Area 3: Process Studies

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.

  Significant Results:  Thompson et al. (2024) found that chronic saltwater intrusion produced significant, widespread changes in marsh structure and function, and that recovery varied from months (e.g., porewater nutrients) to years (e.g., elevation) (Fig. 1).

Fig. 1. Patterns of recovery for the various parameters evaluated in the Saltex experiment. Source: Thompson et al. 2024.

• 3A.2 - Continue the PredEx Experiment

  Activities:  We sample the PredEx experiment annually.

  Significant Results:  Morton et al. (2024) found that mesopredator release prevented trophic cascades from occurring, resulting in little effect of nekton exclusion on Spartina biomass and density.

• 3A.3 - Continue the High Marsh manipulation

  Activities: The high marsh experiment has been decommissioned but we are maintaining the groundwater wells.

  Significant Results:  We are using data from the groundwater wells to help evaluate seasonal variations in salinity, drought, and plant zone migration in the high marsh.

• 3A.4 - Establish Disturbance manipulation

  Activities:  We continued the DRAGNET distributed disturbance experiment, which began in 2021, as well as our standardized experiments along elevation (2022) and the estuarine salinity gradient (2023) (Fig. 2).

  Significant Results:  After 3 years, the rototilling treatment in DRAGNET had strong effects on soils, plants, and invertebrates whereas the herbicide treatment only affected plants and invertebrates; fertilizer had no effects (Fig. 3). Plants in the disturbance across elevation took between 9 & 28 months to recover, and recovery rate was best predicted by redox potential.

Fig. 2. The disturbance experiments along elevation and salinity were created by deploying 2 x 3 m tarps for 3 months to kill underlying vegetation, creating a standardized disturbance. We are monitoring recovery in 0.5 x 0.5 m plots within paired disturbed and control plots.

Fig. 3. In the DRAGNET experiment, the two disturbance treatments that involved rototilling (DIST and LTND) greatly reduced soil shear strength, redox, plant cover and snail density. The herbicide treatment (HERB) did not affect soil shear strength or redox, but did reduce plant cover and snail density. The two fertilization treatments (LTNUT and STNUT) had no effect on any variable except possibly reducing snail density (LTNUT). Source: K. Madushani and S. Pennings.

B) Focused Studies

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

  Activities:  We are using a combination of field observations, flux tower data, remote sensing, and modeling to evaluate Spartina production and its relationship to drivers.

  Significant Results:  Hawman et al. (2024) synthesized 8 y of flux tower data (2014-2022) and found the lowest annual C uptake occurred in 2016, when the Palmer Drought Severity Index had its lowest annual mean. They also found spatial heterogeneity within the marsh: areas on the marsh edge were flooded more frequently and responded to variation in river discharge whereas those in the interior relied on in situ precipitation (Fig.4; See Key Accomplishments).

Fig. 4. a) Interannual variability in NEE measured at the flux tower shows a strong drought response in 2016. Different portions of the marsh show differing patterns: b) the marsh interior is related to precipitation and the marsh edge is related to c) river discharge and d) drought. Source: Hawman et al. 2024.

• 3B.2 - Investigate marsh fauna interactions

  Activities:  We continue to conduct focused studies to understand the relationships between marsh fauna and environmental variables. This past year we examined effects of distance from marsh edge as a factor controlling invertebrate density and predation risk of the periwinkle snail; monitored Littoraria survival in relation to temperature measurements from biomimics; analyzed densities of the invasive fiddler crab (Leptuca thayeri); evaluated food sources of estuarine consumers in marshes with differing channel density (Fig 5); and conducted investigations of the effects of large grazers in east coast marshes.

  Significant Results:  Although large grazers suppressed plant height, cover and flowering, suppressed snail density, and reduced soil organic C (Sharp et al. 2024), moderate levels of grazing did not harm salt marsh invertebrates (Pennings and Lugar, subm.). Ziegler et al. (in press) found that reduced seascape connectivity can influence trophic relationships in marsh consumers. Williams et al. (2024) found that aggregations of mussels can stimulate N removal in a positive, density-dependent manner (Fig. 6).

Fig. 5. Common marsh invertebrates a) killifish and b) grass shrimp were sampled in areas with contrasting channel densities: c) Dean Creek and d) Belle Marsh as part of a stable isotope study of trophic relationships. Source: Ziegler et al., in press.

Fig. 6.Conceptual diagram illustrating the effects of mussel presence on N pools and processes. Source: Williams et al. 2024.

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

  Activities:  We collected cores this past year from marshes throughout coastal Georgia to assess marsh accretion rates and carbon content and conducted a study to evaluate the use of tea bags to assess organic matter decay in salt marsh sediments.

  Significant Results:  Reddy et al (in press) demonstrated that the tea bag index for organic matter decay is sensitive to position in the tidal frame (faster at lower elevations compared) and soil depth (faster at the surface), and found that Roobibos tea may be useful as a standardized material to study decay dynamics in coastal wetlands. We also contributed to a large distributed study characterizing methane dynamics across the continental US (Koontz et al. 2024).

Area 3 Publications from GCE-IV

Yang, Z., Alexander, C.R. Jr. and Alber, M. 2024. The dynamics of marsh-channel slump blocks: an observational study using repeated drone imagery. Biogeosciences. 21:1757-1772. (DOI: 10.5194/bg-21-1757-2024)

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: https://doi.org/10.1007/s12237-023-01265-z)

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: https://doi.org/10.1007/s12237-023-01265-z)

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: https://doi.org/10.1029/2020JG006213)

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: https://doi.org/10.1016/j.agrformet.2020.108133)

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: https://doi.org/10.1029/2019GL082374)

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: https://doi.org/10.1038/s41561-019-0435-2)

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: https://doi.org/10.1016/j.scitotenv.2019.133779)

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. 138:137–154. (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: https://doi.org/10.1016/j.aquabot.2017.04.003)

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.