The Dynamical response of Salinity to Freshwater Discharge and Wind Forcing in adjacent estuaries on the georgia coast

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Density-driven circulation, tides, and wind-driven currents are responsible for horizontal exchange between estuaries and the coastal ocean, which, in turn, have a major impact on the ecology, chemistry, water quality, and sedimentary processes in estuarine and coastal environments (Geyer and Signell, 1992).
Circulation, mixing, and transport processes are evaluated in terms of freshwater discharge, ocean inundation, atmospheric forcing, and frictional effects over tidal, subtidal, and seasonal time scales.We currently have a good understanding of estuary/ocean exchange in isolated systems, but how buoyancy forcing from one estuary propagates into adjacent estuaries is not well known.

The Georgia Coastal Ecosystems Long
Term Ecological Research (GCE LTER) site is located along three adjacent estuaries on the Georgia coast (Altamaha, Doboy, Sapelo), and it encompasses upland (mainland, barrier islands, marsh hammocks), intertidal (fresh, brackish, salt marsh), and submerged (river, estuary, continental shelf) habitats (see color infrared image of the domain in Figure 1).characterized by strong seasonal variability (Weber and Blanton, 1980).

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Long-term observations and highly idealized numerical model simulations reveal a large degree of water exchange between interconnected estuaries in the Georgia Coastal Ecosystem LTER domain.This highly complex estuarine system, with sounds connected by a network of channels, creeks, and intertidal areas, is expected to also present large spatial and temporal variability in residence time.The residence time-the average time a water particle spends within the estuary or in some portion thereof (Geyer and Signell, 1992)-is one of the most important factors influencing water contamination and nutrient levels, distributions of organics, and their spatiotemporal variations in bays and estuaries (Aikman and Lanerolle, 2004).
In fact, model results show that the fraction of nutrients entering an estuary that is exported or denitrified can often be predicted from the freshwater residence time (Dettmann, 2001).Additionally, large residence times have been implicated in the outbreak of harmful algal blooms (Bricelj and Lonsdale, 1997).
Processes that influence the transport of nutrient-rich riverine waters (approximately represented by the dye distribution shown in Figure 6) are crucial for understanding water quality in estuaries.
To date, several studies have quantified residence and flushing times in Altamaha Sound (Alber and Sheldon, 1999a;Sheldon and Alber, 2002), including lowfrequency variability possibly associated with surface water withdrawal (Alber and Sheldon, 1999b).However, those studies did not address the potential for circulation to the Doboy and Sapelo estuaries.

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Figure 2. (a) Forcing within the domain is shown as river discharge and sea surface height (SSh, relative to mean sea level) associated predominately with the alongshore winds.(b) Salinity is shown along the longitudinal and meridional gradients within the gce domain.See Figure 1 for station locations.
wind data from the offshore National Data Buoy Center (NDBC 41008) located at Grays Reef National Marine Sanctuary was collected for the period 2002-2012 and daily averaged quantities were binned into seasonal time frames.Strong Nor' easter storms dominate in the fall (September/ October/November), with winds blowing predominantly alongshore, causing downwelling-favorable conditions and onshore transport (Figure 3).In winter (December/January/February), westerly winds are strengthened, but strong storms from the northeast are still relatively frequent.Spring (March/April/ May) and summer (June/July/August), on the other hand, are characterized by winds predominantly from the southwest, which are upwelling favorable and promote offshore transport.Throughout the year, alongshore winds dominate over cross-shore winds.Subtidal sea surface height (SSH) from the USGS station at Meridian, GA, is positively correlated (r = 0.53) with alongshore winds (positive winds from the NNE) and negatively correlated (r = -0.37)with cross-shore winds (positive winds from the WNW) both with p-values < 0.05.The change in inundation in the estuary as a result of changing prevailing winds can be as much as 0.6 m (Figure 2a).During the fall and winter seasons, Nor' easters create downwellingfavorable conditions that promote onshore transport and, hence, inundation in the estuaries.These conditions could cause the Altamaha River plume to thicken and converge on the Georgia coast and move southward.During spring and summer, winds are primarily from the southwest and promote offshore transport and thus reduced sea surface heights.The offshore transport during spring could also be enhanced because of increased river discharge during this time.During these upwelling-favorable winds, the Altamaha River plume could thin and move northward off the Georgia coast.In fact, Blanton and Atkinson (1983) showed that the river-generated low-salinity plume is carried offshore in spring and alongshore and southward in autumn.Winter storms from the west bring the most significant cross-shore winds, and the negative correlation seems to imply that water is forced out of the estuary, causing decreased sea surface heights.In shallow waters where friction is important, cross-shelf winds can result in cross-shelf velocities in the upper few meters of the water column that are similar in magnitude to those generated by alongshelf winds

FigureFigure 3 .Figure 4 .Figure 6 .Figure 5 .
Figure 2b shows daily averaged salinity over the GCE domain at each of the long-term monitoring sites.GCE1 in the headwaters of Sapelo Sound shows large changes in salinity that are mainly due to local rain and groundwater input, which is presumably recharged by precipitation.GCE7 in the Altamaha River, located approximately 20 km upstream from the ocean, is almost always fresh except for times of drought conditions.

Figure 7 .
Figure 7. Binned scatterplot of subtidal total dye flux at the mouth of Doboy Sound as a function of subtidal coastal sea surface height (SSh).positive values represent flux up the estuary.
Ten years of oceanic and meteorological monitoring data were collected in order to understand the spatial and temporal patterns of salinity distribution across three adjacent estuaries in the Georgia Coastal Ecosystems Long Term Ecological Research domain.Empirical orthogonal function analysis shows that 95% of the subtidal salinity variability can be explained by two principle modes.
Associate Professor and Renato M. Castelao is Assistant Professor, Department of Marine Sciences, University of Georgia, Athens, GA, USA.aBStr act.