Groundwater as a nutrient and carbon source to coastal systems

Serious declines in coastal water quality and ecosystem health have resulted from population growth and agricultural, commercial, and industrial activities in coastal watersheds and from increased loading of anthropogenic wastes (organics and nutrients) originating at localized (e.g., sewage, industrial effluent) and diffuse (e.g., agricultural run-off) sources. Ultimately, when excessive amounts of anthropogenic materials arrive in coastal waters, they lead to eutrophication, which can be loosely defined as the increase in labile organic matter supply to an ecosystem. Visible signs of eutrophication, including increased frequency of harmful algal blooms, water column hypoxia/anoxia, fish kills, and reduced water quality, are apparent in coastal environments across the globe. Understanding the causes of eutrophication and documenting an ecosystem's response to eutrophication are two key research challenges facing coastal ecologists today.

Predicting the response of coastal ecosystems to land use change and eutrophication requires robust models that include all relevant sources of nutrients and organic materials. Groundwater is an important, but poorly understood, source of nutrients and organic materials to coastal waters. Along the coast of Georgia, groundwater inputs are thought to be important, but this input term has not been quantified. The lack of information regarding groundwater flux and groundwater quality makes it impossible to predict how the quality (chemistry) or quantity (freshwater flux) of this 'source term' will respond to increased development pressures in coastal regions.

As part of the and the Georgia Coastal Ecosystems LTER project and NOAA Sea Grant Land Use-Coastal Ecosystem Change program, we are examining groundwater geochemistry and sediment biogeochemistry in coastal ecosystems in Georgia (Dover Bluff along the Satilla River and at several sites on Sapelo Island in the LTER domain) and South Carolina (the Okatee estuary near Bluffton, SC). We are collaborating with Billy Moore (U South Carolina, geochemical tracers), Marj Aelion (U South Carolina, marsh sediment N cycling), and Carolyn Ruppel (Gerogia Tech, hydrology) on various aspeccts of this project. This work represents an integrated effort to quantify the impact of coastal development and land use change on ecological, chemical, and physical processes through the study of both groundwater and surface water components. We are quantifying the flux and chemical signature of shallow coastal groundwater and the data will be used to contrast the chemical composition of groundwater and surface waters. The impact of groundwater on biological processes is also being evaluated in the marsh and in estuarine tidal creeks.

In Joye's lab, PhD students Bill Porubsky and Nat Weston and Technician Matt Erickson work on these projects. Undergraduate students Ray Dixon and Lori Jarrell are conducting indepent study projects supported by NSF REU funds.

Study Sites

The Okatee River estuary
The Okatee River estuary (32.34º N, 80.89º W), SC, is a tributary of the Colleton River and out flow from the system makes its way eventually to Port Royal Sound. The Okatee headwaters lie adjacent to the Sun City housing development and recreational complex. Currently, 2200 housing units are occupied and an additional 6,400 units are under development (Sun City Public Relations Office, pers. com.). Construction of these units is almost complete and the watershed population will quadruple (from ca. 5,000 individuals to ca. 20,000 individuals) when the new homes are occupied (Sun City Public Relations Office and Fred Holland, SC Dept. Natural Resources, Marine Resources Research Institute, pers. comm.). As a result, the septic inputs to the surficial groundwater, and to Okatee system, are likely to increase. The Okatee is surrounded by an extensive marsh of Spartina alterniflora but Salicornia and Juncus are found in saltpans and near the upland boundary, respectively.

Sites T1, 278 and GD were sampled along a salinity gradient in the Okatee Estuary. Salinity in the Okatee is highly dependent on freshwater discharge. Site T1 in the upper reaches of the Okatee can have salinities ranging from 0 to 20 ppt. The GD site further downstream in the Okatee is less influenced by freshwater discharge, with salinities typically near seawater levels. The PB site on Malind Creek, a small tidal creek feeding the Okatee, typically has salinities comparable to the T1 site. In January 2003, an additional five sites between the 278 and GD sites were sampled to evaluate spatial variability in porewater biogeochemistry.

Two transects of groundwater monitoring wells, installed by Dr. Carolyn Ruppel as part of a collaborative NOAA Sea Grant project, are sampled bimonthly to evaluate spatial and temporal trands in groundwater biogeochemistry.

Sapelo Island

Sapelo Island (middle right panel on the figure above) is a pristine barrier island that lies within the domain of the Georgia Coastal Ecosystems LTER project (see the GCE web site for detailed information about the project). The pristine marshes on Sapelo Island are usually saline, but can be influenced by freshwater discharge from the Altamaha River. Our primary study site, Moses Hammock (MH on map), is located at the upper reaches of the Duplin River, within the Sapelo Island National Estuarine Research Reserve. Existing well fields (installed by LTER investigator Dr. Carolyn Ruppel of Georgia Tech) border small marsh area to northwest. Additional wells were installed to south end of hammock where marsh is more extensive and permanent plots are located. The sites contains a transition from upland to salt marsh to tidal creek and the marsh is dominated by Spartina alterniflora. We are also working at Dean Creek (DC) in collaboration with the Sapelo Island Microbial Observatory (SIMO) to study linkages between sediment micobial community structure and sediment biogeochemical dynamics.

The Satilla River Estuary

The Satilla River (bottom right panel on the figure above) is a coastal plain blackwater river that drains a 9143 km² watershed. The average flow rate is 65 m³ sec^-1 and the narrow floodplain is bordered largely by cypress swamps and bottomland forests.

The Satilla River carries a high dissolved organic load (25 mg L^-1) and a low sediment load, and has a low pH (ca. 6). The Satilla River has received minimal human impact along its floodplain and within its watershed although development pressure is expected to increase in the coming years. Marshes within the tidally influenced portion of the estuary are exposed daily, implying that significant groundwater-derived inputs could be expected, particularly at low tide. Dover Bluff marsh (DB, 30.99º N, 81.50º W) lies on Umbrella Creek along the brackish intertidal portion of the Satilla River estuary, GA.

The Dover Bluff residential community (~50 homes) lies immediately adjacent to a Spartina alterniflora salt marsh and tidal creek (Umbrella Creek). The homes employ septic systems to process household waste. A network of upland groundwater monitoring wells and marsh wells and piezometers was installed along two transects extending from the upland across the marsh to the tidal creek by Dr. Carolyn Ruppel at the Georgia Institute of Technology as part of a collaborative NOAA Sea Grant project. Monitoring well depth varies from 3 to 5 m. Bundled marsh piezometers are installed at three depths (0.5, 1.0 and 2.0 m) providing access to groundwater within the marsh.

Representative thermal IR images from the Okatee estuary. Panel on the left is an orthophoto on which the locations of groundwater inputs (determined from thermal IR imaging; details below). The panels on the right provide examples of thermal IR snapshots from different sites along the Okatee. Similar images show extensive groundwater inputs at the Dover Bluff site along the Satilla estuary.

Cold ground water (white) contrasts against the warm (black) tidal creek water (images from August 2001).

This remote sensing component relies on the use of thermal IR imaging to visually determine where cold groundwater discharges into warm surface waters. This technique is most successful in summer when temperature differences between warm surface and colder groundwater are at their maximum (Portnoy et al. 1998).

Numerous sub-marsh groundwater flows were documented in the Okatee (note the cooler areas in images B, C, D, and E). Additionally, discrete plumes entering a tidal creek to the N of the N well transect. [Joye et al. unpublished data; a manuscript is in preparation].

To study the factors controlling microbial metabolism in coastal sediments, we collect sediment cores from marsh and creekbank locations. The figure to the right shows Ph.D. candidate Nat Weston collecting cores from Dover Bluff (Satilla River). The figure below shows Nat discussing his work with Dover Bluff residents.

Joye's groundwater related research strives to:

> Document with and between site spatial and temporal trends in groundwater biogeochemical signatures.

> Document rates and pathways of microbially-mediated transformations of groundwater-derived C, N and P that occur in sediments as groundwater transits the upland-marsh-tidal creek ecotone.

> Determine the importance of groundwater as a source for labile nutrients and organic matter to coastal waters.

Specific questions regarding groundwater dynamics:

1. How does groundwater biogeochemistry vary between developed and pristine sites?

2. What is the impact of tidal pumping of coastal aquifers on biogeochemical processes, like nitrification and denitrification, within those aquifers?

3. Is groundwater a source of labile nutrients and perhaps organic matter to coastal waters?

4. How do coastal sediment microbes alter groundwater-derived C, N and P?

We thank the National Science Foundation's LTER program and NOAA Sea Grant LU-CES programs in GA and SC for supporting this work.

**Disclaimer** The content of this page is based in part on work supported by the National Science Foundation and NOAA Sea Grant programs in GA and SC. Any opinions, findings, and conclusions or recommendations expressed here are those of the author (Mandy Joye) and do not necessarily reflect the views of the National Science Foundation or NOAA Sea Grant.