[1] Tidal fluctuations drive groundwater flow in salt marsh sediments. This flow could cause significant chemical exchange across the sediment-water interface and could affect marsh ecology. Numerical models of a generalized tidal creek and marsh were constructed to calculate flow patterns and solute exchange between the marsh and creek. The governing equation for saturated/unsaturated flow was modified to account for tide-related changes in total stress. Groundwater flow occurred primarily in the creek bank, even when the marsh platform was inundated at high tide. For marsh sediments with a hydraulic conductivity of 10 À4 m s À1 , groundwater ages in simulations lasting 60 days were on the order of days near the creek bank and increased to 50-60 days with distance into the marsh. The volume of water that discharged between high and low tide was 0.22-0.31 m 3 per meter length of channel, which, for a creek drainage density of 0.012 m À1 , corresponds to 10-14 L m À2 d À1 . Sediment permeability and capillarity were important controls on flow and groundwater age in the marsh. Sediment compressibility affected groundwater age for compressible sediments representative of mud but not for sediments with lower compressibilities representative of sand. Simulations were relatively insensitive to dispersivity. A comparison of simulation results with other estimates of groundwater exchange from the North Inlet, South Carolina, suggest that tides could drive observed exchange there only if the Pleistocene sands underlying muddy marsh sediments outcrop within the tidal range.

Global climate change is expected to affect temperature and precipitation patterns, oceanic and atmospheric circulation, rate of rising sea level, and the frequency, intensity, timing, and distribution of hurricanes and tropical storms. The magnitude of these projected physical changes and their subsequent impacts on coastal wetlands will vary regionally. Coastal wetlands in the southeastern United States have naturally evolved under a regime of rising sea level and specific patterns of hurricane frequency, intensity, and timing. A review of known ecological effects of tropical storms and hurricanes indicates that storm timing, frequency, and intensity can alter coastal wetland hydrology, geomorphology, biotic structure, energetics, and nutrient cycling. Research conducted to examine the impacts of Hurricane Hugo on colonial waterbirds highlights the importance of long‐term studies for identifying complex interactions that may otherwise be dismissed as stochastic processes. Rising sea level and even modest changes in the frequency, intensity, timing, and distribution of tropical storms and hurricanes are expected to have substantial impacts on coastal wetland patterns and processes. Persistence of coastal wetlands will be determined by the interactions of climate and anthropogenic effects, especially how humans respond to rising sea level and how further human encroachment on coastal wetlands affects resource exploitation, pollution, and water use. Long‐term changes in the frequency, intensity, timing, and distribution of hurricanes and tropical storms will likely affect biotic functions (e.g., community structure, natural selection, extinction rates, and biodiversity) as well as underlying processes such as nutrient cycling and primary and secondary productivity. Reliable predictions of global‐change impacts on coastal wetlands will require better understanding of the linkages among terrestrial, aquatic, wetland, atmospheric, oceanic, and human components. Developing this comprehensive understanding of the ecological ramifications of global change will necessitate close coordination among scientists from multiple disciplines and a balanced mixture of appropriate scientific approaches. For example, insights may be gained through the careful design and implementation of broad‐scale comparative studies that incorporate salient patterns and processes, including treatment of anthropogenic influences. Well‐designed, broad‐scale comparative studies could serve as the scientific framework for developing relevant and focused long‐term ecological research, monitoring programs, experiments, and modeling studies. Two conceptual models of broad‐scale comparative research for assessing ecological responses to climate change are presented: utilizing space‐for‐time substitution coupled with long‐term studies to assess impacts of rising sea level and disturbance on coastal wetlands, and utilizing the moisture‐continuum model for assessing the effects of global change and associated shifts in moisture regimes on wetland ecosystems...

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Abstract. We use 228Ra and 226Ra to determine the mass balance of dissolved inorganic nitrogen (DIN) and dissolved reactive phosphorus (DRP) in the North Inlet salt marshestuarine system. While this system has only minor freshwater inputs of nutrients or radium, it is an extremely productive ecosystem. In addition, there are significant exports of these dissolved species to the coastal ocean. Saline groundwater in this estuarine system contains nutrient and radium concentrations more than an order of magnitude greater than surface waters. Using a radium mass balance, we estimate the groundwater discharge necessary to support the export of radium to the coastal ocean and the corresponding flux of nutrients from the groundwater. From these calculations, we show that the underlying aquifer supplies nutrients sufficient to support the net primary productivity of the salt marsh ecosystem and to account for the known export of nutrients from the marsh. We conclude that the major nutrient source to the North Inlet, South Carolina, salt marsh is the saline aquifer lying just beneath the surface of the marsh. Furthermore, extrapolation of the nutrient export to include other South Carolina marshes suggests that nutrient fluxes from salt marshes to the coastal ocean rival riverine nutrient fluxes for the region.

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