The Mass Balance of Salt and Water in Intertidal Sediments: Results from North Inlet, South Carolina

  • Morris J
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Salinity can be used as a conservative tracer of porewater turnover in circumstances when evapotranspiration
is great enough to concentrate porewater salts in intertidal sediments. At two intertidal sites situated at mean high
tide at North Inlet, South Carolina, porewater drainage was estimated by this method to be 9.41 m-2 d-1 and 16.6 1 m-2
d-1, depending on physical soil properties and assuming that solute losses occur by simple diffusion across the sediment
surface, by uptake and excretion by vegetation, and by drainage. Mass balance simulations indicated that sediment
physical properties, evapotranspiration, and elevation are important determinants of seasonal salinity extremes. At sites
situated near mean high tide, small differences in elevation significantly affect salinity and drainage rate. As site elevation
increases, losses of solutes by drainage and diffusiona ndde crease the variability of porewater salinity increases. This
is significant because interannual changes in mean sea level, which average ?2.9 cm on the South Carolina coast, can
have a great impact on the structure and function of estuaries due to changes in the solute balance of intertidal zone
sediments. Mass balance simulations that used reduced evapotranspiration rates typical of colder climates significantly
reduced the mean and variability of porewater salinity, which suggests that at lower latitudes salinity becomes a more
dominant determinant of biological processes. This should influence a number of processes including primary productivity,
strategies of water conservation and osmoregulation, and community structure. This conclusion is consistent with
published data that show tropical mangroves to have lower photosynthetic rates, and presumably lower gas exchange
rates in general, than mid- and high-latitude salt marsh grasses.

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  • James T. Morris

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