Area 3: Process Studies

Objectives Progress Report Publications Show All  

Process Studies

We conduct long-term manipulations as well as focused investigations designed to develop a mechanistic understanding of ecosystem function and responses to both long-term and episodic changes.

Research Objectives

A) Long-Term manipulations

  • 3A.1 - Track recovery in the SALTEx Experiment
  • 3A.2 - Continue the PredEx Experiment
  • 3A.3 - Continue the High Marsh manipulation
  • 3A.4 - Establish Disturbance manipulation

B) Focused Studies

  • 3B.1 - Investigate controls of S. alterniflora production
  • 3B.2 - Investigate marsh fauna interactions
  • 3B.3 - Enhance our understanding of coastal carbon dynamics

Current Progress Report

Below is an update for each of the Area 3 objectives as reported in the most recent annual report. For a list of all reports click here (Annual Reports).

A) Long-Term manipulations

  • 3A.1 - Track recovery in the SALTEx Experiment

      Activities:  Monitoring of the SALTex experiment ended in 2022 and we will be removing experimental infrastructure.

      Significant Results:  We are writing up the results of the 4 years of dosing and 5 years of recovery in the SALTex experiment.

  • 3A.2 - Continue the PredEx Experiment

      Activities:  We sample the PredEx experiment annually. This year we evaluated effects of treatments on snail parasites and set up an experiment to disentangle the effects of mesopredators and nekton on marsh community composition.

      Significant Results:  Morton et al. (in review) used structural equation modeling to describe the combined, cascading effects of both nekton predators and mud crabs on marsh grazers in the Predex experiment (Fig. 1).

Area 3 Figure 1

Fig. 1. Structural equation model depicting the effect of nekton predators and mud crab mesopredators in the PREDEX manipulation. Arrow width is proportional to the standardized effect size given next to the black (positive effect) and red (negative effect) arrows. Nonsignificant relationships (P > 0.05) are omitted for clarity. Source: Morton et al. , sub.

  • 3A.3 - Continue the High Marsh manipulation

      Activities: This experiment has been decommissioned.

      Significant Results:  We are using data from the high marsh wells to evaluate the groundwater numerical model (Obj 2A4). Simon et al. (2023) used computer vision methods to identify high marsh plant species (Fig. 2).

Area 3 Figure 2

Fig. 2. Distribution of six plant species present in a photo-mosaic (~10,000 photographs) of a high marsh plant community (3200 m2). The approach shows promise as a method of efficiently generating and statistically analyzing community data for sessile species at scales not previously possible. Source: Simon et al. 2023.

  • 3A.4 - Establish Disturbance manipulation

      Activities:  We continued the DRAGNET distributed disturbance experiment, which began in 2021 (Fig 3). In 2022 we set up a standardized disturbance experiment across the natural elevation gradient in the salt marsh, and in 2023 we set up a similar manipulation along the estuarine salinity gradient.

      Significant Results:  After two years, the disturbance treatments in DRAGNET (roto-tilling and herbicides) are having strong effects on plants, invertebrates, and soil variables. In the disturbance across elevation experiment, Spartina has almost totally recovered in places where it was initially productive, but has not recovered where it was initially stunted.

Area 3 Figure 3

Fig. 3. The GCE DRAGNET experiment is part of a globally distributed experiment. It includes a roto-tilling treatment (inset) that kills plants and disrupts soil. Aerial view of plots. Source: S. Pennings.

B) Focused Studies

  • 3B.1 - Investigate controls of S. alterniflora production

      Activities:  We used pulse-amplitude modulation(PAM) measurements to study the photosynthetic performance of Spartina at different inundation levels (Fig 4).

      Significant Results:  Mao et al. (2023) suggest that when submerged, PSII reaction centers in S. alterniflora leaves are still active and able to transfer electrons, but only at ~20% of the typical daily rate (Fig. 5).

Area 3 Figure 4

Fig. 4. In situ measurements of chlorophyll through pulse-frequency-modulation (PAM) fluorescence: a) frame showing units at top and bottom of canopy and b) close-up. Source: Mao et al. 2023.

Area 3 Figure 5

Fig. 5. Diurnal dynamics of chlorophyll fluorescence parameters observed at 5-min intervals during a high tide period that occurred in the middle of the day (flooded points). Submerged leaves at the bottom of the canopy had large reductions in PSII operating efficiency (30-41%) whereas exposed leaves (top of canopy) showed only small reductions (7-8.3%). Source: Mao et al. 2023.

  • 3B.2 - Investigate marsh fauna interactions

      Activities:  We conduct focused studies to understand relationships between marsh fauna and environmental variables. This year we deployed biomimics to monitor ecologically relevant temperatures of invertebrates and sediment, collected fish samples to process for stable isotopes to compare with other coastal LTER sites, initiated studies of the invasive mangrove crab, and participated in an investigation of the effects of large grazers in east coast marshes.

      Significant Results:  Sharp et al. (subm.) found that large herbivore grazing affects both plants and soil carbon content. Biomimic results show large variations in temperature that could affect organismal performance: max temperature of snails exceeded 40 C only 10% of the time at the sediment surface compared to half the time at 40 cm.

  • 3B.3 - Enhance our understanding of coastal carbon dynamics

      Activities:  This year we collected 25 cores from marshes throughout coastal Georgia to assess carbon accumulation rates, analyzed carbon isotopes (d13C-DIC) of estuary samples to distinguish marsh vs. oceanic sources, and conducted a cross-site field experiment using marsh organs to test how sea level rise and changing redox conditions will affect soil carbon cycling. We also analyzed NEE dynamics at the flux tower (Obj 1A2).

      Significant Results:  Soil core data demonstrate that vertical accretion rates are comparable to sea level rise and that terrestrial organic matter accounts for a substantial fraction of C burial in GA marshes (Manns 2023; Giordano 2023). In lab manipulations, Spivak et al. (2023) found evidence for substantial rates of chemolithoautotrophy in wetland soil (Fig. 6). GCE C data are part of multiple synthetic efforts (See Key Accomplishments).

Area 3 Figure 6

Fig. 6.Comparison between calculated and measured DIC production after experimental pulses of bioavailable C incubated under anaerobic (N2) and aerobic (O2) conditions. Ratios >1 in the aerobic treatments provide evidence for substantial rates of chemolithotrophy. Source: Spivak et al. 2023.

Area 3 Publications from GCE-IV

Lynn, T., Alber, M., Shalack, J. and Mishra, D. 2023. Utilizing Repeat UAV Imagery to Evaluate the Spatiotemporal Patterns and Environmental Drivers of Wrack in a Coastal Georgia Salt Marsh. Estuaries and Coasts. (DOI:

Lynn, T., Alber, M., Shalack, J. and Mishra, D. 2023. Utilizing Repeat UAV Imagery to Evaluate the Spatiotemporal Patterns and Environmental Drivers of Wrack in a Coastal Georgia Salt Marsh. Estuaries and Coasts. (DOI:

Hawman, P., Mishra, D., O'Connell, J.L., Cotten, D.L., Narron, C. and Mao, L. 2021. Salt Marsh Light Use Efficiency is Driven by Environmental Gradients and Species-Specific Physiology and Morphology. Journal of Geophysical Research: Biogeosciences. 126. (DOI:

Hensel, M.S., Silliman, B.R., von de Koppel, J., Hensel, E., Sharp, S., Crotty, S.M. and Byrnes, J. 2021. A large invasive consumer reduces coastal ecosystem resilience by disabling positive species interactions. Nature Communications. 12(1). (DOI: 10.1038/s41467-021-26504-4)

O'Connell, J.L., Mishra, D., Alber, M. and Byrd, K.B. 2021. BERM: A belowground ecosystem resilience model for estimating Spartina alterniflora belowground biomass. New Phytologist. (DOI: 10.1111/nph.17607)

Mobilian, C., Wisnoski, N., Lennon, J., Alber, M., Widney, S. and Craft, C.B. 2020. Differential effects of press vs. pulse seawater intrusion on microbial communities of a tidal freshwater marsh. Limnology and Oceanography Letters. (DOI: 10.1002/lol2.10171)

Nahrawi, H.B., Leclerc, M.Y., Pennings, S.C., Zhang, G., Singh, N. and Pahari, R. 2020. Impact of tidal inundation on the net ecosystem exchange in daytime conditions in a salt marsh. Agricultural and Forest Meteorology. 294:108133. (DOI:

Solohin, E., Widney, S. and Craft, C.B. 2020. Declines in plant productivity drive loss of soil elevation in a tidal freshwater marsh exposed to saltwater intrusion. Ecology. 101(12):13. (DOI: 10.1002/ecy.3148)

Alber, M. and O'Connell, J.L. 2019. Elevation drives gradients in surface soil temperature within salt marshes. Geophysical Research Letters. 46:5313-5322. (DOI:

Spivak, A.C., Sanderman, J., Bowen, J.L., Canuel, E.A. and Hopkinson, C.S. 2019. Global-change controls on soil-carbon accumulation and loss in coastal vegetated ecosystems. Nature Geoscience. 12:685692. (DOI:

Widney, S., Smith, D., Herbert, E., Schubauer-Berigan, J.P., Li, F., Pennings, S.C. and Craft, C.B. 2019. Chronic but not acute saltwater intrusion leads to large release of inorganic N in a tidal freshwater marsh. Science of the Total Environment. 695. (DOI:

Wang, Y., Castelao, R. and Di Iorio, D. 2017. Salinity Variability and Water Exchange in Interconnected Estuaries. Estuaries and Coasts. (DOI: 10.1007/s12237-016-0195-9)

Schalles, J.F., Hladik, C.M., O'Donnell, J., Miklesh, D.M., Pudil, T. and Nealy, N. 2021. Presentation: Satellite and drone remote sensing to study decadal scale and high resolution spatial-temporal patterns and declines of Spartina alterniflora above-ground biomass in Georgia, USA salt marshes. Session 2. 1st International Symposium on Coastal Ecosystems and Global Change (CoEco1), April 18, 2021, Xiamen University, Xiamen, China.

Schalles, J.F., Hladik, C.M., O'Donnell, J., Miklesh, D.M., Pudil, T., Nealy, N. and Currin, H. 2021. Presentation: Serious multidecadal declines in aboveground biomass of the keystone salt marsh species, Spartina alterniflora, are related to climate change in coastal Georgia, USA. Wetlandscapes: Understanding the Large-scale Wetland Functions in the Landscape Symposium. 11th INTECOL International Wetlands Conference, October 14, 2021, Christchurch, New Zealand (virtual, prerecorded).

Kunza Vargas, A.E. and Pennings, S.C. 2005. Poster: Plant diversity of Texas and Georgia salt marshes. Ecological Society of America 2005 Meeting - Ecology at multiple scales, August 7-12, 2005, Montreal, Canada.

Area 3 Publications from GCE-III

Journal Articles

Li, F., Angelini, C., Byers, J., Craft, C.B. and Pennings, S.C. 2022. Responses of a tidal freshwater marsh plant community to chronic and pulsed saline intrusion. Journal of Ecology. 110:1508-1524. (DOI: 10.1111/1365-2745.13885)

Simon, J., Hopkinson, B.M. and Pennings, S.C. 2022. Insights into Salt Marsh Plant Community Distributions Through Computer Vision and Structural Equation Modeling. Estuaries and Coasts. 46:431-449. (DOI: 10.1007/s12237-022-01147-w)

Li, F. and Pennings, S.C. 2019. Response and Recovery of Low-Salinity Marsh Plant Communities to Presses and Pulses of Elevated Salinity. Estuaries and Coasts. 42:708-718. (DOI: 10.1007/s12237-018-00490-1)

Herbert, E., Schubauer-Berigan, J.P. and Craft, C.B. 2018. Differential effects of chronic and acute simulated seawater intrusion on tidal freshwater marsh carbon cycling. Biogeochemistry. 138:137154. (DOI: 10.1007/s10533-018-0436-z)

Li, F. and Pennings, S.C. 2018. Responses of tidal freshwater and brackish marsh macrophytes to pulses of saline water simulating sea level rise and reduced discharge. Wetlands. 38:885-891. (DOI: 10.1007/s13157-018-1037-2)

Alexander, C.R. Jr., Hodgson, J. and Brandes, J. 2017. Sedimentary processes and products in a mesotidal salt marsh environment: insights from Groves Creek, Georgia. Geo-Marine Letters. 37:345-359. (DOI: 10.1007/s00367-017-0499-1)

Jung, Y. and Burd, A.B. 2017. Seasonal changes in above- and below-ground non-structural carbohydrates (NSC) in Spartina alterniflora in a marsh in Georgia, USA. Aquatic Botany. 140:13-22. (DOI:

Craft, C.B., Herbert, E., Li, F., Smith, D., Schubauer-Berigan, J.P., Widney, S., Angelini, C., Pennings, S.C., Medeiros, P.M., Byers, J. and Alber, M. 2016. Climate change and the fate of coastal wetlands. Wetland Science and Practice. 33(3):70-73.

Hawkes, A., Kemp, A., Donnelly, J., Horton, B., Peltier, W., Cahill, N., Hill, D., Ashe, E. and Alexander, C. 2016. Relative Sea-Level Change in Northeastern Florida (USA) During the Last ~8.0 KA. Quaternary Science Reviews. (DOI: 10.1016/j.quascirev.2016.04.016)

Herbert, E., Boon, P., Burgin, A.J., Neubauer, S.C., Franklin, R.B., Ardon, M., Hopfensperger, K.N., Lamers, L. and Gell, P. 2015. A global perspective on wetland salinization: Ecological consequences of a growing threat to freshwater wetlands. Ecosphere. 6(10)(206):1-43. (DOI: 10.1890/ES14-00534.1)

Wieski, K. and Pennings, S.C. 2014. Latitudinal variation in resistance and tolerance to herbivory of a salt marsh shrub. Ecography. 37:763-769. (DOI: 10.1111/ecog.00498)

Schalles, J.F., Hladik, C.M., Lynes, A.R. and Pennings, S.C. 2013. Landscape estimates of habitat types, plant biomass, and invertebrate densities in a Georgia salt marsh. Special Issue: Coastal Long Term Ecological Research. Oceanography. 26:88-97. (DOI: 10.5670/oceanog.2013.50)

Porubsky, W.P., Joye, S.B., Moore, W.S., Tuncay, K. and Meile, C. 2011. Field measurements and modeling of groundwater flow and biogeochemistry at Moses Hammock, a backbarrier island on the Georgia coast. Biogeochemistry. 104:69-90. (DOI: 10.1007/s10533-010-9484-8)

Meile, C., Porubsky, W.P., Walker, R.L. and Payne, K. 2009. Natural Attenuation Of Nitrogen Loading From Septic Effluents: Spatial And Environmental Controls. Water Research. 44(5):1399-1408. (DOI: 10.1016/j.watres.2009.11.019)

Theses and Dissertations

Jung, Y. 2018. Modeling Growth and Production Dynamics of Spartina Alterniflora. Ph.D. Dissertation. University of Georgia, Athens, GA. 148 pages.

Ledoux, J.G. 2015. Drivers of groundwater flow at a back barrier island - marsh transect in coastal Georgia. M.S. Thesis. The University of Georgia, Athens. 104 pages.

Conference Papers (Peer Reviewed)

Porubsky, W.P. and Meile, C. 2009. Controls on groundwater nutrient mitigation: Natural attenuation of nitrogen loading from septic effluents. In: Hatcher, K.J. (editor). Proceedings of the Georgia Water Resources Conference. Athens, Georgia.

Conference Posters and Presentations

Widney, S., Smith, D., Schubauer-Berigan, J.P., Herbert, E., Desha, J. and Craft, C.B. 2017. Poster: Changes in sediment porewater chemistry in response to simulated seawater intrusion in tidal freshwater marshes, Altamaha River, GA. Society of Wetland Scientists Annual Meeting, June 5-8, San Juan, Puerto Rico.

Smith, D., Herbert, E., Li, F., Widney, S., Desha, J., Schubauer-Berigan, J.P., Pennings, S.C., Angelini, C., Medeiros, P.M., Byers, J., Alber, M. and Craft, C.B. 2016. Poster: Seawater Addition Long Term Experiment (SALTEx). Georgia Department of Natural Resources Coastal Resources Division 2016 Climate Conference, November 2-3, 2016, Jekyll Island, GA.

Ledoux, J.G., Alexander, C.R. Jr. and Meile, C. 2015. Poster: Groundwater flow at the Georgia coast: Magnitude and drivers across a back barrier island marsh transect. LTER All Scientists Meeting, Aug 30-Sept 2, Estes Park, CO.

Miklesh, D.M., McKnight, C.J., Di Iorio, D. and Meile, C. 2015. Poster: Controls on porewater salinity distributions in a southeastern salt marsh. LTER All Scientists Meeting, Aug 30-Sept 2, Estes Park, CO.

Ledoux, J.G., Alexander, C.R. Jr. and Meile, C. 2014. Poster: Delineating groundwater flow along a marsh transect at a back barrier island on the coast of Georgia. Southeastern Estuarine Research Society Fall meeting, November 6-8, Carolina Beach, NC.

Alexander, C.R. Jr., Alber, M., Hladik, C.M. and Pennings, S.C. 2010. Presentation: Physical-Biological Interactions in Coastal Settings: The Georgia Coastal Ecosystem LTER Example. American Geophysical Union - Meeting of the Americas, 9-13 August 2010, Foz do Iguacu, Brazil.

Alexander, C.R. Jr. 2008. Presentation: Stratigraphic Development of Holocene and Pleistocene Marsh Islands. Tidalites 2008 - Seventh International Conference on Tidal Environments, 25th-27th September, 2008, Qingdao, China.


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.