Louisiana Coastal Wetlands Research Articles

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.

MAXIMUM LIKELIHOOD ALGORITHM DETECTS COASTAL WETLAND CHANGES IN TWO CONTRASTING COASTAL WETLANDS IN LOUISIANA

Abstract. Louisiana coastal wetlands contain about 37 percent of the estuarine herbaceous marshes in the conterminous United States. However, the combined effect of sea level rise and other anthropogenic factors have altered land use land cover over the last few years. This is true for two wetlands in coastal Louisiana, Barataria bay and Wax Lake delta. Barataria Bay, Louisiana, USA has experienced significant land loss. Updated information on the dynamics of change in these wetlands is limited and poorly documented. This information is necessary to develop strategies that will contribute to reversing and halting degradation. Thus, this study employed the Maximum Likelihood classifier on Landsat satellite imagery to assess land use and land cover changes in Barataria Bay and Wax Lake Delta, southeastern Louisiana, USA. The analysis revealed notable alterations in the land cover patterns over the study period. In Barataria Bay, there was a decrease in salt marsh areas with a corresponding increase in open water and Built-up area. In contrast, Wax Lake Delta demonstrated substantial land/wetland growth, with significant expansion of vegetation cover. The Maximum Likelihood classifier demonstrated high accuracy in classifying the land cover types, with an overall accuracy of 86% for Barataria Bay and 92% for Wax Lake Delta. These results highlight the effectiveness of the classifier in accurately identifying and mapping land cover changes in coastal environments. The findings contribute valuable insights for understanding the dynamics of coastal ecosystems and can inform decision-making processes for coastal management and conservation efforts.

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.

A 5200-year paleoecological and geochemical record of coastal environmental changes and shoreline fluctuations in southwestern Louisiana: Implications for coastal sustainability

This study presents a 5200-year history of coastal environmental changes and shoreline fluctuations based on a multi-proxy sedimentary record developed from the eastern end of Louisiana's Chenier Plain. Palynological, loss-on ignition, grain-size, and X-ray fluorescence data for a 525 cm sedimentary core indicate five stages of ecosystem development in the Rockefeller Wildlife Refuge since the mid-Holocene. Between 5190 and 4240 cal yr BP, the relative sea-level was much lower than the present level, and our study area was occupied by a baldcypress swamp. When the Sale-Cypremont subdelta complex formed at a western position approximately 4600 years ago, our study area was transformed to a maritime forest about 4240–3440 cal yr BP, suggesting shoreline progradation. When the Mississippi River delta lobe switched to more easterly positions during the Teche, St. Bernard, Lafourche, and Plaquemine-Balize complexes, our study site was transformed back to a freshwater swamp between 3440 and 1520 cal yr BP and then to a coastal marsh between 1520 and 300 cal yr BP. After the Atchafalaya-Wax Lake subdelta complex formed at a more westerly position again, the modern-day beach form at 300 cal yr BP. These coastal ecosystem developments suggest that shoreline retreat started at ~3440 cal yr BP and persisted to the present day. In addition, evidence of at least four possible paleohurricane events was found during the late Holocene. Our multi-proxy record demonstrates that shoreline fluctuation in southwestern Louisiana was tied to the Mississippi River Delta Switch since the mid-Holocene. Shoreline progradation occurred during the Sale-Cypremont subdelta phase ~5200–3500 years ago, when the sediment supply was high. Retrogradation occurred after ~3500 cal yr BP when the Mississippi River subdelta complexes occupied more easterly position, resulting in diminished sediment supply. A significant increase in the delivery of water and total suspended sediment load from the Atchafalaya River to the Gulf of Mexico is needed in order to sustain the southwestern Louisiana coastal wetlands.

Life Cycle of Oil and Gas Fields in the Mississippi River Delta: A Review

Oil and gas (O&G) activity has been pervasive in the Mississippi River Delta (MRD). Here we review the life cycle of O&G fields in the MRD focusing on the production history and resulting environmental impacts and show how cumulative impacts affect coastal ecosystems. Individual fields can last 40–60 years and most wells are in the final stages of production. Production increased rapidly reaching a peak around 1970 and then declined. Produced water lagged O&G and was generally higher during declining O&G production, making up about 70% of total liquids. Much of the wetland loss in the delta is associated with O&G activities. These have contributed in three major ways to wetland loss including alteration of surface hydrology, induced subsidence due to fluids removal and fault activation, and toxic stress due to spilled oil and produced water. Changes in surface hydrology are related to canal dredging and spoil placement. As canal density increases, the density of natural channels decreases. Interconnected canal networks often lead to saltwater intrusion. Spoil banks block natural overland flow affecting exchange of water, sediments, chemicals, and organisms. Lower wetland productivity and reduced sediment input leads to enhanced surficial subsidence. Spoil banks are not permanent but subside and compact over time and many spoil banks no longer have subaerial expression. Fluid withdrawal from O&G formations leads to induced subsidence and fault activation. Formation pore pressure decreases, which lowers the lateral confining stress acting in the formation due to poroelastic coupling between pore pressure and stress. This promotes normal faulting in an extensional geological environment like the MRD, which causes surface subsidence in the vicinity of the faults. Induced reservoir compaction results in a reduction of reservoir thickness. Induced subsidence occurs in two phases especially when production rate is high. The first phase is compaction of the reservoir itself while the second phase is caused by a slow drainage of pore pressure in bounding shales that induces time-delayed subsidence associated with shale compaction. This second phase can continue for decades, even after most O&G has been produced, resulting in subsidence over much of an oil field that can be greater than surface subsidence due to altered hydrology. Produced water is water brought to the surface during O&G extraction and an estimated 2 million barrels per day were discharged into Louisiana coastal wetlands and waters from nearly 700 sites. This water is a mixture of either liquid or gaseous hydrocarbons, high salinity (up to 300 ppt) water, dissolved and suspended solids such as sand or silt, and injected fluids and additives associated with exploration and production activities and it is toxic to many estuarine organisms including vegetation and fauna. Spilled oil has lethal and sub-lethal effects on a wide range of estuarine organisms. The cumulative effect of alterations in surface hydrology, induced subsidence, and toxins interact such that overall impacts are enhanced. Restoration of coastal wetlands degraded by O&G activities should be informed by these impacts.

Mapping 30 Years of Change in the Marshlands of Breton Sound Basin (Southeastern Louisiana, U.S.A.): Coastal Land Area and Vegetation Green Cover

Potter, C. and Amer, R., 2020. Mapping 30 years of change in the marshlands of Breton Sound basin (southeastern Louisiana, U.S.A.): Coastal land area and vegetation green cover. Journal of Coastal Research, 36(3), 437–450. Coconut Creek (Florida), ISSN 0749-0208.This study analyzes the 30-year record of Landsat satellite imagery to better understand the patterns and processes of land loss/gain and recent changes in marshland vegetation green cover in coastal wetlands of Breton Sound basin of southeastern Louisiana. Impacted by both Hurricane Katrina in 2005 and Hurricane Gustav in 2008, this vast estuary area just south of New Orleans was mapped at 30 m resolution in 5-year intervals from 1985 to 2017 using the Landsat Normalized Difference Water Index (NDWI) and the Normalized Difference Vegetation Index (NDVI). Results revealed an entirely new picture of marshland change prior to Hurricanes Katrina and Gustav in the Breton Sound basin. Following decades of extensive wetland loss resulting largely from gas and oil well drilling and canal construction, this analysis estimated gains in land area between 1985 and 2005 (pre-Katrina) of 247 km2 across all subbasins of Breton Sound. The largest proportional increases in marshland areas were mapped from dNDWI analysis in the subbasins of Bayou la Loutre, Bayou Pointe-en-Pointe, and Bayou Terre Aux Boeufs, all surrounding the outer Mississippi River Gulf Outlet Canal. Brackish salinity areas were among the first of Louisiana's coastal wetlands to succumb to the hurricane storm surges of 2005 and 2008, as Landsat dNDWI analysis estimated that about 245 km2 of wetlands in the Breton Sound basin were lost to open water from the impacts of Hurricanes Katrina and Gustav. However, combined dNDWI and dNDVI analysis indicated that gains in marshland area and green vegetation cover have occurred at a yearly rate of increase after 2008 comparable to the period between 1985 and 2005. Specific areas of rapid land gain were detected in River aux Chenes and Bayou Bienvenue subbasins, and Bayou Plaquemines and Bayou la Loutre in brackish to saltwater marshland types on the outer margins of Breton Sound basin.

Resource competition model predicts zonation and increasing nutrient use efficiency along a wetland salinity gradient

A trade-off between competitive ability and stress tolerance has been hypothesized and empirically supported to explain the zonation of species across stress gradients for a number of systems. Since stress often reduces plant productivity, one might expect a pattern of decreasing productivity across the zones of the stress gradient. However, this pattern is often not observed in coastal wetlands that show patterns of zonation along a salinity gradient. To address the potentially complex relationship between stress, zonation, and productivity in coastal wetlands, we developed a model of plant biomass as a function of resource competition and salinity stress. Analysis of the model confirms the conventional wisdom that a trade-off between competitive ability and stress tolerance is a necessary condition for zonation. It also suggests that a negative relationship between salinity and production can be overcome if (1) the supply of the limiting resource increases with greater salinity stress or (2) nutrient use efficiency increases with increasing salinity. We fit the equilibrium solution of the dynamic model to data from Louisiana coastal wetlands to test its ability to explain patterns of production across the landscape gradient and derive predictions that could be tested with independent data. We found support for a number of the model predictions, including patterns of decreasing competitive ability and increasing nutrient use efficiency across a gradient from freshwater to saline wetlands. In addition to providing a quantitative framework to support the mechanistic hypotheses of zonation, these results suggest that this simple model is a useful platform to further build upon, simulate and test mechanistic hypotheses of more complex patterns and phenomena in coastal wetlands.

Nearshore exposure to Deepwater Horizon oil

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 576:111-124 (2017) - DOI: https://doi.org/10.3354/meps11811 Theme Section: Response of nearshore ecosystems to the Deepwater Horizon oil spill Nearshore exposure to Deepwater Horizon oil Shahrokh Rouhani1,*, Mary C. Baker2, Marla Steinhoff2, Mengni Zhang1, Jacob Oehrig1, Ian J. Zelo2, Stephen D. Emsbo-Mattingly3, Zachary Nixon4, Jonathan M. Willis5, Mark W. Hester5 1NewFields Companies LLC, 1349 W. Peachtree Street, Suite 2000, Atlanta, GA 30309, USA 2National Oceanic and Atmospheric Administration, Assessment and Restoration Division, 7600 Sand Point Way NE, Seattle, WA 98115, USA 3NewFields Companies LLC, 300 Ledgewood Place, Suite 305, Rockland, MA 02370, USA 4Research Planning Inc., 1121 Park St., Columbia, SC 29201, USA 5Institute for Coastal and Water Research, Department of Biology, University of Louisiana at Lafayette, Lafayette, LA 70504, USA *Corresponding author: srouhani@newfields.comAdvance View was available online October 23, 2016 ABSTRACT: The Deepwater Horizon (DWH) oil spill affected more than 2000 km of shoreline. DWH oil entered the nearshore environment, stranding on shorelines as tar balls and/or emulsified oil, or forming submerged oil mats and integrating into nearshore sediments. The available chemistry data showed submerged sediments, especially within the first 50 m from oiled shorelines, displayed patchy distributions of elevated polycyclic aromatic hydrocarbon (PAH) concentrations in excess of ambient concentrations, which were quantified based on forensic findings establishing their source as being from the Macondo oil. Consistent with observed shoreline oiling conditions, PAH concentrations in the soils of affected Louisiana coastal wetlands were orders of magnitude higher than ambient concentrations, especially in locations along the seaward edge of the marsh. Both total and petrogenic PAHs decreased with distance from the shore in both inland and offshore directions. Although PAHs exhibited evidence of weathering over time, in the most heavily oiled areas, they continued to exceed ambient concentrations by orders of magnitude through fall of 2013. KEY WORDS: Deepwater Horizon oil spill · National resources damage assessment · NRDA · Salt marsh · Nearshore · Submerged oil · PAH · Louisiana Full text in pdf format Supplementary material PreviousNextCite this article as: Rouhani S, Baker MC, Steinhoff M, Zhang M and others (2017) Nearshore exposure to Deepwater Horizon oil. Mar Ecol Prog Ser 576:111-124. https://doi.org/10.3354/meps11811 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 576. Online publication date: August 03, 2017 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2017 Inter-Research.

InSAR-Based Mapping of Tidal Inundation Extent and Amplitude in Louisiana Coastal Wetlands

The Louisiana coast is among the most productive coastal areas in the US and home to the largest coastal wetland area in the nation. However, Louisiana coastal wetlands have been disappearing at an alarming rate due to natural and anthropogenic processes, including sea level rise, land subsidence and infrastructure development. Wetland loss occurs mainly along the tidal zone, which varies in width and morphology along the Louisiana shoreline. In this study, we use Interferometric Synthetic Aperture Radar (InSAR) observations to detect the extent of the tidal inundation zone and evaluate the interaction between tidal currents and coastal wetlands. Our data consist of ALOS and Radarsat-1 observations acquired between 2006–2011 and 2003–2008, respectively. Interferometric processing of the data provides detailed maps of water level changes in the tidal zone, which are validated using sea level data from a tide gauge station. Our results indicate vertical tidal changes up to 30 cm and horizontal tidal flow limited to 5–15 km from open waters. The results also show that the tidal inundation is disrupted by various man-made structures, such as canals and roads, which change the natural tidal flow interaction with the coast.

Distribution and recovery trajectory of Macondo (Mississippi Canyon 252) oil in Louisiana coastal wetlands

We measured the concentration of petroleum hydrocarbons in 405 wetland sediment samples immediately before the April 2010 Deepwater Horizon disaster led to their broad-scale oiling, and on nine trips afterwards. The average concentrations of alkanes and PAHs were 604 and 186 times the pre-spill baseline values, respectively. Oil was distributed with some attenuation up to 100m inland from the shoreline for alkanes, but increased for aromatics, and was not well-circumscribed by the rapid shoreline assessments (a.k.a. SCAT) of relative oiling. The concentrations of target alkanes and PAHs in June 2013 were about 1% and 5%, respectively, of the February 2011 concentrations, but remained at 3.7 and 33 times higher, respectively, than in May 2010. A recovery to baseline conditions suggests that the concentration of alkanes may be near baseline values by the end of 2015, but that it may take decades for the PAH concentrations to be that low.

Contribution of tropical cyclones to the sediment budget for coastal wetlands in Louisiana, USA

The storm surge from a single hurricane can deposit tens of millions of tons of sediment on coastal wetlands within 100 km of landfall, but the distribution and cumulative amount from hurricanes at a centurial timescale is unknown. Here we use a model calibrated by three storms to estimate the average deposition on the deteriorating Louisiana coast from 1851 to 2008. The total deposition on Louisiana coastal wetlands, exclusive of open water, averages 5.6 million tons of inorganic sediment per year, equivalent to 3.8 % of the modern annual Mississippi River sediment load. Seventy nine percent of this sediment is deposited in a 20 km strip along the Gulf of Mexico (7,400 km2 wetlands) comprised primarily of salt marshes, and this distribution matches spatial and temporal patterns described in modern surficial deposits and sediment cores. We estimate that surge-induced deposition of sediment is attributable to at least 65 % of the inorganic content of the top 24 cm of soils in abandoned delta lobes, and 80 % in the chenier plain. While the most sedimentation from a given event results from the most intense storms, 78 % of the long-term hurricane sedimentation results from moderate storms (930–990 mb) that comprise 51 % of tropical cyclone events. Furthermore, we estimate that the 47 % of storms that make landfall with an internal barometric pressure above 990 mb account for only 7 % of the tropical cyclone sedimentation on wetlands.

Development of a Reproducible Method for Determining Quantity of Water and its Configuration in a Marsh Landscape

ABSTRACT Suir, G.M.; Evers, D.E.; Steyer, G.D., and Sasser C.E., 2013. Development of a reproducible method for determning quantity of water and its configuration in a marsh landscape. In: Brock, J.C.; Barras, J.A., and Williams, S.J. (eds.), Understanding and Predicting Change in the Coastal Ecosystems of the Northern Gulf of Mexico, Journal of Coastal Research, Special Issue No. 63, pp. 110–117, Coconut Creek (Florida), ISSN 0749-0208. Coastal Louisiana is a dynamic and ever-changing landscape. From 1956 to 2010, over 3,734 km2 of Louisiana's coastal wetlands have been lost due to a combination of natural and human-induced activities. The resulting landscape constitutes a mosaic of conditions from highly deteriorated to relatively stable with intact landmasses. Understanding how and why coastal landscapes change over time is critical to restoration and rehabilitation efforts. Historically, changes in marsh pattern (i.e., size and spatial distribution of marsh landmasses and water bodies) have been disti...

Monitoring Vegetation Response to Episodic Disturbance Events by using Multitemporal Vegetation Indices

Steyer, G.D.; Couvillion, B.R., and Barras, J.A., 2013. Monitoring vegetation response to episodic disturbance events by using multitemporal vegetation indices In: Brock, J.C.; Barras, J.A., and Williams, S.J. (eds.), Understanding and Predicting Change in the Coastal Ecosystems of the Northern Gulf of Mexico, Journal of Coastal Research, Special Issue No. 63, pp. 118–130, Coconut Creek (Florida), ISSN 0749-0208.Normalized Difference Vegetation Index (NDVI) derived from MODerate-resolution Imaging Spectroradiometer (MODIS) satellite imagery and land/water assessments from Landsat Thematic Mapper (TM) imagery were used to quantify the extent and severity of damage and subsequent recovery after Hurricanes Katrina and Rita of 2005 within the vegetation communities of Louisiana's coastal wetlands. Field data on species composition and total live cover were collected from 232 unique plots during multiple time periods to corroborate changes in NDVI values over time. Aprehurricane 5-year baseline time series clearly identified NDVI values by habitat type, suggesting the sensitivity of NDVI to assess and monitor phenological changes in coastal wetland habitats. Monthly data from March 2005 to November 2006 were compared to the baseline average to create a departure from average statistic. Departures suggest that over 33% (4,714 km2) of the prestorm, coastal wetlands experienced a substantial decline in the density and vigor of vegetation by October 2005 (poststorm), mostly in the east and west regions, where landfalls of Hurricanes Katrina and Rita occurred. The percentage of area of persistent vegetation damage due to long-lasting formation of new open water was 91.8% in the east and 81.0% and 29.0% in the central and west regions, respectively. Although below average NDVI values were observed in most marsh communities through November 2006, recovery of vegetation was evident. Results indicated that impacts and recovery from large episodic disturbance events that influence multiple habitat types can be accurately determined using NDVI, especially when integrated with assessments of physical landscape changes and field verifications.

Freshwater river diversions for marsh restoration in Louisiana: Twenty-six years of changing vegetative cover and marsh area

[1] The restoration of Louisiana's coastal wetlands will be one of the largest, most costly and longest environmental remediation projects undertaken. We use Landsat data to show that freshwater diversions, a major restoration strategy, have not increased vegetation and marsh coverage in three freshwater diversions operating for ∼19 years. Two analytic methods indicate no significant changes in either relative vegetation or overall marsh area from 1984 to 2005 in zones closest to diversion inlets. After Hurricanes Katrina and Rita, these zones sustained dramatic and enduring losses in vegetation and overall marsh area, whereas the changes in similar marshes of the adjacent reference sites were relatively moderate and short-lived. We suggest that this vulnerability to storm damage reflects the introduction of nutrients in the freshwater diversions (that add insignificant amounts of additional sediments), which promotes poor rhizome and root growth in marshes where below-ground biomass historically played the dominant role in vertical accretion.

Denitrification Enzyme Activity as an Indicator of Nitrate Movement through a Diversion Wetland

The Davis Pond freshwater diversion is intended to help restore Louisiana's coastal wetlands by reintroducing Mississippi River water to Barataria Basin. We hypothesized that the high NO3− concentration (2.0 mg NO3–N L−1) of the Mississippi River water would control the rate of denitrification in the receiving marsh given that the soils are saturated, anaerobic, and contain high C. Therefore, areas of high denitrification enzyme activity (DEA) in the marsh would represent soils exposed to river NO3− and actively involved in denitrification. Data from 88 soil samples (0–10 cm) collected throughout the marsh revealed significantly higher rates of DEA in a 715‐ha area adjacent to the diversion inflow. This area of generally high DEA contained >80% of all DEA observed while representing only 19% of the total marsh area at the low discharge rate of 39.5 m3 s−1 The area of high DEA coincided with the highest surface water NO3− and indicated that the marsh has a greater aerial capacity for NO3− removal than is utilized. A laboratory experiment suggested that soils loaded with external NO3− typically had higher DEA rates than soils receiving no added NO3− The DEA was strongly dependent on soil depth (92% of DEA occurred at 0–5 cm) and internal N cycling was substantial in this wetland soils. This study demonstrates the applicability of using soil DEA to map where denitrification activity is greatest, the aerial extent of soils involved in denitrification, and the general flow path of introduced nutrients in large wetlands where NO3− is the limiting factor for denitrification.

Denitrification potential and its relation to organic carbon quality in three coastal wetland soils

Capacity of a wetland to remove nitrate through denitrification is controlled by its physico-chemical and biological characteristics. Understanding these characteristics will help better to guide beneficial use of wetlands in processing nitrate. This study was conducted to determine the relationship between soil organic carbon (SOC) quality and denitrification rate in Louisiana coastal wetlands. Composite soil samples of different depths were collected from three different wetlands along a salinity gradient, namely, bottomland forest swamp (FS), freshwater marsh (FM), and saline marsh (SM) located in the Barataria Basin estuary. Potential denitrification rate (PDR) was measured by acetylene inhibition method and distribution of carbon (C) moieties in organic C was determined by 13C solid-state NMR. Of the three wetlands, the FM soil profile exhibited the highest PDR on both unit weight and unit volume basis as compared to FS and SM. The FM also tended to yield higher amount of N 2O as compared to the FS and SM especially at earlier stages of denitrification, suggesting incomplete reduction of NO 3 − at FM and potential for emission of N 2O. Saline marsh soil profile had the lowest PDR on the unit volume basis. Increasing incubation concentration from 2 to 10 mg NO 3 −-N L − 1 increased PDR by 2 to 6 fold with the highest increase in the top horizons of FS and SM soils. Regression analysis showed that across these three wetland systems, organic C has significant effect in regulating PDR. Of the compositional C moieties, polysaccharides positively influenced denitrification rate whereas phenolics (likely phenolic adehydes and ketonics) negatively affected denitrification rate in these wetland soils. These results could have significant implication in integrated assessment and management of wetlands for treating nutrient-rich biosolids and wastewaters, non-point source agricultural runoff, and nitrate found in the diverted Mississippi River water used for coastal restoration.

Microbial food web contributions to bottom water hypoxia in the northern Gulf of Mexico

Nutrients from the Mississippi/Atchafalaya Rivers greatly stimulate biological production in the ‘classical’ food web on the inner shelf of the northern Gulf of Mexico. Portions of this production, especially large diatoms and zooplankton fecal pellets, sink and decompose in the bottom water, consuming oxygen and contributing to the annual development of an extensive zone of bottom water hypoxia, typically >15,000 km 2 since 1993. The microbial food web is also active in the Mississippi River plume, but consists of small organisms that sink slowly. This ‘recycling’ food web has not been considered as a significant contributor to vertical flux and hypoxia. However, gelatinous zooplankton, especially pelagic appendicularians such as Oikopleura dioica, mediate the conversion of microbial web organisms to organic particles with high sinking rates. When pelagic appendicularians are abundant in coastal regions of the northern Gulf of Mexico, they stimulate the rapid vertical transfer of microbial web productivity in the surface layer, which is only 5–15 m thick in the coastal hypoxic region, to the sub-pycnocline layer that becomes hypoxic each summer. In this paper we present results from two studies examining the significance of this pathway. In both 2002 and 2004, we observed high production rates of appendicularians in coastal waters. Discarded gelatinous houses and fecal pellets from the appendicularian populations often provided more than 1 g m −2 d −1 of organic carbon for the establishment and maintenance of hypoxia in the northern Gulf of Mexico. This source of organic matter flux is especially important in regions far from the river plumes and during periods of low river discharge. Autotrophic elements of this food web are primarily supported by recycled inorganic nutrients originating in the Mississippi and Atchafalaya Rivers. Sources of dissolved organic matter (DOM) supporting the heterotrophic components of this microbial food web may include in situ production, the Mississippi/Atchafalaya Rivers, and Louisiana's coastal wetlands. If significant, the latter source provides a possible link between Louisiana's high rates of coastal land loss and the large hypoxic zone observed along the coast during summer. Both of the latter DOM sources are independent of phytoplankton production stimulated by inputs of riverine inorganic nutrients.

Export of Dissolved Organic Carbon from a Ponded Freshwater Marsh Receiving Diverted Mississippi River Water

A series of diversion projects has been implemented to reintroduce Mississippi River water into Louisiana's coastal wetlands in order to reduce wetland loss. The export of dissolved organic carbon (DOC) was measured in a 3,700-ha ponded freshwater marsh that receives diverted Mississippi River water. Results show that highly organic marsh soil and plant material are a source of DOC. DOC, on average, was 3 mg/l greater in outlet water as compared to the concentration in river water entering the wetland. DOC in water leaving the marsh was higher in summer months, with a concentration up to 18 mg/l. Based on a discharge of 1,000 ft/sec (28.3 m/sec), it was estimated that the equivalent of 7,335 kg/day of DOC would be exported from the marsh into Lake Cataouatche, located in the northern portion of the Louisiana Barataria Basin estuary. Results suggest that river diversion would likely increase the export of DOC from the marsh as compared to normal transport associated with rainfall and tidal exchange.

Louisiana Coastal Wetlands and Louisiana Coastal Grey Literature: Vanishing Treasures

Over the last century Louisiana has lost an alarming amount of coastal wetlands to coastal erosion. Natural disasters and manmade solutions to problems alike have contributed to this national tragedy. A vast amount of grey literature documenting the history of land loss in Louisiana has been produced, but never collocated for researchers’ use. The Louisiana Coastal Grey Literature Project endeavored to locate, organize, and provide access to these valuable hidden treasures.

A screening of the capacity of Louisiana freshwater wetlands to process nitrate in diverted Mississippi River water

Mississippi River water is currently being diverted into Louisiana coastal wetlands for slowing or reversing marsh deterioration attributed to the rapid subsidence and accompanying saltwater intrusion. In this study, nitrate processing and removal was quantified in a 3700 ha ponded freshwater wetland through which the diverted Mississippi River water enters Louisiana Barataria Basin estuary. Nitrate removal rates using a mass balance approach (measuring changes in nitrate content between the inlet and the outlets) showed that the fresh water wetland removed practically all the nitrate in the diverted river water (≈35 m 3 s −1) during a discharge event in April 2003. Denitrification was a major process in removing nitrate in diverted river water. The capacity of the ponded wetland to remove all nitrate in diverted river water was strongly influenced by discharge rate. The ponded wetland did not remove all the NO 3 in river water at high discharge rate (>100 m 3 s −1) in December 2003. Results demonstrate that freshwater diversion through Louisiana coastal wetland can effectively process nitrogen in diverted Mississippi River water. However, discharge or pulsing rate should be regulated where possible to maximize nitrogen removal for limiting amount of nitrogen reaching lower reaches of Louisiana's Barataria Basin estuary.