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

Area3 Figure 1

Fig. 1. Alpha diversity of the total DNA community in each treatment group of the SALTEx experiment. * indicates Press was significantly different from Control (p<0.02). Source: Mobilian et al. 2020, L&O Letters

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

      Activities and Accomplishments:  We continue to track recovery in the SALTEx experiment following cessation of experimental dosing in December 2017. We are monitoring porewater, sediment elevation, and vegetation. Although porewater nutrients returned to background levels within a few weeks, there is evidence of residual salinity in several of the press plots. Widney et al. (2019) published a synthesis of the biochemical effects of the SALTEx experiment showing that three years of saltwater intrusion increased porewater Cl-, SO4, HS and inorganic N (NH4 and NO3) and decreased plant N storage (see key findings). Solohin et al. (2020) found that declining soil surface elevation and associated carbon loss was due to reduced belowground biomass in press plots, and Mobilian et al. (2020) showed that decreased carbon inputs from plants resulted in reduced microbial diversity and decreased microbial carbon cycling (Fig. 1).

  • 3A.2 - Continue the PredEx Experiment

      Activities and Accomplishments:  We continue the predator exclusion experiment initiated in summer 2016, with annual sampling of the invertebrate and plant communities. In 2019 we added camera traps to evaluate the presence of avian and mammalian consumers in the plots, began measuring benthic microalgae, pore water and decomposition, and conducted tethering experiments with fiddler crabs and Littoraria. Although sampling was limited in 2020 due to travel restrictions associated with COVID, we expect to resume our regular schedule of observations in 2021. Results to-date have revealed top-down control of invertebrate populations that cascades down to affect ecosystem processes such as enhanced consumption and decomposition rates. It appears that mesopredators (mud crabs) have increased in the predator exclusion treatment and have prevented other invertebrates from increasing as dramatically as we expected (Fig. 2). Camera traps showed that terrestrial predators were not major visitors to the experimental plots, and both soil pore water and snail mortality varied with elevation as opposed to predator exclusion. We did not see effects on S. alterniflora biomass, which we attribute to mesopredator release and/or compensatory facilitation by fiddler crabs. A manuscript on these results is in preparation.

  • 3A.3 - Continue the High Marsh manipulation

      Activities and Accomplishments:  During GCE-III we initiated a large-scale experiment wherein we attempted to manipulate surface and shallow groundwater flow from the upland border into the high marsh. Although we continue to sample the plots annually, data from groundwater wells indicates that this manipulation was largely ineffective at altering groundwater flow and porewater salinities, and we have seen no consistent changes between treatments in benthic micro-algae, invertebrates or vascular plants. However, we anticipate that data from the wells will be useful for investigating links between major perturbations (high rainfall events, very high tides), groundwater level and porewater salinity, and vegetation changes. Data from the wells are also being analyzed to calculate hydraulic gradients for groundwater flow models.

Area 3 Figure 2

Fig. 2. Densities of a) Littoraria b) fiddler crab burrows and c) mud crab burrows observed in the predator exclusion experiment. By year 3 both Littoraria and fiddler crabs exhibited an apparent release from predation when nekton was excluded, but this effect disappeared in year 4 when the abundance of mud snails (mesopredators) increased inside the treatments. Source: J. Morton and B. Silliman

  • 3A.4 - Establish Disturbance manipulation

      Activities and Accomplishments:  We are planning two experiments in which we will implement a standardized 4 m2 disturbance across natural gradients of 1) salinity and 2) elevation to test the hypothesis that underlying abiotic gradients affect the response of a marsh to a disturbance. These will be initiated in the second half of the project based on our ongoing observations of natural perturbations (Area 4). We are also participating in a distributed "DragNet" experiment, set to begin in summer 2021, aimed at understanding how grasslands around the world recover from disturbances under different nutrient regimes.

B) Focused Studies

Area3 Figure 3

Fig. 3. Annual cycle of Gross Primary Production, GPP (a) and Light Use Efficiency, LUE (b) calculated from the GCE flux tower. LUE is GPP/PAR. Source: Hawman et al. 2021, JGR

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

      Activities and Accomplishments:  Temperature and flooding are two key variables known to affect S. alterniflora production, and both are experiencing long-term change (winter temperatures are increasing; sea-level rise is expected to increase flooding of low-lying areas). We have multiple efforts underway to collect information on these variables and how they affect plant production (see Area 2 Objective 4). We are planning a greenhouse experiment to evaluate how winter soil temperature interacts with salinity and nutrients to affect belowground processes and S. alterniflora phenology. (Our planned trials for this work were postponed due to COVID, but we developed experimental apparatus in 2020-1.) Hawman et al. (2021) evaluated the annual cycle of GPP and light use efficiency measured at the flux tower (Fig. 3) and found that the cloudiness index and daily maximum tide height are the primary factors that explain deviation in S. alterniflora light use efficiency. Nahrawi et al. (2020) reported that tidal flooding depressed NEE and that this effect varied seasonally as a function of plant phenology.

  • 3B.2 - Investigate marsh fauna interactions

      Activities and Accomplishments:  Marsh fauna can often drive marsh response to perturbations, and we are conducting several investigations to understand their effects on marsh structure and function. For example, Angelini et al. (2016) demonstrated that there is a mutualism between S. alterniflora and mussels in which S. alterniflora patches associated with mussels are more resilient to drought because mussels enhance water storage and reduce soil stress. In 2019 we began field measurements to evaluate how interactions between cordgrass and ribbed mussels influence tide water chemistry and the net import/export of materials from marshes. Sharp and Angelini (in press) found that birds and nekton can enhance S. alterniflora resilience to drought by increasing soil aeration via probing as well as their suppression of snail grazers and transmission of disease to snails. GCE researchers have also conducted several studies examining top-down effects of megafauna such as feral hogs on marsh ecosystems (see key findings).

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

      Activities and Accomplishments:  The sources and sinks of carbon in coastal ecosystems are an important component of the global carbon budget. In GCE-III Wang et al. (2017) published a complete budget of CO2 exchange in the Duplin River estuary (based on DIC exchange in a creek in combination with estuary metabolism measurements and data from the flux tower), which showed that although the marsh is a net sink for CO2 it is also a source of C to estuarine and coastal water.. High-frequency monitoring conducted with funds provided through an ROA supplement showed evidence for DIC export during spring tides, which is consistent with the Duplin River budget. We are following up on these observations with additional sampling this summer. In the marsh, our eddy covariance data provide estimates of vertical flux for the marsh carbon budget. We have evidence that S. alterniflora photosynthesizes when fully submerged, suggesting that part of the fixed C is likely exchanged with the water rather than the atmosphere. Spivak et al. (2019) wrote a synthesis paper highlighting the importance of understanding the key biogeochemical mechanisms within the marsh that control decomposition of soil organic matter when evaluating the effects of changing environmental conditions on coastal wetland C storage (Fig. 4), and we are using this to guide sampling and analysis of cores collected along elevation, salinity, and disturbance gradients. (See also cross-site research.)

Area 3 Figure 4

Fig. 4. Effects of changing environmental conditions on decomposition. A. Sea level rise alters plant communities, destabilizes soils and affects porewater chemistry. Red crosses indicate dieback. B. Warming enhances respiration more than production, promoting C loss. Species range shifts may alter soil inputs and processing. C. Eutrophication increases plant productivity, soil OM content and electron acceptor availability (SR, sulfate reduction; DNF, denitrification, DNRA, dissimilatory nitrate reduction to ammonia). D. Landscape alterations often reduce plant productivity and destabilize soils. Source: Spivak et al. 2019, Nature Geoscience

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. 12:685692. (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. 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.