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Publications Journal Articles Tidal wetland Gross Primary Production across the continental United States, 2000-2019
Abstract - We mapped tidal wetland gross primary production (GPP) with unprecedented detail for multiple wetland types across the continental United States (CONUS) at 16‐day intervals for the years 2000–2019. To accomplish this task, we developed the spatially explicit Blue Carbon (BC) model, which combined tidal wetland cover and field‐based eddy covariance tower data into a single Bayesian framework, and used a super computer network and remote sensing imagery (Moderate Resolution Imaging Spectroradiometer Enhanced Vegetation Index). We found a strong fit between the BC model and eddy covariance data from 10 different towers (r2 = 0.83, p < 0.001, root‐mean‐square error = 1.22 g C/m2/day, average error was 7% with a mean bias of nearly zero). When compared with NASA's MOD17 GPP product, which uses a generalized terrestrial algorithm, the BC model reduced error by approximately half (MOD17 had r2 = 0.45, p < 0.001, root‐mean‐square error of 3.38 g C/m2/day, average error of 15%). The BC model also included mixed pixels in areas not covered by MOD17, which comprised approximately 16.8% of CONUS tidal wetland GPP. Results showed that across CONUS between 2000 and 2019, the average daily GPP per m2 was 4.32 ± 2.45 g C/m2/day. The total annual GPP for the CONUS was 39.65 ± 0.89 Tg C/year. GPP for the Gulf Coast was nearly double that of the Atlantic and Pacific Coasts combined. Louisiana alone accounted for 15.78 ± 0.75 Tg C/year, with its Atchafalaya/Vermillion Bay basin at 4.72 ± 0.14 Tg C/year. The BC model provides a robust platform for integrating data from disparate sources and exploring regional trends in GPP across tidal wetlands.
(contributed by R. A. Feagin, 2020)
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    Effects of ten years of nitrogen and phosphorus fertilization on carbon and nutrient cycling in a tidal freshwater marsh
Abstract - Tidal freshwater marshes can protect downstream ecosystems from eutrophication by intercepting excess nutrient loads, but recent studies in salt marshes suggest nutrient loading compromises their structural and functional integrity. Here, we present data on changes in plant biomass, microbial biomass and activity, and soil chemistry from plots in a tidal freshwater marsh on the Altamaha River (GA) fertilized for 10 yr with nitrogen (+N), phosphorus (+P), or nitrogen and phosphorus (+NP). Nitrogen alone doubled aboveground biomass and enhanced microbial activity, specifically rates of potential nitrification, denitrification, and methane production measured in laboratory incubations. Phosphorus alone increased soil P and doubled microbial biomass but did not affect microbial processes. Nitrogen or P alone decreased belowground biomass and soil carbon (C) whereas +NP increased aboveground biomass, microbial biomass and N cycling, and N, P, and C assimilation and burial more than either nutrient alone. Our findings suggest differential nutrient limitation of tidal freshwater macrophytes by N and microbes by P, similar to what has been observed in salt marshes. Macrophytes outcompete microbes for P in response to long‐term N and P additions, leading to increased soil C storage through increased inputs of belowground biomass relative to N and P added singly. The susceptibility of tidal freshwater marshes to long‐term nutrient enrichment and, hence their ability to mitigate eutrophication will depend on the quantity and relative proportion of N vs. P entering estuaries and tidal wetlands.
(contributed by Ellen Herbert, 2020)
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    Historical Changes in the Vegetated Area of Salt Marshes
Abstract - Salt marshes are valuable ecosystems, and there is concern that increases in the rate of sea level rise along with anthropogenic activities are leading to the loss of vegetated habitat. The area of vegetated marsh can change not only through advance and retreat of the open fetch edge, but also due to channel widening and contracting, formation and drainage of interior ponds, formation and revegetation of interior mud flats, and marsh migration onto upland areas, each of which is influenced by different processes. This study used historical aerial photographs to measure changes in the extent of vegetated marsh over approximately 70 years at study marshes located in three long-term ecological research (LTER) sites along the US East coast: Georgia Coastal Ecosystems (GCE), Virginia Coast Reserve (VCR), and Plum Island Ecosystems (PIE). Marsh features were categorized into vegetated marsh, ponds, interior mud flats, and channels for three time periods at each site. The three sites showed different patterns of change in vegetated marsh extent over time. At the GCE study site, losses in vegetated marsh, which were primarily due to channel widening, were largely offset by channel contraction in other areas, such that there was little to no net change over the study period. The study marsh at VCR experienced extensive vegetated marsh loss to interior mud flat expansion, which occurred largely in low-lying areas. However, this loss was counterbalanced by marsh gain due to migration onto the upland, resulting in a net increase in vegetated marsh area over time. Vegetated marsh at PIE decreased over time due to losses from ponding, channel widening, and erosion at the open fetch marsh edge. Digital elevation models revealed that the vegetated areas of the three marshes were positioned at differing elevations relative to the tidal frame, with PIE at the highest and VCR at the lowest elevation. Understanding the patterns of vegetation loss and gain at a given site provides insight into what factors are important in controlling marsh dynamics and serves as a guide to potential management actions for marsh protection.
(contributed by Christine Burns, 2020)
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    Ecosystem stability and Native American oyster harvesting along the Atlantic Coast of the United States
Abstract - The eastern oyster (Crassostrea virginica) is an important proxy for examining historical trajectories of coastal ecosystems. Measurement of ~40,000 oyster shells from archaeological sites along the Atlantic Coast of the United States provides a long-term record of oyster abundance and size. The data demonstrate increases in oyster size across time and a nonrandom pattern in their distributions across sites. We attribute this variation to processes related to Native American fishing rights and environmental variability. Mean oyster length is correlated with total oyster bed length within foraging radii (5 and 10 km) as mapped in 1889 and 1890. These data demonstrate the stability of oyster reefs despite different population densities and environmental shifts and have implications for oyster reef restoration in an age of global climate change.
(contributed by Victor D. Thompson, 2020)
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    Sea-level rise and the emergence of a keystone grazer alter the geomorphic evolution and ecology of southeast US salt marshes
Abstract - Keystone species have large ecological effects relative to their abundance and have been identified in many ecosystems. However, global change is pervasively altering environmental conditions, potentially elevating new species to keystone roles. Here, we reveal that a historically innocuous grazer—the marsh crab Sesarma reticulatum—is rapidly reshaping the geomorphic evolution and ecological organization of southeastern US salt marshes now burdened by rising sea levels. Our analyses indicate that sea-level rise in recent decades has widely outpaced marsh vertical accretion, increasing tidal submergence of marsh surfaces, particularly where creeks exhibit morphologies that are unable to efficiently drain adjacent marsh platforms. In these increasingly submerged areas, cordgrass decreases belowground root:rhizome ratios, causing substrate hardness to decrease to within the optimal range for Sesarma burrowing. Together, these bio-physical changes provoke Sesarma to aggregate in high-density grazing and burrowing fronts at the heads of tidal creeks (hereafter, creekheads). Aerial-image analyses reveal that resulting “Sesarma-grazed” creekheads increased in prevalence from 10 ± 2% to 29 ± 5% over the past <25 y and, by tripling creek-incision rates relative to nongrazed creekheads, have increased marsh-landscape drainage density by 8 to 35% across the region. Field experiments further demonstrate that Sesarma-grazed creekheads, through their removal of vegetation that otherwise obstructs predator access, enhance the vulnerability of macrobenthic invertebrates to predation and strongly reduce secondary production across adjacent marsh platforms. Thus, sea-level rise is creating conditions within which Sesarma functions as a keystone species that is driving dynamic, landscape-scale changes in salt-marsh geomorphic evolution, spatial organization, and species interactions.
(contributed by Sinead M. Crotty, 2020)
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    Does predator-driven, biotic resistance limit the northward spread of the non-native green porcelain crab, Petrolisthes armatus?
Abstract - Biotic resistance by native predators can limit the geographic range and abundance of nonnative species following introduction into an ecosystem. Here we tested the hypothesis that the strength ofpredation pressure varies with latitude and limits the abundance and northward expansion of the non-native green porcelain crab, Petrolisthes armatus, whose northern range is also hypothesized to be limited by physical tolerances to cold temperatures. We quantified the predation risk of P. armatus across 400 km of the crab’s invasive range along the coastline of the southeastern US. In addition, we measured the density of large P. armatus, habitat quality, and other environmental factors that may affect the crab’s predation risk. Finally, we conducted a size-selective predator exclusion experiment to determine the predator species and size classes that may be consuming P. armatus. Results indicated that neither the density of large P. armatus nor its predation risk varied systematically with latitude. Instead, variation in predation risk was best explained by local site-level differences in habitat quality, the density of large P. armatus, and the mean abundance of predators. The predator exclusion experiment indicated that both small and large size classes of predators are capable of equallystrong rates of predation on P. armatus. Together, our results suggest that although native predators readily consume P. armatus, they do not provide biotic resistance against its northward expansion. Instead, it seems likely that other latitudinally differential factors like low winter temperatures that decrease P. armatus survival are more influential in limiting the crab’s northern expansion.
(contributed by Kaitlin A. Kinney, 2019)
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    Elevation drives gradients in surface soil temperature within salt marshes
Abstract - Elevation differences in salt marshes result in numerous ecological consequences as a result of variation in tidal flooding. We demonstrate here that elevation differences are also negatively correlated with soil temperature on the marsh platform, irrespective of tidal flooding. Field observations of soil temperature at 10‐cm depth in a Georgia marsh showed that elevation increases of 0.5 m corresponded to decreases in average soil temperature of 0.9–1.7°C during both winter and summer. Landsat 8 estimates of land surface temperatures across the marsh in dry (nonflooded) scenes also showed that temperature decreased with increasing elevation, which was consistent with soil observations. Similar satellite results were also found in a test marsh in Virginia. Biological reactions are temperature‐dependent, and these findings indicate that metabolic processes will vary over short distances. This is important for accurately estimating marsh metabolism and predicting how changes in temperature will affect future productivity.
(contributed by Merryl Alber, 2019)
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    Global-change controls on soil-carbon accumulation and loss in coastal vegetated ecosystems
Abstract - Coastal seagrass, mangrove and salt-marsh ecosystems—also termed blue-carbon ecosystems—play an important role in theglobal carbon cycle. Much of the organic carbon they store rests in soils that have accumulated over thousands of years. Rapidly changing climate and environmental conditions, including sea-level rise, warming, eutrophication and landscape development, will impact decomposition and thus the global reservoir of blue soil organic carbon. Yet, it remains unclear how these disturbances will affect the key biogeochemical mechanisms controlling decomposition—mineral protection, redox zonation, water content and movement, and plant–microbe interactions. We assess the spatial and temporal scales over which decomposition mechanisms operate and how their effectiveness may change following disturbances. We suggest that better integration of decomposition mechanisms into blue-carbon models may improve predictions of soil organic carbon stores and facilitate incorporation of coastal vegetated ecosystems into global budgets and management tools.
(contributed by Amanda C. Spivak, 2019)
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    Chronic but not acute saltwater intrusion leads to large release of inorganic N in a tidal freshwater marsh
Abstract - Sea level rise is expected to increase inundation and saltwater intrusion into many tidal freshwater marshes and forests. Saltwater intrusion may be long-term, as with rising seas, or episodic, as with low river flow or storm surge. We applied continuous (press) and episodic (pulse) treatments of dilute seawater to replicate 2.5 × 2.5 m field plots for three years and measured soil attributes, including soil porewater, oxidation-reduction potential, soil carbon (C), and nitrogen (N) to investigate the effects of continuous and episodic saltwater intrusion and increased inundation on tidal freshwater marsh elemental cycling and soil processes. Continuous additions of dilute seawater resulted in increased porewater chloride, sulfate, sulfide, ammonium, and nitrate concentrations. Plots that received press additions also had lower soil oxidation-reduction potentials beginning in the second year. Episodic additions of dilute seawater during typical low flow conditions (Sept.-Oct.) resulted in transient increases in porewater chloride and sulfate that returned to baseline conditions once dosing ceased. Freshwater additions did not affect porewater inorganic N or soil C or N. Persistent saltwater intrusion in freshwater marshes alters the N cycle by releasing ammonium-N from sorption sites, increasing nitrification and severely reducing N storage in macrophyte biomass. Chronic saltwater intrusion, as is expected with rising seas, is likely to shift tidal freshwater marshes from a sink to a source of N whereas intermittent intrusion from drought may have no long term effect on N cycling.
(contributed by Sarah Widney, 2019)
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    Microspatial differences in soil temperature cause phenology change on par with long-term climate warming in salt marshes
Abstract - Phenology studies mostly focus on variation across time or landscapes. However, phenology can vary at fine spatial scales, and these differences may be as important as long-term change from climate warming. We used high-frequency ‘‘PhenoCam’’ data to examine phenology of Spartina alterniflora, afoundation species native to salt marshes on the US East and Gulf coasts, and a common colonizer elsewhere. We examined phenology across three microhabitats from 2013 to 2017 and used this information to create the first spring green-up model for S. alterniflora. We then compared modern spatial variation to that exhibited over a 60-year climate record. Marsh interior plants initiated spring growth 17 days earlier than channel edge plants and spent 35 days more in the green-up phenophase and 25 days less in the maturity phenophase. The start of green-up varied by 17 days among 3 years. The best spring green-up model was based on winter soil total growing degree days. Across microhabitats, spring green-up differences were caused by small elevation changes (15 cm) that drove soil temperature variation of 0.8C. Preliminary evidence indicated that high winter belowground biomass depletion triggered early green-up. Long-term change was similar: winter soil temperatures warmed 1.7 ± 0.3C since 1958, and green-up advanced 11 ± 6 days, whereas contemporary microhabitat differences were 17 ± 4 days. Incorporating local spatial variation into plant phenology models may provide an early warning of climate vulnerability and improve understanding of ecosystem-scale productivity. Microscale phenology variation likely exists in other systems and has been unappreciated.
(contributed by Jessica O'Connell, 2019)
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    Coastal dynamics and adaptation to uncertain sea level rise: Optimal portfolios for salt marsh migration
Abstract - The sustainability of dynamic natural systems often depends on their capacity to adapt to uncertain climate-related changes, where different management options may be combined to facilitate this adaptation. Salt marshes exemplify such a system. Marsh sustainability under rapid sea level rise requires the preservation of transgression zones - undeveloped uplands onto which marshes migrate. Whether these uplands eventually become marsh depends on uncertain sea level rise and natural dynamics that determine migration onto different land types. Under conditions such as these, systematically diversified management actions generally outperform ad hoc or non-diversified alternatives. This paper develops the first adaptation portfolio model designed to optimize the benefits of a migrating coastal system. Results are illustrated using a case study of marsh conservation in Virginia, USA. Results suggest that models of this type can enhance adaptation benefits beyond those available through current approaches.
(contributed by O. D. Vinent, 2019)
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    Generalizing Ecological Effects of Shoreline Armoring Across Soft Sediment Environments
Abstract - Despite its widespread use, the ecological effects of shoreline armoring are poorly synthesized and difficult to generalize across soft sediment environments and structure types. We developed a conceptual model that scales predicted ecological effects of shore-parallel armoring based on two axes: engineering purpose of structure (reduce/slow velocities or prevent/stop flow of waves and currents) and hydrodynamic energy (e.g., tides, currents, waves) of soft sediment environments. We predicted greater ecological impacts for structures intended to stop as opposed to slow water flow and with increasing hydrodynamic energy of the environment. We evaluated our predictions with a literature review of effects of shoreline armoring for six possible ecological responses (habitat distribution, species assemblages, trophic structure, nutrient cycling, productivity, and connectivity). The majority of studies were in low-energy environments (51 of 88), and a preponderance addressed changes in two ecological responses associated with armoring: habitat distribution and species assemblages. Across the 207 armoring effects studied, 71% were significantly negative, 22% were significantly positive, and 7% reported no significant difference. Ecological responses varied with engineering purpose of structures, with a higher frequency of negative responses for structures designed to stop water flow within a given hydrodynamic energy level. Comparisons across the hydrodynamic energy axis were less clear-cut, but negative responses prevailed (>78%) in high-energy environments. These results suggest that generalizations of ecological responses to armoring across a range of environmental contexts are possible and that the proposed conceptual model is useful for generating predictions of the direction and relative ecological impacts of shoreline armoring in soft sediment ecosystems.
(contributed by J.E. Dugan, 2018)
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    Effects of small-scale armoring and residential development on the salt marsh-upland ecotone
Abstract - Small-scale armoring placed near the marsh-upland interface to protect single-family homes is widespread but understudied. Using a nested, spatially blocked sampling design on the coast of Georgia, USA, we compared the biota and environmental characteristics of 60 marshes adjacent to either a bulkhead, a residential backyard with no armoring, or an intact forest. We found that marshes adjacent to bulkheads were at lower tidal elevations and had features typical of lower elevation marsh habitats: high coverage of the marsh grass Spartina alterniflora, high density of crab burrows, and muddy sediments. Marshes adjacent to unarmored residential sites had higher soil water content and lower porewater salinities than the armored or forested sites, suggesting that there may be increased freshwater input to the marsh at these sites. Deposition of Spartina wrack on the marsh-upland ecotone was negatively related to elevation at armored sites and positively related at unarmored residential and forested sites. Armored and unarmored residential sites had reduced densities of the high marsh crab Armases cinereum, a species that moves readily across the ecotone at forested sites, using both upland and high marsh habitats. Distance from the upland to the nearest creek was longest at forested sites. The effects observed here were subtle, perhaps because of the small-scale, scattered nature of development. Continued installation of bulkheads in the southeast could lead to greater impacts such as those reported in more densely armored areas like the northeastern USA. Moreover, bulkheads provide a barrier to inland marsh migration in the face of sea level rise. Retaining some forest vegetation at the marsh-upland interface and discouraging armoring except in cases of demonstrated need could minimize these impacts.
(contributed by Alyssa Gehman, 2018)
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    Controls on porewater salinity in a Southeastern salt marsh
Abstract - In coastal marsh ecosystems, porewater salinity strongly affects vegetation distribution and productivity. To simulate marsh porewater salinity, an integrated, spatially explicit model was developed, accounting for tidal inundation, evaporation, and precipitation, as well as lateral and vertical exchanges in both surface waters and the subsurface. It was applied to the Duplin River marsh, Sapelo Island, USA, over a 3-year period, which covered both drought and wet conditions. Simulated porewater salinity in the low and high marsh correlated with Duplin River salinity, withevapotranspiration and precipitation leading to substantial variations in porewater salinities across seasons, in particular in the high marsh. The model revealed substantial interannual variability in marsh soil conditions, and—due to its processbased approach linked to external forcings—can be used to explore effects of sea level rise and changes in hydrological forcings on marsh soil conditions.
(contributed by David Michael Miklesh, 2018)
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    Inorganic carbon and oxygen dynamics in a marsh-dominated estuary
Abstract - We conducted a free-water mass balance-based study to address the rate of metabolism and net carbon exchange for the tidal wetland and estuarine portion of the coastal ocean and the uncertainties associated with this approach were assessed. We measured open water diurnal O2 and dissolved inorganic carbon (DIC) dynamics seasonally in a salt marsh-estuary in Georgia, U.S.A. with a focus on the marsh-estuary linkage associated with tidal flooding. We observed that the overall estuarine system was a net source of CO2 to the atmosphere and coastal ocean and a net sink for oceanic and atmospheric O2. Rates of metabolism were extremely high, with respiration (43 mol m−2 yr−1) greatly exceeding gross primary production (28 mol m−2 yr−1), such that the overall system was net heterotrophic. Metabolism measured with DIC were higher than with O2, which we attribute to high rates of anaerobic respiration and reduced sulfur storage in salt marsh sediments, and we assume substantial levels of anoxygenic photosynthesis. We found gas exchange from a flooded marsh is substantial, accounting for about 28% of total O2 and CO2 air–water exchange. A significant percentage of the overall estuarine aquatic metabolism is attributable to metabolism of marsh organisms during inundation. Our study suggests not rely on oceanographic stoichiometry to convert from O2 to C based measurements when constructing C balances for the coastal ocean. We also suggest eddy covariance measurements of salt marsh net ecosystem exchange underestimate net ecosystem production as they do not account for lateral DIC exchange associated with marsh tidal inundation.
(contributed by Shiyu Wang, 2018)
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200 Records

This material is based upon work supported by the National Science Foundation under grants OCE-9982133, OCE-0620959, OCE-1237140 and OCE-1832178. Any opinions, findings, conclusions, or recommendations expressed in the material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.