Intensive hammock characterization Dr. Clark R. Alexander Skidaway Institute of Oceanography

10 Ocean Science Circle Skidaway Institute of Oceanography Savannah Georgia 31411 USA

clark.alexander@skio.uga.edu https://www.skio.uga.edu/people/faculty/506-2/ https://orcid.org/0000-0002-3904-1091 Dr. Merryl Alber University of Georgia

Dept. of Marine Sciences University of Georgia Athens Georgia 30602-3636 USA

malber@uga.edu https://marsci.uga.edu/directory/people/merryl-alber https://orcid.org/0000-0002-9467-4449 Georgia Coastal Ecosystems LTER Project

Dept. of Marine Sciences University of Georgia Athens Georgia 30602-3636 USA

gcelter@uga.edu https://gce-lter.marsci.uga.edu/ Dr. Clark R. Alexander Skidaway Institute of Oceanography

10 Ocean Science Circle Skidaway Institute of Oceanography Savannah Georgia 31411 USA

clark.alexander@skio.uga.edu https://www.skio.uga.edu/people/faculty/506-2/ https://orcid.org/0000-0002-3904-1091 Principal investigator Dr. Merryl Alber University of Georgia

Dept. of Marine Sciences University of Georgia Athens Georgia 30602-3636 USA

malber@uga.edu https://marsci.uga.edu/directory/people/merryl-alber https://orcid.org/0000-0002-9467-4449 Principal investigator Alana R. Lynes University of Houston

Department of Biology and Biochemistry 369 Science and Research Bldg 2 University of Houston Houston Texas 77204-5001 USA

alynes76@sbcglobal.net Graduate research assistant Dr. Christine M. Hladik Georgia Southern University

Department of Geology and Geography Georgia Southern University Statesboro Georgia 30460 USA

chladik@georgiasouthern.edu https://www.georgiasouthern.edu/cosm/sees/christine-hladik/ https://orcid.org/0000-0002-0265-6412 Graduate research assistant 2026

In 2008 we selected two hammocks for more detailed study: HN_I_1 is of Holocene origin and is located adjacent to Blackbeard Island to the north of Sapelo; PC_I_29 is also of Holocene origin and is located adjacent to the south end of Sapelo Island. These are of similar size, with similar vegetation zones in the high marsh. We set up transects in each hammock that run from the nearby upland (Blackbeard and Sapelo Islands, respectively), through the marsh, and up and over each hammock to the marsh adjacent to the Sound (Sapelo and Doboy, respectively) (See Figure 1 under the Study Area tab). In 2009, we vibracored a transect on PN_i_12, to include a Pleistocene hammock in our detailed core analysis.

C. Alexander (SKIO) led a field team that took vibracore samples along the transects on each hammock to get detailed information about stratigraphy. These cores show that the sedimentology of these features argues for distinctly different physical and stratigraphic processes dominating their formation. HNi1 formed as a storm berm, on top of pre-existing salt marsh approximately 3,000 years old, burying Holocene saltmarsh under a normally graded, coarse to fine grained sand. PCi29 formed rapidly, in less than a few hundred years, at the very energetic paleo-sound margin of Doboy. PN_i_12 is underlain by a hard-pan (humate) deposit, below which a Pleistocene mud is observed. In many cases, Pleistocene and Holocene hammocks appear to rest upon older salt marsh, with the level of consolidation determining the substrate character. Sound Grain size and x-radiography illustrate the differences in sedimentary processes important in the genesis of each of these features (See Figure 2 under the Study Area tab).

On HN_i_1 and PC_i_29, groundwater wells were installed along the transects in the locations sampled by vibracore. Wells were placed in a contiguous sand layer in order to prevent clogging and to allow sampling of a continuously connected groundwater source. In addition to the vibracore stations, two additional lateral wells were installed at HN_I_1 approximately 5 meters to either side of the main transect near well #4 (HN_i_1) and well #6 (PC_i_29) in order to get data on lateral inflow of groundwater. An additional upland well was also installed at both sites in order to obtain a true zero salinity end member. The wells range in depth from 41 to 168 cm below a constant zero datum (99 to 247 cm below the surface). (see Figure 3 under the Products tab). Pressure, salinity, and temperature loggers were placed into select wells at both sites to collect data at 10-minute intervals; these data will be used in the groundwater modeling efforts. Groundwater biogeochemistry and radium samples were collected in August and September 2008. Sample collection will continue on monthly intervals, with additional samples planned to capture spring/neap tide and storm event signals

Area 3: Responses to Salinity and Inundation (more information) Question 4: What are the underlying mechanisms by which proximity of marshes to upland habitat drives ecosystem change along lateral gradients in the intertidal zone? (more information)

LTER NSF Blackbeard Island Georgia Sapelo Island GCE hammock Holocene marsh upland Overall geographic extent of the research project -81.295506 -81.195661 31.546868 31.395295 2008-01-01

National Science Foundation grant number OCE-0620959

estuary marsh complex Hunt Camp - Barrier island/marsh site on western Sapelo Island. This site is located at the upper reaches of the Duplin River, and is within the Sapelo Island National Estuarine Research Reserve. Existing well fields border small marsh area to northwest, some wells have been installed to south end of hammock where marsh is more extensive and permanent plots are located. Two existing hydrographic sondes and weather stations within this site are operated by SINERR (Hunt Camp dock) and UGAMI (flume dock). -81.295506 -81.258908 31.497026 31.462320 North Sapelo - High salinity marsh/Sapelo Sound site. Few or no small creeks, but one large creek is easily accessed. Further upstream upland drainage is affected by culvert at Reynolds' duck pond. Upland is heavily forested. Hydrographic sonde is deployed adjacent to this site attached to a channel marker (under U.S. Coast Guard permit). A huge mud flat is present at the northern tip of Sapelo Island. -81.233249 -81.195661 31.546868 31.515188 UGA Marine Institute - The University of Georgia Marine Institute is located on the southern end of Sapelo Island, a barrier island off the central Georgia coast. -81.282950 -81.278834 31.398877 31.395295 Figure 1. Hammocks selected for intensive characterization https://gce-lter.marsci.uga.edu/public/resources/projects/hammockFig1Q4.png Figure 2. Vibracore sampling https://gce-lter.marsci.uga.edu/public/resources/projects/hammockFig2Q4.png 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) 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) http://www.sciencedirect.com/science/article/pii/S0277379116301275 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. HYD-GCES-1401b - Groundwater well pressure, temperature, conductance, salinity and oxygen measurements for GCE-LTER study hammock PC_i_29 from 15-Aug-2008 to 13-Dec-2013 https://gce-lter.marsci.uga.edu/public/app/dataset_details.asp?id=567 HYD-GCES-1401a - Groundwater well pressure, temperature, conductance, salinity and oxygen measurements for GCE-LTER study hammock HN_i_1 from 30-Oct-2008 to 03-Dec-2013 https://gce-lter.marsci.uga.edu/public/app/dataset_details.asp?id=566 POR-GCES-1401 - High frequency sediment surface temperature measurements at GCE-LTER study hammock PC_i_29 from 15-Aug-2008 to 24-Sep-2012 https://gce-lter.marsci.uga.edu/public/app/dataset_details.asp?id=568 MLT-GCEM-0807 - GCE-LTER Hammock Well Vegetation and Invertebrate Monitoring - July 2008 https://gce-lter.marsci.uga.edu/public/app/dataset_details.asp?id=465 INS-GCES-0706 - Survey of grasshopper abundance in GCE LTER hammock sites and five GCE LTER study sites https://gce-lter.marsci.uga.edu/public/app/dataset_details.asp?id=462 INV-GCES-0706a - Summer 2007 crab population survey based on crab hole counts in fifty-four GCE LTER Hammock sites and four GCE LTER study sites https://gce-lter.marsci.uga.edu/public/app/dataset_details.asp?id=463 INV-GCES-0706b - Summer 2007 mollusc population survey in fifty-five GCE LTER Hammock sites and five GCE marsh monitoring sites https://gce-lter.marsci.uga.edu/public/app/dataset_details.asp?id=464 PLT-GCES-0706 - Summer 2007 survey of plant presence and abundance at fifty-five GCE LTER Hammock sites and five GCE marsh monitoring sites https://gce-lter.marsci.uga.edu/public/app/dataset_details.asp?id=470 Figure 3. Vibracore profiles. https://gce-lter.marsci.uga.edu/public/resources/projects/vibracoreFig3Q4.png