A Long Term Ecological Research (LTER) site has been established on the central Georgia coast in the vicinity of Sapelo Island (MAP A). The study area includes three contiguous estuarine systems separated from each other by marshes and separated from the coastal ocean by barrier islands.

The three estuaries chosen for this study differ dramatically in the degree of riverine influence. Salinity structure results from the interaction of river discharge and mixing of oceanic/coastal water by relatively large semi-diurnal tides with ranges of 2-3 meters.

The most southerly estuary is Altamaha Sound, which lies at the mouth of the Altamaha River, the largest and one of the least developed rivers in Georgia. Altamaha Sound is strongly river-dominated and encompasses a complex delta made up of low islands, marshes and tributaries. Extremely strong horizontal and vertical salinity gradients are common in the Altamaha estuary, especially during high discharge periods.

Doboy Sound, located to the north of Altamaha Sound, connects to the coastal ocean via a deep pass. It has no large upland source of freshwater. Low salinity water from the Altamaha River is transported into Doboy Sound through the connecting IntraCoastal Waterway (ICW) and marsh channels. Tidal exchange with the Altamaha's plume in the coastal ocean can also deliver low salinity water to Doboy Sound.

The third estuary, Sapelo Sound, is at the northern edge of the study area; it is also a lagoonal estuary with no large streams discharging directly into it. Fresh water enters as precipitation, groundwater or as small volumes of surface inflow. The highest salinities of the LTER domain are found in Sapelo Sound because it is farthest from the Altamaha River freshwater source.

Altamaha River discharge is measured at Doctortown, Georgia, located approximately 90 km from the ocean. Discharge varies over annual and inter-annual cycles and is typically less than 400 m3 s-1 during low flow but can exceed 3000 m3 s-1 during high flow (Fig. 1).

Information on salinity distribution is available from several projects (see Acknowledgments). A survey on 4 April 2000 that occurred after a moderately high discharge period showed brackish water extending from the Altamaha estuary northward through the ICW to near Sapelo Sound (Fig. 2b). The contrast during low discharge (Fig. 2a, 2c) is remarkable with low salinity water confined mainly in the vicinity of Altamaha Sound. Relatively fresh water is distributed through the Altamaha River (which splits into three branches ~20 km from the ocean) and many inter-connecting tidal creeks (Fig. 2d).

The salinity regime in the Altamaha estuary is extremely compressed (Fig. 3). During low water (LW), surface salinity can change by 20 PSU in less than 20 km. On the rise to high water (HW), the isohalines typically shift inland about 10 km. The surveys of 29 August 1998 occurred during neap tide after an extremely high discharge period; vertical stratification was strong at the mouth of the estuary, particularly at LW (Fig. 3b). The 15 September 2000 surveys occurred just after a spring tide and after a prolonged drought (Fig. 3b). Salinity values were elevated and vertical stratification was much lower compared to August 1998, even though the axial salinity gradient was about the same for the two sets of surveys.

The tide also shifts the salinity regime in the ICW connecting Doboy and Altamaha Sounds (Fig. 4). At HW (Fig. 4a), there is a small zone of low salinity water trapped between the ICW and Altamaha Sound. As the tide ebbs (Fig. 4b-c), Altamaha Sound water enters the ICW and salinity drops almost 20 PSU. At LW, a mixture of water between 20 and 25 PSU is advected northward almost to Doboy Sound (Fig. 4d-e). On the rise to HW (Fig. 4f), the salinity throughout the ICW rises to about 28 PSU, except for the brackish water zone that appears in almost the same area as in the previous HW (Fig. 4a).

Salinity varies significantly over a tidal cycle, depending upon position relative to the ocean and the Altamaha River. Compare a site under oceanic influence with one in the upper reaches of the Altamaha estuary (Fig. 5a; see MAP B for site locations). Not only is the level of salinity different, but also the shape of the salinity curve during the tidal cycle. Unusually high salinity values were recorded at the riverine site (Site 7a) due to prolonged drought conditions in the region.

Large temporal differences in salinity also occur at inland locations (Fig. 5b1), illustrating the ability of the tidal creek network to distribute low salinity water from the Altamaha estuary. Site 4 salinity data for the shaded time period is compared with simultaneous data from the mouth of the Altamaha River (Fig. 5b2). The transport of the strong axial salinity gradient in Altamaha Sound is manifested as a large salinity excursion at a point (Site 9). A much smaller gradient at the inland site (Site 4) indicates that a high degree of mixing occurs between the two locations.

The LTER domain may be described as a "horizontal" estuary with freshwater discharge entering the southern area and high salinity coastal water entering the northern area (Fig. 6). The heavier salty coastal water seeks deeper routes of entry while the lighter low salinity water exits through Altamaha Sound, tidal creeks and the IntraCoastal Waterway. The resulting salinity in the domain reflects the tidal mixing of these two end-members. The presence of low salinity water at the mouth of the Altamaha several months after seasonally higher discharge suggests that the intertidal zone, which contains vast areas of freshwater and brackish tidal marshes, retains a significant portion of the river discharge over a several-month period.

In view of the high temporal and spatial variability of the salinity regime, the Georgia Coastal Ecosystems Long Term Ecological Research (GCE-LTER) project is installing a network of monitoring stations. Conductivity, temperature and subsurface pressure will eventually be measured at nine long-term monitoring sites (MAP B). Moored instruments have already been deployed at four sites.

At quarterly intervals throughout the year, hydrographic (CTD) and other water properties will be measured along the axes of the three estuaries and in selected tidal creeks. Repeated synoptic sampling, especially after a large discharge event, will help define the major routes of freshwater distribution within the domain. Other LTER collaborators will use the long-term observations to interpret ecosystem processes and responses to perturbations.


• Salinity fluctuations throughout the LTER domain are directly linked to fluctuations of Altamaha River discharge.
• Freshwater from the Altamaha River primarily affects the southern half of the LTER domain (Altamaha and Doboy Sounds).
• Long-term sampling of hydrographic properties in the three estuaries will further the understanding of salt and freshwater exchange within the system.
• Other project collaborators will use data from the monitoring program to define how the ecosystem responds and adapts to salinity fluctuations.