Tidal influence on soil conditions in an intertidal creek-marsh system

To understand better how tides affect the soil conditions of salt marshes, pore-water flow in a modeled 3-D creek-marsh system was simulated under the influence of both monochromatic and spring-neap tides. In analyzing the simulation results, we examined six indices, viz., marsh surface elevation, hydroperiod, shortest distance to creeks, mean soil water saturation, soil saturation index and net water flux across the marsh surface. The results demonstrated that mean soil water saturation, soil saturation index and net water flux exhibited corresponding patterns of spatial variations, which were related to marsh surface elevation, hydroperiod, shortest distance to creeks and tidal regime. While the relationships could be described by a simple function under monochromatic tides, under spring-neap tides it was more complex with different behaviors in the lower and upper marsh areas. The shortest distance to creeks was a critically important factor affecting soil conditions. Well aerated and drained zones co-existed near the creek under both monochromatic and spring-neap tides. Spring-neap tides led to improved soil aeration in the upper marsh, including the interior section (away from the creek) where the hydroperiod and inundation frequency were reduced during neap tides. However, the local flushing condition did not improve correspondingly. The study suggests that single morphological index cannot describe the soil conditions adequately. A proper representation of the soil conditions for plant growth and distribution should take into account not only the aeration condition but also the flushing condition. For that purpose, a composite index is needed to combine the soil water saturation index and the net flux index.