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 Estuarine-Ocean Exchange 

Processes that govern exchange of water between estuaries and oceans
Research is being focused on the net transport through tidal inlets and the controlling dynamics. Present studies suggest that axial transport of water and salt is achieved through a competition of tidally-induced and discharge-induced flows. In a multiple inlet system sych as found in Georgia, this competition varies from inlet to inlet. Interannual cariations in freshwater discharge to Georfia estuaries greatly affect the inland penetration of salt to Georgia's estuarine system.
Competition and Lateral Distribution of Tide- and Discharge- Induced Flows in a Coastal Lagoon with Multiple Inlets
Chunyan Li and Jack Blanton
Skidaway Institute of Oceanography
Introduction
Most studies on exchange processes between an estuarine/lagoonal system and adjacent coastal ocean are conducted at a single opening. Along Georgia's coast, rarely is an estuarine/lagoonal system connected to the coastal ocean through a single opening. Several openings are often seen to connect the same system in which a network of channels interleave into each other bounded by intertidal salt marsh and shallow shoals that may be exposed at low tides. Within such a system, little is known about how the transport of water and salt is accomplished. Which opening would be flood/ebb dominated? What are the main factors/mechanisms of the transport processes? The present study is aimed at a preliminary investigation of these questions.

Study Area
The study is conducted in a coastal lagoon system (the Georgia LTER site): the Altamaha Sound, Doboy Sound, and Sapelo Sound area (Figure 1). The main freshwater input is from the Altamaha River. The three main Sounds are connected through a network of narrow channels. Tidal forcing is very strong with a tidal range of more than 2.5 meters.
Observations

A 4-day observation was conducted in the area on R/V Blue Fin (Figure 2), a 72 ft vessel, from September 13 to September 17, 2000. A vessel mounted ADCP, a thermosalinograph, and a SBE 25 CTD were used. Two triangular routes were designed to repeatedly sample at the mouth of Altamaha Sound and Sapelo Sound. Each of the triangles were occupied for 20 times within 13 hours. The ADCP was an RDI 600 KHz Broadband direct reading model mounted on one side of the vessel. Velocity data were averaged every 30 seconds and the vessel was moving at a speed of about 6 knots. The vertical bin of velocity was chosen to be 0.5 m. Since the depth was less than 15 m in the area, the ADCP was operated in bottom tracking mode all the time.

Data Analysis
The ADCP data were calibrated for compass error before analyzed. The data from the two triangular routes were selected to extract M2 tidal information based on a statistical-harmonic analysis technique [Li et al ., 2000]. Different horizontal grids were used to bin the data either along the planned track or over the whole area covering the triangles. Within each horizontal bin, the statistical-harmonic analysis was applied to the velocity and water depth to yield information about tide and tidal velocity as well as salinity. The M2 tide appeared to be the main tidal component and amplitude, phase, and mean of tide, tidal velocity, and salinity were obtained. Statistics of the analysis was calculated to provide errors and/or uncertainties of the estimates.

Summary
Altamaha Sound had both a strong tidal forcing and a significant amount of freshwater discharge. As a result, the Altamaha Sound was mostly vertically well mixed while allowing a significant horizontal salinity and density variations. In contrast, Sapelo Sound had no significant variation in salinity and density over the tidal cycles. It was both vertically and horizontally well mixed. Our analysis showed that Altamaha Sound had a net outward transport while the Sapelo Sound had a net inward transport. The narrow channels connecting the north and south portions of the lagoon system showed strong two layer flow and variation in salinity, indicating a strong mixing of salt water with freshwater. With the existence of multiple inlets and a network of narrow channels connecting different parts of the system of different tidal and saline characteristics, the water transport and salt flux becomes much more complicated. This study shows that the key to the transport of water and salt in such a system is most likely among the competition of tidal forcing and density pressure gradient under the strong influence of the network of connecting channels as well as the geometry and bathymetry of the inlets.

Acknowledgements
LTER-NSF (OCE-9982133) and CZM (RR100-279-9262764). Many people contributed to the field work and technical support (Captain Jay Fripp, Captain Raymond Sweatte, Cheryl Burden, Trent Moore, Julie Amft, Sue Elston, Daniella DiIorio, Alan Barton, and Michael Richter.)

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Sponsors:

  • Cite LTER grant
  • Cite CZM grants

Overview of System
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.

Variations in River Discharge
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).
Seasonal Salinity Distribution
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).
Tidal Excursion of the Salinity Field
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).
Temporal Variations at Moorings
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.

Discussion
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.

Conclusions

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.

Additional information can be obtained at
http://gce-lter.marsci.uga.edu/lter
(GCE-LTER Project Web site)

Acknowledgements
We thank the following individuals and agencies for generous support of this work: R/V BLUE FIN and GANNET crews and numerous field assistants, including Trent Moore and Cheryl Burden Ross.
U.S. Geological Survey for Altamaha discharge data and Site 4 (Meridian) salinity and water level data.
Wade Sheldon and Sue Elston for salinity data.
NSF Long Term Ecosystem Research Grant Number: OCE:9982133.
NSF Land Margin Ecosystem Research Grant Number: DEB-9412089.
Georgial Coastal Zone Management Grant Number: RR100-279/9262764.

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