Louisiana Marshes Research Articles

Feedback of coastal marshes to climate change: Long-term phenological shifts.

Coastal marshes are important carbon sinks facing serious threats from climatic stressors. Current research reveals that the growth of individual marsh plants is susceptible to a changing climate, but the responses of different marsh systems at a landscape scale are less clear. Here, we document the multi‐decadal changes in the phenology and the area of the extensive coastal marshes in Louisiana, USA, a representative of coastal ecosystems around the world that currently experiencing sea‐level rise, temperature warming, and atmospheric CO 2 increase. The phenological records are constructed using the longest continuous satellite‐based record of the Earth's ecosystems, the Landsat data, and an advanced modeling technique, the nonlinear mixed model. We find that the length of the growing seasons of the intermediate and brackish marshes increased concomitantly with the atmospheric CO 2 concentration over the last 30 years, and predict that such changes will continue and accelerate in the future. These phenological changes suggest a potential increase in CO 2 uptake and thus a negative feedback mechanism to climate change. The areas of the freshwater and intermediate marshes were stable over the period studied, but the areas of the brackish and saline marshes decreased substantially, suggesting ecosystem instability and carbon storage loss under the anticipated sea‐level rise. The marshes' phenological shifts portend their potentially critical role in climate mitigation, and the different responses among systems shed light on the underlying mechanisms of such changes.

Legacy effects of Hurricane Katrina influenced marsh shoreline erosion following the Deepwater Horizon oil spill

Disturbance interactions occur when one perturbation influences the severity and perhaps the baseline state of succeeding disturbances. Natural and anthropogenic disturbances are frequent in dynamic coastal ecosystems and can often be linked. We evaluated potential for disturbance interactions associated with the 2010 Deepwater Horizon (DWH) oil spill, which was preceded by disturbance from Hurricane Katrina in 2005, by quantifying marsh shoreline retreat across both events. Our goal was to determine the degree to which Hurricane Katrina altered baseline rates of erosion prior to the DWH spill. We quantified erosion rate and fetch from aerial images of northern Barataria Bay, Louisiana marsh shorelines classified as reference, moderately-oiled, and heavily-oiled over three pre-spill time periods (1998–2004, prior to Hurricane Katrina; 2004–2005, during Katrina; 2005–2010, post-Katrina but pre-oil spill) and a post-spill period from 2010 to 2013. Prior to Hurricane Katrina, marsh shoreline erosion rates were low (from 0.38 to 1.10 m yr−1). In contrast during Hurricane Katrina (2004–2005), erosion increased by 661% and 756%, respectively, for shorelines that would subsequently become moderately and heavily-oiled; reference shoreline erosion increased by 59%. These high erosion rates were associated with increased fetch and higher wave action due to loss of protective geomorphic features such as small islands and spits and persisted during the post-Katrina/pre-spill period of 2005–2010 (0.62, 1.38, and 2.07 m yr−1 for reference, moderately, and heavily-oiled shorelines, respectively). Erosion rates increased modestly after the DWH event (reference = 1.13 m yr−1, moderate oiling = 1.45 m yr−1; heavy oiling = 2.77 m yr−1), but not significantly, compared to the post-Katrina period. Consequently, we could not detect a post-spill increase in marsh shoreline erosion. Rather, we concluded that Hurricane Katrina reset the erosion baseline, thereby connecting the two disturbances, and was the major driver of marsh shoreline erosion at our research sites during the study period.

Restoration Affects Sexual Reproductive Capacity in a Salt Marsh

Plant sexual reproduction is an important driver of plant community maintenance, dispersal, and recovery from disturbance. Despite this, sexual reproduction in habitats dominated by clonally spreading perennial species, such as salt marshes, is often ignored. Communities dominated by long-lived perennial species can still depend on sexual reproduction for recolonizing large disturbed patches or for establishing in new patches, such as restored sites. We investigated the influence of restoration and elevation on flowering phenology, potential seed and seedling production, and insect flower damage of the dominant salt marsh grass, Spartina alterniflora, in reference and restored marshes in southeastern Louisiana, USA. We additionally tested whether elevation gradients or soil parameters could explain differences in sexual reproduction between sites. We demonstrate that sediment-slurry amendment restoration may not affect flowering phenology or insect flower damage at ecologically relevant levels, but that restoration activity increases sexual reproductive output at the patch scale. Restoration activity affected reproductive dynamics more often than changes in elevation alone. Restoration of subsiding salt marsh habitat by altering the soil environment may increase sexual reproductive capacity of these wetlands.

Foraging Movements of Black Skimmers (Rynchops niger) Nesting on the Isles Dernieres Barrier Island Refuge, Louisiana, USA

Several populations of Black Skimmers (Rynchops niger), colonial waterbirds with a tactile foraging strategy, are declining. Improving our limited knowledge of Black Skimmer foraging ecology has become critical to inform conservation decisions. Black Skimmers were GPS-tracked from the Isles Dernieres Barrier Island Refuge in coastal Louisiana, USA, from 27 June-1 July 2013 and 30 May-2 June 2016, to shed light on their foraging movements during the breeding season. Black Skimmers foraged almost exclusively at night, as far as 16 km from the colony in the coastal marshes of Louisiana, in a home range of 86-256 km2. Additional movement data are needed to identify areas of conservation need and provide a basis for future studies of skimmer response to environmental changes.

An investigation of environmental concentrations of atrazine, chlorothalonil, and fipronil in a Mississippi River-influenced marsh in southern Louisiana

The pesticides atrazine, chlorothalonil, and fipronil are common surface water contaminants due to their high rates of application and their chemical properties. This study investigates the introduction of contaminants into coastal wetlands via the Bayou Lamoque Mississippi River control structure in Plaquemines Parish, Louisiana; results serve as a model for future studies regarding contaminant introduction from freshwater diversions aimed at Gulf Coast restoration. A protocol for environmental sampling using Continuous Low-level Aquatic Monitoring (C.L.A.M.) C-18 disks is described, along with effective solid phase extraction (SPE) and gas chromatography mass spectrometry (GC/MS) methodology for the determination of atrazine, chlorothalonil, and fipronil in surface water. Average concentrations of atrazine and chlorothalonil inside the Bayou Lamoque freshwater diversion structure for the months of June – October 2016, were 33.3 ng l−1 and 3.5 ng l−1, respectively. Fipronil was not detected in concentrations exceeding its method detection limit of 1.26 ng l−1. The detected concentrations are innocuous to non-target organisms based on current ecotoxicity data, and are likely a result of runoff from nearby upstream agricultural and residential land.

Gulf of Mexico estuarine blue carbon stock, extent and flux: Mangroves, marshes, and seagrasses: A North American hotspot

The Gulf of Mexico blue carbon habitats (mangroves, seagrass, and salt marshes) form an important North American blue carbon hot spot. These habitats cover 2,161,446 ha and grow profusely in estuaries that occupy 38,000 km2 to store substantial sedimentary organic carbon of 480.48 Tg C. New investigations around GoM for Mexican mangroves, Louisiana salt marshes and seagrasses motivated our integration of buried organic carbon to elucidate a new estimate of GoM blue carbon stocks. Factors creating this include: large GoM watersheds enriching carbon slowly flowing through shallow estuarine habitats with long residence times; fewer SE Mexican hurricanes allowing enhanced carbon storage; mangrove carbon productivity enhanced by warm southern basin winter temperatures; large Preservation reserves amongst high anthropogenic development. The dominant total GoM mangrove blue carbon stock 196.88 Tg from total mangrove extent 650,482 ha is highlighted from new Mexican data. Mexican mangrove organic carbon stock is 112.74 Tg (1st sediment meter) plus USA 84.14 Tg. Mexican mangroves vary greatly in storage, total carbon depositional depths and in sediment age (to 3500 y). We report Mexican mangrove's conservative storage fraction for the normally-compared top meter, whereas the full storage depth estimates ranging above 366.78 Tg (high productivity in very deep sediment along the central Veracruz/Tabasco coast) are not reflected in our reported estimates. Seagrasses stock of 184.1 Tg C organic is derived from 972,327 ha areal extent (in 1st meter). The Louisiana marshes form the heart of GoM salt marsh carbon storage 99.5 Tg (in 1st meter), followed by lesser stocks in Florida, Texas, finally Mexico derived from salt marsh extent totaling 650,482 ha. Constraints on the partial estuarine fluxes given for this new data are discussed as well as widespread anthropogenic destruction of the GoM blue carbon. A new North American comparison of our GoM blue carbon stocks versus Atlantic coastal blue carbon stock estimates is presented.

Recovery of horse fly populations in Louisiana marshes following the Deepwater Horizon oil spill

The Deepwater Horizon oil spill in April 2010 had unprecedented impact on the Gulf of Mexico. We established the greenhead horse fly (Tabanus nigrovittatus Macquart) as a bioindicator of marsh health. This species is bound to coastal marshes, since its larvae develop as top invertebrate predators in the marsh soil. Immediately after the oil spill (2010–2011), populations of this horse fly declined in oiled areas of Louisiana marshes with significant impacts on genetic structure. In this follow-up study five years after the catastrophic event (2015–2016), we now report signs of recovery of populations in formerly oiled areas. Fly numbers increased compared to previous counts. Previously detected genetic bottlenecks in oiled populations have disappeared. Migration into oiled areas began to replenish formerly depleted horse fly populations in impacted regions with populations from non-oiled areas as an important source of migrants. Parameters of family structure that had been impacted by the oil spill (number of breeding parents, effective population size, number of family clusters) rebounded to levels similar to or exceeding those in non-oiled control areas.

Persisting responses of salt marsh fungal communities to the Deepwater Horizon oil spill

The plant microbiome, composed of diverse interacting microorganisms, is thought to undergird host integrity and well-being. Though it is well understood that environmental perturbations like oil pollution can alter the diversity and composition of microbiomes, remarkably little is known about how disturbance alters plant-fungal associations. Using Next-Generation sequencing of the 18S rDNA internal transcribed spacer (ITS1) region, we examined outcomes of enduring oil exposure on aboveground leaf and belowground endophytic root and rhizosphere fungal communities of Spartina alterniflora, a highly valued ecosystem engineer in southeastern Louisiana marshes affected by the 2010 Deepwater Horizon accident. We found that aboveground foliar fungal communities exhibited site-dependent compositional turnover with consequent loss in diversity according to oiling history. Rhizosphere soil communities also exhibited shifts in community composition associated with oiling history, whereas root endophytic communities did not. Oiling did not increase or decrease similarities among aboveground and belowground communities within an individual host, indicating that host plant characteristics exert stronger control than external factors on fungal community composition. These results show that fungal community responses to oiling vary within tissues of the same host plant, and that differences in the local environment, or alternatively, site-specific differences in residual oil constrain the magnitude of exposure responses. Our study offers novel perspectives on how environmental contaminants and perturbations can influence plant microbiomes, highlighting the importance of assessing long-term ecological outcomes of oil pollution to better understand how shifts in microbial communities influence plant performance and ecosystem function. Our findings are relevant to coastal management programs tasked with responding to oil spills and increasing pressures arising from intensifying development and climate change. Understanding how modification of plant-microbiome associations influences plant performance, particularly of ecosystem engineers like S. alterniflora, can help guide efforts to protect and restore at-risk coastal ecosystems.

Flooding Alters Plant‐Mediated Carbon Cycling Independently of Elevated Atmospheric CO2 Concentrations

AbstractPlant‐mediated processes determine carbon (C) cycling and storage in many ecosystems; how plant‐associated processes may be altered by climate‐induced changes in environmental drivers is therefore an essential question for understanding global C cycling. In this study, we hypothesize that environmental alterations associated with near‐term climate change can exert strong control on plant‐associated ecosystem C cycling and that investigations along an extended hydrologic gradient may give mechanistic insight into C cycling. We utilize a mesocosm approach to investigate the response of plant, soil, and gaseous C cycling to changing hydrologic regimes and elevated atmospheric carbon dioxide (CO2) concentrations expected by 2100 in a coastal salt marsh in Louisiana, USA. Although elevated CO2 had no significant effects on C cycling, we demonstrate that greater average flooding depth stimulated C exchange, with higher rates of labile C decomposition, plant CO2 assimilation, and soil C respiration. Greater average flooding depth also significantly decreased the soil C pool and marginally increased the aboveground biomass C pool, leading to net losses in total C stocks. Further, flooding depths along an extended hydrologic gradient garnered insight into decomposition mechanisms that was not apparent from other data. In C‐4 dominated salt marshes, sea level rise will likely overwhelm effects of elevated CO2 with climate change. Deeper flooding associated with sea level rise may decrease long‐term soil C pools and quicken C exchange between soil and atmosphere, thereby threatening net C storage in salt marsh habitats. Manipulative studies will be indispensable for understanding biogeochemical cycling under future conditions.

Assessing biomass of diverse coastal marsh ecosystems using statistical and machine learning models

The importance and vulnerability of coastal marshes necessitate effective ways to closely monitor them. Optical remote sensing is a powerful tool for this task, yet its application to diverse coastal marsh ecosystems consisting of different marsh types is limited. This study samples spectral and biophysical data from freshwater, intermediate, brackish, and saline marshes in Louisiana, and develops statistical and machine learning models to assess the marshes’ biomass with combined ground, airborne, and spaceborne remote sensing data. It is found that linear models derived from NDVI and EVI are most favorable for assessing Leaf Area Index (LAI) using multispectral data (R2 = 0.7 and 0.67, respectively), and the random forest models are most useful in retrieving LAI and Aboveground Green Biomass (AGB) using hyperspectral data (R2 = 0.91 and 0.84, respectively). It is also found that marsh type and plant species significantly impact the linear model development (P < .05 in both cases). Sensors with coarser spatial resolution yield lower LAI values because the fine water networks are not detected and mixed into the vegetation pixels. The Landsat OLI-derived map shows the LAI of coastal mashes in Louisiana mostly ranges from 0 to 5.0, and is highest for freshwater marshes and for marshes in the Atchafalaya Bay delta. The CASI-derived maps show that LAI of saline marshes at Bay Batiste typically ranges from 0.9 to 1.5, and the AGB is mostly less than 900 g/m2. This study provides solutions for assessing the biomass of Louisiana’s coastal marshes using various optical remote sensing techniques, and highlights the impacts of the marshes’ species composition on the model development and the sensors’ spatial resolution on biomass mapping, thereby providing useful tools for monitoring the biomass of coastal marshes in Louisiana and diverse coastal marsh ecosystems elsewhere.

Importance of Site History and Environmental Setting on Soil Properties in Restored Louisiana Back-Barrier Island Salt Marshes

ABSTRACT Feher, L.C.; Willis, J.M., and Hester, M.W., 2018. Importance of site history and environmental setting on soil properties in restored Louisiana back-barrier island salt marshes. Although previous studies have identified significant differences between the soils of salt marshes restored with dredged material and natural marshes, it is unclear how these differences might affect plant community composition, soil development, and the overall stability of the marsh through time. Using soil shear strength as a metric of soil integrity and resistance to erosion, this study utilized field surveys of one natural and two restored back-barrier island salt marshes in coastal Louisiana to examine differences in shear strength and other soil properties in relation to site history and environmental setting. Soil properties at both restored sites were largely dependent on the source of the fill material in addition to site-specific restoration approaches. Soils at the younger restored marsh were significantly m...

Effects of isolation on ant assemblages depend on microhabitat

AbstractHow isolation affects biological communities is a fundamental question in ecology and conservation biology. Local diversity (α) and regional diversity (γ) are consistently lower in insular areas. The pattern of species turnover (β diversity) and the influence of isolation on competitive interactions are less predictable. Differences in communities across microhabitats within an isolated patch could contribute to the variability in patterns related to isolation. Trees form characteristically dense and sparse patches (low vs. high isolation) in floating marshes in coastal Louisiana, and canopy and root areas around these trees could support distinct ant communities. Consequently, trees in floating marshes provide an ideal environment to study the effects of isolation on community assemblages in different microhabitats. We sampled ant communities in 120 trees during the summer of 2016. We found ant α diversity was not different between the canopy and roots, and the magnitude and directional effects of isolation on ants were inconsistent between the canopy and root areas. In the roots of sparse sites, ant diversity (α, β, and γ) was lower, species composition was changed, and the signature of interspecific competition was more prominent compared to dense sites. In the canopy, however, significant differences between dense and sparse sites were only detected in α and γ diversity, and ant species co‐occurrence was not significantly different from a random distribution. The inconsistent responses of ants in canopy and root areas to isolation may be due to the differences of species pool size, environmental harshness, and species interactions between strata. In addition, these findings indicate that communities in distinct microenvironments can respond differentially to habitat isolation. We suggest incorporating organisms from different microhabitats into future research to better understand the influence of isolation on the assembly of biological communities.

Assessing methods for estimating roughness coefficient in a vegetated marsh area using Delft3D

A Delft3D-FLOW model was used to simulate tidal flow in Davis pond marsh in Louisiana, USA. The study area is a freshwater marsh consisting of one main channel and floodplain. Vegetation-induced flow resistance greatly influences tidal flow dynamics in the marsh. This study evaluated eight approaches to estimate vegetation roughness, including two constant Manning's n values, four empirical relations for calculating n, and two methods for calculating Chezy's C values originally embedded in the Delft3D model. Simulated results of water surface elevation (WSE) were compared with the corresponding field observation at eleven stream gauges in the study area. We concluded that the roughness coefficient for vegetated area varies with time as flow depth changes. Among the selected empirical relations for the vegetation roughness, the ones accounting for the effect of the vegetation frontal area and the degree of submergence have closely matched the measurements.

Microbial processes linked to soil organic matter in a restored and natural coastal wetland in Barataria Bay, Louisiana

Louisiana contains 40% of the lower 48 United States coastal and estuarine wetlands and also has the highest rates of wetland loss,∼80% of the nation’s total. Approximately, 4900km2 of coastal land in Louisiana has been lost since 1930. Our study location, Barataria Bay, has one of the highest coastal wetland erosion rates in the nation, at ∼41km2y−1 and therefore is a high priority for wetland restoration activities. Sandy sediment dredged from the bed of the Mississippi River was placed at targeted locations in Barataria Bay in an effort to restore and protect coastal Louisiana marshes. This study sought to determine how comparable these dredged river-sediment marsh soils are to those of the surrounding natural marshes in Barataria Bay. Percent organic matter in the created marsh was 11.6% of the natural marsh. Extractable ammonium and extractable nitrate in the created marsh soils were 15.4% and 8.75% of the natural marsh. In the created marsh, the β-glucosidase enzyme activity was 10.3% and soil oxygen demand was 11.2% of the natural marsh, reflecting significantly lower soil microbial processes within the created marsh sites. Associated ecosystem services including carbon sequestration and nutrient availability for support of primary production, are consequently significantly lower in the newly created wetland soil. These data provide a baseline for monitoring and eventual determination of the trajectory for development of microbial-driven ecosystem services of created wetlands using dredged river sediment.

A Comparison of Two Insect Collection Techniques in Oiled and Non-Oiled Salt Marshes in Louisiana

Insects are important components in coastal marsh ecosystems and can be used as an indicator of marsh health. Collections of insects in coastal marshes are usually made using either sweep nets or insect vacuums. Differences in these 2 methods have not been compared within the marsh ecosystem. Therefore, we compared collections made using these methods in oiled and non-oiled marshes along multiple transects on the Louisiana coast. We found that sweep net collections accounted for 5.9 times more individual arthropods and more total insect taxa when compared with the insect vacuum collections. Oiled marshes had greater total insect richness but similar abundance and average richness as non-oiled sites; however, we found some changes in insect community composition between sites. These results indicate that sweep net collections are an effective means to sample insects in marsh environments and that oil intrusion can lead to changes in arthropod community structure in coastal marshes.

Drought‐associated phenological changes of coastal marshes in Louisiana

AbstractDrought poses serious threats to the valuable coastal marsh ecosystems, especially considering that the frequency, intensity, and acuteness of drought may increase with prospective climate change. We study the drought‐associated phenological changes of Louisiana coastal marshes through combining remote sensing‐derived phenological record (i.e., Landsat‐derived Normalized Difference Vegetation Index, or NDVI) and field‐based environmental data during the past 30 yr. We find that drought condition in southeast Louisiana is characterized by decreased discharge of the Mississippi River and reduced precipitation, which correlate with the continental weather patterns over the conterminous United States influenced by La Niña. The peak NDVI day of saline marshes delays for two months (P &lt; 0.01), from mid‐July to mid‐September, and their growth duration consequently shortens for two months (P &lt; 0.01) in drought years. Such phenological changes might result from the inhibition of the growth of Spartina alterniflora, a dominant species in saline marshes whose biomass peaks earlier than the coexisting species, perhaps due to reduced freshwater input and increased water deficit. Drought might also cause Spartina dieback via similar mechanisms. Our results underline the correlation between drought in southeast Louisiana and La Niña and highlight the sensitivity of saline marshes to drought, thereby providing valuable information for coastal marsh management and conservation facing climate change.

Effect of Habitat Complexity on Predator—Prey Relationships: Implications for Black Mangrove Range Expansion into Northern Gulf of Mexico Salt Marshes

ABSTRACT The tropically associated black mangrove (Avicennia germinans) is experiencing a climate-induced range shift and expanding into salt marshes of northern Florida, southern Louisiana and most recently, Horn Island, MS. To date, little is known about how black mangroves function as nursery habitat for important fishery species such as shrimps or how their increase may affect survival of such species. The main objective of our study was to determine habitat preference and survival rates of common, economically important penaeid shrimps in the presence and absence of the increasingly abundant tropical predator, the gray snapper (Lutjanus griseus). We also examined the effects of habitat identity and structure on juvenile white shrimp (Litopenaeus setiferus) and brown shrimp (Farfantepenaeus aztecus) behavior, using preference experiments in indoor mesocosms both with and without the predatory gray snapper. Results showed that shrimp prefer Spartina over Avicennia with or without predation risk. Surviv...

Relationships Between Salinity and Short-Term Soil Carbon Accumulation Rates from Marsh Types Across a Landscape in the Mississippi River Delta

Salinity alterations will likely change the plant and environmental characteristics in coastal marshes thereby influencing soil carbon accumulation rates. Coastal Louisiana marshes have been historically classified as fresh, intermediate, brackish, or saline based on resident plant community and position along a salinity gradient. Short-term total carbon accumulation rates were assessed by collecting 10-cm deep soil cores at 24 sites located in marshes spanning the salinity gradient. Bulk density, total carbon content, and the short-term accretion rates obtained with feldspar horizon markers were measured to determine total carbon accumulation rates. Despite some significant differences in soil properties among marsh types, the mean total carbon accumulation rates among marsh types were not significantly different (mean ± std. err. of 190 ± 27 g TC m−2 year−1). However, regression analysis indicated that mean annual surface salinity had a significant negative relationship with total carbon accumulation rates. Based on both analyses, the coastal Louisiana total marsh area (1,433,700 ha) accumulates about 2.7 to 3.3 Tg C year−1. Changing salinities due to increasing relative sea level or resulting from restoration activities may alter carbon accumulation rates in the short term and significantly influence the global carbon cycle.

Is Exposure to Macondo Oil Reflected in the Otolith Chemistry of Marsh-Resident Fish?

Genomic and physiological responses in Gulf killifish (Fundulus grandis) in the northern Gulf of Mexico have confirmed oil exposure of resident marsh fish following the Macondo blowout in 2010. Using these same fish, we evaluated otolith microchemistry as a method for assessing oil exposure history. Laser-ablation inductively-coupled-plasma mass spectrometry was used to analyze the chemical composition of sagittal otoliths to assess whether a trace metal signature could be detected in the otoliths of F. grandis collected from a Macondo-oil impacted site in 2010, post-spill relative to pre-spill, as well as versus fish from areas not impacted by the spill. We found no evidence of increased concentrations of two elements associated with oil contamination (nickel and vanadium) in F. grandis otoliths regardless of Macondo oil exposure history. One potential explanation for this is that Macondo oil is relatively depleted of those metals compared to other crude oils globally. During and after the spill, however, elevated levels of barium, lead, and to a lesser degree, copper were detected in killifish otoliths at the oil-impacted collection site in coastal Louisiana. This may reflect oil contact or other environmental perturbations that occurred concomitant with oiling. For example, increases in barium in otoliths from oil-exposed fish followed (temporally) freshwater diversions in Louisiana in 2010. This implicates (but does not conclusively demonstrate) freshwater diversions from the Mississippi River (with previously recorded higher concentrations of lead and copper), designed to halt the ingress of oil, as a mechanism for elevated elemental uptake in otoliths of Louisiana marsh fishes. These results highlight the potentially complex and indirect effects of the Macondo oil spill and human responses to it on Gulf of Mexico ecosystems, and emphasize the need to consider the multiple stressors acting simultaneously on inshore fish communities.

Population Dynamics and Community Composition of Ammonia Oxidizers in Salt Marshes after the Deepwater Horizon Oil Spill.

The recent oil spill in the Gulf of Mexico had significant effects on microbial communities in the Gulf, but impacts on nitrifying communities in adjacent salt marshes have not been investigated. We studied persistent effects of oil on ammonia-oxidizing archaeal (AOA) and bacterial (AOB) communities and their relationship to nitrification rates and soil properties in Louisiana marshes impacted by the Deepwater Horizon oil spill. Soils were collected at oiled and unoiled sites from Louisiana coastal marshes in July 2012, 2 years after the spill, and analyzed for community differences based on ammonia monooxygenase genes (amoA). Terminal Restriction Fragment Polymorphism and DNA sequence analyses revealed significantly different AOA and AOB communities between the three regions, but few differences were found between oiled and unoiled sites. Community composition of nitrifiers was best explained by differences in soil moisture and nitrogen content. Despite the lack of significant oil effects on overall community composition, we identified differences in correlations of individual populations with potential nitrification rates between oiled and unoiled sites that help explain previously published correlation patterns. Our results suggest that exposure to oil, even 2 years post-spill, led to subtle changes in population dynamics. How, or if, these changes may impact ecosystem function in the marshes, however, remains uncertain.