors contribute to this loss, one major driver is decreased wetland elevation stemming from disconnection of recurrent sediment rich influxes of Mississippi River floodwater. The decrease in sediment delivery, in conjunction with natural subsidence processes, reduce marsh elevations and increase flooding stress on vegetation, leading to marsh collapse. River sediment diversions are large-scale restoration techniques intended to rehabilitate Louisiana's degraded coastal wetlands by mimicking historic crevasse splays, thereby stimulating land building and vegetation growth. However, such large-scale restoration techniques require an appropriate knowledge base of vegetation and soil biogeochemical responses to altered environmental conditions. In this research, monospecific sods of three foundational marsh species (Spartina patens, Spartina alterniflora, Typha latifolia) were exposed to inundation depth (20 and 40 cm), duration (8 and 16 weeks), and salinity (0 and 5 psu) treatments consistent with a range of anticipated sediment diversion scenarios in a mesocosm setting. Additionally, sods were monitored through week 20 after being removed from their experimental conditions. Both Spartina alterniflora and Typha latifolia displayed relatively consistent increases in stem densities over time across inundation depth-duration treatments, whereas Spartina patens displayed a sharp decline in stem densities at 40 cm flooding depths. Additionally, both Spartina alterniflora and Spartina patens had the least observed amount of total aboveground biomass at 40 cm flooding depths. Spartina patens sods displayed a significant loss in surface elevation with increased flooding stress through study week 20, whereas Typha latifolia demonstrated a significant gain in surface elevation. These results suggest that restoration projects increasing inundation within the limits of the depths employed herein and the anticipated duration of river diversion induced inundation are unlikely to negatively impact the aboveground growth responses of Spartina alterniflora and Typha latifolia, but could reduce stem densities and aboveground biomass of Spartina patens.

Temporal variability in soil organic matter accretion rates in coastal deltaic wetlands under changing depositional environments

AbstractThe Mississippi River Deltaic Plain experiences high relative sea level rise, limited sediment supply, and high marsh edge erosion, leading to the substantial coastal wetland and stored soil organic matter (SOM) loss. The objective of this study was to understand the SOM accumulation rates over the past 1000 years related to the changes in the depositional environment in these highly eroding coastal wetlands. Soil cores (2 m) were collected from four sites in Barataria Basin, LA and analyzed for proportion of organic and mineral matter, total C, N, P, particle size, and stable isotopic composition (δ13C and δ15N), as well as 14C and 137Cs dating. The soil carbon stock in the 2 m depth (62.4 ± 2 kg m−2) was approximately 88% greater than the carbon stock in just the 1 m depth (33.1 ± 0.6 kg m−2) indicating a need for considering deeper soil profiles (up to 2 m) to estimate blue carbon stock in deltaic environments. The average vertical accretion rate for Barataria Basin was 8.1 ± 0.6 mm year−1 over 50 years. The long‐term (1000‐year time scale) C accumulation rate (39 g C m−2 year−1) was ∼14% of the short‐term accumulation rate (254 ± 19 g C m−2 year−1). Wetlands in Barataria Basin started as fresh marsh and transitioned over time to intermediate to brackish. These marshes were able to maintain relative elevation through the accumulation of organic matter and mud despite high relative rates of sea‐level rise. However, the high rates of edge erosion may limit these marshes to continue to sequester atmospheric carbon under accelerating sea level in the absence of restoration efforts.

A Model of the Spatiotemporal Dynamics of Soil Carbon Following Coastal Wetland Loss Applied to a Louisiana Salt Marsh in the Mississippi River Deltaic Plain

AbstractThe potential for carbon sequestration in coastal wetlands is high due to protection of carbon (C) in flooded soils. However, excessive flooding can result in the conversion of the vegetated wetland to open water. This transition results in the loss of wetland habitat in addition to the potential loss of soil carbon. Thus, in areas experiencing rapid wetland submergence, such as the Mississippi River Delta, coastal wetlands could become a significant source of carbon emissions if land loss is not mitigated. To accurately assess the capacity of wetlands to store (or emit) carbon in dynamic environments, it is critical to understand the fate of soil carbon following the transition from vegetated wetland to open water. We developed a simple soil carbon model representing soil depths to 1 m using the data collected from a Louisiana coastal salt marsh in the Mississippi River Deltaic Plain to predict soil carbon density and stock following the transition from a vegetated salt marsh to an open water pond. While immediate effects of ponding on the distribution of carbon within the 1‐m soil profile were apparent, there were no effects of ponding on the overall, integrated, carbon stocks 14 years, following wetland submergence. Rather, the model predicts that soil carbon losses in the first meter will be realized over long periods of time (∼200 years) due to changes in the source of carbon (biomass vs. mineral sediment) with minimal losses through mineralization.

Geomorphologic change and patchy mud infilling in a sandy dredge pit in eastern Ship Shoal, Louisiana shelf, USA

To restore degraded barrier shorelines of the Mississippi River Deltaic Plain of Louisiana, sand is dredged from borrow areas in estuaries and continental shelves. The dredge pit in South Pelto Blocks 12 & 13 on eastern Ship Shoal used for Caminada-Moreau Headland restoration project is one of several sand dredge pits for barrier island restoration in Louisiana. Ship Shoal Sand Complex is in a sand-dominated, energetic, continental shelf environment. During energetic events, transient mud can blanket Ship Shoal, and then bypasses the shoal and deposits in deeper water. In this study, we collected and interpreted a suite of geophysical data from South Pelto Dredge Pit which is located in water depths of ∼9 m on eastern Ship Shoal. Repeat bathymetric survey data show that South Pelto Dredge Pit is presently infilling at an average rate of approximately 27,480 m3/year, which is slower than the predicted rate presented by an earlier numerical model. Side-scan mosaic maps indicate low-backscatter (muddy) sediments were transported into this pit and deposited in many patches within two years after dredging. The seabed inside this pit became smoother over time and the slope of the pit walls became gentler. Infilling of the South Pelto Dredge Pit seem to be controlled by sediment availability, which is volumetrically dominated by (a) far-field fluvial suspended sediment, (b) the adjacent seabed sediment resuspended by tides, waves, and currents, but presumably less impacted by (c) hurricanes or tropical storms during our study period of 2017–2018. The South Pelto Dredge Pit thus displays a unique mixing and redistribution of cohesive mud and non-cohesive sand. Initial spatial control on sediment infilling appears to be dictated by micro-bathymetry on the pit bottom created by the dredging activities. Bathymetric, side scan and grain-size data all indicate that mud was prone to deposit in the troughs.

Nitrogen Mineralization in Mississippi River Deltaic Plain Freshwater Floating Marshes

ABSTRACTThe mineralization of soil organic nitrogen in two floating marshes types in Louisiana, USA with different vegetation and mat thickness (and biomass) was measured by incubating peat soil in situ in polyethylene bottles at depths of 10 and 25 cm. The size of the extractable inorganic N (NH4+ – N) pool was related to mat thickness and soil bulk density. The N mineralization rates were twice as great at the thick mat site (1.5 mg N/kg soil/day) compared to the thin mat floating marsh (0.71 mg N/kg soil/day). The amount mineralized on a volume basis to 30 cm depth were 22 mg N/m2/day for the thick mat marsh and 6 mg N/m2/day for the thin mat marsh. The nitrogen mineralization rates observed in this study reflect plant biomass differences between the thick mat and thin mat marshes, with the former exhibiting approximately 7-fold greater biomass. These low overall mineralization rates suggest that nitrogen entering these freshwater floating marshes from Mississippi River water percolating through the root zone, would result in increased plant productivity-increasing marsh stability and also providing a natural mechanism contributing to the processing and removal of dissolved inorganic nitrogen in river water entering these wetlands.

A review of sediment diversion in the Mississippi River Deltaic Plain

One of the proposed methods for restoring the disappearing Mississippi Delta is sediment diversion which uses channels and structures to divert water and sediment from the Mississippi and Atchafalaya Rivers into adjacent basins. This study presents a comprehensive review of geological and physical aspects of sediment dynamics in the Mississippi River Deltaic Plain (MRDP), with special reference to diversion studies over the past two decades. We synthesize these studies, present the current understanding of sediment diversions in the context of sediment dynamics, identify multiple key knowledge gaps, and make recommendations for future studies.To maximize net land building in the MRDP, management strategies should be focused on (a) enhancing river sediment delivery (both mud and sand), (b) increasing sediment retention in receiving basins and (c) minimizing erosion in bays and estuaries. Compared with extensive studies of land building, there have been relatively fewer studies of erosional processes. A heterogeneous coastal geological framework, cohesive sediment erodibility and subsidence together play complicated yet critical roles in future sediment dynamics in bays and estuaries of the MRDP. Sediment retention rates are highly sensitive to spatial and temporal scales, types of sediments and delivery season. Sediment diversions to seaward receiving basins provide more surge protection but tend to have lower sediment retention due to active coastal processes. Structures and devices that improve sediment retention, trap sediments, dissipate waves, and build living shorelines should be explored and cost-to-benefit analysis is needed. Long-term planning should consider more landward diversions, strategic community relocation, and nonlinear response of the complex sedimentary system of the MRDP.

Consequences of Mississippi River diversions on nutrient dynamics of coastal wetland soils and estuarine sediments: A review

Coastal Louisiana is dominated by the Mississippi River Deltaic Plain, which is composed of a series of overlapping and truncated sub-delta lobes that formed an extensive coastal wetland – open water shallow bay and low relief upland mosaic across the entire coastal zone (∼25,000 km2). These coastal wetlands have been eroding at an alarming rate during the past century, coincident with major modifications to the deltaic landscape. One of these former modifications included extensive leveeing of the Mississippi River, essentially isolating the river from the coastal basins. One restoration technique involves re-connecting the previously isolated coastal basins to the river via diversions, which would once again allow the influx of freshwater, sediments and nutrients into the coastal basins. We review issues on potential impacts of nutrient loading from the river as a consequence of river diversions, focused on nitrogen (N) and phosphorus (P) dynamics including denitrification related to carbon dynamics. We also examined how water residence times in these shallow systems, compared to deeper, often seasonally stratified, coastal systems, are more likely to lead to greater N removal potential in the context of a large scale river diversion helping to reduce N transport to the coastal ocean.

Differences in long-term vs short-term carbon and nitrogen sequestration in a coastal river delta wetland: Implications for global budgets

Coastal wetlands can serve as a considerable sink for carbon (C) gases. However, the capacity for wetlands to serve as more permanent C and N sinks over the long term is less clear given the time dependence of sediment deposition, organic matter decomposition, and anthropogenic land use change. In this study, we compare the short-term (decadal scale) and long-term (millennial scale) C and N accumulation rates estimated using 137Cs and radiocarbon dating of vibracores collected from a freshwater coastal wetland in the Louisiana Mississippi River deltaic plain (Atchafalaya River delta). The mean short-term (60 yrs) sediment accumulation rate was 1.4 cm/yr while the mean rate of long-term (100–1000 yrs) sediment accumulation was an order of magnitude lower at 0.12 cm/yr. Annual rates of C and N accumulated over the past several thousand years were approximately 10% of that over the past 60 years after correcting for bulk density. These results are similar to other coastal wetlands and suggest that time scale must be considered in determining the relative permanence of C and N storage in coastal wetland soils. This difference is especially important for assessing the role of C cycling in relation to global change models and N cycling related to water quality in accurately quantifying the role of coastal deltaic fresh water wetlands in regulating these biogeochemical cycles.

Mercury adsorption in the Mississippi River deltaic plain freshwater marsh soil of Louisiana Gulf coastal wetlands

Mercury adsorption characteristics of Mississippi River deltaic plain (MRDP) freshwater marsh soil in the Louisiana Gulf coast were evaluated under various conditions. Mercury adsorption was well described by pseudo-second order and Langmuir isotherm models with maximum adsorption capacity of 39.8 mg g−1. Additional fitting of intraparticle model showed that mercury in the MRDP freshwater marsh soil was controlled by both external surface adsorption and intraparticle diffusion. The partition of adsorbed mercury (mg g−1) revealed that mercury was primarily adsorbed into organic-bond fraction (12.09) and soluble/exchangeable fraction (10.85), which accounted for 63.5% of the total adsorption, followed by manganese oxide-bound (7.50), easily mobilizable carbonate-bound (4.53), amorphous iron oxide-bound (0.55), crystalline Fe oxide-bound (0.41), and residual fraction (0.16). Mercury adsorption capacity was generally elevated along with increasing solution pH even though dominant species of mercury were non-ionic HgCl2, HgClOH and Hg(OH)2 at between pH 3 and 9. In addition, increasing background NaCl concentration and the presence of humic acid decreased mercury adsorption, whereas the presence of phosphate, sulfate and nitrate enhanced mercury adsorption. Mercury adsorption in the MRDP freshwater marsh soil was reduced by the presence of Pb, Cu, Cd and Zn with Pb showing the greatest competitive adsorption. Overall the adsorption capacity of mercury in the MRDP freshwater marsh soil was found to be significantly influenced by potential environmental changes, and such factors should be considered in order to manage the risks associated with mercury in this MRDP wetland for responding to future climate change scenarios.

Using ecosystem modeling to evaluate trade-offs in coastal management: Effects of large-scale river diversions on fish and fisheries

A coupled ecosystem modeling approach was used to evaluate how select combinations of large-scale river diversions in the lower Mississippi River Deltaic Plain may affect the distribution, biomass, and landings of fish and shellfish over decades relative to a future without action. These river diversions are controlled openings in the riverbank of the Mississippi River designed to reintroduce sediment, water, and nutrients into hydrologically isolated coastal wetlands in order to mitigate wetland loss. We developed a spatial ecosystem model using Ecopath with Ecosim (EwE) software, and prepared it to receive output from a Delft3D hydrodynamic model coupled to primary production models. The Delft3D model provided environmental drivers including salinity, temperature, Chl a, total suspended solids, and change in wetland cover as a result of simulated river diversions over decadal model runs. Driver output was averaged either daily, monthly, or annually depending on the parameter. A novel oyster-specific subroutine is introduced in this paper to incorporate information at daily intervals in Ecospace, while Ecospace runs on a monthly time step. The ecosystem model simulates biomass and distribution of fish and shellfish species, and landings of targeted fisheries species, as a result of environmental changes projected for a preliminary set of management scenarios designed to evaluate and screen select combinations of river diversions. Abundant local field samples and landings data allowed for model calibration and validation. The results of simulations indicate that inflow of Mississippi River water in estuaries may cause local shifts in species assemblages. These changes were in some cases direct effects of decreased salinity, such as locally reduced Spotted Seatrout biomass. Changes in some other species in the affected areas resulted from indirect effects; for example, reduced Chl a (as a result of increased TSS) resulted in near-field reductions of Gulf Menhaden. The simulations also showed that local biomass reductions were mostly the result of redistribution, since the scenario with the proposed diversions open had minimal impact on the total biomass or landings of species simulated in the Mississippi River Delta as compared to a future without action. The model and its output were used as a decision support tool to help evaluate and compare alternative management actions. The results of this study played a role in the decision by the Coastal Protection and Restoration Authority to prioritize moving forward to conduct more detailed analyses through engineering and design of the two middle diversions but not the two lower diversions that were tested in this study.

Predicting the impacts of Mississippi River diversions and sea-level rise on spatial patterns of eastern oyster growth rate and production

There remains much debate regarding the perceived tradeoffs of using freshwater and sediment diversions for coastal restoration in terms of balancing the need for wetland restoration versus preserving eastern oyster (Crassostrea virginica) production. Further complicating the issue, climate change-induced sea-level rise (SLR) and land subsidence are also expected to affect estuarine water quality. In this study, we developed a process-based numerical modeling system that couples hydrodynamic, water quality, and oyster population dynamics. We selected Breton Sound Estuary (BSE) (∼2740km2) in the eastern Mississippi River Deltaic Plain since it is home to several of the largest public oyster seed grounds and private leases for the Gulf coast. The coupled oyster population model was calibrated and validated against field observed oyster growth data. We predicted the responses of oyster population in BSE to small- (142m3s−1) and large-scale (7080m3s−1) river diversions at the Caernarvon Freshwater Diversion structure planned in the 2012 Coastal Master Plan (Louisiana) under low (0.38m) and high (1.44m) relative sea-level rise (RSLR=eustatic SLR+subsidence) compared to a baseline condition (Year 2009). Model results showed that the large-scale diversion had a stronger negative impact on oyster population dynamics via freshening of the entire estuary, resulting in reduced oyster growth rate and production than RSLR. Under the large-scale diversion, areas with optimal oyster growth rates (>15mg ash-free dry weight (AFDW) oyster−1wk−1) and production (>500g AFDWm−2yr−1) would shift seaward to the southeastern edge of the estuary, turning the estuary into a very low oyster production system. RSLR however played a greater role than the small-scale diversion on the magnitude and spatial pattern of oyster growth rate and production. RSLR would result in an overall estuary-wide decrease in oyster growth rate and production as a consequence of decreased salinities in the middle and lower estuary because rising sea level likely causes increased stage and overbank flow downstream along the lower Mississippi River.

Mangrove expansion and contraction at a poleward range limit: climate extremes and land-ocean temperature gradients.

Within the context of climate change, there is a pressing need to better understand the ecological implications of changes in the frequency and intensity of climate extremes. Along subtropical coasts, less frequent and warmer freeze events are expected to permit freeze-sensitive mangrove forests to expand poleward and displace freeze-tolerant salt marshes. Here, our aim was to better understand the drivers of poleward mangrove migration by quantifying spatiotemporal patterns in mangrove range expansion and contraction across land-ocean temperature gradients. Our work was conducted in a freeze-sensitive mangrove-marsh transition zone that spans a land-ocean temperature gradient in one of the world's most wetland-rich regions (Mississippi River Deltaic Plain; Louisiana, USA). We used historical air temperature data (1893-2014), alternative future climate scenarios, and coastal wetland coverage data (1978-2011) to investigate spatiotemporal fluctuations and climate-wetland linkages. Our analyses indicate that changes in mangrove coverage have been controlled primarily by extreme freeze events (i.e., air temperatures below a threshold zone of -6.3 to -7.6°C). We expect that in the past 121yr, mangrove range expansion and contraction has occurred across land-ocean temperature gradients. Mangrove resistance, resilience, and dominance were all highest in areas closer to the ocean where temperature extremes were buffered by large expanses of water and saturated soil. Under climate change, these areas will likely serve as local hotspots for mangrove dispersal, growth, range expansion, and displacement of salt marsh. Collectively, our results show that the frequency and intensity of freeze events across land-ocean temperature gradients greatly influences spatiotemporal patterns of range expansion and contraction of freeze-sensitive mangroves. We expect that, along subtropical coasts, similar processes govern the distribution and abundance of other freeze-sensitive organisms. In broad terms, our findings can be used to better understand and anticipate the ecological effects of changing winter climate extremes, especially within the transition zone between tropical and temperate climates.

Co-evolution of wetland landscapes, flooding, and human settlement in the Mississippi River Delta Plain.

River deltas all over the world are sinking beneath sea-level rise, causing significant threats to natural and social systems. This is due to the combined effects of anthropogenic changes to sediment supply and river flow, subsidence, and sea-level rise, posing an immediate threat to the 500–1,000 million residents, many in megacities that live on deltaic coasts. The Mississippi River Deltaic Plain (MRDP) provides examples for many of the functions and feedbacks, regarding how human river management has impacted source-sink processes in coastal deltaic basins, resulting in human settlements more at risk to coastal storms. The survival of human settlement on the MRDP is arguably coupled to a shifting mass balance between a deltaic landscape occupied by either land built by the Mississippi River or water occupied by the Gulf of Mexico. We developed an approach to compare 50 % L:W isopleths (L:W is ratio of land to water) across the Atchafalaya and Terrebonne Basins to test landscape behavior over the last six decades to measure delta instability in coastal deltaic basins as a function of reduced sediment supply from river flooding. The Atchafalaya Basin, with continued sediment delivery, compared to Terrebonne Basin, with reduced river inputs, allow us to test assumptions of how coastal deltaic basins respond to river management over the last 75 years by analyzing landward migration rate of 50 % L:W isopleths between 1932 and 2010. The average landward migration for Terrebonne Basin was nearly 17,000 m (17 km) compared to only 22 m in Atchafalaya Basin over the last 78 years (p < 0.001), resulting in migration rates of 218 m/year (0.22 km/year) and <0.5 m/year, respectively. In addition, freshwater vegetation expanded in Atchafalaya Basin since 1949 compared to migration of intermediate and brackish marshes landward in the Terrebonne Basin. Changes in salt marsh vegetation patterns were very distinct in these two basins with gain of 25 % in the Terrebonne Basin compared to 90 % decrease in the Atchafalaya Basin since 1949. These shifts in vegetation types as L:W ratio decreases with reduced sediment input and increase in salinity also coincide with an increase in wind fetch in Terrebonne Bay. In the upper Terrebonne Bay, where the largest landward migration of the 50 % L:W ratio isopleth occurred, we estimate that the wave power has increased by 50–100 % from 1932 to 2010, as the bathymetric and topographic conditions changed, and increase in maximum storm-surge height also increased owing to the landward migration of the L:W ratio isopleth. We argue that this balance of land relative to water in this delta provides a much clearer understanding of increased flood risk from tropical cyclones rather than just estimates of areal land loss. We describe how coastal deltaic basins of the MRDP can be used as experimental landscapes to provide insights into how varying degrees of sediment delivery to coastal deltaic floodplains change flooding risks of a sinking delta using landward migrations of 50 % L:W isopleths. The nonlinear response of migrating L:W isopleths as wind fetch increases is a critical feedback effect that should influence human river-management decisions in deltaic coast. Changes in land area alone do not capture how corresponding landscape degradation and increased water area can lead to exponential increase in flood risk to human populations in low-lying coastal regions. Reduced land formation in coastal deltaic basins (measured by changes in the land:water ratio) can contribute significantly to increasing flood risks by removing the negative feedback of wetlands on wave and storm-surge that occur during extreme weather events. Increased flood risks will promote population migration as human risks associated with living in a deltaic landscape increase, as land is submerged and coastal inundation threats rise. These system linkages in dynamic deltaic coasts define a balance of river management and human settlement dependent on a certain level of land area within coastal deltaic basins (L).

Will Mississippi River diversions designed for coastal restoration cause harmful algal blooms?

Large-scale river diversions have been proposed as a way to transport sediment to the deteriorating Mississippi River deltaic plain. These diversions would also transport immense nutrient loads to shallow receiving estuaries, including open water areas where there may be potential for blooms of toxin-producing cyanobacteria. We examined environmental variables and cyanobacteria dynamics in the Lake Pontchartrain Estuary during the spring-summer period of two years: (a) 2011 when the system received a massive inflow of diverted Mississippi River water from the Bonnet Carré Spillway, and (b) 2012 when no diversion occurred. Cluster analysis of 180 samples collected over the course of both years identified five distinct water types grouped according to salinity, temperature, clarity, bioavailable nutrients, and chlorophyll a. Three of these water types were unique to the 2011 diversion event. We found no evidence of immediate, direct cyanobacteria bloom stimulation by diverted nutrients in the “high phytoplankton biomass” cluster associated with the 2011 diversion. Substantial nitrate assimilation by non-harmful phytoplankton occurred as the plume of diverted freshwater traveled through the system that year. The “summer” cluster characterized by nitrogen-limited conditions and including data from both years was characterized by significantly greater cyanobacteria abundance than all other clusters. There was no difference in cyanobacteria abundance between years within the summer cluster. Microcystin levels detected suggested low probability of acute health risks for recreational waters and were not correlated to cyanobacteria abundance. Site-specific understanding of the following is needed to guide diversion management: (i) interannual phytoplankton community variability in non-diversion years, (ii) cyanobacteria inputs from local tributaries, (iii) interactions between plume physics, chemistry, and phytoplankton dynamics, (iv) internal phosphorus feedbacks that may indirectly link diversions to summer blooms, and (v) controls on cyanotoxin production.