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:  We are tracking recovery in the SALTEx experiment, measuring SETs every 6 mo and the plants every summer.

      Significant Results:  Li et al. (2022) synthesized the cascading effects of above-ground plant biomass in the Saltex treatments (Fig. 1). As of summer 2022, both Press and Pulse treatments had regained elevation lost during dosing.

Area 3 Figure 1

Fig. 1. Cascading effects of treatments on benthic microalgae and fiddler crabs include the effects of plant above-ground biomass on (a) proportion of light penetrating the canopy and (b) fiddler crab burrow density, and the effects of proportion of light penetrating the canopy on (c) abundance of cyanobacteria and (d) abundance of diatoms. Data are from fall 2016. Additional hypothesized mechanisms linking plant above-ground biomass to benthic algae and fiddler crab abundance include (e) effects on porewater nutrients, structural support for crab burrows, and access for predators. Source: Li et al. 2022.

  • 3A.2 - Continue the PredEx Experiment

      Activities:  We sample the predator exclusion experiment annually. This year we evaluated fish usage across treatments, conducted a predation assay, and analyzed porewater data. A post-bac student (COVID supplement) evaluated mud crab predation on snails.

      Significant Results:  There was no effect of predator exclusion either on mummichog use or in predation assays. However, porewater ammonium concentrations at 15 cm depth were slightly higher in the exclosures compared to the open treatments. We also found that mud crabs can control snail numbers, but multiple mud crabs have antagonistic interactions (Fig. 2).

Area 3 Figure 2

Fig. 2. Effect of varying mud crab density (predator) on snails. Although mud crabs reduce snail densities, multiple crabs have agonistic interactions and the effect on snails is reduced. Source: B. Silliman.

  • 3A.3 - Continue the High Marsh manipulation

      Activities:  We are decommissioning the high marsh experiment because it was ineffective at altering groundwater flow, but the wells have been useful for documenting hydraulic gradients.

      Significant Results:  We are using well data to better understand fluxes of groundwater in the high marsh (see Objective 2A4).

  • 3A.4 - Establish Disturbance manipulation

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

      Significant Results:  Disturbance is showing large effects in the DRAGNET experiment: Spartina density is almost zero, soil temperature and ammonium levels are elevated, and soil shear strength is reduced (Fig. 4).

Area 3 Figure 3

Fig. 3. GCE REU student Zahrria Johnson doing the disturbance manipulation (rototilling) in the Dragnet experiment. Source: S. Pennings.

Area 3 Figure 4

Fig. 4. Soil ammonium levels (millimolar) in Dragnet Treatments after one year. Treatment codes: CONT control; DIST disturbed by rototilling; HERB herbicide; LTND long term nutrient addition + disturbance; STNUT short term (3 y) nutrient addition. Ammonium levels are elevated in treatments without plants (DIST, LTND), which likely reflects a combination of decomposition of roots and rhizomes plus no uptake by plants. HERB=herbicide, which reduced plants by about half in the first year, led to slightly elevated ammonium but it was highly variable. The 2022 application of herbicide killed the plants off completely, and we expect high ammonium in the HERB treatment in 2023. Source: S. Pennings.

B) Focused Studies

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

      Activities:  We conducted a hydroponic experiment to evaluate how winter soil temperatureinteracts with salinity and nutrients to affect belowground processes and Spartinaphenology. We completed field sampling of vertical profiles of leaf area index inSpartina canopies to quantify the effects of tidal flooding.

      Significant Results:  Preliminary analyses suggest that change in Spartina belowground biomass was lower over the course of the winter greenhouse experiment as temperature and salinity increased, and higher as nutrients increased.

  • 3B.2 - Investigate marsh fauna interactions

      Activities:  We conducted focused studies on the thermal performance of Littoraria and salinityeffects on oyster predation. We continue to test new designs for biomimic sensors.

      Significant Results:  Atkins et al. (2022) found variation in respiration and feeding among different size-classes of Littoraria, which can lead to divergent population-level responses to temperature (Fig. 5). GCE results on how mussels improve soil conditions was part of a review paper on the effects of animals on coastal restoration projects (Sievers et al. 2022).

Area 3 Figure 5

Fig. 5. Thermal performance curves fit to data from the respiration experiment for Littoraria irrorata, using the best Gaussian model with effects of mass on the thermal trait Eh (deactivation energy). Blue, red, and black lines represent predicted performance for an individual of median size within the (A) large (86 mg), (B) medium (31 mg), and (C) small (8 mg) size classes, respectively. Points represent measurements of snail respiration rates (ppm CO2 min-1) for 24 snails, each measured at all 8 temperatures. Dashed gray lines represent the temperature at peak respiration. Confidence intervals (95%) were bootstrapped using 500 runs. Source: Atkins et al. 2022.

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

      Activities:  We sampled DIC to evaluate mixing between Altamaha and Doboy Sounds (ROAsupplement). In the lab, we tested soil metabolism responses to root exudates,including assessing whether specific exudates increase vulnerability of 'old' carbon.

      Significant Results:  Craft (subm.) found that a formerly impounded marsh had greater vertical accretion, carbon sequestration, and nitrogen burial than a control marsh 64 y after reintroduction of tidal inundation. Reddy (GCE intern) found that organic matter decay rates vary predictably with relative elevation in the tidal frame (Fig. 6). Temmink et al. (2022) synthesized information on carbon sequestration in coastal wetlands and highlighted how they can shift from carbon sinks to sources when feedbacks between geomorphology and vegetation are disrupted.

Area 3 Figure 6

Fig. 6.Decomposition rates of tea bag organic matter buried at (a) 10 and (b) 50 cm, and stabilization of tea bag organic matter buried at (c) 10 and (d) 50 cm vs relative elevations within the tidal frame (Z*). Significant correlations are denoted with a solid line (p<0.05). Source: S. Reddy, 2020, undergraduate honors thesis.

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. (DOI:

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)

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.