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 continue to track recovery in the SALTEx experiment following cessation of experimental dosing in December 2017.

      Significant Results:  Analyses of the SALTEx experiment show differential response curves to salinity: porewater NH4 increased linearly and DOC concentrations decreased as a power function, whereas other constituents showed no significant trends (Fig. 1).

Area 3 Figure 1

Fig. 1. Salinity-response relationships for porewater constituents observed during the SALTEx experiment, in which salinity was artificially increased. Source: C. Craft and M. Alber.

  • 3A.2 - Continue the PredEx Experiment

      Activities:  We continue sampling the predator exclusion experiment initiated in summer 2016. This past year we conducted additional mesopredator tethering trials.

      Significant Results:  Mesopredators (mud crabs) have increased in the predator exclusion treatment after several years, and are likely the reason that other invertebrates have not increased dramatically as we expected. We are planning to conduct a mesopredator x nekton presence experiment in 2022 to tease apart their relative effects.

  • 3A.3 - Continue the High Marsh manipulation

      Activities:  The high marsh experiment was largely ineffective at altering groundwater flow, but the data are useful for understanding hydraulic gradients. We plan to decommission this experiment in 2022.

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

  • 3A.4 - Establish Disturbance manipulation

      Activities:  We started the DRAGNET distributed disturbance experiment in 2021 and are planning experiments in which we will implement a standardized disturbance across natural gradients of salinity and elevation to test the hypothesis that underlying abiotic gradients affect marsh recovery from disturbance.

      Significant Results:  None to date

B) Focused Studies

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

      Activities:  We have multiple efforts underway to collect information on how temperature and flooding affect plant production. In June 2021 we began monthly sampling of vertical profiles of leaf area index in S. alterniflora canopies to quantify the effects of tidal flooding; in December we set up a hydroponic experiment to evaluate how winter soil temperature interacts with salinity and nutrients to affect belowground processes and S. alterniflora phenology (Fig. 2).

      Significant Results:  Hawman and Mishra (in prep) found that NEE decreased sharply under flooded conditions, and that accounting for changes in emergent leaf area index due to tidal flooding improved relationships with the Sentinel-2 near-infrared reflectance index.

Area 3 Figure 2

Fig. 2. Greenhouse experiment set up in Dec. 2021 to determine how over-wintering Spartina plants respond to different levels of temperature, salinity and nutrients. Plants are being grown hydroponically to allow us to follow both above- and below-ground production. Source: J. O’Connell and S. Pennings.

  • 3B.2 - Investigate marsh fauna interactions

      Activities:  We continue to conduct focused studies to understand the relationships between marsh fauna and environmental variables. Activities this past year included studies of detritivores, variation in metabolic rate of snails, and the effects of herbivory by megafauna on salt marsh invertebrates. We also continue to refine designs for biomimic sensors.

      Significant Results:  Seer et al. (2021) found that the colonization of litter by infauna shifts during decomposition as litter becomes less labile (Fig. 3).

Area 3 Figure 3

Fig. 3. Conceptual model (a) and experimental data (b) showing the shift from litter characteristics to habitat type in explaining the variance in the faunal composition of the decomposer community. Source: Seer et al. 2021.

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

      Activities:  We are collaborating with the Univ. of Wisconsin to collect pCO2 data at the GCE flux tower to understand CO2 export by tidal waters. We also completed a lab experiment on decomposition controls under varying oxygen conditions and bioavailable C inputs.

      Significant Results:  GCE was part of a cross-site study that highlighted the significant role of transport in organic matter dynamics (Harms et al. 2021).

Area 3 Publications from GCE-IV

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. 12:685–692. (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. (DOI: 10.1111/1365-2745.13885)

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. 138:137–154. (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.